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BIOLOGY
ESSAYS
COMPLETE
FOR
KCSE
M 1-
4 REVISION
STUDY GUIDE

Kanzika K.R
 Form 1: Essays
1. Explain the various ways in which a typical cell is adapted to its
functions

Has a cell membrane; with pores; that regulates substances entering and
leaving the cell;

Cytoplasm; contain sugars and salts; for maintaining its osmotic pressure;
also has a liquid medium; for all biochemical reactions;

Nucleus; contain chromosomes having hereditary material; and controls all

the activities of the cell;

Ribosomes; are sites for protein synthesis;

Golgi bodies/apparatus; for secretion of hormones and enzymes; formation of

lysosomes;

Lysosomes; contain lytic enzymes for breaking down worn-out organelles;

Secretory vesicles; formed from golgi apparatus for secreting substances;

Smooth endoplasmic reticulum; synthesizes and transports lipids;

Rough endoplasmic reticulum; transport proteins;

Nucleolus; controls the activities of the nucleus; produces ribosomes;

Mitochondria; form sites for energy production;

Centrioles; formation of cilia and flagella; forms spindle fibres used in cell
division;

Plant sap vacuoles; store salts and other dissolved substances; controls
osmotic pressure and turgidity of cells;

Food vacuoles; involved in digestion of engulfed food;

Chloroplasts; form sites for photosynthesis in plant cells.

2.Explain how the various specialized cells are modified to carry out
their functions in plants and animals

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 Animal cells:

Sperm cell;

Has acrosome containing lytic enzymes; that digest the egg membranes for
penetration during fertilization;

Has a long tail; containing numerous mitochondria; to generate maximum

energy for propulsion/swimming in the vaginal fluid after ejaculation;

Red blood cells;

Are flattened, circular/spherical biconcave in shape; to increase the surface

area for packaging of haemoglobin;

Has haemoglobin; that combines with respiratory gases; for transport to and
from body tissues;

White blood cells;

Are amoeboid in shape hence able to change shape; to engulf pathogens

through phagocytosis;

Lymphocytes produce antibodies to fight pathogens;

Nerve cell;

Has extensions/dentrites; to receive and send information for

sensatio ;Ciliated epithelial cells;

Have cilia for propulsion of mucus that traps dust and micro-organisms in the
respiratory tract;

Muscle cells;

Elongated, striated and contractile; to bring about movement;

Plant cells:

Guard cells;

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Bean-shaped; to regulate the size of the stomata allowing gaseous
exchange; and control water loss;

Has chloroplasts with chlorophyll; for photosynthesis;

Root hair cell;

Elongated; thin-walled; with dense cytoplasm for absorption of water and

mineral salts;

Epidermal cell;

Thin; for protection of inner tissues from mechanical and micro-organism

attack;

Palisade cell;

Contains numerous chloroplasts with chlorophyll; for photosynthesis;

elongated; to increase surface area for trapping maximum amounts of light
energy;

Meristematic cell;

Thin-walled; with dense cytoplasm; for primary and secondary growth;

3.How are the leaves of higher plants adapted to their functions

Broad and flattened lamina; to increase surface area; for absorption of light;

Thin blade; to reduce distance for diffusion of gases and penetration of light
waves;

Transparent epidermis and cuticle; to allow light to penetrate to tissues;

Cuticle layer absent on stomata; to allow for gaseous exchange;

One-cell thick epidermal layer; to reduce the distance over which sunlight
penetrates;

Palisade cells have numerous chloroplasts containing chlorophyll; to trap

maximum amounts of light energy;

Have stomata on the epidermis; to allow for gaseous exchange; and control
of water loss through transpiration;
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Palisade layer have elongated cells located at right angles to the leaf
surface; for maximum absorption of light energy;

Spongy mesophyll consists of spherical and loosely-packed cells; to create

air spaces; which communicate with the atmosphere through stomata; for
purposes of gaseous exchange and control of water loss;

Veins have conducting tissues: xylem; for movement of water and dissolved
mineral salts; phloem; for translocation of manufactured food;

4.Explain how the various teeth adapt mammals for nutrition

Incisor; sharp; chisel-shaped; for biting; and cutting food; one root for
support in the jaw bone;

Canines; long; sharp; pointed; for holding prey; piercing; and tearing flesh
from prey; single root; for support in the jaw bone;

Premolars; large/wide; to increase surface area for grinding food; highly

cusped; to increase surface area for grinding food; two roots; for firm
support/anchorage in the jaw bone;

Molars; large/wide; to increase surface area for grinding food; highly cusped;
to increase surface area for grinding food;

5.Describe what happens to a meal of oily beans and maize from the
time of ingestion up to the time of absorption

In the mouth;

Starch in maize; is digested by salivary amylase/ptyalin/diastase into

maltose; Food is chewed and mixed by teeth and the tongue; rolled into
boluses by peristalsis; it enters into the stomach via the cardiac sphincter;

In the stomach,
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Gastric juice containing pepsinogen that is activated to pepsin; digests
proteins in the beans; into shorter peptides; food is churned and allowed into
the duodenum; via the pyloric sphincter muscle;

In the duodenum;

Bile juice secreted by the gall bladder; emulsifies the oils in the beans into
tiny oil droplets; pancreatic juice;secreted by the pancrease; contains
pancreatic amylase; that digests starch to maltose; pancreatic lipase; that
digests the oil in the beans to fatty acids and glycerol; trypsin; digests
proteins into shorter peptides; food enters into the ileum; where succus
entericus is secreted; it contains maltase enzyme; that digests the maltose
into glucose; that is absorbed; peptidase; digests peptides into amino acids;
lipase digests the remaining lipids (oil) into fatty acids and glycerol; which is
absorbed through the lacteals of the villi;

6.How are the small intestines in mammals adapted to their

functions?

