BIOLOGY

Malawian Student

FORM THREE NOTES

Compiled by Samuel W. Mwitana

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Preface

Mwitana’s Biology form 3 Notes is a down to earth copy with an up to date text as dictated by
the Malawian MSCE Biology Syllabus. It has been developed as teaching notes for teachers with
well explained summaries and carefully selected diagrams that enhance quick student’s
understanding. This copy was produced taking on board student’s feedback in the course of my
teaching MSCE Biology since 2014 to 2022 at Bwabwali CDSS and currently Dzumila
Secondary School in Chikwawa. It is therefore user friendly and mostly useful to contemporary
averagely performing learners who find it difficult following usual big texts. This copy is
produced as a first draft to help 2023 candidates as they prepare for their MSCE examinations. If
you know any, please kindly share so that they can also benefit from the same. For feedback,
please contact the writer, Samuel W. Mwitana (Bed, Dip HRM) of Dzumila Secondary School,
Box 170, Nchalo; email mwitanas_10@yahoo.com or samuelmwitana@gmail.com

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Table of contents

Page Topic
4 of 97 Living things and their Environment
17 of 97 Human Interaction with their Environment
24 of 97 Plant Structure and Function
39 of 97 Vertebrates and Invertebrates
50 of 97 Human Digestive System
62 of 97 Human Circulatory System
75 of 97 Reproduction
86 of 97 Genetics

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 LIVING THINGS AND THEIR ENVIRONMENT
Living things comprise of plants and animals that live in an environment. The place of living is
called a habitat. Ecosystem is a natural habitat consisting of all organisms interacting with each
other and with the physical environment around them.

Sampling Methods
When counting organisms we use a sample. This is a small representative group of organisms.
The method of using a sample in counting organisms is known sampling. Sampling is a
technique of obtaining a representative group in a population for study.

Effect of using a large sample and a small sample when estimating organisms
A large sample uses a large field of the study area when making sampling. It therefore covers a
larger part of the population of organisms. This produces a better estimate of the population than
when using a smaller sample in the field.

ECOLOGICAL METHODS USED TO STUDY POPULATIONS

1. Quadrat
A quadrat is a square frame made of wood or metal (usually 1 m2).
This frame is thrown to land randomly at an area in which organism population is being sampled.
To estimate the population the following steps are taken:
a. Choose an area to study e.g. star grass in a school ground.
b. Estimate the size of the selected area in square meters.
c. Throw the quadrat randomly into the selected area and identify and count the number of
organisms being studied.
d. Repeat the process above and record the number of organisms against quadrat throws.
Quadrat throw No. of organisms per quadrat Quadrat throw No. of organisms per quadrat
1st 20 4th 0
nd th
2 2 5 50
rd th
3 100 6 80
The number of times a quadrat is thrown depends on how big the study area is.
e. Calculate the average number of organisms in per quadrat and then the approximate
number of organisms in the whole area;
𝐸𝑠𝑡𝑖𝑚𝑎𝑡𝑒𝑑 𝑝𝑙𝑎𝑛𝑡 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛
𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑙𝑎𝑛𝑡 𝑠𝑝𝑒𝑐𝑖𝑒𝑠 𝑖𝑛 𝑎 𝑞𝑢𝑎𝑑𝑟𝑎𝑡 𝑋 𝑡𝑜𝑡𝑎𝑙 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑒𝑙𝑑
=
𝑎𝑟𝑒𝑎 𝑜𝑓 𝑡ℎ𝑒 𝑞𝑢𝑎𝑑𝑟𝑎𝑡
The quadrat method is most suitable for organisms which do not move like plants.

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Advantages of Quadrat Method
a. A quadrat is easy to make
b. A quadrat can be used to determine percentage distribution of organisms in an area.
c. It is easy to count organisms found in a small area than counting all organisms in the
whole area.

Disadvantages of Quadrat Method

a. Quadrats can only be used to sample small plants and not big trees
b. Quadrats cannot be used to sample moving animals because they can move away from it.
c. Quadrats cannot be used in steep slopes

2. Line transect
Line transect involves the use of a straight line cutting across an area in which the organisms to
be sampled are to be found. Quadrats are then placed at regular intervals which can be 1 – 3
metres apart, along the line. Each quadrat placed is known as a station.

To estimate the population of plant species in a given area, you firstly estimate the size of the
study area. Then you count the number of plants in each station. Afterwards, you sum up the
total plants in all the section and then divide by the number of stations made. This produces an
average number of plant species per station. Then the average number of plants per station is
multiplied to the total area of the study field to get the estimated population of the plants chosen.

For example:
Width of study area: 25 metres
Length of the study area 40 metres
Total area of the field: 25 m × 40 m = 1000 m2
Plant population in stations:
Station 1 2 3 4 5
No. counted 12 10 18 5 5
Average plants per station = 12+10+18+5+5÷5
= 50÷5
= 10 plants per station
Total estimated population for the area will be: 1000 × 10 plants which is 10, 000 plants.

3. Belt transect
This method involves the use of two parallel lines, one metre apart. These lines cut across an
area in which the organisms to be studied are found. They make a belt transect.

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To estimate the population of plant species in a given area, you firstly estimate the size of the
whole study area. You also estimate the area of the belt transect. Then you count the number of
plants in the belt transect. The total population in the belt transect is multiplied by the total study
area and divided by the area of the belt transect.

For example:
Width of study area: 25 metres
Length of the study area 40 metres
Total area of the field: 25 m × 40 m = 1000 m2
Width of the belt transect 1m
Length of the belt transect 60 m
Area of the belt transect 60 m2
Plant population in the belt transect: 240
Total estimated population = 1000 m2 × 340 plants ÷ 60 m2
= 240 000 plans ÷ 60
= 4, 000 plants for the total area

4. Mark – recapture Method

This method is suitable for capturing some small animals like crabs, grasshoppers or fish which
constantly move around in the habitats. Mark-recapture is done as follows:
a. Choose a habitat in which the animal is found.
b. Many animals in the area are captured, counted, marked and then released back into the
area.
The animals are given time to disperse and mix with the remaining population. After some time,
one returns to the habitat and capture as many organisms as possible. The marked and unmarked
animals in the second capture are counted. The following formula is then used:
𝐹𝐶𝑀 𝑋 𝑆𝐶
𝐸𝑠𝑡𝑖𝑚𝑎𝑡𝑒𝑑 𝑛𝑜. 𝑜𝑓 𝑜𝑟𝑔𝑎𝑛𝑖𝑠𝑚𝑠 = , Where
𝑆𝐶𝑀

FCM – Number of animals in the first capture – marked and released.

SC – Total number of animals in second capture (both marked and unmarked)
SCM – Number of marked animals in the second capture.

Advantages of the Mark – Recapture method

a. It is easy to estimate population of moving animals over a given area.
b. It can be used to show distribution of animals in a given area.
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Disadvantages of Mark – Recapture Method
a. The method cannot be used to estimate plant population.
b. It interferes with the environment of the animals.
c. Can cause harm to animals when trapping them.

FOOD CHAIN
A food chain is a sequence used to describe a feeding relationship between producers and
consumers.
Grass Grasshopper Bird Wild dog Mongoose
The arrows in the food chain indicate the flow of energy. There are three main levels that the
energy must pass through a food chain. Those levels are called trophic or nutritional levels.
1. Producers – make their own food by trapping light energy from the sun, e.g. plants.
2. Consumers – depend on producers for their food. Direct consumers are called primary
consumers. Secondary and tertiary consumers depend on primary consumers.
3. Decomposers – saprophytes that act on the dead remains of organisms in all the other
levels.

FOOD WEB
This is a series of interconnected food chains showing feeding relationships between various
species of organisms in a given community. A food web is composed of all possible food chains
in a given ecosystem. A food web is easily made by arranging organisms in their trophic level.

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FOOD PYRAMIDS
A food pyramid is a means of presenting quantity of animals or energy at a trophic level. It
shows transfer of energy through trophic levels as well as pictures how some energy made by
plants or consumed by animals is lost through respiration, excretion and in indigestible material
in faeces. This lost energy does not make it to the next trophic level. There are Pyramids of
numbers and Pyramids of biomass

Pyramid of Numbers
It is a pyramid that shows the number of all organisms at each trophic level of a food chain. A
pyramid of numbers can be upright or inverted depending on the nature of organisms in the
feeding relationship.

An inverted pyramid of numbers is produced where the producer is bigger (like 1 tree) and able
to feed many consumers (like caterpillars or a swarm of grasshoppers).

Pyramid of Biomass
A pyramid of biomass is drawn to show dry
mass or weight of organisms in each trophic
level of a food chain. The weight is found by
drying all the organisms in the trophic level
and weighing their total mass.
A pyramid of biomass will always be upright because
in any ecosystem, the producers transfer some of its
energy to primary consumers, then some of the energy
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is also transferred form primary consumers to secondary consumers and so on. As such, the dry
mass of all the producers is much higher than that of primary consumers for the ecosystem to be
sustained.

Pyramid of energy
A pyramid of energy is drawn to show the amount of energy in the bodies of all organisms in
each trophic level in a food chain. Energy is expressed in joules (j) or kilocalories (Kcal) per unit
area at a specified period.

Pyramids of energy are always upright. This means that

trophic levels at the bottom of the pyramid have more energy
compared to those on top because some energy is lost as it is
being transferred to upper trophic levels.

ENERGY FLOW IN ECOSYSTEMS

In an ecosystem, organisms are mainly interrelated through feeding relationships where they pass
chemical energy in food from one organism to another. The source of that energy is the sun.
Plants are called primary producers because they trap light energy from the sun and use it to
form chemical energy (carbohydrates). Consumers feed on the energy in plants either directly
(herbivores) or indirectly (carnivores). Energy in animals is lost as heat energy in breath, urine
and faeces. When an organism dies, the energy in it is released into the ecosystem through decay.
Energy flows in an ecosystem through two feeding relationships: food chain and food web.

NUTRIENT CYCLES IN AQUATIC AND TERRESTRIAL ECOSYSTEMS

Nitrogen Cycle
Nitrogen is an essential element that is used in the formation of proteins in both plants and
animals. Nitrogen is also found free in nature as gas (N2). However, the nitrogen in the
atmosphere is not absorbed by organisms in its gaseous form. This is because nitrogen gas is
inert or difficult to react chemically and hence a lot of energy is required to break its bonds.
However, some bacteria split the nitrogen molecule so that it combines with other compounds to
be taken up or used by plants. Therefore, bacteria help the nitrogen change between states so it

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can be used. Plants use nitrogen either in the form of ammonium (NH4) or as nitrates (NO3).
They use these forms to make proteins. Animals feed on plants to get proteins.
The Nitrogen cycle is as follows:

Processes in the nitrogen cycle

1. Fixation. Fixation is the first step in the process of making nitrogen usable by plants.
Here bacteria change nitrogen from the air into ammonium. Lighting energy also
contributes to nitrogen fixation into the soil.
2. Nitrification. This is the process by which ammonium gets changed into nitrates by
bacteria. Nitrates are what the plants can then absorb.
3. Absorption. This is how plants gets nitrates from the soil to assimilate them into the cells
of the plants in photosynthesis.
4. Feeding. This is where animals get nitrogen from plants. Animals loose nitrogen into the
soil through decomposition or decay and through excretion.
5. Ammonification. This is part of decaying process. When a plant or animal dies,
decomposers like fungi and bacteria turn the nitrogen back into ammonium so it can
reenter the the nitrogen cycle.
6. Dentrification. Extra nitrogen in the soil gets back into the air. There are special bacteria
that perform this task as well.

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THE CARBON CYCLE
Carbon cycle is the flow of carbon atoms in nature.
Below is the diagram representing the carbon cycle

Carbon atoms are found in the atmosphere in form of carbondioxide and in all living organisms
in form of organic substances such as food.

From the atmosphere, carbondioxide is absorbed by plants and used for making organic food.
When plants are eaten by animals carbon atoms are transferred into animals. When plants and
animals respire or decompose, carbon atoms are released back into the atmosphere in form of
carbondioxide. Plants and animals may also form fossil fuels such as petroleum and coal which
when burnt, they will release carbondioxide into the atmosphere.

THE WATER CYCLE

The water cycle is also known as the hydrological cycle. It describes the movement of water
from the atmosphere as rains to the earth’s surface where it is distributed into water bodies, free
flow, down into the soil and bodies of living things.

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The Hydrological cycle mainly involves the following processes:
a. Evaporation – Formation of water vapour from water bodies and bodies of organisms
into the atmosphere.
b. Condensation – Cooling of water vapour in the atmosphere to form clouds.
c. Precipitation – Falling of condensed water back to the earth’s surface as rain.
d. Infiltration – Sinking of water into the ground. Some excess water which can not sink as
ground water flow on the ground to the rivers and lakes as run off.

Components of an ecosystem (The Physical World)

There are three components: Physical Factors, Plant Communities and Animal Communities.

1. Physical factors (Abiotic Factors)

These are non – living factors that influence the lives of living organisms. They include
soil, rainfall, oxygen concentration, light, atmospheric pressure, humidity, wind speed,
salinity and power of hydrogen (pH).
2. Plant communities
These are populations of different species of plants growing in a given area including all
other organisms that carry out the process of photosynthesis such as green algae,
spirogyra and planktons found in aquatic habitats. Plants use light and simple compounds
such as carbon dioxide, water and mineral salts to make food substances and are therefore
referred to as producers.

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 3. Animal communities
These are populations of different animal species (organisms that cannot carry out the
process of photosynthesis) in a given area. Animal species are heterotrophs in that they
obtain food nutrients from the bodies of other organisms and they are divided into several
groups:
a. Herbivores… they obtain food by feeding directly on plants.
b. Carnivores… they obtain food by feeding on other animals
c. Omnivores… they obtain food from both plants and animals
d. Parasites… they obtain food and shelter from the bodies of other animals (host) and
they cause harm to the host.
e. Saprophytes/decomposers… they utilize the energy stored in the remains and dead
bodies of other organisms. They cause decay e.g. bacteria and fungi.
f. Detrivores… they feed on decomposing plant materials e.g. beetles, earthworms and
cockroaches.
g. Scavengers … carnivores that feed on dead animals killed by others e.g. vultures and
hyena

PHYSICAL FACTORS INFLUENCING LIVING THINGS IN AN ECOSYSTEMS

a. Soil - (Soil is a natural covering found on the earth’s surface) Soil factors are also
known as edaphic factors and influence an ecosystem in several ways
i. Soil is a habitat (where organisms get food and shelter)
ii. Soil is an important component for growth of plants (provides support, water
& nutrients)
iii. Soil determines the amount of food an ecosystem should produce depending
on its fertility
b. Light Intensity – Amount of light in a fresh water ecosystem depends on the depth of
light penetration into the water. Some communities are on shores which receive much
light while others are in deep lakes which receive reduced insolation. In tropical Savanna,
light intensity is moderate to high because it is a region that receives adequate sunshine.
c. Water – In fresh water ecosystems, water is fresh meaning has low salt concentration
and a relatively neutral pH. For example, Lake Malawi water is slightly alkaline. In
tropical Savanna region, water is mostly present during rainy seasons which alternate
with dry seasons every year.

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 d. Temperature – In fresh water lakes, temperatures vary depending on how deep the
waters are. Shallow waters have high temperatures while deep waters have low
temperatures. In tropical ecosystems, temperatures depend on altitude but are generally
moderate.
e. pH – Fresh water ecosystems have neutral pH. In tropical ecosystems, pH varies but most
organisms survive on neutral pH habitats.
f. Humidity – Humidity is high in Fresh water ecosystems since water molecules are
mostly dilute and hence easy to evaporate. In tropical Savanna humid conditions are
mainly found during rainy season.
1. Mineral Salts – In fresh water ecosystems nutrient concentration depends on depth.
Fertile soils are available near the shore due to sediments and deposits. In tropical
Savanna, fertile soils are also available and these support abundant vegetation.
g. Amount of Oxygen – In fresh water ecosystems, Oxygen concentration decreases down
the lake. However, in tropical Savanna Woodland ecosystems oxygen is abundantly
available everywhere.

LIFE FORMS IN AQUATIC AND TERRESTRIAL ECOSYSTEMS

Aquatic ecosystem is established in water bodies and mostly moist environments. The ecosystem
includes animals like earthworms, water insects, fish, crabs, snakes, crocodiles, hippo and others.
Aquatic ecosystems have also plants like planktons, reeds, water hyacinth and water lilies.
Terrestrial ecosystems are established on land. They inhabit all plants and animals that survive
on rain or under desert conditions.

ADAPTATIONS OF PLANTS AND ANIMALS TO VARIOUS ENVIRONMENTS

a. Mesophytes – Mesophytes are plants that grow under average conditions of water supply
and temperature. These plants develop into grass and forests.
(i) Have thin leaves to ensure rapid diffusion of gases from stomata to the cells used
in photosynthesis.
(ii) Have broad and flat leaf blades to provide a large surface area for absorption of
light and carbon dioxide.
(iii) There is mosaic arrangement of leaves for each to receive enough sunlight.