-Small intestines consists of the duodenum and the ileum; most digestion of
food occurs in the duodenum;-Bile from the gall bladder of the liver is
secreted through the bile ducts; and it is used to emulsify fats/break fat
particles into tiny droplets; to increase the surface area for enzyme action;-
The pancrease secretes pancreatic juice to the duodenum; the juice contains
pancreatic amylase; that helps to breakdown the remaining starch into
maltose; trypsin; (that is secreted in its inactive form, trypsinogen, and
activated by enterokinase enzyme); hydrolyses proteins into shorter
peptides;

-Pancreatic lipase; converts lipids into fatty acids and glycerol; sodium
hydrogen carbonate is also produced; to neutralize the acidic chyme from
the stomach; and provide a suitable alkaline medium for pancreatic and
other intestinal enzymes;

-The ileum is long; and narrow; to increase the surface area for complete
digestion of food; and maximum absorption of digested food; highly-coiled;
to reduce speed of food flow; for maximum digestion; and absorption;

-Presence of villi; and microvilli; to increase surface area; for maximum

absorption;

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-Dense network of capillaries; to transport blood; for efficient transport of
absorbed food;

-Presence of lacteals in the villi; for absorption of fatty acids and glycerol
molecules;

-Presence of enzymes: Lipase; for digestion of lipids into fatty acids and
glycerol; maltase; for digestion of maltose to glucose molecules; peptidase;
for breakdown of peptides into amino acids; sucrase; for digestion of sucrose
into glucose and fructose; lactase; for digestion of lactose into glucose and
galactose;

-Goblet cells; produce mucus; to lubricate the walls of the ileum; for smooth
flow of food; coats the walls of ileum to prevent digestion by peptidase
enzyme;

7.Explain why the following conditions are necessary for

photosynthesis

a. Carbon (IV) Oxide

Required in the dark stage of photosynthesis; it combines with the hydrogen

ion from the light stage; to form glucose, proteins and lipids; low
concentrations reduces the rate of production of energy and food; while high
concentrations leads to an increase in the amount of energy and food
formed;

b. Light

It is used to break down water molecules (through photolysis); into hydrogen

ions, oxygen and energy;the energy and hydrogen ions formed are used in
the dark stage;

c. Chlorophyll

Green pigment that traps light energy from the sun; that is used in
photolysis of water molecules;

d. Suitable temperature and pH

Temperature affects the enzymes involved in photosynthesis;

suitable/optimum temperatures activate enzymes; for maximum production
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of food; while extremely low temperatures inactivate enzymes;leading to
less or no production of food; high temperatures denature enzymes;
stopping the process of photosynthesis; photosynthetic enzymes work well in
low pH;

so the rate is high; while higher pH reduces enzyme activity; lowering the
rate of photosynthesis;

e. Water

Forms a medium for the chemical reactions; it is split to yield hydrogen ions,
oxygen and energy for use in the dark stage; solvent for the materials used
in photosynthesis;

8(a.)What is digestion?

Digestion is (mechanical and chemical) the process by which large complex

food molecules; are broken down into soluble molecules (for absorption
across intestinal wall to bloodstream);

b. What is the importance of a balance diet in human nutrition?

A balanced diet consists of all the food types in their right proportions; it
includes proteins,carbohydrates, minerals, lipids, water, vitamins and
roughage;

Proteins; used for growth and repair of worn out tissues; provide energy
incase of acute shortage of carbohydrates/starvation; bind and transport
specific molecules from one part of the body to another;structural proteins
support tissues in the body e.g. bone and cartilage tissues; act as metabolic
regulators such as enzymes and hormones;

Carbohydrates; used to produce energy/store energy;

Lipids produce energy; form of storage of energy; insulate the body; major
structural components of the cell membrane; when oxidised, it provides
metabolic water;

Vitamins; defense against infections; form coenzymes which activate

enzymes;

Water; a universal solvent; medium for chemical reactions; used as coolant

because of its high specific heat capacity; maintains the shape of cells;
hydrolysis of many substances; transport medium in the body;
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Mineral salts; formation of teeth and bones; formation of hormones (e.g.
thyroxine); formation of blood; maintenance of osmotic pressure of body
fluid; transmission of nerve impulses;

Roughage; makes food to be bulk; promotes peristalsis; and absorption of

water in the large intestines; induces mucus production;

9. Explain the factors that affect enzyme activity

Temperature; enzymes are protein in nature; and hence sensitive to

temperature changes; as temperature increases, enzyme activity also
increases until optimum/maximum; above this optimum the reaction
decreases sharply; due to the destruction of the enzyme structure/become
denatured; making the enzyme ineffective/nonfunctional; most enzymes
have optimum temperature of between 35oC and 40oC; when temperature
decreases, the rate of enzyme reaction decreases as the enzyme becomes
inactivated;

pH/acidity or alkalinity; most enzymes have optimum pH of close to 7/neutral

which is the intracellular pH;however some enzymes work best in an alkaline
medium while others work best in an acidic medium/condition; as the pH
exceeds optimum, the enzyme activity decreases; extreme acidity or
alkalinity denatures most enzymes;

Substrate concentration and enzyme concentration; enzyme reaction

increases with increase in substrate concentration; up to a certain level
where further increase in substrate concentration does not increase the rate
of enzyme reaction; this is because when substrate concentration is
increased, all the active sites of the enzyme are occupied; however, when
the enzyme molecules are increased, there is a proportional increase in the
maximum rate of enzyme action; enzymes are however required in small
amounts hence; they speed up the rate of biochemical reactions without
altering the equilibrium;

Enzyme cofactors/coenzymes; these are non-proteinous substances which

activate the enzymes; most enzymes will not work without them; examples
of cofactors are metallic ions such as iron, magnesium, zinc,copper and also
vitamins as enzyme coenzymes; these substances are required in small
amounts and areused repeatedly/can be recycled;

Enzyme inhibitors; these are substances that inhibit enzyme action by

competing with the normal substrate for the active sites; there are two
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types: competitive and non-competitive; competitive inhibitors have no
permanent effect on the enzyme action; while non-competitive inhibitors
combine permanently with theenzyme molecules thus distorting or blocking
the active sites permanently; examples of these inhibitors include cyanides,
mercury, silver; inhibition can be reduced by reducing the concentration of
the inhibitors; or by increasing the substrate concentration;

Form 2:essays
1.Describe how the mammalian body protects itself against

Pathogenic microbes are found on the skin, respiratory tract, mouth, vagina
and the intestinal tract;

The skin; has a keratinised and waterproof cornified outer layer; that
provides a mechanical barrier to microbes/prevents entry of microbes.

Sebaceous gland; produces sebum; which has antiseptic properties.

The respiratory tract; produce mucus secretions that trap dust.

Cilia sweep/waft/propel the microbes to the pharynx for swallowing or to be

coughed out;

Reflex actions of coughing/sneezing/vomiting help remove foreign materials

from the respiratory tract/digestive tract;

Lysozymes/enzymes in saliva/nasal secretions/tears; digest walls of bacteria

destroying them.

Gastric secretions such as hydrochloric acid lowers the pH in the stomach

killing micro-organisms;
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Clotting of blood; prevents entry of microbes after damage of blood vessels.

Phagocytosis; by phagocytes engulf and destroy microbes and other foreign

bodies.

Lymphocytes are stimulated to produce antibodies; by proteins present in

microbes protecting the body;

Antibodies destroy/kill micro-organisms through various ways:

Agglutinins; bind to pathogens making them clump together; killing them;

Lysins; bind to pathogens and make them burst or disintegrate;

Opsonins; bind to pathogens making them easily recognized hence be

engulfed/destroyed by other lymphocytes;

Anti-toxins; bind and neutralize toxins produced by micro-organisms;

Vagina is acidic; hence making it not conducive for growth and reproduction
of micro-organisms;

2.Outline and explain the various homeostatic functions of the liver

in mammals

Deamination

Process of removal of an amino group from an amino acid molecule; the

process gets rid of excess amino acids in the body; as the body is not able to
store them; the amino group enters the ornithine cycle; where it is combined
with carbon (IV) oxide to form urea; which is excreted in urine through the
kidney;

Heat production;

Many metabolic activities take place in the liver; releasing heat energy; that
is distributed by the blood to other parts of the body; this helps in
thermoregulation; Storage of vitamins and mineral salts; Vitamins A, B, D, E
and K; are stored in the liver; worn-out red blood cells, are broken down to
yield iron; which is stored in the liver in form of ferritin; this is used later in
case of shortage;

Formation of red blood cells;

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Occurs in the liver of the foetus; the liver also breaks down old/exhausted
red blood cells; leading to formation of more in the bone marrow to replace
the worn-out cells; to enhance oxygen and carbon (IV) oxide distribution;

Regulation of blood sugar level;

Liver cells convert excess glucose into glycogen and fats under the influence
of insulin hormone; the stored glycogen is however converted back to
glucose; when glucose levels are low; by the liver cells; under the influence
of glucagon hormone;

Regulation of plasma proteins;

Plasma proteins such as prothrombin and fibrinogen are manufactured in the

liver using the amino acids found in the liver; they play a major role in blood
clotting; that prevents excessive blood loss and infection at the injured area;
other plasma proteins produced by the liver such as serum and albumen;
contribute to the

maintenance of osmotic pressure in the body; non-essential amino acids are

also synthesized by the liver; for use by the body;

Storage of blood;

The liver is highly vascularised; hence it is capable of holding a large volume

of blood when the blood vessels dilate during hot conditions; when the
temperatures are low, the blood vessels constrict under the influence of the
endocrine and nervous systems; hence less blood is stored in the liver; this
contributes to thermoregulation;

Detoxification;

This is the process where harmful compounds such as drugs and poisons; are
converted to less toxic compounds in the liver; toxicity is caused by
medication, drugs and microorganisms; the toxic compounds are later
excreted in urine; detoxification prevents the accumulation of toxins in body
cells; which could lead to death or malfunctioning of the body cells;

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3 a. What is homeostasis?

Mechanisms of control and maintenance of a constant internal environment

regardless of the external conditions;

b. Name any three factors that must be maintained constant in

mammalian bodies

Temperature; Water; Salt or ion content; Carbon (IV) oxide; Glucose; amino
acids;

c. Explain how endotherms respond to heat and cold conditions in

their environment

Heat/hot conditions:

Increased sweating; to lose heat through latent heat of vaporization;

Dilation of arterioles under the skin; to bring more blood to the skin surface
to lose heat to the atmosphere;

Decreased body metabolism; to reduce heat generation;

Erector pili muscles relax; making hair follicles to relax hence hair lies flat on
skin, no air is trapped; to lose heat;

Slow/reduced muscular activity due to slow metabolism; to reduce heat

production;

Panting to expose tongue and mouth; to release heat;

Moving to shades to avoid direct heat;

Aestivation; to escape the extreme heat; flapping of ears to create currents

to carry away heat;

Cold conditions:

Stamping of feet; to generate heat;

Basking in the sun to gain heat directly;

Less production of sweat; to reduce water loss through latent heat of

vaporization;

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Vasoconstriction of arterioles; hence less blood flow to the skin surface to
reduce heat loss;

increased metabolism through release of more thyroxine hormone; to

generate heat;

Erector pili muscles contract; pulling hair follicles hence hair is raised; to trap
a layer of moist air; to prevent heat loss;

Shivering/rapid contraction of muscles; to yield heat to warm body;

4. Describe the route taken by water from the soil up to the

evaporating surface of a plant

Water is drawn into the root hair cells by osmosis; due to the presence of
dissolved substances in the cell sap of root hairs, the concentration of cell
sap is greater than that of the surrounding solution in the soil/concentration
gradient; this exerts a higher osmotic pressure, thus drawing the water
molecules across the cell wall and cell membrane into the root hair cells;

More water drawn into the root hair cells dilutes the cell sap; making it less
concentrated than that in the adjacent cortex cell of the root;

Due to osmotic gradient, water moves from the adjacent cells to the next by
osmosis; until it enters the xylem vessels located in the center of the root;

The xylem vessels of the root then conduct the water up into the xylem
vessels in the stem into the leaves;

There is a force in the roots which pushes water up the stem; this force is
known as root pressure; and can be considerably high in some plants; energy
from the endodermal cells of the root is responsible for driving this force; in
the xylem vessels, water would rise up by capillarity; to some extent
because the vessels are narrower and there is a high attractive force
between the water molecules and the cell walls; the cohesive; and adhesive
forces are important in the maintenance of a continuous and uninterrupted
water column in the xylem vessels up the tree to the leaves;

Water vaporizes from the spongy mesophyll cells; their cell sap becomes
concentrated than the adjacent cells.