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 (iv) There is presence of stomata on the upper and lower leaf epidermis for efficient
gaseous exchange.
(v) Have mesophyll layer with air spaces (spongy mesophyll) to allow free
circulation of gases.
(vi) Have leaves with chlorophyll to attract light energy for photosynthesis.
(vii) Have thick, transparent cuticles to prevent water loss.
(viii) Have a well-developed root system (long tap root or fibrous root) and root hairs
for absorbing water.
b. Xerophytes – These are plants that grow in dry ecosystems. These plants are found in
deserts.
(i) Have more developed roots to absorb water from the soil.
(ii) Have water storage tissues like succulent (swollen) stems, for example in cacti,
aloe and baobab.
(iii) Have structures that reduce evaporation like, thick waxy cuticle, hairy leaves to
keep damp air, small leaves to reduce the surface area, thorny stems to scare away
insects, and others have few stomata.
(iv) Have life cycles that enable them to resist dry seasons like producing seeds that
remain dormant in dry periods and germinate only when the rains start.
c. Hydrophytes – These are plants that grow in moist conditions
(i) Has very thin or no cuticle at all to enable the plant absorb water, minerals and
carbon dioxide over its whole body.
(ii) Their roots are not well developed because the plants absorb water over their
whole body. The roots are used mainly for anchorage.
(iii) They have many air spaces in the stem and leaf tissue which enables the plants to
float (buoyance) and for gaseous exchange.
(iv) They contain little xylem and support tissue because they have special tissue
called aerenchyma which provides plant support and hence in buoyancy of the
water.
(v) Submerged plant leaves do not have stomata and floating ones have many stomata
on the upper part.
d. Camels – Camels are animals that are found in deserts. They travel long distance without
finding water.
(i) Camels take up a lot of water (about 120 litres) per drinking session. This makes
them survive long distance without drinking water.

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 (ii) Camels store fat in the hump. The fat can be used during drought times to provide
energy and metabolic water.
(iii) Camels’ bodies change temperatures during the day in order to prevent sweating
thereby conserving water.
(iv) Camel’s feet are wide so that they can walk on sand without sinking.
(v) Camels have thick fur to provide warmth during cold desert nights and insulating
against daytime heat.
e. Polar bears – Polar bears live in very cold areas under freezing temperatures.
(i) Polar bears have a thick layer of fat under their skin to insulate from cold
temperatures.
(ii) The skin under fur is black to ensure that the polar bear has a better heat retention
rate.
(iii) They have long and stiff hair between pads of their feet to protect them from cold.
(iv) They have small and rounded ear lobes to prevent water from entering the ears
and freezing eardrums.
f. Goats
(i) Have hooves with soft spongy inner pads to enable them climb high cliffs with
greater speed.
(ii) The hind legs are heavily muscular to assist in jumping greater distances.
(iii) They have a rectangular pupil in their eyes instead of round to enable sharp night
vision.
(iv) They have four chambered stomachs with a large rumen for digestion of fibres in
plants.
g. Shark
(i) Their bodies are pointed on both ends making a streamlined shape for efficient
swimming.
(ii) Their skins are covered with sharp scales (denticles) for protection.
(iii) They have cartilage and not bone inside their body. This makes them much lighter
and flexible for swimming.
(iv) They have several means of sensing prey and danger. These include snouts, ears
and lateral lines which have senses.
(v) Male sharks have modified pelvic fins which transfer sperms to female for
internal fertilization. The eggs that are laid are few but very large.

End of topic on Living things and their Environment – We have to suit in our Environment

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HUMAN INTERACTION WITH THE ENVIRONMENT
Humans interact with their environment in significantly different ways. While almost all
organisms live in the environment without tampering it, humans manipulate their environment to
suit their needs.

Human activities on the environment

Mostly, humans cut trees for several reasons. Some of the reasons for deforestation are clearing
land for settlement, for agriculture and for urban expansion or infrastructural developments.
Humans also apply chemicals like fertilizers and pesticides apart from disposing wastes to the
environment. All these have effects on the environment.
What is the impact of human interaction with the environment?
1. Causes land degradation
2. Causes pollution
3. Contributes to global warming and climate change
4. Makes extinction of various species of plants and animals

LAND DEGRADATION
Land degradation is the temporary or permanent lowering of the productivity of land. Such land
does not have the quality required to sustain plant and animal growth and development.

Human activities that cause land degradation

(i) Deforestation
(ii) Poor farming methods
(iii) Overgrazing
(iv) Quarrying and mining

The above factors lead to soil erosion. The topsoil that is rich in organic matter and nutrients,
capable to hold and provide nourishment to plants is washed away. Without it the land becomes
less fertile and its agricultural productivity reduces. Quarrying and mining activities reduce land
available for agriculture and human habitation.

Reasons for humans engaging in activities leading to land degradation

(i) Poverty – People cut down trees for sale or burn charcoal to earn a living
(ii) Illiteracy – Some people do not understand the impact of wanton cutting down of
trees on their environment and consequently their lives

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 (iii) Overpopulation – Overpopulation results in pressure on land for settlement,
agricultural land and construction materials.
(iv) Attitude – Other people have a negative attitude towards care for the environment.
They think it is the government’s responsibility to care for their land.

POLLUTION
Pollution is the addition of substances to the environment in quantities that are harmful to
organisms and destructive to an ecosystem. The substances that cause pollution are called
pollutants. These pollutants are released into the environment as a result of human activities such
as combustion of fuels, use of pesticides and disposal of domestic sewage and industrial wastes.

Examples of pollutants
Pollutants range from toxic chemicals, noise from factories and vehicles, untreated sewage from
homes, fertilizers from farms and heat from nuclear plants among others.

Types of pollution
There are several types of pollution that include air pollution, water pollution, soil pollution and
sound pollution.

Air Pollution
Air pollution is the addition of waste substances from human activities into the air.
Causes of air pollution
Mostly, air pollution is caused by emission of different gases into the atmosphere which may be
harmful in several ways. Some of the gases include.
a. Carbon dioxide, sulphur dioxide and carbon monoxide which is released as gaseous
wastes from factories. Burning of fossil fuels like oil, petrol and coal also produce carbon
dioxide and carbon monoxide gases.
b. Lead and carbondioxide are also emitted from exhausts of motor vehicles which burn
leaded petrol.
c. Nitrogen oxide and hydrocarbons are also released from motor vehicle exhausts.
In addition to the gases, air pollution can also be caused by soot from burning fossil fuel, smog
due to smoke produced by factories combining with fog and use of aerosols. Aerosols are liquid
substances put in cans under high pressure so as to release them as sprays for example, in
pesticides.

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Effects of air pollution

a. Causes acid rain – acid rain is rain that contains large amounts of oxides of sulphur and
nitrogen from carbon dioxide, sulphur dioxide and nitrogen dioxides from factories.
These gases dissolve in the water droplets to form acidic solutions that fall as acid rain.
Acid rain damages plants, kills fish together with their eggs and also causes the corrosion
of buildings made from limestone.
b. Affects breathing – Polluted air affects respiratory systems of humans and other animals
when inhaled.
c. It causes greenhouse effect – The earth gets its heat from the sun. Most of the heat that
reaches the earth is reflected back to outer space. The atmosphere acts as an insulating
layer, absorbing some of this heat thereby maintaining the temperature of the earth. Some
gases like carbon dioxide and methane are very good at trapping most of this heat. They
are called greenhouse gases because they act as greenhouses. The increased concentration
of these gases in the atmosphere raises the global temperatures by slowing down the loss
of heat from the earth’s surface. This can affect weather patterns around the earth.
d. Depletion of the ozone (O3) layer – The ozone layer is made up of atmospheric gas
called ozone that surrounds the earth. Ozone is a gas that has three oxygen atoms in its
molecule. It is important because it filters out harmful radioactive rays, which are
destructive to life on earth. Air pollution by gases called chlorofluorocarbons (CFCs)
react with and depletes this ozone layer. Chlorofluorocarbons are released from aerosol
(spray) cans, refrigerators and foam plastics. Depletion of ozone may lead to more
harmful ultraviolet radiation penetrating to the earth. This may increase the rate of skin
cancers and eye defects in humans and damage to crops.
e. Causes respiratory diseases such as emphysema and bronchitis – Emphysema is a
condition in which the alveoli walls are weakened and made inelastic. It is caused by long
term irritation of the lungs by cigarette smoke, air pollution or industrial dust. Bronchitis
is caused by cigarette smoke and air pollutants. In bronchitis there is secretion of excess
mucus from the respiratory system as a result of irritation of the respiratory system by the
pollutants. Coughing occurs to get rid of the excess mucus. The second symptom is
breathlessness due to reduced gaseous exchange.

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Control of air pollution
Air pollution can be controlled through different ways that prevents excess release of gases to the
atmosphere. It can also be controlled by biological means like:
1. Planting trees – Trees and other vegetation use up carbon dioxide for photosynthesis
thereby reducing its levels in the atmosphere.
2. Using non-fossil energy such as biogas – They do not emit harmful gases into the
atmosphere.

Water pollution
Water pollution is the addition of harmful substances into water as a result of human activities.
These substances include toxic metals, pesticides and excess fertilizers. Some sources of water
pollution include:
a. Wastes and sewage from homes – The release of organic wastes such as human faeces
and sewage into water bodies like lakes and rivers causes water pollution which results
disturbing the aquatic ecosystem. For instance,
 Fish and other aquatic organisms may die due to lack of oxygen. This is because
microorganisms use up oxygen in water when decomposing the wastes.
 Causes eutrophication. This is the process whereby a water body becomes too rich
in nutrients thereby promoting excessive growth of algae which thus diminish
oxygen and sunlight penetration into the waters for the benefit of other organisms.
As a result, the organisms are unable to carry out photosynthesis and they die.
 Untreated sewage introduces into the water organisms that cause diseases to
humans such as typhoid, cholera and dysentery.
b. Industrial wastes – Industrial wastes for example from breweries, tanneries, textile and
paper industries contain toxic chemicals. These chemicals are harmful to organisms like
fish and other aquatic organisms. The toxic chemicals can also be transferred along the
food chain to other organisms like humans and birds that eat fish.
c. Agricultural practices – Pesticides are chemical compounds that are used to kill pests
that damage crops. Pesticides and excess fertilisers applied to crops may enter into rivers
and lakes through runoff water after the rains. Some of the pesticides contain toxic metals
such as copper. Excess fertilisers containing nitrates in water causes eutrophication which
in turn causes death of aquatic organisms.

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 d. Oil spills – Water gets polluted by oil that spills off from oil tankers in the sea as a result
of accidents. Such oil is toxic to aquatic life and kills many types of bacteria. Water birds
die when they ingest the oil. Oil also floats on water thereby reducing entry of oxygen
into the water.

Control of water pollution

The methods used to control water pollution include:
a. Treatment of sewage before releasing it into water bodies to kill micro-organisms and
other pollutants. Pit latrines should also be constructed away from water sources.
b. Avoiding excessive use of chemicals such as pesticides in pest control. Biological control
methods should instead be adopted.
c. Encouraging use of farmyard manure to replace artificial fertilisers. This would help
reduce eutrophication and hence save aquatic life.

Soil pollution
Soil pollution is the addition of harmful substances into the soil. The main sources of soil
pollution are agricultural chemicals and disposal of solid wastes.

a. Agricultural chemicals
Some pesticides such as copper sulphate which are used to control fungi in fruit orchards are
insoluble. Therefore, they accumulate in the soil and reach levels that kill soil organisms
especially the nitrogen fixing bacteria. When fertilisers are used in the farms frequenty, they
degrade the productivity of agricultural land.

b. Disposal of Solid wastes

There are two main types of solid wastes, biodegradable and non-biodegradable. Biodegradable
wastes such as potato or fruit peelings and cabbage pieces can rot. Non-biodegradable wastes
such as plastic containers, scrap metal among others cannot rot. When biodegradable waste is not
properly disposed of, it rots and attracts flies such as houseflies which spread pathogens. Non-
biodegradable wastes compromise the productivity of the land like reduction in porosity of soil.

Control of soil pollution

a. Using pesticides that break down easily to harmless substances before accumulating in
the soil.

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 b. Controlling pests biologically like use of natural enemies (predators) of pests and crop
rotation to periodically remove the food of pests.
c. Using biodegradable wastes for making manure
d. Recycling and re-use – because non-biodegradable wastes cannot be broken down by
bacteria, they can be managed through reuse and recycling.
(i) Reuse of wastes – This involves using a product or packaging material more than
once in the same system for example, re-usable glass milk bottles.
(ii) Recycling – this is the reuse of material to make similar new products or something
different for example, PVC bottles are recycled into garden furniture or insulation
material.

CLIMATE CHANGE
One of the direct impacts of human interaction on their environment is climate change. Climate
change is experienced mainly in three ways:
(i) Changing in rainfall patterns like having short rainfall periods with heavy outpour
(ii) Changing in global temperatures, i.e. rise in temperatures (global warming)
(iii)Increased wind (storms) in a given area

Causes of climate change

 Increased emission of greenhouse gases into the atmosphere i.e. carbon dioxide from
burning of fossil fuels and methane gas from fermentation of organic substances. The
atmosphere around the earth acts as an insulating layer, absorbing heat. Carbon dioxide
and methane gases are very good at trapping most of the heat, therefore increased
emission of these gases leads to climate change as it raises temperatures globally.
 Clearing of forests (deforestation) to give way land for farming, establishment of urban
centres and mining activities, leads to destruction of vegetation. Plants consume large
proportions of carbon dioxide in the atmosphere for photosynthesis. Their destruction
causes accumulation of carbon dioxide in the atmosphere.

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Impact of climate change
Floods
Increased temperatures have led to increased rain intensity causing heavy rains within a very
short period of time. The rain falls heavily not giving the soil enough time to absorb it. This leads
to speedy runoff forming destructive floods within a short period of time.

Droughts
Climate change has resulted increased temperatures that lead to massive loss of water from the
soil, making crops to fail. Unreliable rainfall patterns have also made it difficult plan crop
growing seasons hence problems in food production.

Stormy winds
Occurrence of strong winds called stormy winds is attributed to climate change. These winds
destroy houses, cause wind erosion and increases evaporation of water from the soil. Destruction
of forests has led to increase of the storm winds since forests act as windbreaks.

Global warming
Carbon dioxide in the atmosphere traps the radiant energy from the sun. This raises the global
temperatures by slowing down the loss of heat from the earth to outer space. This affects weather
patterns around the earth.

Increase of atmospheric temperatures has resulted in melting of ice caps leading to rise in the
levels of seas and oceans. This causes flooding in the low lands. It also affects growth and
development of plant and animal communities.

Ways of mitigating climate change

Climate change is caused by a combination of factors. It cannot be eliminated because it has
taken a long time to develop, but it can be mitigated. To mitigate means to make something less
severe.

Reafforestation
This is the planting of trees in areas that once had trees. Planting fast growing trees on steep
slopes and along riverbanks will preserve the banks of rivers and hold soil on land to minimize
erosion. It will also slow down speed of runoff water hence allow infiltration of water into the
soil.

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Use of renewable energy
Increase use of renewable energy to reduce dependence on non-renewable energy such as the
fossil fuels. This will in turn reduce greenhouse gas emissions.

Use of improved crop varieties

Improved crop varieties can grow and mature under unfavourable rainfall patterns. It is important
to adopt growing of modern varieties of maize that can grow and mature within the prevailing
weather conditions. The use of traditional maize varieties used to old weather patterns is no
longer viable.

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PLANT STRUCTURE AND FUNCTION

Plants are food producers. This means they are able to harness inorganic compounds from its
surrounding into food substances that support all living organisms in an ecosystem. The main
centre for food production in a plant is the leaf.

Transverse Section of a leaf

The leaf is made of a flat and thin blade (lamina) attached to the stem by a leaf stalk (petiole).
The leaf stalk has a main function of exposing the leaf to sunlight by holding it away from the
stem. The leaf blade is used for absorption of sunlight and carbon dioxide. The leaf veins and
the midrib house the vascular bundle consisting of xylem and phloem. As such, they provide a
transport system for substances in a plant. Xylem transports water and mineral salts to the leaf
while phloem transports manufactured food from the leaves to other parts of the plant.