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This increases the osmotic pressure of the spongy mesophyll cells; as a
result of this, water flows into the cell from other surrounding cell, which in
turn takes in water from xylem vessels within the leaf veins; this creates a
pull/suction force that pulls a stream of water from xylem vessels in the stem
and roots.

This force, known as transpiration pull; helps in maintaining a continuous

column of water from the roots to the leaves; water flows from the midrib
into leaf veins from where it enters leaf cells; from the mesophyll cells, it
enters the airspace.

5.How is the mammalian heart adapted to its functions?

Heart is enclosed in a pericardial membrane/pericardium; that produces a

fluid; to lubricate it; the membrane also keeps the heart in position;

It is covered in a fatty layer; that acts as a shock absorber;

Made up of cardiac muscles; which are interconnected/interacted hence

contract and relax without fatigue or nervous stimulation/myogenic; for
continuous pumping of blood throughout the lifespan of the animal;

The muscles are supplied by nutrients and oxygen; by the coronary arteries;
and the coronary veins take away wastes and carbon (IV) oxide;

Heart is divided into 4 chambers; for efficient double circulation/ avoid

mixing of oxygenated and deoxygenated blood/carry large volume of blood;

Has interventricular septum; to separate oxygenated and deoxygenated

blood;

Ventricles are thick/muscular; to generate high pressure to pump blood out

of the heart;

Left ventricle has thick muscles/more muscular; to pump blood to all body
tissues;

Heart has bicuspid; and tricuspid valves; to prevent back flow of blood to left
auricle; and right auricle respectively;

Valves have tendinous cords/valve tendons; to prevent them from turning

inside out;

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Semi lunar valves located at the beginning of major arteries; prevent
backflow of blood into the ventricles;

Has sino-artrio node located in the muscles of the right auricle; to initiate
heart beat/contractions of heart muscles/cardiac muscles, rate of heart beat
is controlled by nerves; vagus nerve; slows down heartbeat;while
sympathetic nerve; speeds up the heartbeat;

Has aorta; to transport oxygenated blood to all body parts;

Has pulmonary artery; that transports deoxygenated blood from right

ventricles to lungs for oxygenation;

Has pulmonary vein; that transports oxygenated blood from lungs to the left
ventricles; for distribution to all body parts;

Has the venacava; that receives deoxygenated blood from all body parts to
right ventricles;

6.Describe double circulation in mammals

Deoxygenated blood from body tissues (except lungs); enters the heart via
the right auricle; through the venacava; it flows to the right ventricle; via the
tricuspid valve; the right ventricle contracts; pumping blood; via the semi
lunar valves; through the pulmonary artery; to the lungs for oxygenation; the
oxygenated blood from the lungs; flow through the pulmonary vein; to the
left auricle; via the bicuspid valve; to the left ventricle; the left ventricle
contracts; pumping blood via the semi lunar valves; through the aorta; to the
rest of the body tissues;

7.Describe the process of urine formation in the mammalian kidneys

The afferent arteriole which is a branch of the renal artery supplies blood to
the glomerulus; the afferent arteriole has a wider lumen/diameter than the
efferent arteriole; which takes away blood from the glomerulus;the
differences in the diameter of the afferent and the afferent vessels causes
high pressure; leading to ultrafiltration of blood; the walls of the blood
capillaries are one-cell thick; hence glucose, amino acids,vitamins,
hormones, salts, creatine, urea and water filter into the Bowman’s
capsule; to form glomerular filtrate;

White blood cells, red blood cells, plasma proteins such as globulin and
platelets are too large to pass through the capillary wall; hence remain in the
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blood capillaries; useful substances in the human body are selectively
reabsorbed; back into the blood stream at the proximal convoluted tubule;
the tubule is highly coiled; to increase the surface area for reabsorption of
the substances; the useful substances include amino acids,glucose, vitamins,
hormones, sodium chloride and water;

Many mitochondria found at the proximal convoluted tubule; provide energy

for reabsorption of these substances against a concentration gradient; the
glomerular filtrate flows into the descending and the ascending limb of the
loop of Henle; blood in the capillaries and the glomerular filtrate in the loop
of Henle move in opposite directions/counter-current flow; this provides a
steep concentration gradient that leads to maximum absorption of water
through osmosis;

Sodium chloride is actively absorbed from the ascending limb into the blood
capillaries; under the influence of aldosterone hormone; the glomerular
filtrate flows into the collecting tubule from where, more water is reabsorbed
into the blood stream; antidiuretic hormone influences the amount of water
to be reabsorbed depending on the osmotic pressure of the blood; the
glomerular filtrate from several collecting tubules now referred to as urine; is
emptied into the collecting duct;

The urine passes through pyramid, pelvis and ureter into the bladder; where
it is stored for some time. The sphincter on the urethra relaxes to allow urine
to be released from the body;

8.Explain the role of the following hormones during homeostasis

a. Antidiuretic Hormone (ADH)

Secreted by the (posterior lobe/end) pituitary gland; in response to an

increase in the osmotic pressure of blood; the hormone stimulates the distal
convoluted tubules and the collecting ducts; to increase their permeability to
water; this increases the reabsorption of water into the bloodstream;
concentratedand less urine is excreted; when the osmotic pressure
decreases, less or no hormone is produced;hence the tubules become
impermeable to water; less water is reabsorbed into the bloodstream;hence
more dilute urine is excreted; fluctuations in the osmotic pressure is
detected by the hypothalamus;

b. Insulin

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Secreted by the pancrease; in response to a rise in blood sugar level; it
stimulates liver cells to convert the excess glucose into glycogen and fats for
storage in the liver and muscle cells; increases the oxidation of glucose in
respiration to yield water energy and carbon (IV) oxide/increases metabolism
in the body; this leads to a fall in blood glucose to normal level;

c. Glucagon

Secreted by the pancrease; in response to a decline in blood glucose level; it

stimulates liver cells to convert the stored glycogen and fats back to glucose;
stimulates the conversion of amino acids to glucose; and stops the oxidation
of glucose in the body cells; the glucose formed is released to the
bloodsteream causing a rise of blood glucose level to normal;

9(a.) Distinguish between Diabetes mellitus and Diabetes insipidus

Diabetes mellitus is a condition/disease caused by failure of the pancrease to

produce adequate insulin hormone; leading to excess glucose levels in the
body some of which is released in urine while diabetes insipidus is a
condition caused by failure/inability of the kidney tubules to control the
amount of water in urine as a result of a defect in production of antidiuretic
hormone (ADH) leading to production of more dilute urine;

b. Explain how mammalian bodies regulate glucose and protein

levels in their blood

When glucose level is high (above 90mg/100cm3), the brain sends impulses
to the (β cells of islets of Langerhans) pancrease cells; to release insulin
hormone; the hormone stimulates liver cells to convert the excess glucose
into glycogen and fats for storage in the liver and muscle cells; increases the
oxidation of glucose in respiration to yield water energy and carbon (IV)
oxide/increases metabolism in the body; this leads to a fall in blood glucose
to normal level;

However, when the glucose level falls below normal (below 90mg/100cm3);
the brain sends impulses to the (α cells of the islets of Langerhans)
pancrease cells; which are stimulated to release glucagon hormone; the
hormone stimulates liver cells to convert the stored glycogen and fats back
to glucose; stimulates the conversion of amino acids to glucose; and stops
the oxidation of glucose in the body cells to avail more glucose; the glucose
formed is released to the bloodstream causing a rise of blood glucose level
to normal;
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The level of plasma proteins such as prothrombin, globulins, albumins and
fibrinogen; which play a major role in osmoregulation and blood clotting; are
controlled by the liver; which manufactures them using the amino acids
found in the liver; when their levels reduce,more is produced; but when the
level is high, less of the proteins is produced in the liver; excess amino acids
are deaminated; as the body is not able to store them; the process involves
removal of an amino group from an amino acid molecule; the amino group
enters the ornithine cycle; where it is combined with carbon (IV) oxide to
form urea; which is excreted in urine through the kidneys;

10. Describe the structure and function of the mammalian skin

It has a cornified layer ;made up of dead cells and is tough and impermeable
to water; to protect the skin against mechanical damage; bacterial infections
and water loss; granular layer; whose cells divide to form the cornified layer;

Malpighian layer; which is made up of diving cells that give rise to a new
granular layer;

Contains melanin; to protect skin against ultra-violet rays/radiations;

Sebaceous glands; which secrete sebum; to make the skin supple/soft and
waterproof; sebum is also antiseptic;

Blood vessels; dilate during hot weather; increasing blood flow near the skin
surface; heat loss is enhanced;constrict; in cold weather; less blood flow;
minimize heat loss;

Sensory nerve endings and receptors; enable detection of external

environmental changes;

Highly coiled sweat glands; secrete sweat; to control body temperature;

when hot sweat evaporates cooling the body; sweat contains excretory
products;

Subcutaneous fat/adipose tissue in dermis; for insulation; hair; to regulate

body temperature; in cold weather erector pili muscles contract; hair is
raised, air trapped to insulate the body; in hot weather, erector pili muscles
relax; hair lies flat reducing insulation;

Dense network of blood capillaries; supply nutrients/oxygen to skin tissues;

as well as carrying away wastes and carbon (IV) oxide away from the skin

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tissues; adipose tissue/sub-cutaneous layer; serves as an insulator;helping in
temperature control; helps in manufacture of vitamin D;

11. Discuss the various mechanisms of opening and closing of

stomata

Photosynthetic theory;

During the day, guard cells carry out photosynthesis manufacturing glucose;
This increases the osmotic pressure of the sap vacuole; which becomes
higher than that of the neighbouring epidermal cells; guard cells therefore
take in water by osmosis; and become turgid; the outer thin wall stretches
easily; pulling the thicker inner wall outwards; thus the stomata opens;

At night, there is no light hence no photosynthesis takes place; plant cells

respire using up more glucose; the osmotic pressure of the sap vacuole of
the guard cells reduces; becoming lower than the neighbouring epidermal
cells; the guard cells lose water by osmosis; to adjacent epidermal cells; they
then become flaccid; pulling together the thick inner walls; and stomata
closes;

Enzymatic inter-conversion between starch and glucose/sugar;

At day time, plants continuously use carbon (IV) oxide for photosynthesis;
leading to an increase in the pH of the guard cells; this causes starch to be
converted to sugar/glucose; the glucose increases the osmotic pressure of
the guard cells; hence water is taken in by osmosis; the cells become turgid
and bulge outwards; causing the stomata to open;

At night, no photosynthesis occurs but respiration takes place; carbon (IV)

oxide accumulates in guard cells;lowering the pH; the low pH favours
conversion of glucose into starch; starch is osmotically inactive;this lowers
the osmotic pressure of guard cells; guard cells therefore lose water by
osmosis to the adjacent epidermal cells; become flaccid; pulling together the
thick inner walls; and the stomata closes;

Active ion exudation;

19
During the day, there’s an accumulation of potassium and sodium
ions; as aresult of active pumping of the ions by the ATP formed through
photosynthesis; carbon (IV) oxide fixation occurs in the guard cells; the
guard cells become turgid; and stomata open;