The Internal structure of a leaf

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Functions of the Parts of the leaf
1. Cuticle – This is thin waxy layer at the surface of the leaf. Cuticle is transparent to allow
light to enter into the leaf.
Function: To prevent excess loss of water from the leaf. Since cuticle is a barrier, it also
protects the inner cells of the leaf.
2. Epidermis – Epidermal cells are divided into the upper epidermis and the lower
epidermis.
Function: To protect the inner cells of the leaf
3. Palisade mesophyll
Palisade mesophyll tissue is found below the upper epidermis. They are cylindrical in
shape to increase the surface area for photosynthesis.
Function: Used for photosynthesis (food making in plants). Palisade mesophyll cells
have chloroplasts that enable them to carry out the process of photosynthesis.
4. Spongy mesophyll
Spongy mesophyll is made up of cells which are irregular in shape. Their shapes leave air
spaces in between them to enable carbon dioxide reach all photosynthetic cells.
Function: They contain chloroplasts for photosynthesis.
5. Guard cells
Guard cells are found at the openings of the leaf called Stomata.
Function: They open and close stomata for gas exchange in the leaf. Guard cells also
contain chloroplasts for photosynthesis.
6. Stomata Stomata are pores through the leaf.
Function: They allow carbon dioxide to enter the leaf during photosynthesis. Stomata
also allow removal of oxygen from the leaf during photosynthesis.
7. Veins (Vascular bundle)
Veins contains vascular bundle. Vascular bundle is made up of xylem and phloem.
Vascular tissue runs from the roots, through the stems to the leaves.
Function: Vascular tissue acts as a transport system. Xylem transports water and mineral
salts from the roots to the leaves. Phloem transports manufactured food from the leaves to
all parts of the plant.
8. Chloroplasts
Chloroplasts are oval shaped parts of all cells that manufacture food.

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 Function: Chloroplasts trap light energy from the sun used for photosynthesis.

INTERNAL STRUCTURE OF THE STEM

The internal structure of a dicotyledonous stem is different from the one of the
monocotyledonous stem

The difference between the internal structure of a stem of Dicotyledon and Monocotyledon
The stems differ in that the vascular bundles of a stem of a dicotyledon are arranged in a ring
while the vascular bundles of a stem of a monocotyledon are scattered.

INTERNAL STRUCTURE OF THE ROOT

The root also differs in dicotyledonous and monocotyledonous plant.

The difference between the internal structure of a root of Dicotyledon and Monocotyledon
The roots differ in such a way that in a dicotyledonous root the xylem occupies the centre of the
pith while in a monocotyledonous root the vascular bundles are arranged in a ring.

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ADAPTATIONS OF LEAVES FOR PHOTOSYNTHESIS
Leaves are a main centre of a plant used for photosynthesis. The leaf has various adaptations that
enable it to efficiently make food.
1. The leaf has a broad and flat surface – This increases the surface area for absorption of
sunlight.
2. The leaf has a thin lamina (blade) – This reduces distance for sunlight to reach all
photosynthetic cells. It also enables efficient gas exchange between the leaf and the
atmosphere
3. The leaf has a branching network of veins – Veins contains vascular bundle which
provides an efficient transport of substances in the leaf. Xylem brings water to the leaf for
photosynthesis while phloem prevents accumulation of manufactured food in the leaf by
transporting it to all parts of the plant.
4. The leaf has stomata – Stomata allow diffusion of carbon dioxide into the leaf as a raw
material for photosynthesis. They also allow oxygen to move out of the leaf as a by-
product of food making process. In general, stomata are there for gas exchange.
5. The leaf has airspaces – Air spaces allow gases to circulate within the cell. This enables
carbon dioxide to reach all mesophyll cells for photosynthesis.

Gaseous Exchange in leaves

The leaves have tiny pores on their surface that are called stomata. Mostly, a lot of stomata are
found on the lower part of the leaf as compared to the upper part. Stomata are used for exchange
of gases between the leaf and the atmosphere. Exchange of gases is made possible when there is
a difference between the gas in the leaf and the atmosphere.

During the day, plants make food. As such, they use up carbon dioxide in their leaves and
produce oxygen. This reduces the amount of carbon dioxide in the leaf and increases oxygen in
it. As a result, through diffusion carbon dioxide move from the atmosphere to the leaf and
oxygen move from the leaf to the atmosphere.

At night, plants do not make food. However, the leaf cells continuously produce energy in
respiration. As such, they use up oxygen and produce carbon dioxide. This reduces amount of
oxygen in the leaf and increases carbon dioxide in it. As a result, through diffusion oxygen move

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from the atmosphere to the leaf and carbon dioxide move from the leaf to the atmosphere during
the night.

TRANSPORT IN PLANTS
Transport in plants is a process whereby substances move from one part of a plant to another.

A need for a transport system in plants

Plants make their own food in the process of photosynthesis. This process uses raw materials like
carbondioxide and water. It produces glucose. These substances need to be transported from one
place to another. For example, water and mineral salts are transported from the roots to the
leaves. Glucose also is transported from the leaves through all parts of the plant. Plants therefore
have a system that is used to transport substances within it.

Tissues used for transport in plants

Raw materials and products for photosynthesis are transported by specialized tissues which are
called vascular bundles. Vascular bundles are also called conducting structures. They are made
of the xylem and phloem. Xylem and phloem runs through the roots, stems and leaves of plants.

The Xylem
Xylem is made of dead long hollow cells joined end to end. The vessels run from the roots of the
plant up through the stem. The function of the xylem tubes is to transport water and mineral salts
from the roots to the leaves. Water passes through them easily because they have no cellular
contents that would otherwise cause obstruction. Xylem tube consists of two types of cells
namely tracheids and vessel elements. Vessels usually are shorter and broader than tracheids.

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Tracheids are found in all plants. Apart from the tracheids, flowering plants also have vessel
elements. The walls of xylem tissue have tiny pores known as pits. Water passes from one cell to
another through the pits.

Some characteristics of xylem tubes

a. They are made of dead cells
b. They do not contain cytoplasm
c. Do not have nuclei
d. Their walls are made of cellulose and lignin. The lignin is very strong so the xylem
vessels help to keep plant upright.
Phloem tube
Phloem tube consists of living cells. Its function is to translocate food. Phloem tissue is made up
of sieve elements (sieve tubes) and companion cells. Companion cells have nuclei and many
other organelles responsible for manufacturing and secreting substances into sieve element.

Characteristics of phloem tube

a. They are made of living cells called sieve elements.
b. They have cytoplasm
c. Their walls are not lignified
d. They have ends not completely broken down. As such, they form sieve plates which have
small holes in them.
e. Each sieve tube element has a companion cell next to it.

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Summary of the differences between the xylem and Phloem tissue
 Structural differences
1. Xylem tube has dead cells while phloem tube has living cells.
2. Xylem tube does not contain nucleus while phloem tube contains nucleus in its
companion cells.
3. Xylem tube has lignified walls while phloem tube has no lignified walls.
4. Xylem tube has no cross walls between adjacent cells while phloem has cross walls
perforated into sieve pores.
 Functional differences
1. Xylem tissue conduct water and mineral salts absorbed from the soil while phloem
tissue conduct manufactured food from the plants.
2. xylem tissue help to provide support to the plant

TRANSLOCATION
Translocation is the movement of food substances from leaves to other tissues throughout the
plant. From leaves, manufactured food passes through sieve tubes of phloem through active
transport. Energy from the companion cell is used for translocation of food from leaves.

What experiment can be done to show that phloem tissue translocate food substance?
Movement of manufactured food can be shown to occur in phloem tissue through the ringing
experiment. In this experiment a ring of bark of tree is removed and the tree is observed for
several days. After some time, the region above the ring bulges out (swells) showing excessive
growth of the plant. This occurs because when the bark is removed, phloem tubes are removed
too since they within the bark. Xylem vessels remain intact and are able to conduct water
through to the leaves. However, the manufactured food fails to be translocated down the
removed ring of bark. As such, the food accumulated at the top of the ring and causes excessive
growth.

TRANSPORT SYSTEMS IN PLANTS

In plants, there are three processes that make up the transport system. These are Diffusion,
Osmosis and Active transport.

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Diffusion

Diffusion is the movement of substances from a region of high concentration to a region of low
concentration. Concentration gradient is the different between the concentrations of substances
in different regions. It is also known as diffusion gradient.

Factors that affect the rate of diffusion

1. Size of particles – Small particles move or diffuse faster than large ones.
2. Temperature – Increasing the temperature makes the particles in a solution or gas move
faster.
3. Difference in concentration – The bigger the difference in concentration between two
points the faster the rate of diffusion.
4. Surface area of membranes – For a molecule to get into an organism’s body it has to
pass through a cell membrane. Thin membranes enhance faster diffusion than thick
membranes.
5. Distance a particle has to move – If the distance to be covered between the two regions
of different concentrations is small, diffusion occurs faster.

Significance of diffusion
1. Helps in the movement of manufactured food within the plant.
2. Uptake and removal of oxygen and carbon dioxide by plants.

Osmosis
Osmosis is the movement of water from a region of higher water concentration to a region of
lower water concentration across semi-permeable membrane. A semi-permeable membrane is
the one that allows small particles to pass through it. Larger particles do not pass through it.

Osmotic Potential
Osmotic potential is the pressure that draws water molecules across a semi-permeable membrane
to a region of low water concentration. Solutions that have high concentration of substances like
sugars contain low levels of water molecules. They are called hypertonic solutions. They have
high osmotic potential. If a cell is placed in such a solution, it loses its water by osmosis.

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Solutions that have a lot of water molecules are dilute solutions. They have a low osmotic
potential. They are called hypotonic solutions. A cell placed in such solution gains water by
osmosis. Isotonic solutions have the same osmotic potential. As such they contain same
concentration of water between them.
Significance of osmosis
i. Helps in the absorption of water from the soil into the root.
ii. Helps in turgidity where cells obtain water and become firm. This provided support to
the plant tissues.
iii. Helps in opening and closing of stomata through guard cells which lose and obtain
water by osmosis to open and close the stomata

Active transport
Active transport is the movement of particles from a region of low concentration to a region of
high concentration. Active transport uses a lot of energy to move substances to regions where
they are already many. The energy is produced by the process of respiration.
Examples of processes that involve active transport
a. Phloem translocation
b. Absorption of ions by roots. This is the case because the concentration of mineral
nutrients required by the plant is always greater in the roots than in the soil. Thus ions
move into the root against a concentration gradient, hence requiring s lot of energy.
Nitrates and magnesium ions are readily absorbed by active transport.
Significance of active transport
a. Helps in the movement of manufactured food within plant.
b. Helps in the active uptake of mineral salts
c. Helps in accumulation of substances in storage tissues like tubers, seeds and fruits.

Investigations on osmosis
There are a number of investigations that can be done to demonstrate osmosis. Some of them are
as follows:
Investigation 1
Aim: To investigate osmosis in living tissue (potatoes)
Materials: Fresh potatoes, water, sugar, large beakers or a dish, water, scapel

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Procedure:
1. Choose two fresh potatoes and cut their end so that they can stand. They can be either
irish or sweet potatoes.
2. Peel the sides near the cut ends of the potatoes. Then scoop out the cavity at the other end
3. Stand two potatoes, A and B in a dish (or beaker) containing a little water. Put some
water in the cavity of the potato A and sugar in the cavity of potato B.
4. Leave the set-up for 24 hours as follows:

Results
Potato A No water movement

Potato B Water will go to sugar cavity (osmosis)

Conclusion
In living cells, water moves from an area of high water concentration (or a dilute solution) to an
area of low water concentration (more concentrated solution). In potato A, the level of water
remained the same in the potato and the beaker because there is a balance in osmotic potential. In
beaker B, water moved from the beaker to the cavity of the potato because the cavity is a region
of low water level concentration due to the sugar. The water that moved to the cavity moved
through the living cells. Therefore, osmosis occurs in living cells.

Investigation 2
Aim: To find out the effect of osmosis on living cells
Materials: Irish potato, two beakers, scalpel, distilled water, salt.
Procedure:
1. Using the scalpel peel 2 pieces of potatoes to a strip of 2cm by 2cm wide and 4 cm long
each.
2. In beaker A, put 30ml of 50% salt solution. Leave beaker B with just distilled water (0%
added salt solution)

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 3. In each beaker, put one piece of the potato strip as shown in the diagram below

4. Leave the set-up for 20 minutes and there after observe what will happen to the length and
flexibility of each strip
Results
Potato strip A Potato strip B

Length Reduced length Increased length

Flexibility More flexible (soft) Less flexible/tough/stiff

Discussion
The potato cells in A lost water to the salt solution in the beaker by osmosis. The cells are said to
be flaccid. Flaccidity is the condition in which the cytoplasm of a plant cell shrinks to the centre
of the cell such that the cell membrane tears away from the cell wall. Flaccidity is also called
Plasmolysis. It occurs if the solution in the plant cell is less concentrated than the solution
around the cell. A lot of water will diffuse out of the cell and the vacuole and cytoplasm goes on
shrinking as shown in the diagram below. The shrinking causes the cells to reduce in size and
become more flexible (soft).

The potato strip B gained water through osmosis. This is because the cell sap in the vacuole and
cytoplasm was a more concentrated solution (created by mineral salts and sugars stored in the
potato) as compared to the pure water in the beaker. There is a lot of free water around the cell
than in the cell such that water moves from the beaker into the cytoplasm and vacuole by

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osmosis. As more and more water enters the cytoplasm and the vacuole, they swell. Such a cell
is said to be turgid. Turgidity is a condition when cell is in state of blown up, tight and firm.
That is why the potato strip in B became more firm (hard) and increased in size. The turgidity of
the plant cells helps a plant that has no wood in it to stay upright and keeps the leaves firm.
Turgor pressure is the outward pressure that the cytoplasm exerts on to the cell wall in a plant
cell. However, the plant cell has very strong cell wall so that it prevents the cell from bursting.

Transpiration
Transpiration is the evaporation of water from the plants into the atmosphere mostly through
leaves. The rate of transpiration is the speed at which water in form of vapour is lost per unit
time by a given plant.

Transpiration stream
Transpiration stream is the movement of water within a plant. When water is lost from the
surface of cell wall of spongy mesophyll cells, water is then drawn from the adjacent cells by
osmosis. The adjacent cells continue to draw water from the xylem. This creates a sucking effect
of water from the xylem creating a continuous flow from the xylem through the mesophyll cells
and out of the stomata.

Forces that contribute to transpiration stream

In plants, water is transported to the leaves against the force of gravity. Some trees are very tall
requiring enough force to pull the water up the plant. Some forces that help in movement of
water in plants up against the force of gravity are: Capillarity, cohesion and adhesion, root
pressure and transpiration pull.
1. Capillarity – Capillarity is the tendency of water to rise inside a narrow tube. This is
because water molecules have the ability to cling to the surface of the tube. Water rises
more in tubes with small diameters than in wide tubes. In plants, xylem forms narrow tubes

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 through which water moves because of strong forces of attraction between the water
molecules and the cell walls of the tubes or xylem vessels.
2. Cohesion and adhesion – Cohesion is the tendency of water molecules to get attracted to
each other. Adhesion is tendency of water molecules to get attracted to unlike molecules.
Water rises up in the xylem tissue as it gets attracted to each other (cohesion) or by being
attracted to the walls of xylem vessel (adhesion). Cohesive and adhesive forces prevent the
column of water from breaking.
3. Root pressure – Root pressure is the pressure that holds up the column of water against the
pull of gravity. When the stem of a plant that is well supplied with water is cut off near the
ground level, sap flows from the cut-stem. The Root pressure is due to an increase of
osmotic pressure caused by accumulation of solutes in the xylem of roots. In the root, the
endodermis moves solutes by active transport into the xylem. As a result, the osmotic
pressure of sap in the root xylem increases. This causes water from the soil to diffuse into
the root xylem by osmosis, causing an increase in pressure. This pressure in the root xylem
causes water to move upwards in the xylem of the stem. As mentioned before, root pressure
can only raise water to a height of about one metre.
4. Transpiration pull – Transpiration pull is another force that transports water through the
plant. This process begins in the leaf which sets up conditions that cause water to be drawn
all the way from the roots in a continuous stream. Water evaporates into the air spaces from
the surrounding mesophyll cells in the leaf. This causes water to move by osmosis from the
cells away from the air to the cells next to the atmosphere. This continues up to the water in
the xylem which also moves by osmosis into the mesophyll cells.

Importance of transpiration
Transpiration is important to a plant for the following reasons:
1. Cooling of plants – In hot climates or on hot days, direct sunlight causes the surface of the
plants to heat up. In such situations, transpiration is important because the plant gets cooled
down as the water is evaporating.
2. Distribution of mineral salts throughout the plant – When transpiration occurs, it causes
water to flow through a plant. This is because when water evaporates through the stomata,
more water is drawn from the leaf cells to replace it. As the water flows through the plant,
it carries with it the mineral salts dissolved in it which are distributed throughout the plant.

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 3. Uptake of water – The water lost by transpiration is replaced by water absorbed from the
soil.