At night, before the stomata close, the ions diffuse out of the guard cells into
epidermal cells; the osmoticpressure of guard cells is lowered; they lose
water to epidermal cells by osmosis; and become flaccid; thereby closing the
stomata;

12a. Explain how the following blood cells are adapted to their
functions

i. Red Blood Cell

Presence of haemoglobin molecules; with a high affinity to combine with

oxygen as/to form oxyhaemoglobin;bi-concave shape; to increase the
surface area for packaging of haemoglobin;absence of nucleus; to
accommodate maximum/more haemoglobin molecules;thin membrane; for
faster diffusion of gases;

ii. White blood cell

Irregular in shape/amoeboid; which changes to enable the cell to squeeze

through the capillaries;lymphocytes produce antibodies; which help to
prevent diseases;phagocytes are amoeboid-shaped; to change shape and
engulf bacteria cells;

b. Explain the different ways in which Carbon (IV) Oxide is

transported by blood

-Carbon (IV) oxide diffuses out of the tissues into the red blood cells where it
reacts with water; in the presence of carbonic anhydrase enzyme; to produce
carbonic acid;

-The acid dissociates into hydrogen and hydrogen carbonate ions; the
hydrogen carbonate ions then diffuse out of the red blood cells into the
plasma; where it further dissociates to produce carbon (IV) oxide on reaching
the alveolar cavities of the lungs and diffuses into the alveoli; some carbon
(IV)oxide combines with the amine group in the haemoglobin molecule
20
forming carbaminohaemoglobin; which dissociates in the lungs producing
carbon (IV) oxide;

-Some carbon (IV) oxide dissolves in the blood plasma forming carbonic acid,
which dissociates to carbon (IV) oxide on reaching the lungs;

13. a. What is transpiration?

Process by which plants lose water to the atmosphere; in form of vapour

(through lenticels, stomata and cuticle);

b. Explain how the various environmental factors affect the rate of

transpiration

Temperature; an increase in temperature increase the water vapour holding

capacity of air in the spaces between mesophyll cells due to increased
evaporation; therefore more water diffuses from the cells increasing the
water vapour pressure; this causes an increase in the diffusion gradient
between the intercellular spaces and the atmosphere; hence increasing the
rate of transpiration; low temperature decreases water vapour pressure and
the diffusion gradient hence; lowering the rate of transpiration;

Light intensity; high light intensity increases the rate of photosynthesis in the
guard cells; causing the opening of stomata; leading to increased water loss;
it also increases the internal temperature of the leaf; that increases the
evaporation rate in the intercellular spaces; leading to a higher rate of
transpiration; low light intensity; reduces the rate of water loss;

Humidity; a humid atmosphere lowers the water vapour diffusion gradient;

hence lowering the rate of water loss; in a less humid/dry atmosphere, water
diffusion gradient is high/steep; hence the rate of transpiration increases;

Availability of water in the soil; more water will diffuse to the atmosphere
when there is adequate or excess water; as more will be absorbed;
increasing the rate of water loss; the guard cell will also remain turgid; hence
stomata are open; leading to more water loss; however, less water in the soil
leads to a reduced diffusion gradient between the mesophyll cells and the
atmosphere; thereby reducing the rate of transpiration;

21
Wind/air currents; wind blowing over a leaf surface carries pockets of
moisture away from the leaf;creating a steep diffusion gradient between the
atmosphere and the leaf; increasing the

rate of water loss; in still air/on a calm day however, water vapour at the leaf
area becomes saturated; and the diffusion gradient is lowered; reducing the
rate of transpiration;

Atmospheric pressure; low atmospheric pressure leads to a high rate of

diffusion of water vapour;since air molecules move at a faster rate; and this
increases the rate of water loss; in high atmospheric pressure conditions
however, there is low rate of diffusion of water vapour; hence the rate of
water loss is lowered;

14. Discuss the various ways in which anaerobic respiration is

utilized in industries and homes .

Bread making; yeast is used to ferment sugar in wheat flour into carbon (IV)
oxide and energy; the carbon (IV) oxide is produced in form of bubbles that
causes the dough to rise and become porous;

Beer making; yeast is used to ferment sugars in malt/grapes/fruits; to form

beer, wines and spirits;

Sewage treatment; anaerobes break down raw sewage and harmful

industrial effluents; to harmless products of water, energy/heat and carbon
(IV) oxide;

Silage formation; vegetation is fermented by bacteria to produce nutritious

and good-scented/flavoured animal feed that increases production/yields;

Production of acids and strong liquors; special bacteria and fungi ferment
food products; to produce acids such as citric acid, oxalic acid and vinegar;
the products are used as food preservatives and flavouring agents;

Manufacture of dairy products; under controlled environments; anaerobes

help in fermentation hence manufacture of milk products such as butter,
cheese, ghee and yogurt;

Production of fuels such as biogas and gasohol; cane sugar is fermented by

yeast; to produce gasohol for running engines or operating machinery;
animal wastes such as guano and cow dung; can be used to produce a
mixture of methane and carbon (IV) oxide gas; by exposing it to
22
fermentation agents; methane is used to run simple machines such as water
pumps and for cooking;

Production of fermented porridge and milk; maize or wheat flour and milk is
exposed to microbes in the air which ferment it; to produce sour and sweet
tasting porridge or milk;

15. Discuss the composition and functions of mammalian

Mammalian blood consists of two main components: Blood plasma; and the
blood cells; (Red bloodcells/Erythrocytes, White blood cells/Leucocytes and
Platelets/Thrombocytes);

Blood plasma transport nutrients (glucose, amino acids, vitamins, fatty acids
and glycerol, dissolved oxygen)to tissues; transports hormones,
enzymes/metabolic regulators to target organs and tissues; Transport
excretory substances/wastes from the cells; to excretory organs for
elimination from the body; Distribute heatenergy; helping in
thermoregulation; Transports/contains water, plasma proteins and dissolved
mineral salts;important in osmoregulation; Suspends blood cells;

Red blood cells transport oxygen; and dissolved carbon (IV) oxide; helps in
regulation of pH;

White blood cells help in protection/immunity; by engulfing or producing

antibodies to kill/destroy invading micro-organisms/pathogens;

Platelets help in blood clotting; preventing excessive blood loss; entry of

pathogens; and promotes healing of wounds;

23
24
 Form 3:essays
1.Explain how the various abiotic factors may affect plants

Temperature; affects soil formation and distribution of plants; affect

transpiration rate as high temperatures lead to high rates of transpiration; It
also affects the rate of photosynthesis with the direct influence on enzyme
activity;

Light intensity; affects the rate of photosynthesis;

Wind; increase the transpiration rates; affects dispersal of seeds and fruits;
agents of pollination; affect distribution in terms of wind storms/breakages;

Atmospheric pressure; high atmospheric pressure leads to low rates of

transpiration; high oxygen and carbon (IV) oxide concentration; high
photosynthetic rates; while low atmospheric pressure leads to high
transpiration rates; less concentration of oxygen and carbon (IV) oxide;
leading to low rates of photosynthesis;

Water/Rainfall; forms a raw material for photosynthesis; helps in support in

plant tissues; affects distribution of plants; Humidity; low humidity leads to
high transpiration rates; while high humidity leads to low rates of
transpiration; pH; affects distribution of plants; some grow in acidic soils;
others in alkaline soils; Edaphic/soil factors; affects plant distribution; in
terms of being sources of water and mineral salts; provide a substratum for
anchorage of plants;

2. Discuss the causes, effects and control measures for water

pollution

Causes of water pollution are varied:

Industrial effluents; have heavy metals that poison aquatic organisms;

Untreated organic matter has phosphates/sulphates/nitrates/salts;

that cause eutrophication; causing algal bloom that deprives the water of
nutrients; when the algae die,they lead to an increase in putrefying bacteria
whose decomposition activities lead to the release of awful smells/odours;

25
Oil effluents clog respiratory surfaces of aquatic organisms/death due to
suffocation;

Domestic effluents/sewage; form a habitat of pathogens that spread

water borne diseases; decomposing sewage promotes eutrophication leading
to algal bloom; death promotes/attracts saprophytic bacteria that use up
oxygen in water; causing suffocation/death to aquatic organisms;

Agrochemicals/fertilizers; phosphates/nitrates; cause eutrophication;

heavy metals in agrochemicals (herbicides/pesticides); affect respiratory
surfaces/cause breathing problems;

Hot water; raise temperature of water; killing organisms; dissolves less

oxygen; reducing its content in water;

Oil spillage; in oceans from tanks/refineries; soak feathers of marine birds

preventing flight; clogs respiratory surfaces leading to death; coats
photosynthetic phytoplanktons; reduces light penetration hampering
photosynthesis;

Sediments; from soil erosion makes water dirty; making it unfit for
consumption; clogs respiratory surfaces hindering gaseous exchange;
reduces light penetration hindering photosynthesis;

Control methods:

-Enforcement of environmental laws;

-Use of unleaded fuel/petroleum products;

-Proper treatment and disposal of sewage wastes;

-Treatment of industrial effluents before release;

-Public education on correct use of inorganic fertilizers and agrochemicals;

and use of alternatives such as biological control of weeds/pests/organic
manure;

-Use of undersea pipelines instead of tankers to transport oil products;

-Cooling hot water before release to water bodies;

26
3.How are xerophytes and hydrophytes adapted to their habitats

Xerophytes:
Thick waxy cuticle; minimize water loss;

Leaves are folded and reduced in size; to minimize stomatal

transpiration;

Sunken stomata; to reduce rate of transpiration;

Thick/succulent leaves, side branches or stems; for water storage;

Shedding of leaves during the dry periods; to reduce surface area

exposed for transpiration;

Reversed stomatal rhythm; prevent excessive loss of water;

Deep penetrating roots; to absorb water from deep below the surface;

Superficial roots; to absorb surface water run-off;

Leaves covered in scales/hairs; to trap a moist layer of air; to reduce the

rate of transpiration;

Drought-resistant seeds; that remain dormant till favourable weather

resumes;

Underground organs (corms/bulbs); for storage of water and

reproduction;

Most stomata located on the lower leaf surface; to avoid exposure to

direct light; to reduce evaporation;

Reduced number of stomata; to reduce the rate of transpiration;

Hydrophytes:
Stomata on the upper surface of leaves; to provide a large surface area
for gaseous exchange; and loss of excess water;

27
Poorly-developed roots that lack root hairs; to reduce/avoid absorption
of water;

Aerenchyma tissue in leaves, stems and roots; to store air; and for
buoyancy;

Deeply-dissected leaves; to provide a large surface area for absorption of

light;

Highly-sensitive; and numerous chloroplasts; for photosynthesis;

Greatly reduced vascular bundle; to avoid absorption of water;

Flowers raised above the water; to allow for pollination;

Lack of a cuticle or very thin cuticle; for faster loss of water;

4. Outline the differences between wind and insect pollinated

flowers

-Flowers of wind pollinated plants are small; with no bracts, sepals or petals;
if present the petals are small,inconspicuous; often white or green in colour;
while insect pollinated flowers are large; often with brightlycoloured petals,
bracts or inflorescence; to attract insects.