Factors affecting the rate of transpiration

Rate of transpiration is the speed at which a plant loses water through transpiration. The factors
affecting transpiration rate include temperature, air movements, humidity and Light intensity.
1. Temperature – High temperatures cause faster evaporation of water from the leaf and
therefore transpiration is very fast. Low temperatures cause little evaporation of water from
the leaf occurs causing transpiration to occur very slowly.
2. Humidity – Humidity is the amount of water vapour in the air. High humidity means the
air is saturated with water vapour. This leads to reduced transpiration as there is very little
space available in the air for water vapour from the leaf to occupy. However, when the air
is dry, that, is humidity is very low, there is plenty of space for water vapour being
transpired from the leaf to occupy. Therefore the rate of transpiration is high.
3. Air movement – Moving air carries with it moisture that has evaporated from the leaf
surface. This prevents the air surrounding the stomates from becoming saturated with water
vapour from the leaf. This creates room for vapour transpiring from the leaf, thus
increasing the rate of transpiration.
4. Light intensity – Light intensity is the strength of light received by the earth’s surface. It
varies in the course of the day depending on the position of the sun. For example, at dawn,
there is very little light, and the light intensity is low. As the sun rises in the sky, the
strength of sunlight increases. Light intensity affects transpiration because it has an effect
on the opening of stomates. The rate of transpiration is high when there is high light
intensity because the stomates open more. When the intensity of light is low, the rate of
transpiration is reduced because stomates open less.
5. Water supply – If water is in short supply, then the plant will close its stomata. This will
cut down the rate of transpiration. Transpiration rate decreases when water supply
decreases below the certain level.

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VERTEBRATES AND INVERTEBRATES
Animals are classified into two broad groups: Vertebrates and Invertebrates. Vertebrates are
animals with a backbone while invertebrates are animals without a backbone.

Groups of invertebrates
Invertebrates are also put in various groups. Some of the groups of invertebrates include insects,
nematodes, annelids, crustaceans, arachnids and molluscs.

Insects
Insects are small invertebrates that mostly live on land. Most insects have two pairs of wings
while other insects like most ants and termites do not have wings. Other examples of insects
include grasshopper, butterfly, cockroach, lice, bees and wasps.

All Insects have the following external features:

1. The body of insects is divided into three parts: Head, thorax and abdomen
2. They have three pairs of legs attached to the thorax
3. They have one pair of antennae
4. They have compound eyes

Nematodes
Nematodes have soft cylindrical bodies which are not segmented. Nematodes are also called
roundworms. Nematodes possess both male and female sexes. When they mate, they exchange
the genitals to allow cross breeding and each nematode can then reproduce. As such they are said
to be bi-sexual because they reproduce both sexually and asexually. Examples of nematodes
include ascaris worms, hookworms and filarial worms. Some nematodes are parasitic. This
means they need a host animal to live in. They can even cause various diseases to the host
animal, e.g. elephantiasis which is caused by Wuchereria bancrofti.

Annelids
Annelids also have cylindrical bodies. However, their bodies are segmented, and are therefore
called segmented worms. Annelids can regenerate if their bodies are broken. Examples of
annelids are earthworms, lugworm and leeches. Most annelids like the earthworm live in moist
soils. They make tunnels making the soil loose. This helps plant roots to grow easily. It also

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improves drainage of water and aeration of the soil. Other annelids, like leeches suck blood from
the foot of humans.

Crustaceans
Crustaceans are invertebrates that have bodies covered with a hard shining coat called
exoskeleton. Crustaceans mostly live in water. They include crabs, prawns, crayfish, woodlouse,
lobsters and shrimps. Crustaceans are mainly aquatic.
Characteristic of crustaceans
1. Their bodies are divided into two parts which are cephalothorax and abdomen. A
cephalothorax is made up of the head and thorax fused together.
2. They have two pairs of antennae.
3. They have many legs, usually ten or more. The first pair of thoracic legs ends in pincers,
which are used to grab and hold food.
4. They have a pair of stalked eyes

Arachnids
Arachnids are also animals which have the body divided into two parts: Cephalothorax and
abdomen. They mostly live on land. Some arachnids like the scorpion and the spider are
carnivorous (feeding on other animals like insects). Others like ticks and mites are ectoparasites.
They live on the body of other organism (host) finding shelter and sucking blood.
Arachnids have the following characteristics
1. The body is divided into two parts: Cephalothorax and abdomen. The head and thorax are
joined into one part to form cephalothorax.
2. They have four pairs of jointed legs attached to the cephalothorax
3. They have no antennae
4. They have simple eyes in clusters

Molluscs
These are animals with soft bodies which are enclosed in a shell for protection. They have a
slimy muscular foot. Molluscs have eyes and have tentacles (feelers). Examples include snails,
slugs, oyster, octopus, catfish and cuttlefish. The octopus has no shell and has eight tentacles.
The snail has four tentacles and two eyes attached on large tentacles. Molluscs live both on moist
land and in water.

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Characteristics of vertebrates
Fish
All fish live in water. Fish show external fertilization. When male and female fish mate, the eggs
meet the sperms in the water. Examples of fish include tilapia (chambo), lungfish, mudfish,
sharks and eel. Fish are ectothermic or cold-blooded animals. This means their body temperature
changes with the change of the temperature of the environment.
Other characteristics of fish
1. Their bodies are covered with scales
2. They have fins that help movement through the water
3. They use gills for gaseous exchange. The gills take oxygen out of the water.

Amphibians
The word amphibian comes from ‘amphi’ which means ‘dual’ or two. They live both on land
and in water. They have mucous glands under their skin to keep the skin moist. Amphibians
breathe using their skin because their lungs are very small. In addition, young amphibians have
gills. Fertilization of eggs in amphibians is external. In here, male amphibians release sperms in
water where a female amphibian laid eggs. The eggs are covered with a jelly to protect them
until they hatch. Examples of amphibians include frogs, toads, newts and salamanders.
Amphibians do not have scales. They are cold blooded animals.

Reptiles
The word reptile comes from the Latin ‘reptilis’ which means to crawl. Reptiles move by
crawling. They live on land, and many of them swim well and feed in water. Their bodies are
covered in hard scales. Reptiles breathe using lungs. They have sexual reproduction where
fertilization takes place inside the body of the female. The female then lays eggs which are often
buried in sand to protect them as the embryo develops inside. Examples of reptiles are snakes,
turtles, tortoises, crocodiles and lizards. Reptiles are cold – blooded animals.

Birds
Birds have bodies covered with feathers. Their legs are covered with scales. Their front limbs are
in the form of wings. They have beaks for feeding. Fertilization in birds is internal after which
the female lays eggs which are incubated. There are many examples of birds which include
chicken, hawks, sparrows, ostirches, parrots, sea gulls, penguins and many others. Birds are

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endothermic or warm-blooded animals. This means they keep their body temperature constantly
warm regardless of the changes in temperature of the surroundings.

Mammals
The word mammals come from the Latin word mammalia which means the mammary glands. In
female it’s the organ that secretes milk. Some examples of mammals include human, cats, dogs,
cows, sheep, goats, whales, polar bears, bats and kangaroos. Mammals have internal fertilization
and the female one keeps the offspring inside the body until birth.
Characteristics of mammals
1. The presence of mammary glands
2. Their bodies are covered with hairs or fur
3. They have external ears (pinna)
4. They have differentiated teeth according to the type of food they feed on.

IDENTIFYING ANIMALS USING DICHOTOMOUS KEY

A dichotomous key is a set of instructions used to identify unknown organisms by using
description of observable features. The key is made by having steps that are numbered. Each step
has two statements describing a single and common characteristic of organisms given.
For example: Study the animals below and use them to construct a dichotomous key.

1. Animal without wings ……………………… see 2

Animal with wings …….…………………… bird
2. Animal with legs ………….…………………see 3
Animal without legs ………………………… fish
3. Animal with body divided in segments …….. see 4
Animal with no body segments …………...… lizard
4. Animal with 2 body segments ………………. spider
Animal with 3 body segments ……………….. ant

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INTERNAL STRUCTURE OF VERTEBRATES
Vertebrates have distinguishable systems inside them. Some of the clear systems that are found
in various vertebrates include the locomotory system, reproductive system, circulatory system
and respiratory system. Some of the unique features of these systems are:
Fish

Locomotory Fish uses fins for movement (swimming in water). Internally, fish also has a
system swim bladder containining gases. This swim bladder enables the fish to change
its density thereby enabling them to move up and down in water.

Circulatory Have the heart, blood vessels and the blood. The heart consists of 4 chambers in
system a roll

Respiratory Fish has gills for gas exchange. Each gill has row of thin projections called
system filaments used as a surface for gas exchange.

Reproductive The female fish produces eggs located above the intestines. The male fish
system produces milt (fish semen) for fertilizing the eggs. The male organs are much
smaller.

Amphibians

Circulatory Have heart, blood and blood vessels used for locomotion. The heart consists of
system three chambers (2 auricles and one ventricle)

Respiratory Adult amphibians respire through the lungs and the skin because they have a
system small surface area in lungs alone for respiration.

Reptiles
Circulatory Have heart, blood and blood vessels. The heart consists of three chambers. In
system here, there is some mixing of oxygenated and deoxygenated blood. Some large
reptiles like the crocodile have a four chambered heart with a septum.

Respiratory Have lungs used as a gas exchange surface. Aquatic reptiles do not breathe
system inside the water and divert blood away from the lungs to conserve energy that
could be lost through pumping blood to the lungs.

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 Mammals
Circulatory Have heart, blood vessels and blood. The heart has two atria and two ventricles
system that are completely separated.

Respiratory Consist of nostrils, trachea, two bronchi, bronchioles and alveolus enclosed in
system lungs. Have diaphragm for efficient respiration

Birds

Locomotory Have thin and hollow bones that makes a bird lighter in air
system

Respiratory Have a trachea, lungs and air sacs. Air sacs are used for storage of air.
system

Circulatory Have a heart with two separate atria and ventricles separating oxygenated from
system deoxygenated blood

LOCOMOTION IN FISH
Structures in fish that are used for locomotion are fins. Fish has various fins as shown below:

Fish also has internal structures that help in locomotion. These structures are the swim bladder
and the strong block muscles.

What happens to fish during locomotion?

Fish move in water through swimming. Swimming involves two things:
(i) Forward movement (propulsion)
(ii) Control of body position in water

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Forward Movement
Forward movement in fish is also known as propulsion. Propulsion is brought about by powerful
muscles of the tail of the fish. The muscles on each side of the body contract alternately. The
contractions pull the body from side to side. This produces wavelike movement which is
transmitted backwards to the tail. The sideways movement of the tail produces a forward thrust
(push) of the fish against the waters. The muscles of the fish are known as myotomes or block
muscles because they are arranged in segments. The water that enters the mouth of the fish also
provides a forward push to the fish as it is expelled through the fish’s gill covers.

Control of body position in water

Moving through the water is difficult. The fish need to control body position during swimming.
Fish use fins for controlling body stability and direction. When the fish is swimming, it meets the
following challenges that affect its stability:
1. Yawing – Yawing is the sideways movement of fish in water. It is controlled by the
median fins (dorsal and ventral).
2. Rolling – Rolling is the tendency of fish to fall on its sides. Fish is kept stable and
upright by the paired fins (pectoral and pelvic) and median fins, which prevent the fish
from rolling.
3. Pitching – Pitching is the tendency of the fish to move its head up and down. Pitching is
controlled by the paired fins which act as hydroplanes causing the fish to swim
downwards or upwards according to the angle to the water at which they are held.

Fins also help the fish to reduce speed (brake) or increase speed. This is controlled by the paired
fins. When they lie flat on the body surface they help maintain streamlined shape of the fish.
This steers the fish further and make it move fast. In order to brake (stop moving forward) the
fish spreads out the pectoral and pelvic fins at 90° to the body creating resistance against the
water. Most fish are also assisted in changing depth by the presence of a swim bladder which,
when full with air allows the fish to rise and when less full allows the fish to sink.

Adaptations of fish for swimming in water

Water is much more difficult to move through. When a fish moves in water, it is usually slowed
down. The slowing down is called drag. Drag is caused by:

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  Water resistance (friction) between fish’s surface and water.
 Turbulence – formation of currents and irregular pattern of flow in the water.
As such, fish has to be structurally adapted for locomotion in water.

Some of the adaptations of fish for locomotion are:

1. Streamlined in shape that help reduce resistance of their body through the water.
2. Presence of scales which overlap backward to reduce drag during locomotion.
3. The body of the fish is smeared with layers of mucus to overcome drag.
4. Fish has swim bladder which helps the fish to have a light weight and less dense than
water (or makes them buoyancy)
5. Fish has fins which help in maintaining stability and propelling the fish forward in water.
6. Fish has flexible vertebral column which allows the fish’s body to curve.

LOCOMOTION IN BIRDS
Feathers are locomotory structures in birds. Feathers are only found in birds. They are extremely
light and yet strong enough to withstand heavy winds. They also insulate the body of the bird.

Types of feathers
There are four types of feathers and these are flight feathers, down feathers, contour feathers and
filopume.
1. Flight feathers – These are feathers that are used for flying. They have a broad and flat
shape. They prevent air from passing through them. They are also called quill feathers.
2. Down feathers – These are feathers close to the layer of the skin of the bird. They are
small and soft forming an insulating layer of the bird.
3. Contour feathers – These are feathers that cover the body and give the bird its shape and
coloration. Their structure a more similar to that of flight feathers.
4. Filoplume feathers – These are hair-like feathers that can be seen after the bird has been
plucked. They are fine with a long shaft performing sensory function when air pressure
changes.

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The structure of a feather

The Quill attaches to muscles. This enables the feather to change the angle. Shaft is used for
attachment of the vane. The vane makes the edges of a feather.

What happens during locomotion in birds?

Locomotion in birds is shown in three ways; walking in Ostrich and Kiwi, swimming in Ducks
and Penguins and flying in most birds like Eagles. When the bird is flying, it remains in air being
carried by the forces of air below it just like a balloon. These forces are called lift. Lift helps the
bird to overcome the force of gravity thereby allowing it to remain afloat in air.

Flight movements in birds

Birds have two main types of flight movements: flapping and gliding (or soaring).

Flapping in birds
During this flight, wings are flapped up and down. This is called an upstroke and a down stroke.
Muscles that hold the wing to the pectoral girdle control flapping. The muscles are a pair of
antagonistic muscles called pectoralis (or flight muscles).
a. A downstroke is the movement when wings are flapped down. During a downstroke
Pectoralis major contracts and pectoralis minor relaxes. This pulls the wing downwards
and the air below produces a force that pushes the bird upwards. The down stroke allows
the birds to gain height.
b. An upstroke is the movement when the wings are flapped up. During an upstroke,
pectoralis minor contracts and the pectoralis major relaxes. This pulls the wing upwads
and the bird downwards. The upstroke is more of a recovery stroke as the wings are
returning to a position ready for another stroke.

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Gliding in birds
In gliding, the wings of a bird are out-spread. Gliding is used when the bird slides down a steep
slope in order to lose height while gaining forward momentum. The bird may also spread out its
wings to allow upwards air currents or wind to lift the bird allowing it to gain height without
moving its wings (soaring). In here, wings are used as aerofoils. An aerofoil is a special
streamlined shape of the bird’s wing. The shape of an aerofoil helps air to move on the upper
part of the wing faster than air that moves on its lower surface. Then, the faster moving air has
lower pressure than slower moving air. As a result, the faster moving air creates a zone of low
pressure while the slow moving air creates a zone of high upward pressure which helps the bird
overcome the force of gravity.

How birds are adapted for locomotion

1. Birds have streamlined body which reduces the effect of drag by the resistance of air.
2. Birds have large and powerful pectoral muscles that provide power to flap wings in
flight. This helps to overcome gravity.
3. Birds contain air sacs attached to the lungs and these make them lighter hence
overcoming gravity.
4. Birds have lighter bones that make them to be lighter in weight as such, they easily
overcome gravity.
5. Birds have flight feathers of a bird provide an aerofoil effect which generates light
weight hence overcoming gravity.
6. Birds have down feathers which provide enough insulation to the bird so that the flight
muscles can work more efficiently.
7. Birds have good eye sight to detect changes in the environment during locomotion.

LOCOMOTION IN MAMMALS
Mammals have four limbs used as their locomotory structures. The limbs are modified for
different locomotory movements like legs for walking, running and jumping. A bat has wings for
flying and whales have flippers for swimming. The above locomotory structures in mammals are
made up bones, muscles and tendons. Ligaments connect bones at joints and this provides
stability during movement. Tendons connect bones to muscles to pull bones when muscles
contract.

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What happens during locomotion in mammals?
During locomotion, mammals create a propulsion force that moves the body forward. This force
is provided by the contraction of muscles of the legs. As the muscles contract, they move the
bone back and forth.
Adaptations of mammals for movement
Mammals are adapted for locomotion in the following ways:
1. They have a rigid skeleton to provide support and a surface for muscle attachment.
2. Mammals have skeletal muscles that appear in pairs on either side of the bone. These are
antagonistic muscles that help to bend and straighten the joints.
3. Mammals have various joints that allow movement
4. Mammals have tendons that are tough used to connect muscles to a bone for movement
5. Mammals have a vertebral column with small bones that allow movements making it
flexible.

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The Human Digestive System
Digestion is the breaking down of large food particles into small particles which can be absorbed
by the body. There are two types of digestion, physical and chemical.

Physical Digestion
Physical digestion involves breaking down of food using mechanical or physical force. It mostly
uses the action of teeth to break the food. Physical digestion is important because it makes food
particles smaller for easy swallowing. It also increases the food’s area where chemical digestive
enzymes can work on
Chemical Digestion
Chemical Digestion is the process where food is broken down by the use of chemicals in the
body which are called enzymes.
Parts of the Human Digestive System

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Digestion in humans takes place in a special system called the digestive system which is also
called the alimentary canal or the gut. The alimentary canal of a human being is composed of
the mouth, gullet, stomach, small intestines, large intestines, rectum and anus. The organs
associated with the alimentary canal are the liver and the pancreas. The associated organs
produce various chemicals that aid in the digestive process.

Functions of parts of the digestive system

The Process of digestion takes place in various centres. Firstly, food is taken into the body
through the mouth in a process called ingestion. The lips, teeth and tongue are used in ingestion.
1. The mouth
In the mouth, both physical and chemical digestions take place. Teeth break down food
physically by the action of mastication. The saliva produced by the salivary gland contains an
enzyme called Ptyalin or Salivary amylase which chemically breaks down starch into maltose.
After chewing, the tongue pushes the food against the soft palate into pharynx. At the pharynx,
food is directed into the oesophagus by epiglottis in the process called Swallowing.
2. Epiglottis
This is a flap of tissue at the back of the mouth to the stomach. Epiglottis closes the entrance of
trachea during swallowing preventing the food from entering the windpipe. The food then is
directed through the oesophagus to the stomach.
3. Oesophagus
The oesophagus is a tube running from the back of the mouth to the stomach. It transports food,
which is now called the bolus, to the stomach in a process called Peristalsis. In peristalsis,
muscles of the oesophagus contract and relax rhythmically thereby pushing the food through the
tube.
4. The stomach
The stomach is a thick muscular bag that stretches as it fills with food. When the food reaches
the stomach, the stomach walls begin wave-like contractions which churn or mix the food into a
semi liquid form called chyme. The stomach walls secrete gastric juice which contains enzymes
like Pepsin which is used to break down proteins to polypeptides. It also contains Renin that
breaks a protein in milk (casein) into caseinogen for pepsin to work on. Gastric juice also
contains hydrochloric acid which destroys germs that enter through the mouth, dissolve bones
swallowed in food and also creates an acidic environment for enzyme pepsin to work on.

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Cardiac Sphincter muscle closes the stomach when food enters it making the food not to
forcibly move back to the oesophagus. The lower part of the stomach is closed by Pyloric
Sphincter muscle to prevent food from moving to the duodenum before time. When ready, the
stomach releases the food slowly by slowly to the duodenum so that the small intestines can
work on the chime easily.

5. Small intestines
The small intestine is about 6–7 m long in a human. It is made up of two parts: The duodenum
and the ileum.
a. The duodenum – The duodenum is the first part. It is short and wider than the ileum. At
the duodenum, pancreatic duct empties pancreatic juice and bile.
 Pancreatic juice is produced by the pancreas. The juice contains an enzyme
pancreatic amylase which digests starch into maltose. It also contains an enzyme
trypsin which digests polypeptides to peptides. The pancreatic juice also contains
sodium bicarbonate (sodium hydrogen carbonate) that neutralises the chyme. This
creates an alkaline environment where enzymes in the small intestines can work
properly. It also neutralise the acidic content of food from the stomach.
 Bile contains salts for emulsification of fats and neutralisation of acidic chyme
from stomach. Emulsification is the physical breaking down of fats into small fat
droplets by bile salts. Bile is made in the liver and stored for a short while in the
gall bladder. It passes through the bile duct which joins the pancreatic duct and
forms a common duct that empties its contents into the duodenum.
b. The ileum – The ileum is the second part of the small intestine. The ileum has two main
functions: The first one is to complete the breaking down of food. The second function
is to act as the site for absorption of digested food into the blood. From duodenum, the
food enters ileum. The walls of the ileum produce a juice called intestinal juice. The juice
contains various enzymes that complete chemical digestion. They include maltase which
digests maltose to glucose, sucrase which digests sucrose to glucose and fructose, lactase
which digests lactose to glucose and galactose, peptidase which digest peptides into
amino acids and lipase which breaks down lipids into fatty acids and glycerol. Once

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 digestion is completed the soluble food is then able to pass through the walls of the small
intestines to the body for use.
6. Large intestines
The remaining substances move to the colon (large intestines) where water and mineral salts are
also absorbed into the body. The colon is about 1.5 metres long. Absorption of water leaves the
undigested material in a semi solid form and it is stored at the rectum, pending egestion. Egestion
is the removal of undigested material through the anus.

Effects of enzymes on food substances

Enzymes break complex food molecules of carbohydrates, proteins and lipids into small soluble
products. Different enzymes catalyse different food types which would otherwise take a long
time to dissolve. Below is a summary of the main enzymes in the human body
No Name of The gland the The juice it Where it Food it End
Enzyme enzyme is produced is found in works acts on product

1. Salivary Salivary gland Saliva Mouth Starch Maltose

Amylase
(Ptyalin)
2. Pepsin Walls of Stomach Gastric Stomach Protein Polypeptide
(Gastric juice) Juice
3. Renin Walls of Stomach Gastric Stomach Caseinogen Casein
(Gastric juice) Juice
4. Pancreatic Pancreas Pancreatic Duodenum Starch Maltose
amylase Juice
5. Trypsin Pancreas Pancreatic Duodenum Polypeptide Peptides
Juice
6. Maltase Walls of the small Intestinal Ileum Maltose Glucose
intestines Juice
7. Lactase Walls of the small Intestinal Ileum Lactose Glucose +
intestines Juice Galactose
8. Sucrase Walls of the small Intestinal Ileum Sucrose Glucose +
intestines Juice Fructose
9. Lipase Walls of the small Intestinal Ileum Lapids Fatty acids
intestines Juice and glycerol
10. Peptidase Walls of the small Intestinal Ileum Peptides Amino acids
intestines Juice

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Properties of Enzymes
1. Enzymes are proteins in nature – All enzymes are proteins in nature since they have
long and folded polypeptide chains.
2. Enzymes work best at a particular temperature – Enzymes are affected by
temperature. Low temperatures makes enzymes inactive while higher temperatures
denature (destroy) the enzymes. The temperature at which an enzyme works best is called
optimum temperature. The optimum temperature for enzymes that work in a human being
is around 37ºC
3. Enzymes work best at a particular pH – Some enzymes work best in acidic conditions
like pepsin (which works in pH of around 2) while other enzymes work best in alkaline
conditions like trypsin (pH of around 8). The pH at which the enzyme works best is
called optimum pH.
4. Enzymes are biological catalysts – A catalyst is a substance that speeds up a chemical
reaction without itself becoming part and parcel of the reaction.
5. Enzymes are specific – This means each kind of enzyme only catalyses one kind of a
substrate. A substrate is a substance (or food) on which an enzyme acts on during
chemical digestion.

Investigations on enzymes

Experiments are on enzymes are conducted in order to have a clear understanding of how they
work in the body of an organism.

Experiment 1
Aim: To investigate the presence of enzyme catalase in living cells/tissue
Procedure: Fresh liver, distilled water, Hydrogen Peroxide (H2O2), growing splint, mortar and
pestle, test tubes
1. Crush fresh liver using mortar and pestle
2. Add small amount of water to the crushed liver
3. Filter the contents into the test tube to collect the extract
4. Add Hydrogen Peroxide and shake the contents
5. Use glowing splint to test for the presence of oxygen.

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Results
The enzymes in liver will break the hydrogen peroxide into hydrogen and oxygen molecules.
Presence of oxygen is observed by the glowing splint that burst into flames.

Conclusion
The glowing splint proves that gas produced is oxygen from the breaking down of hydrogen
peroxide by enzymes in liver cells (catalase). Therefore, living cells contain enzyme catalase.

Experiment 2

Aim: To investigate the effect of temperature on enzyme activity

Materials: Starch solution, ptyalin enzyme (1%), iodine solution, Benedict’s solution, two
beakers, test tubes, thermometer and means of heating.
Procedure:
1. Place 2 cm3 each of starch solution into three different test tubes labelled A, B and C.
2. To each test tube, add 1cm3 of ptyalin enzyme.
3. Immerse test tube A into a beaker of cold water (preferably with ice-cubes)
4. Put test tube B in a water-bath maintained at 37°C.
5. Boil the contents of test tube C, as shown in the diagram below

6. Leave the set up for about 10 minutes and then test the contents from each test tube with
iodine and Benedict’s solutions. Observe and record your results.

Results:
The results of the above investigation can be summarized in the table below:
Iodine solution Benedict’s solution
Test tube A Blue-black No colour change (remains blue)
Test tube B No colour change (remains Brown) Brick-red
Test tube C Blue-black No colour change (remains blue)

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In test tube A and C, the contents showed blue-black colour when tested using iodine solution
meaning starch was available. When using Benedict’s solution, the colour of the solutions did
not change from blue. This means there was no reducing sugar (maltose) in test tubes A and C.
The contents of test tube B remained brown after a test for starch. This means no starch was
found there. However, using Benedict’s solution the contents changed to Brick-red meaning
there was a reducing sugar.
Conclusion:
Test tube B showed absence of starch. Its starch was digested into maltose by salivary amylase
(ptyalin) enzyme. That is why no starch was found there but only maltose. In test tubes A and C,
the contents showed presence of starch. Starch remained intact because in A the enzyme was
inactive due to low temperatures and in C boiling of the contents led to the denaturing of an
enzyme making it not able to work on the starch. Therefore 37º C is an appropriate temperature
for optimum enzyme activity.

Experiment 3
Aim: To investigate effect of pH on enzyme activity
Materials: Test tubes, iodine solution, starch solution, dilute Hydrochloric acid, sodium
hydroxide solution, ptyalin enzyme
1. Put 1cm3 of the starch solution in three test tubes labeled A, B and C
2. Also add 1cm3 ptyalin enzyme solutions to test tubes B and C. Shake for the contents in
each test tube to mix. Test tube A should be left with starch only as a control test
3. Add 3 drops of dilute hydrochloric acid to test tube B and a few drops of sodium
hydroxide to test tube C, as shown below

4. Leave the set-up in a water bath maintained at 37º C for 20 minutes and then test the
contents from each test tube for starch.
Results:
The contents in test tube A showed blue-black colour after applying drops of iodine solution.
This means starch was available. In test tube B, the colour of iodine solution also turned blue-

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black showing presence of starch. In test tube C, iodine solution remained brown as an indication
of absence of starch.

Conclusion:
Test tube A contained starch. It was a control test and no other solution was added. Test tube B
also showed presence of starch. This means the enzyme ptyalin did not catalyze it to maltose.
The enzyme failed to work due to the presence of hydrochloric acid. However, in test tube C, no
starch was available. The enzyme ptyalin digested the starch to maltose. The enzyme managed to
work properly in the presence of an alkaline environment created by the presence of sodium
hydroxide. Therefore, an enzyme works best at a particular pH.

End Products of Digestion

End products are simple soluble substances that are readily absorbed by human body for use.
They are incorporated in all metabolic processes, a process called assimilation. The following
table summarises end products of food digestion
Food nutrient End product
Carbohydrates Starch Glucose (as a main product)
Sucrose (ordinary sugar) Glucose and Fructose
Lactose (milk sugar) Glucose and Galactose
Proteins Amino Acids
Lipids Fatty acids and glycerol

Food absorption in the body

End products of digestion are soluble substances small enough to pass through the walls of the
small intestines into the blood system. All amino acids and monosaccharides are absorbed into
the blood capillaries of the villi through diffusion and active transport. Some fatty acids are
absorbed into the blood capillaries while some are absorbed into the lacteals at the villi. Lacteals
of the villi are also used to absorb some fat soluble vitamins. Some vitamins are absorbed in the
stomach and they then enter into the blood capillaries of the stomach walls. Other vitamins like
vitamin B12 and vitamin K are synthesized in the colon and are absorbed through the walls of the
colon into the blood capillaries.

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Adaptations of small intestines for efficient absorption of food

1. Small intestines are very long. This gives a large surface area on which food is absorbed.
2. Inside walls of small intestines are highly folded. This also gives a large surface area on
which food is absorbed.
3. The inner surface of small intestines has many finger-like projections called villi. This
also creates large surface area for food absorption.
4. The small intestine also has a thin epithelial lining which is one cell thick. This allows
rapid diffusion food from the intestinal space into the blood.
5. Epithelial cells have numerous mitochondria in them to provide enough energy for active
uptake of digested food into the blood.

Structure of Villi
Villi (singular villus) are very tiny fingerlike projections that are found in the small intestines
and structures to absorb digested food material.

Adaptations of a villus for efficient food absorption

1. The villus has a thin epithelial lining which is one cell thick. This allows rapid diffusion
food from the intestinal space into the blood.
2. The villus has a dense network of blood capillaries. This provides rapid transport of
absorbed food substances.
3. The villus has lacteal. Lacteal provide a surface through which large particles of fatty
acids are absorbed and pass through.

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What happens to food after digestion?
After digestion food is absorbed through the villi in the small intestines. Villi contain blood
capillaries that join to form Hepatic Portal Vein that transport all the food substances to the liver.
The liver determines the right amount of the nutrients to circulate throughout the body for use.
Assimilation is the use of products of digestion for in the body. Various food nutrients are used
up by the body cells for various functions as follows:
a. Glucose
(i) Some glucose is used during respiration to provide energy.
(ii) Glucose is also combined with other compounds containing nitrogen to form
proteins in the body
(iii) Excess glucose is converted to glycogen (or fats) and stored
b. Amino acids – Amino acids are reassembled to form proteins needed in the body.
Afterwards some proteins will be used to form enzymes, hormones, or globulin (stored in
the blood)
c. Fatty acids and glycerol – Fatty acids and glycerol are combined to form fats.
(i) Fats are broken down in the absence of carbohydrates to provide energy
(ii) Some fats are stored in the adipose tissue beneath the skin
(iii) Some fats are deposited in membranes of internal body organs like the kidneys to
absorb shock
FUNCTIONS OF THE LIVER IN RELATION TO DIGESTION
The liver is involved a lot in digestion and usage of food substrates in the following ways:
1. The first function of the liver is control of proteins. This is done through the processes
of deamination and transamination.
a. Deamination – Deamination is the removal of amino group from an amino acid
by the liver. Deamination occurs when too proteins are eaten more than the
required amounts in the body. The amino group is changed to urea and sent to the
kidneys. The remaining part of the amino acid is converted to a carbohydrate.
b. Transamination – Transamination is Transamination is a process by which the
liver converts one form of amino acid into another form of amino acid.

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 2. The other function of the liver is control of sugar (glucose). The liver converts excess
glucose in the blood to glycogen for storage. If there is insufficient glucose in the blood,
the liver converts glycogen back to glucose for immediate use.
3. The other function of the liver is to control of lipids – The liver regulates the level of
lipids circulating in the blood. It converts fatty acids and glycerol into fats for storage
under the skin. The liver also converts fatty acids and glycerol into glucose which can be
used for respiration to release energy.
4. The liver is also used in bile production – Bile emulsifies fats and regulates pH in the
intestines for proper action of pancreatic enzymes. Cells in the liver make bile that is
stored in the gall bladder. The green colour of the bile results from a pigment bilirubin
from the breakdown of haemoglobin of worn out red blood cells.
5. The liver acts as a storage organ – The liver stores fat soluble vitamins like A, D, E, K
and B12 together with some mineral elements such as iron (as red blood cells break the
iron from their haemoglobin), copper and potassium until they are required by the body.

Abnormal conditions associated with the Digestive System

The digestive system experience a number of abnormal conditions some of which include
constipation, diarrhoea, nausea, indigestion, ulcers, vomiting and heart burn.
1. The first problem associated with the human digestive system is constipation.
Constipation is a situation where stools become too hard to be expelled from the body. It
is caused by lack of roughages in the diet and not drinking enough water. Constipation
can be controlled by drinking enough water, doing physical exercises and by eating food
with enough roughage to stimulate peristalsis. It can also be controlled by using drugs
such as laxatives.
2. The other abnormal condition of the digestive system is diarrhoea. Diarrhoea is the
condition where an individual passes out watery stools frequently due to an infection of
the alimentary canal. Diarrhoea is transmitted by germs (bacteria and viruses) that
multiply rapidly in unhygienic conditions. It is treated by antibiotics and replacing the
lost fluids through taking Oral Rehydration Salts.
3. The other abnormal condition associated with the digestive system is nausea. Nausea is a
feeling to vomit. The alimentary canal is upset so much that an individual loses appetite

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 and fails to eat. Nausea may lead to vomiting whereby food is forced violently back into
the mouth.
4. Indigestion is another problem and is caused by eating food too quickly and not chewing
it enough. It can be controlled by ensuring that one eats the food slowly, adequately
chewing it before swallowing.
5. Another abnormal condition of the human digestive system is ulcers. An ulcer is an area
of damage to the lining of the stomach, oesophagus or duodenum. It is caused by over
production of acids in the stomach so that the acid corrodes the stomach walls. Ulcers can
be prevented by eating diets with less acids and spices. It can also be avoided by leading
a worry-free life and a life not burdened by too much work.
6. Heart burn – Another problem is heartburn. This is a burning sensation in the
oesophagus. It is caused by acidic stomach when the contents increase and move upwards
into the oesophagus. It can be controlled by taking anti-acid medications.

End of topic

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THE HUMAN CIRCULATORY SYSTEM
The human circulatory system is a system whereby substances are moved round the body. Useful
substances are transported to body cells and metabolic body wastes are removed from the body
cells. Unlike in small organisms like amoeba where direct diffusion of substances is enough,
large organisms need a circulatory system to enable substances reach all respiratory cells. The
main circulatory systems in a human being are the blood system and the lymphatic system.

Functions of the circulatory System

1. Transport of useful substances – Useful substances like oxygen, food substances and
hormones are transported to body cells for use. Oxygen is transported to body cells from
the lungs to be used for respiration. Soluble food substances like glucose and amino acids
are transported from the small intestines after digestion and used by all cells. Hormones
are also transported from endocrine glands to target organs where they work.
2. Transport of body wastes – During respiration, carbon dioxide is produced. Carbon
dioxide is not stored in the body because it is poisonous to the cells. Therefore, it is
transported from all body cells to the lungs where it is removed in exhaled air. Urea is
also transported to the kidneys where it is removed as urine.
3. Distribution of body heat – The circulatory system transports heat from organs which
produce heat such as the liver to all other parts of the body.
4. Defence against infections – The blood transports white blood cells around the body to
sites of infection to help fight against infections.

COMPONENTS OF THE CIRCULATORY SYSTEM

1. The blood
2. Blood vessels
3. The heart

The Blood
Blood is the liquid which transports materials in mammals. It is a liquid tissue that contains
suspended substances as well as dissolved substances. Blood has three major functions:
1. A medium of transport of materials to and from other tissues
2. Regulation of body temperature and of materials in the body
3. Protection against disease causing micro-organisms

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55 percent of blood is composed of a clear watery fluid called the plasma. The other 45 percent
is composed of blood cells and platelets (suspended in the plasma). Some substances dissolved
in blood plasma include soluble food substances, vitamins and mineral ions, waste products,
hormones, enzymes and proteins such as albumin, fibrinogen and globulins. Blood plasma
without fibrinogen is called serum.

Blood Cells
There are two main types of blood cells: Red blood cells and white blood cells.

1. Red blood cells (erythrocytes)

These are very small cells which look red in colour. They are numerous in the blood as compared
to white blood cells. That is why blood looks red.

The Structure of Red blood cells

Red blood cells have a disc biconcave shape looking thinner in the centre and thicker around the
edges. This shape increases the surface area of red blood cells for oxygen transportation.

The cytoplasm of red blood cells contains an iron rich red pigment called haemoglobin.
Haemoglobin has a high affinity for oxygen meaning it attracts oxygen for transportation to the
body tissues. Red blood cells are made in the bone marrow of short bones like the sternum,
vertebrae and ribs. They stay alive for four months and are destroyed in the liver and spleen. Iron
from destroyed cells is either stored in the liver or re-used in the synthesis of new haemoglobin
in new red blood cells or turned into bile pigment (bilirubin) and excreted.

Functions of the red blood cells

1. Transport oxygen from the lungs to the body tissues. The haemoglobin found in red
blood cells combines with oxygen when blood passes through the lungs to form
oxyhaemoglobin. When the blood reaches a region with low oxygen levels like in the
tissues, the oxyhaemoglobin releases the oxygen it was carrying. The body cells take up
the oxygen and haemoglobin become free to be used again to carry more oxygen.
2. Transport antigens. Antigens are proteins that are found on the surface of red blood
cells. Antigens stimulate the production of antibodies in the body.

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Adaptations of Red blood cells for oxygen transport
a. Red blood cells have haemoglobin which has a high affinity for oxygen
b. Red blood cells have a biconcave shape which increases the surface area for oxygen
transport
c. A red blood cell has no nucleus. This creates space for more cytoplasm and therefore
more haemoglobin in the cell.
2. White blood cells (leucocytes)
The white blood cells are larger than red blood cells. They are colourless and are fewer in
number. The number increases during infections but reduces in the case of HIV infection. White
blood cells have a nucleus.

Functions of white blood cells

White blood cells protect the body by fighting causing organisms.

Types of White Blood Cells

There are two main types of white blood cells. These are Phagocytes and Lymphocyte.
a. Phagocytes - Phagocytes are white blood cells that have a lobed nucleus and a cytoplasm
that contains granules. Phagocytes fight disease causing infections by engulfing and
digesting (dissolving) them. They can change their shapes as they seek germs in infected
tissues through capillary walls to reach infected tissues. They are made in the bone
marrow of long bones.
b. Lymphocytes – Lymphocytes have large rounded nucleus. Lymphocytes protect the
body by producing chemical substances called antibodies that kill germs.

Platelets (thrombocytes)
Platelets are fragments (broken remains) formed from the destruction of larger cells. They are
small and have no nucleus.

Functions of platelets
Platelets are involved in blood clotting when an injury occurs on the skin.

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The Process of blood clotting
Blood platelets help the body in blood clotting. A clot is a mesh made on a cut on a blood vessel
to prevent excess loss of blood. A blood clotting also minimizes entry of germs into the body
through the wound.

When an injury to a blood vessel has occurred, platelets and damaged cells on a wound release a
chemical called thromboplastin. Thromboplastin converts a protein found in plasma called
prothrombin into its active form (as an enzyme) called thrombin. Thromboplastin is produced by
the liver using vitamin K. Then the thrombin converts fibrinogen into fibrin. Fibrinogen is a
soluble protein found in the plasma. Fibrin is an insoluble protein that forms long branching
fibres across the wound. Blood cells and platelets get caught in the mesh forming a clot.

BLOOD VESSELS
There are three types of blood vessel: arteries, veins and capillaries.

Arteries
Arteries are blood vessels whose function is to transport blood from the heart to all the other
body parts. Arteries have thick muscular walls to enable them withstand the high pressure of
blood that flows in them. They also have a narrow lumen (inner space) and elastic walls. The
blood pressure is created by the pumping action of the heart.

All arteries carry oxygenated blood except the pulmonary artery which carries deoxygenated
blood. The biggest artery is the aorta. Arteries branch out to form narrower vessels called
arterioles. The arterioles branch further within the tissues into finer vessels called capillaries.

Veins
Veins are blood vessels which transport blood from the body parts (tissues) back to the heart.
They have thin walls and they carry blood under low pressure. Veins also have a large lumen.

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They have valves that allow blood in them to flow in one direction only. Veins are also located
along skeletal muscles to help push the blood as the muscles contract.

Veins carry deoxygenated blood except the pulmonary vein which carries oxygenated blood
from the lungs into the heart. The biggest vein is the vena cava that empties its blood into the
right atrium. Veins are made from fine vessels called venules which also come from capillaries.

Capillaries
These are small vessels that form a network in tissues joining arteries and veins. Capillaries are
made up of very thin walls. These walls enable exchange of substances between the capillaries
and body tissues.

The function of capillaries

They allow oxygen and nutrients move from the blood to the body tissues for use. They also
allow carbon dioxide and wastes to move from the tissues to the blood for excretion.

THE HUMAN HEART

The human heart lies inside the chest cavity between the two lungs. It is surrounded by a tough
membrane called pericardium. Pericardium protects the heart.

The structure of the human heart

The heart is a muscular organ that lies inside the chest cavity between the two lungs. The human
heart is divided into left and right side separated by the muscle known as Septum. Septum
prevents blood on the right from mixing with those on the left.

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Parts of the heart
The heart is an organ that has been made by various chambers, blood passages (vessels) and
valves that enable blood to flow in one direction and not backwards.
Chambers – The heart has four chambers as follows:
1. Right atrium (plural atria) or right auricle – It receives blood from vena-cava and
pumps it to the right ventricle.
2. Right ventricle – It receives blood from the right atrium and pumps it through the
pulmonary artery.
3. Left atrium (auricle) –It receives blood from the pulmonary artery and pumps it to the
left ventricle.
4. Left ventricle – It receives blood from the left auricle and pumps it through the aorta.
Vessels – The heart is directly connected to four blood vessels as follows:
1. Vena Cava – The vena cava is the blood vessel where all the veins from all body parts
empty blood into. The Vena cava pours into the right atrium of the heart.
2. Pulmonary artery – Pulmonary artery takes de-oxygenated blood from the right
ventricle to the lungs. In the lungs carbon dioxide is removed and oxygen is added to
blood. That process is called gas exchange. It ensures that blood carries oxygen again that
can be used in the body for respiration.
3. Pulmonary vein – The pulmonary vein carries oxygenated blood from the lungs back to
the heart through left atrium.
4. Aorta – The aorta carries oxygenated blood from the left ventricle. The blood is pumped
to the rest of the body parts for use in respiration to produce energy.
Valves – Valves are muscle flaps that are used to prevent backward flow of blood. They open for
blood to move in one direction and are pushed to close when blood flows back. The heart has
three types of valves:
1. Tricuspid valve – Tricuspid valve is found at the right side of the heart. It prevents blood
from the right ventricle to flow back to the right atrium.
2. Bicuspid valve – Bicuspid valve is also known as the mitral valve. It is located at the left
side of the heart. It prevents blood from the left ventricle to flow back to the left atrium.
3. Semilunar Valve – Semilunar valves are found along the pulmonary artery and the aorta.
They are used to prevent backward flow of blood to the ventricles of the heart.

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Functions of the heart
The function of the heart is to pump blood to all body parts through the arteries. The heart pumps
blood by continuous contractions and relaxations of it muscles. This ensures that blood transports
useful substances to all tissues and removes waste products of metabolic processes.

Blood circulation in the human heart

Blood flows from the head, arms and the rest of the body into the right atrium through the vena
cava. The blood is deoxygenated since it is coming from body tissues. Deoxygenated blood is
dark-red in colour.

When the right atrium is filled with blood the tricuspid valve opens allowing the blood to move
into the right ventricle. The muscle of the right ventricle contracts and pushes the blood through
pulmonary artery. At this moment, the tricuspid valves are pushed upwards thereby preventing
the blood from flowing back into the right atrium. As the blood moves through the pulmonary
artery, the semi-lunar valves prevent the blood from slipping back to the right ventricle. In the
lungs, the blood gets oxygenated.

The oxygenated blood from the lungs enters the left atrium through pulmonary vein. The blood
pressure forces the bicuspid valve to open allowing blood into left ventricle. The left ventricle
contracts and pushes the blood into the aorta. The contraction of left ventricle creates blood
pressure that pushes the bicuspid valve upwards preventing blood from going back to the left
auricle. Semi-lunar valves in this case prevent the slipping back of blood from the aorta to the
left ventricle.

Blood pressure
Blood pressure is the force per unit area exerted by the blood against the inner walls of blood
vessels due to the action of the heart. Blood pressure is measured in millimeters of mercury
(mmHg). One’s blood pressure is determined by the ratio of the systolic pressure to diastolic
120𝑚𝑚𝐻𝑔 130𝑚𝑚𝐻𝑔
pressure, normally at or .
80𝑚𝑚𝐻𝑔 90𝑚𝑚𝐻𝑔

𝑆𝑦𝑠𝑡𝑜𝑙𝑖𝑐 𝑝𝑟𝑒𝑠𝑠𝑢𝑎𝑟𝑒
𝐵𝑙𝑜𝑜𝑑 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 =
𝐷𝑖𝑎𝑠𝑡𝑜𝑙𝑖𝑐 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒
Systole is the contraction of the ventricle. Systolic pressure is the pressure or pushing force that
is created when ventricles contract forcing blood out of the heart to the blood vessels. Diastole is

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the relaxation of ventricles. Diastolic pressure is the pressure or sucking force that is created
when ventricles have relaxed enabling the blood to move from atria to the ventricles.

Heart beat (pulse rate)

Heart beat is the number of beats of heart per minute. It occurs due to contraction and relaxation
of ventricles. A healthy adult has a resting heart rate of 60 to 80 beats per minute.

Pulse rate
A pulse is the expansion of the artery that is felt when it passes near a bone or skin. A pulse is a
direct measurement of the heart rate because the pulse rate corresponds to the beating action of
the heart as the heart pumps blood through arteries. The pulse is mostly felt and measured on a
wrist. Pulse rates changes with age and during the day due to physical activities.
Effects of physical exercise on the pulse rate
a. Increases the rate of heart beat/pulse rate. During exercises muscles require more
oxygen to respire faster. The heart rate increases so that more blood flows through the
muscle to provide oxygen for aerobic respiration. This increases the speed of blood flow.
b. Increases the breathing rate. This increases the amount of inhaled oxygen and the
amount of exhaled carbon dioxide.

Abnormal Conditions associated with the Circulatory System

There are a number of abnormal conditions associated with the circulatory system. These include
heart attack, Cardiac arrest, High blood pressure, Fainting and Heart failure

1. Heart attack
Heart attack is one of the problems associated with circulatory system. This is a condition in
which heart muscles gets damaged and fail to contract (function) due to insufficient energy. It is
caused when a coronary artery is blocked mostly due to a deposition of a fatty substance called
cholesterol. A clot in the coronary artery may also cause a heart attack. The part the heart which
is served is by the artery is deprived of oxygen and nutrients.

2. Cardiac arrest
Cardiac arrest occurs when the heart suddenly stops pumping blood. The person becomes
unconscious and response is lost. The individual is also seen gasping for air.

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 3. High blood pressure

High blood pressure is a situation whereby blood is forced to move through the arteries with
force. High blood pressure may be caused by the accumulation of yellow fats (cholesterol) inside
arteries. This cholesterol causes narrowing of arteries thereby generating an increased pressure
for blood to pass through. High blood pressure may damage the kidneys heart and arteries.

4. Fainting
Fainting occurs when blood-flow to the brain is reduced. This deprives the brain cells of oxygen
for them to function properly. Fainting is caused by a blood clot that may form in one of the
blood vessels of the brain. It can also be caused by a problem with part of nervous system that
regulates blood pressure. Fainting results in collapsing and may be fatal.

5. Heart failure
Heart failure is a condition in which a heart muscle is unable to pump enough blood inorder to
meet the body’s needs. This results in a person experiencing shortness of breath and easily
fatigued due to inadequate supply of glucose and oxygen to muscles.

General Prevention measures of problems associated with the circulatory system

1. Taking exercises – Exercises increase the flow of blood through the muscles which may
stop the fatty substance from settling down in the inner lining arteries.
2. Keeping the body weight at reasonable level – Keeping the body weight at reasonable
level by avoiding overeating helps to prevent cholesterol from accumulating in the lining
of arteries.
3. Avoiding smoking. Avoiding smoking removes the possibility of blood clotting caused
by nicotine and carbon monoxide in the coronary artery.
4. Learn to be organized to avoid stress.

LYMPHATIC SYSTEM

The lymphatic system is a system which allows circulation of fluids in the body using medium
called lymph. Lymph is made from the remains of tissue fluid which is drained from blood at a
capillary bed.

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A capillary bed

A capillary is an area in our body where arteries, veins and blood capillaries are connected. An
artery branches into smaller arteries called arterioles that branch further into capillaries. On the
other side of the capillary bed, the capillaries join together to form larger blood vessels called
venules that join further to form a larger vessel called a vein. In this arrangement, blood flows
from the arteries, through the capillaries to the veins.

Tissue Fluid
Blood capillaries are made of cells that are not tightly fitted together. They are one-cell thick.
This leads to some fluids being forced out of the vessels to the body cells through the gaps of the
capillaries. This fluid spread throughout the tissue of the body and it is called tissue fluid. Tissue
fluid moves out of the capillaries at the arteriole end of the bed. This is because the blood there
flows with a very high pressure as created by the pumping effect of the heart through the arteries.
The high pressure is also created by the fact that blood moves from the larger lumens (arteries) to
vessels with smaller lumens (arterioles and then capillaries).

Composition of tissue fluid

Tissue fluid filters all the particles that are small enough to pass through the capillaries to the
surrounding body tissue. This fluid is made up of water, glucose, oxygen, fatty acids, glycerols,
amino acids, vitamins, hormones and some white blood cells. Red blood cells and large proteins
like fibrinogen remain in the blood vessels as they are large and cannot pass through the cells of
the blood capillaries. Even though white blood cells are larger than red blood cells, some of them
escape through their amoebic movements to the body cells.

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Functions of tissue fluid
1. Supply oxygen to the cells
2. Supply food nutrients like glucose to the cells
3. To remove waste products of body metabolism like carbon dioxide away from tissue cells
into the blood system for excretion
4. To remove dead cells around the body tissue and pour them into the lymphatic system
which is more permeable than the blood vessels.
Differences between blood and tissue fluid
Blood Tissue Fluid
1. Has red blood cells No red blood cells
2. It is red in colour Colourless
3. Has fibrinogen No fibrinogen
4. Contain more oxygen Contain less oxygen
5. Contain more glucose Contain less glucose
6. Contain less carbon dioxide Contain more carbon dioxide
7. Contain less water Contain more water

Where does the tissue end into?

Some of the tissue fluid move back to blood vessels at the venous end of the capillary bed. At
this point, there is low pressure of blood in the vessels because the pumping effect of the heart is
not felt here. At the same time, the blood vessels start developing large lumens from the
capillaries to the venules and then a vein. Apart from the creation of low pressure at the venous
end than in the tissues, tissue fluid also moves back to the blood vessels because the blood
becomes more concentrated with dissolved nutrients than the tissue fluid at the venous end. At
this point, tissue fluid has lost most of its dissolved substances to the tissue cells. As such, tissue
fluid moves back to the blood vessels through osmosis.

Formation of Lymph
Not all tissue fluid managed to flow back to the blood vessels. Another part of the fluid remains
in the tissues and is forced to drain into small blindly ended capillaries called lymphatic
capillaries which join to form lymphatic vessels. Tissue fluid that drains into the lymphatic
vessels is called the lymph. The lymphatic system is made of the lymph and the lymph vessels.

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Lymph flow
There is no pump to make lymph flow. The following factors enable lymph to flows in the body:
1. Force of Gravity – The lymph in the upper part of the body like the head and arms is
moved through the lymphatic vessels by the means of gravity.
2. Contraction of skeletal muscles – During activities such as breathing and walking,
contraction of skeletal muscle squeezes the lymph vessels, forcing the lymph to flow.
3. Valves – Presence of valves along the lymph vessels enable lymph to accumulate enough
force to move on. The valves also prevent backward flow of lymph.
The system of lymph vessels returns fluids to the blood system. This prevents the fluids from
accumulating in the tissues. Lacteals are specialized lymph capillaries in the villi of the small
intestines. All the lymph vessels empty into two large lymphatic vessels. These vessels are the
right lymph duct which empties lymph into the right subclavian vein in the neck and the thoracic
duct which empties into the left subclavian vein in the neck. The two subclavian veins empty
their blood into the vena cava.

Lymph glands
At certain pints along the lymphatic vessels, there are some glands which are used to filter
lymph. These glands are lymph nodes, the tonsils, the spleen and thymus.

Lymph nodes
The lymph nodes are accumulated at the neck, arm pit, groin and wall of the intestines. Each
lymph node has tiny spaces in which the lymph is filtered before it goes to the bloodstream. The
nodes consist mainly of lymphocytes that produce antibodies. These antibodies kill germs found

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in the lymph before it goes to blood. The lymph nodes also have stationed monocytes that engulf
and digest germs.

Tonsils
Tonsils produce antibodies to fight infection of the nose, ear and throat regions. These are
pharyngeal (at the back of the nose), palatine (at the back of the mouth) and lingual (at the base
of the tongue).

Spleen

It is the largest lymph gland. It is located in the upper left part of the abdominal cavity. It helps in
producing lymphocytes and antibodies. It also collects damaged and old red blood cells, breaking
them down and releasing the haemoglobin in them.

Thymus
It is found in the anterior thorax. This is the most important gland in children to produce
antibodies because their bodies are not protected fully. Thymus wastes away as an individual
grows through atrophy.

Importance of the lymphatic system

1. Provides oxygen to the organs it encloses e.g. lungs.
2. Provides food substances to the organs it encloses.
3. Acts as a lubricant, preventing friction between surfaces.
4. Provides immunological defence by producing lymphocytes that produce antibodies.

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REPRODUCTION
Reproduction is the process by which living things produce new individuals of their species.
Reproduction enables the survival of species on the earth.

STRUCTURE AND FUNCTION OF THE HUMAN REPODUCTIVE SYSTEM

Female reproductive system
The main structures that make up this system are ovaries, oviducts (fallopian tubes), uterus,
cervix and the vagina.

Ovaries
There are two ovaries in a human located one on the left side and the other on the right side of
the uterus. The function of the ovaries is to make female gametes (eggs or ova). Ovaries also
secrete reproductive hormones (oestrogen and progesterone). Upon birth of a girl, her ovaries are
already full of egg cells that start to develop into maturity. During development, each egg cell is
in the form of a graafian follicle (a fluid – filled structure containing one egg cell surrounded by
a few cells known as follicle cells). In each month (28 days) after puberty, one of the cells
completes its development into an ovum

Oviducts (fallopian tubes)

These are tubes which lead from the ovaries to the uterus. Each tube has an open part which lies
next to the ovary but it is not attached to the ovary.

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The function of the oviduct is to provide a medium where the ova can travel from the ovaries
to the uterus. The lining of the oviducts has cilia to transport an egg cell towards the uterus.
Oviduct also aids in the movement of an egg through its peristaltic (rhythmic contractions and
relaxations) movements. Another function of the oviduct is to provide space where
fertilization takes place.

Uterus (womb)
This is a hollow, thick-walled muscular organ which has a space inside it known as the uterine
cavity. The outer layer of the uterus wall has thick muscles that contract strongly during birth.
The inside layer of the uterus wall (called the endometrium) is made up of many blood vessels.
The uterine cavity leads to the cervic canal which extends to form a ring of muscle in the cervix.
The cervix opens into a space of the vagina. A fertilized egg implants itself in the thickened
endometrium, where it develops into the embryo during pregnancy until birth. The uterus
enlarges during this time to occupy a large space in the abdomen and it shrinks rapidly
immediately after child birth.

Cervix
This is the narrow entrance to the uterus from the vagina. It is also sometimes referred to as the
mouth of the uterus. It has a ring of muscle to close it, and also a mucus plug. During pregnancy,
the mucus plug seals the cervix and prevents entry of harmful microorganisms into the uterus.
The ring of muscles remains contracted to keep the baby in the uterus. During birth, the ring of
muscles relaxes to allow the baby to pass through to the world.

The Male Reproductive System

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 1. Testes (testicle)
Produce sperms (male gametes) and male sex hormone testosterone. The testes have very small,
tightly coiled tubules known as seminiferous tubules whose walls have specialized cells to
produce sperms. Testes hang in the Scrotum to be supported and protected. The scrotal sac also
ensures that testes are located at a lower temperature than that of the body for proper production
of sperms. After sperms are produced, they pass through tiny tubes known as Vas efferentia that
direct the sperms from the seminiferous tubes to the epididymis.

2. Epididymis
This is a muscular coiled tube in which the sperms mature. They are temporarily stored here for
some time. A straight muscular tube known as Vas deferens (deferentia) or sperm duct directs
sperms to the urethra.

3. Urethra
This is the tube that directs urine from the bladder as well as sperm from the vas deferentia
out of the male body via the penis at different times.

4. Penis
The penis conveys urine and sperms at different times. It has specialized tissue known as
erectile tissue that has spaces which fill up with blood during sexual excitement causing the
penis to become rigid and erectile.

5. The cowper’s glands

Secretes a clear, sticky slightly alkaline fluid which cleans the urethra before ejaculation by
neutralizing any urine traces available.

6. Prostrate glands
Secrete mucus and a slightly alkaline fluid during ejaculation. This makes the sperm more
active and neutralize the acidity of the vagina.

7. The seminal vesicles

Produce mucus secretion which aids sperm movement. When the mucus combines with the
sperm they are called semen. Semen is made up of sperms, sugars that nourish the sperms
making them more active, mucus as a medium where sperm swims, alkaline substances and
hormones.

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ROLE OF HORMONES IN THE MENSTRUAL CYCLE
Menstruation is the shedding of the lining of the uterus. The lining of the uterus is shed
together with blood. Menstruation occurs when the released ovum fails to be fertilized and the
process is controlled by various hormones. Menstruation cycle involves the development of an
ovum (oogenesis) to release of the mature ovum (Ovulation) ready form implantation and the
shedding of the uterus lining. These activities repeat in each 28 days.

Main Hormones in the Menstrual Cycle

1. Follicle Stimulating Hormone (FSH)
It is produced by the Pituitary gland. It is secreted after the uterus lining has been shed.
Function: Stimulates the development of follicle cells, including Graafian follicle in
ovary.
2. Luteinizing Hormone (LH): Also secreted by the Pituitary gland after being triggered
by oestrogen
Function: Stimulates ovulation (release of mature ovum). Luteinizing hormone also
stimulates the development of corpus luteum. Corpus luteum is a yellow body from the
remains of the graafian follicle.
3. Oestrogen: It is secreted by the graafian follicle (in the ovary)
Function: Causes growth of the lining that was shed in the previous menstruation. It also
triggers the pituitary gland to release luteinizing hormone.
4. Progesterone: Secreted by Corpus Luteum
Function: Maintains the lining of the uterus to be thick ready for implantation and
pregnancy

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COPULATION, FERTILIZATION AND CONCEPTION
Copulation is the process whereby an erect penis is inserted into a female’s vagina moved
back and forth and the release of semen containing sperms. When a man becomes sexually
aroused, blood is pumped into the penis faster than it can return from it. As such, blood fills the
blood spaces in the spongy tissue of the penis. This causes the penis to become stiff (hard) and
erect. The back and forth movements of the penis stimulates sense organs in the penis and this
makes the sperm duct and the epididymis to contract forcing the sperms out.

Copulation two days before or after ovulation normally results into fertilization. Fertilization is
the fusion of the sperm nucleus with the egg nucleus to form a diploid cell called zygote. After
fertilization, the zygote starts to divide mitotically to form a blastocyst and this continues to
divide to form an embryo. The embryo undergoes rapid cell division and is pushed along the
oviduct to the uterus.

After 6 – 9 days of fertilization, the embryo reaches the uterus and become attached in the
endometrial walls of the uterus. This is called implantation. Conception is taken as the whole
process from fertilization to implantation.

THE PLACENTA

The placenta forms at the point where the embryo gets attached or embedded to the uterus of the
mother. It is a product of the process of mitosis where some cells of the embryo become
differentiated and form villi structures. It is made up of tissues and a large number of blood

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vessels (capilary) connected to the umbilical cord to facilitate transport of substances between
the embryo and the mother.

Functions of the Placenta

1. Place for exchange of materials between the mother’s blood and the embryo’s blood, e.g.
glucose, amino acids, oxygen and salts diffuse into the embryo’s blood while wastes like
urea and carbondioxide diffuse into the mother’s blood for excretion.
2. Forms a barrier separating the blood systems of the embryo and the mother. This helps to:
a. Prevennts the mother’s high blood pressure from damaging the embryo’s blood
vessels.
b. Minimises entry of harmful materials like germs into the embryo’s blood
c. Prevents agglutination when the blood groups of the mother and the embryo are
different
3. It also produces enzyme progesterone which prevents menstruation and further ovulation.
The hormone also stimulates further thickening of the lining of the uterus.

ADAPTATIONS OF PLACENTA FOR EXCHANGE OF MATERIALS

1. It is in close contact with a network of blood capillaries. This ensures sufficient supply of
blood which transport materials to be exchanged.
2. It has finger-like projections called villi which is close to the blood spaces in the walls of
the uterus. This increases the surface area of the placenta for diffusion.
3. It is folded to increase the surface area of the placenta which allows a lot of diffusion to
occur.
4. It has thin membrane so that materials diffuse faster and easier.

THE PROCESS OF BIRTH

Birth is the process whereby the foetus comes out of the mother through the vagina to become a
baby. In the last stages of pregnancy, progesterone hormone levels in the mother’s blood drop.
This stimulates the pituitary gland to release another hormone called oxytocin. Oxytocin
stimulates the muscles of the uterine wall to contract. The waves of contractions of these muscles
are called labour pain. They provide force that pushes the foetus from the uterus to the cervix.

The amnion breaks and releases the amniotic fluid. The foetus is pushed downwards through the
cervix and the birth canal widens for all the baby to be born.

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The baby comes out starting with head. After birth, the baby experiences a sudden change of
temperature and this makes it to cry. The crying makes the baby to take its first breath and the
lungs start to function. When completely out, the umbilical cord is cut and the placenta is
eliminated from the mother’s body as afterbirth.

COMPLICATIONS DURING BIRTH

1. Caesarian Section Operation – Happens when it is difficult for the baby to come out
from the uterus to an extent that the doctor decides to remove the baby by cutting open
the wall of the abdomen and uterus.
2. Premature Baby – Happens when a baby is born before 9 months. If the baby is not too
small and weak, it survices after keeping it warm in an incubator where its also given
specific oxygen supply.
3. Spontaneous Abortion (miscarrriage) – Happens when things go wrong or due to the
defects of the embryo at an early stage of pregnancy, and the embryo is expelled from the
uterus.

MULTIPLE BIRTHS
Twins is a condition whereby two children are born at a single birth. (If they are three, it is called
triplets)

Types of Twins
1. Fraternal Twins - These are non – identical twins that are produced when a woman has
released two eggs and both eggs are fertilised by different sperms to form zygotes. These
children may be of the same or different sex
2. Identical Twins – These are produced from one fertilised egg that divides into two by
mitosis and each becomes a baby. The babies are alike and of the same sex because they
are formed from the same egg and sperm. As such, they have exactly the same genes.
3. Conjoined or Siamese Twins – These are produced from one fertilised egg. In the
course of dividing to form identical twins, the zygote fails to completely divide and
separate hence they are joined at some points. These twins usually share some vital
organs like liver, heart or brain. The twins can be separated through operation after birth.

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BREAST FEEDING
Breastfeeding involves provision of feed to young ones using breast milk. Milk production is
called Lactation. The first milk a baby receives is called colostrum. It is a yellow fluid containing
high amounts of proteins, vitamins and antibodies. These help the baby fight infections before
the baby’s immunity system develops.

IMPORTANCE OF BREASTFEEDING
1. Milk contains all the nutrients needed for growth and development of the baby.
2. Breast milk is pure from diseases and fresh. Its contents also change meeting the needs of
the growing baby.
3. Breast milk is cheap and is available any time when needed.
4. Breast milk is easy to digest than bottled milk which may cause constipation.
5. Breast milk has the right temperature to that of the baby.
6. Breast feeding triggers the production of oxytocin hormone which inhibits thickening of
the endometrium and prevent ovulation. This is taken as a birth control measure.

CONTRACEPTIONS
Contraception is the prevention of conception.

IMPORTANCE OF CONTRACEPTIONS
1. Helps in keeping family sizes small thereby controlling human population.
2. Allows the mother to space the birth of children thereby making her regain strength to
take care of the children.

METHODS OF CONTRACEPTIONS
1. CONDOMS. A condom is a thin rubber rolled over an erect penis. Condoms trap sperms
on ejaculation. They have no side effects but may tear or slip off after ejaculation.
2. DIAPHRAGM. A diaphragm is a thin rubber barrier, dome-shaped fitted over the cervix
to prevent entry of sperms into the uterus. It should be used with spermicide to kill the
sperms and fitted by a medical doctor. A diaphragm may cause abdominal pains and it
should be left in place for some hours after sexual intercourse. A diaphragm is not 100%
reliable.

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 3. SPERMICIDES. A spermicide is a chemical applied or fitted as tablets into the vagina
which kills the sperms. Spermicides are very useful when used with a condom or
diaphragm.
4. INTRA – UTERINE DEVICE (IUD). In this method, a plastic coated copper in the
shape of a loop is inserted in the uterus by a doctor for months or years. It prevents
implantation by irritating the lining of the uterus. The method is highly effective.
However, the device may come out and may cause bleeding or discomfort.
5. CONTRACEPTIVE PILL. Contraceptive pills are tablets that are taken by a woman
and they contain synthetic oestrogen and progesterone that prevents ovulation. They are
highly effective but need to be taken as prescribed. They also have side effects like
nausea, weight gain, headaches and increased risk of blood clotting.
6. VASECTOMY. In a vasectomy, a small operation on men is done whereby the sperm
duct is cut and tied to prevent moving of the sperms outside the body. Men will still have
sexual intercourse but ejaculations will consist semen without sperms. Vasectomy is very
effective, has no side effect, but not reversible.
7. TUBAL LIGATION (TL). In tubal ligation, an operation is done on a woman whereby
oviducts are tied on both ends and cut. This prevents the egg from reaching the uterus and
also sperms from fusing with the egg thereby preventing fertilization. Tubal ligation is
highly effective and not reversible.
8. NORPLANT. Norplant is a small capsule containing oestrogen and progesterone
inserted in the muscles of a woman’s arm to prevent ovulation (mainly egg formation). It
is very effective and can stay for up to five years. It can be removed by a medical doctor
when a woman wants to bear children.
9. ABSTINENCE. In this method, sexual intercourse is avoided. It is a very effective
method.

ABNORMAL CONDITIONS ASSOCIATED WITH REPRODUCTION

1. Sexually Transmitted Diseases (STDs). These include AIDS, Gonorrhea and Syphilis
and they affect and destroy reproductive organs.
2. Maternal Mortality. Maternal mortality refers to all deaths of women that are related to
child bearing. Maternal mortality can be caused by:
a. Excessive bleeding during birth

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 b. Infections during pregnancy, like malaria
c. Unsafe abortion
d. Complications during birth.

To prevent maternal mortality, all pregnant women should ensure that they take nutritious diets,
they regularly visit medical doctors and they avoid giving birth at home where qualified medical
personnel are not available.
3. Sterility. Sterility is also known as infertility and it refers to failure for a couple to
achieve pregnancy.

CAUSES OF INFERTILITY
IN MALES IN FEMALES
Absence of Sperms in Semen – it is caused by Blocked fallopian tubes – this is the most
damage of testes and blockage of sperm ducts frequent cause of infertility in females. Fallopian
as a result of infections tubes can be blocked by infection or
inflammation
Low Sperm Count – it is the most frequent Abnormal Ovulation – ovulation is controlled
cause of infertility in males. It is cause by by hormones. If any hormone is not available,
smoking, drugs, stress, hormonal problems and ovulation will not occur regularly or will not
alcohol. occur at all
Structural abnormalities – these impair or Abnormal Cervical canal – sometimes a woman
prevent fertility. Example of a structural is born with an abnormal formed canal that
abnormality is a tangle of swollen veins prevents the passage of sperms, or lead to
surrounding the testes. miscarriages.
Impotence – inability to achieve and maintain Dangerous Chemicals – certain chemicals can
an erection of penis affect hormonal levels. For example, marijuana
can shorten the menstrual cycle, and cigarette
smoking reduces some hormones’ production or
deplete egg supply.

4. Fistula – A fistula is a hole that is formed between two passages in the body. A vaginal
fistula is an example and is formed between the vagina and the rectum or the vagina and

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 the bladder. The vaginal fistula is formed due to prolonged obstructed labour which
causes an injury thereby creating a hole. The hole makes urine and faeces pass
uncontrollably into the vagina. The situation can be treted through surgery.
5. Anaemia – Anaemia is a deficiency in the number of red blood cells or haemoglobin in
the blood. Anaemia is also an abnormal condition associated with reproduction as it
mostly affect women due to the loss of blood through menstruation or childbirth.
6. Abnormal menses – Menses are menstrual periods that women experience after reaching
puberty age. They come with the flow of blood every 28 days and they last for 4 to 7
days. Menses are said to be abnormal if they become unpredictable such that they start
occurring less than two weeks from the day of one period to another. Menses are also
regarded abnormal if the person misses 3 or more periods, has very heavy flow lasting
more than 7 days, or very light flow in between the periods, or the periods stops
completely among other things.
7. Cervical cancer – Cervical cancer is a cancer that starts in the cervix. It is commonly
caused by Human Papilloma Virus which causes the cells that make the muscle of the
cervix to multiply uncontrollably.
Symptoms of Cervical Cancer include:
 Bleeding in between the periods
 Bleeding after sexual intercourse
 Pelvic pain
 Vaginal discharge mixes with blood

Treatment of Cervical Cancer

 Surgery – also known as operation
 Radiotherapy – therapeutic (remedial) use of ionizing radiation
 Chemotherapy – Use of chemical treatment

End of topic on Reproduction (Contraceptive methods at our disposal)

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GENETICS

Genetics refers to the study of inheritance. Inheritance involves heredity and variations in
organisms. Therefore, genetics is the study of heredity and variations. Heredity is the passing on
of characteristics from parent to offspring. Characteristics are also known as traits.

Definition of terms in genetics

Chromosomes – A chromosome is a threadlike structure found in the nucleus of a cell. It
contains a substance with information for a character known as De-oxyribonucleic acid (DNA).
A chromosome has centres that carry actual traits for an organism. Those centres are known as
loci (singular = locus) or genes.

Genes – Genes are structures found on the DNA molecule on the chromosomes, whose function
is to carry the actual information of characters for heredity. Genes control characters and they are
called hereditary messages.

Allele – This is a pair of genes controlling a character. Alleles are written using letters for
various characters. For example, an allele for hair colour may be written as B for black and b for
brown.

PRINCIPLES OF MENDELIAN GENETICS

Gregory Mendel carried out investigations in inheritance of characteristics. He had experiments
made from his flower and peas’ gardens and came up with some observations. With his findings,
he is taken as the father of genetics. Mendel came up with the following observations that are
taken as principles of Mendelian genetics.
1. Characteristics are controlled by a pair of genes called an allele
2. Alleles of the same gene pass into separate cells during gamete formation. During
meiosis, alleles are separated to form various gametes.
3. Alleles of the same gene do not blend. Alleles continue existing independently, having all
the ability to express itself without being mixed with the other.
4. Alleles of the same gene are inherited independently. Inheritance of an allele is done at
fertilization when male and female gametes fuse. At this point either a dominant or a
recessive allele has a chance of being inherited regardless of the existence of the other.

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A genetic Cross
A cross is a genetic diagram that shows inheritance of genes by offspring from their parents.

Genetic terms used to describe crosses

Genotype – Genotype is the gene combination for a character. There are basically three gene
combinations for a character.
a. Both capital letters, e.g. BB.
b. both small letters, e.g. bb
c. a capital letter and a small letter, e.g. Bb

Phenotype – Phenotype is the final appearance of an organism due to an expression of genes.

Final appearances are what is seen as a trait and depends on the genotype. Examples of
phenotypes include brown skin colour, tallness or shortness, rolling the tongue and curly hair.

Homozygous condition – This is a condition in which a pair of similar genes controls a

character. BB and bb are homozygous condition. An individual with a pair of similar genes
controlling a particular character is called homozygote or pure breed.

Heterozygous condition – This is a condition in which a pair of different genes controls a

character. Bb is an example of heterozygous condition. An organism with a pair of different
genes regulating a particular character is called heterozygote.

Dominant gene – Dominant gene is a gene that expresses itself in both homozygous and
heterozygous condition. By expressing itself, a dominant gene gives the final appearance. A
dominant gene is always written in capital letters. The condition BB is said to be homozygous
dominant.

Recessive gene – This is a gene that expresses itself in a homozygous condition only. A
recessive gene is written in small letters. The condition bb is said to be homozygous recessive.

Example of genetic diagrams (a cross)

Make a cross diagram showing inheritance of genes in offspring whose parents are pure breeds,
one being tall and the other being short. Use B as an allele for tallness and t as an allele for
shortness.

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Solution

The offspring that come from the first cross are called first filial generation (F1 generation).
From the above cross, all the members of F1 generation will be heterozygotes with

 Genotype : Tt
 Phenotype : tall
If the F1 generation are selfied (fertilised themselves), a cross of a second filial generation (F2
generation) will be made as follows:

From the above cross, the second filial generation will be different in such a way that 75% will
be tall and 25% will be short. Out of the three tall offspring, one will be a pure breed

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(homozygous dominant) and the rest will be heterozygous. The short offspring will be a
homozygous recessive (also a pure breed).

The above cross can be used to determine the number of offspring with different phenotypes
from a given total. For example, if the parents had a total of 12 offspring, then the total of short
offspring would be 3 and found as follows:

Total ratio = 3+1 = 4

Ratio of Tall offspring = 3:4
Ratio of Short offspring = 1:4
1
Total number of short offspring = × 12 = 3
4

Co-dominance

Co-dominance is the situation where no gene has an effect over the other. As a result all genes
are able to express themselves in the phenotype of an offspring. An example of co-dominance is
seen in the inheritance of blood groups in humans. Blood groups in a human are A, B, AB and O.
These blood groups are determined by genes A, B and O and every individual possesses two
genes for an allele. Gene O is a recessive gene but genes A and B are dominant genes. The
following table summarises the genotypes and the resulting phenotypes (blood groups).

Genotype Condition Blood group

AA Homozygous A

AO Heterozygous A

BB Homozygous B

BO Heterozygous B

OO Homozygous O

AB Heterozygous AB

Genes A and B are both dominant over gene O. However, they are not dominant over each other.
That is why each of them expresses itself in the final appearance. That is why the blood group

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these genes are called AB, with each gene being seen and mentioned. This is a co-dominance
condition.

Incomplete Dominance

Another circumstance where co-dominance is practical is when a recessive gene is not fully
masked by dominant gene in a character of an organism (i.e. partial dominance). Such genes are
said to be additive genes. An example of those genes are what cause sicke-cell anaemia.

Chromosomes

Chromosomes are threadlike structures made of nucleic acids and protein found in the nucleus of
the cells. They are rod shaped usually found in pairs. A chromosome carries the genes that
determine characteristics of an organism inherited from its parents. A human body cell usually
contains 46 chromosomes arranged in 23 pairs. Each pair is said to have homologous
chromosomes. These are identical chromosomes in gene arrangements that the individual
inherited from each of the two parents. The total number of pairs of homologous chromosomes
is called Diploid Number e.g. 23 homologous pairs of chromosomes in human body cells.

Structure of a chromosome

A chromosome consists of DNA in a protein coat and folded tightly into compact structure with
strands called chromatids. The pair of chromatids is joined at a point called centromere. Each
strand has loci (positions) determining various characters in an individual. Each chromatid has a
copy of the same gene in corresponding positions (allele) to fully control a character.

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Sex chromosomes

For an individual to be male or female, it depends on one particular pair of chromosomes on each
somatic cell. This is the 23rd pair of homologous chromosomes called sex chromosomes since
they determine the sex of an individual.

In females, the two sex chromosomes called X chromosomes are of same size as each other.

In males, the two sex chromosomes are of different sizes, one called X chromosome and the
other smaller one is Y chromosome. Therefore, a female genotype is (XX) and the male
genotype is (XY). As such, sex is determined by X and Y chromosomes.

How sex of a baby is determined at fertilization

The cross diagram below will help describe sex determination which is done at fertilization

Each ovum in a female has one of the X chromosomes, so all ova are the same. In male’s testes
some sperms have an X chromosome and others a Y chromosome. If an X sperm fertilizes the
ovum, the zygote will be (XX) and will grow into a girl. If a Y sperm fertilizes an ovum, the
zygote will (XY) and will develop into a boy. There is an equal chance of an X or Y
chromosome fertilizing an ovum, so the chance of having a boy child is 50:50 to that of having a
girl child.

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Sex–Linked Characteristics

The X and Y chromosomes have other genes on them that cause characters to be inherited either
by males or by females only. Sex linkage result from the fact the X chromosome is longer than Y
chromosome. As such the X chromosome has some genes with no corresponding alleles on the Y
chromosome. Examples of sex-linked characteristics are colour blindness and haemophilia.

Colour Blindness

Colour blindness is the inability to detect and differentiate some colours. It is caused by a
recessive gene found on X chromosome which has no allele on Y, chromosome. Let B represents
the allele for normal colour vision; let b represents the allele for colour blindness.

GENOTYPE PHENOTYPE
XB XB Normal female
XB Xb Normal female but a carrier (heterozygote)
Xb Xb Colour blind female
XB Y Normal male
b
X Y Colour blind male

From the table, it takes up to two genes of colour blindness for a female to be colour blind. In
males, however, it is only one gene that can make one blind. Therefore, the chances of colour
blind are high in males than in females.

Haemophilia

Haemophilia is a condition whereby the blood of an individual takes very long to clot or fails to
clot at all. Haemophilia leads to excessive bleeding in case of an injury and children who are
haemophilic die young due to loss of blood. A gene for blood clotting is also found in the X
chromosome and has no corresponding allele on the Y chromosome. The dominant allele of this
gene H allows the blood to clot normally. But the recessive allele, h, causes haemophilia. There
are three possible genotypes that a woman might have for the haemophilia characteristics. There
are only two possible genotypes for a man. This is because the Y chromosome does not have a
haemophilia or blood clotting gene.

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 GENOTYPE PHENOTYPE
XH XH Normal female
XH Xh Normal female but a carrier (heterozygote)
Xh Xh Haemophiliac female
XB Y Normal male
Xb Y Haemophiliac male
In this case, just a single gene in males makes one haemophiliac. In females, they have to be two
genes for them to be haemophiliac. The case in females is however rare because haemophiliac
individuals die while they are young. As such, they cannot grow up and reproduce. Since females
can be carriers, they grow normally and if the trait is given to a male child, the male will be
haemophiliac. Therefore, haemophilia is also a sex-linked characteristic only found in males.

CELL DIVISION
Cell division is a process in which a cell separate to form new cells. The dividing cell is called a
parent cell and new cells formed from cell division are called daughter cells. Cell division helps
in reproduction, ensures growth in an individual and assists in repairing worn out tissues.

Types of cell division

There are two types of cell division. These are mitosis and meiosis.
Mitosis
Mitosis is the process by which an existing body cell (somatic cell) divides into two daughter
cells. Mitosis occurs during growth and in asexual reproduction (binary fission).
Stages of mitosis
There are five stages of mitosis namely: Interphase, Prophase, Anaphase, Metaphase and
Telophase.
1. Interphase – Interphase is the resting stage. As such, chromosomes are not seen in this
stage using the light microscope. The cell generates energy in preparation for cell
division and the DNA replicates itself whereby each chromosome is doubled into two
sister chromosomes.

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2. Prophase – This is the stage and the cell is about to divide. Chromosomes become
shorter and thicker, hence visible where homologous chromosomes appear in pairs.
Spindle fibres form in the cytoplasm. A pair of centrioles appears at each pole for
attachment of spindle fibres which appear as fibres from pole to pole. At late prophase,
the nuclear membrane disappears.

3. Metaphase – This is the stage where Nuclear membrane completely disappears.

Chromosomes line up on the equator (centre) of the spindle. They become attached to the
spindle fibres at the centromere. The spindle fibres start contracting slowly separating
chromatids apart.

4. Anaphase – At the stage of anaphase, chromatids separate and move to opposite poles of
the cell. This is caused by further shortening of spindle fibres making the centromeres to
split.

5. Telophase – Separated chromatids reach opposite poles of the cell and become
chromosomes. Nuclear membranes form on each pole. Splitting is completed to form two

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 daughter cells each having the same kind and number of chromosomes as the parent cell
from which they were formed.

IMPORTANCE OF MITOSIS

 It brings about growth in multicellular organisms.

 Causes an increase in the number of cells in unicellular organisms.

Meiosis

Meiosis is the cell division in which a body cell divides into four daughter cells. Meiosis occurs
in reproductive organs like testes, ovary, anther and stigma giving rise gametes (reproductive
cells – sperms and ova). Meiosis is also called Reduction Division because every daughter cell
produced has half the number of chromosomes of the parent cell. This number of chromosomes
is called Haploid Number. Meiosis takes place in two divisions, whose stages are given the
same names as in mitosis, but are classified as first meiotic division and second meiotic division.

Stages of Meiosis
A. First meiotic division
1. Interphase – Just like in mitosis, the cell is not dividing at interphase. As such,
chromosomes are not visible.

2. Prophase I – In this stage, homologous chromosomes shorten and appear in nucleus.

Homologous chromosomes pair up forming a bivalent. In this pairing, segments of

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 chromosomes or chromatids may break and re-join at some point called chiasmata
where DNA is shared. This mixing of genetic material for different chromosomes is
called crossing over.
3. Metaphase I – During metaphase I, homologous chromosomes (bivalents) line up on
the centre. The nuclear membrane disappears and Centrioles and spindle fibres form.
4. Anaphase I – In this stage, homologous chromosomes separate due to the contraction
of the spindle. One chromosome from the bivalents move to each pole. Then the cell
starts to split.
5. Telophase I – In the final phase of first meiotic division, separated chromosomes
reach opposite poles of the cell. Spindle fibres disappear and splitting of the cell
continues until it completes. Two new cells form, each with half the number of
chromosomes of the original cell. Nuclear membrane-may not reform because
sometimes prophase II or in other cases metaphase II begins soon after Telophase I.

B. SECOND MEIOTIC DIVISION

6. Prophase II – In prophase II, chromosomes appear for each split daughter cell having
clear chromatids.
7. Metaphase II – At this moment, spindle fibres and centrioles reform in each of the
daughter cell. The chromosomes get connected to the fibres at the centromere and
they line up on each equator.
8. Anaphase II – Then the chromatids of the chromosomes in each new cell separate
and start moving towards the opposite poles of each cell. Each new cell starts to split.
9. Telophase II – Finally, splitting of the cells increases. Nuclear membrane forms on
each pole of each cell. Four new cells form, each with haploid number of
chromosomes. The spindle fibres disappear and the daughter cells undergo a
differentiation process to become well recognised sex cells.

Significance of Meiosis

 Leads to formation of haploid gametes or haploid spores.

 Helps to maintain the diploid chromosome number in successive generations.

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Differences between Mitosis and Meiosis

Mitosis Meiosis

Produces genetically identical cells. Produces genetically different cells.

Chromosomes number of parent cell Chromosomes number of are halved in
retained in daughter cells. parent cell daughter cells.
Two daughter cells are produced. Four daughter cells are produced.
Takes place in somatic cells. Takes place in sex cells.

End of Form Three Notes

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