-Flowers of wind pollinated plants have no nectaries; and no scent; while

flowers of insect pollinated plants are scented; and produce nectar;

-In wind pollinated flowers, the anthers are large; and loosely attached on a
flexible filament; to allow pollengrains to be readily released when wind
blows on the anthers; while anthers of insect pollinated flowers are usually
small; and firmly attached on the filaments; this ensures that the insect rub
against the anther; as they crawl into the flower collecting pollen grains onto
their bodies;

-In wind pollinated flowers, the stigmas are feathery; widely spread; this acts
as nets to catch pollen as it floats through the air; while in insect pollinated
flowers the stigmas are small; smooth; and sticky; and are also enclosed;
this feature ensures that pollen grains from the body of an insect stick onto
it;

28
-In wind pollinated flowers, the flowers are simple with no particular shape;
while some flowers that are insect pollinated have petals with grooves or
dark lines; leading from the petal boarder to the nectaries; some have
tubular or funnel-shaped corolla; and landing platforms; to guide the insect
to the source of the nectar for their food;

-Flowers of wind pollinated plants are either on long stalks above the leaves;
or develop from flower buds that open before the leaf buds; to increase the
flower exposure to air currents; while flowers of insect pollinated plants are
on short stalks; often enclosed by the corolla;

5. Describe what happens in a flower from the time of pollination up

to the time of seed and fruit development

After pollination, the pollen grain absorbs nutrients from the stigma; and
develops a pollen tube; it grows down the style to the embryosac; taking
along the male nuclei;

The tube nuclei initiates and maintains pollen tube growth; while the
generative nucleus divides by mitosis;to form two male gamete nuclei; which
follow behind the tube nucleus as the pollen tube grows down the
style;pollen tube enters the ovule through the micropyle; its tip bursts open;
while the tube nucleus disintegrates;one of the male gamete nucleus fuses
with the egg cell nucleus/oosphere/megaspore; to form the zygote;while the
other fuses with the two polar nuclei; to form a triploid nucleus; called the
primary endospermnucleus;

After fertilization, the zygote undergoes repeated mitotic divisions; to form

an embryo consisting of theplumule, radicle and seed leaves/cotyledons;
primary endosperm nucleus divide repeatedly,become separated by
membranes; to form an (semi-fluid nutritive) endosperm;

Ovary walls change into the pericarp; ovary changes/develops into a fruit;
while ovules lose water and become seeds; the integuments; change into
seed coats/testa; style/filaments/petals/sepals wither and fall off (or may
persist);

6. Discuss the adaptations of the female reproductive system

29
Elastic uterine walls; to expand so as to accommodate the growing foetus;

Muscular foot of the pelvis and bladder support the weight of the growing
foetus;

Funnel-shaped ends of the oviduct; direct the ova released to the uterus;

Muscular uterine walls; contract and relax to expel the foetus at birth;

Long vaginal canal; allow sufficient entry of penis to avoid wastage of

sperms;

The two ovaries maximize chances of releasing ovum after every circle (28
days);

Ovaries are well vascularised/have good blood supply; to ensure

nourishment of cells involved in oogenesis (primordial mother/germ cells) or
egg formation;

High number of potential mother cells; ensures maximum number of ova

which develop to maturity;

Plenty of yolk in egg cells; which nourish the foetus before the placenta
becomes functional;

The vitelline membrane of the ovum thickens after fertilization; preventing

further entry of sperms;

The oviduct wall is able to contract; in order to facilitate movement of ovum

down the oviduct;

Has cilia to waft the ovum forward; wall of the vagina/vulva produce mucus;
to lubricate the penis during copulation;

Clitoris; has many nerve endings; to provide maximum stimulation during

copulation for maximum ejaculation and faster movement of spermatozoa;

7. Describe the process of gaseous exchange in terrestrial plants

Gaseous exchange occurs in the spongy mesophyll;

During the day, air diffuses into large air spaces of the spongy mesophyll;
through the stomata; the carbon (IV) oxide in the air diffuses into
30
photosynthetic cells; in solution form; during photosynthesis, carbon (IV)
oxide is used up; while oxygen is produced; some of the oxygen is used in
respiration; while the rest diffuses out of the leaf; through the stomata;

During the night, air diffuses into the air spaces; through the stomata; the air
dissolves into the film of moisture; oxygen in the air diffuses into the cells;
and is used for respiration; carbon (IV) oxide produced; diffuses out through
stomata; due to a concentration gradient/diffusion gradient; At night, carbon
(IV) oxide accumulates in the leaf since photosynthesis does not occur; some
gaseous exchange also takes place through the cuticle; and through the
epidermis of young leaves, roots and stems; some plants exchange gases
through breathing roots/pneumatophores; older stems exchange gases
through lenticels;

8. How is the mammalian gaseous exchange system adapted to its

functions?

Nasal cavity; has cells that produce mucus; that together with hairs/cilia;
trap and propel dust/microbes to the pharynx to be breathed out/swallowed;

Cavity is supplied with capillaries; that warm the air for faster flow in the
channels;

Epiglottis; covers the trachea during swallowing; so that particles of food and
water may not enter the trachea;

Trachea and bronchi; have cartilage rings; to keep the passages

open/prevent them from collapsing; so that air moves in and out freely and
continuously; are also lined with mucous membranes which have
hairs/ciliated; whose movement/wafting push out dust particles collected in
the passages into the pharynx; richly-supplied with blood vessels; to warm
the air; for faster flow;

Lungs; have numerous alveoli; to increase the surface area for gaseous
exchange;

Alveoli have a thin epithelium; to reduce the distance through which gases
diffuse for easier and faster diffusion; alveoli are moist; to dissolve oxygen
for faster transport;

Lungs are spongy; because of many air sacs that contain a large amount
of/volume of air; Lungs are also supplied with many blood vessels; for
31
transportation of gases; they are also supplied with a network system of
trachea, bronchi and bronchioles; to provide an efficient system/large
surface area for gaseous exchange;

Lungs are enclosed in a pleural membrane; which secrete pleural fluid; that
protect the lung surface; lubricate the chest cavity; allowing smooth
movement of lungs as they change volumes;

Ribs have intercostal muscles; that moves/contracts and relaxes to allow for
inhalation and exhalation; ribs also protect the lungs; has the diaphragm
muscles whose contraction and relaxation leads to inhalation and exhalation
respectively;

9. Describe the role of the following hormones in the menstrual

cycle

a. Luteinising Hormone (LH)

Produced by the pituitary gland; under the influence of oestrogen hormone;

cause the bursting of the Graafian follicle; to release a mature
egg/ovum/causes ovulation; stimulates the change/conversion of the
Graafian follicle; into the corpus luteum; stimulates the corpus luteum; to
secrete progesterone hormone;

b. Follicle Stimulating Hormone (FSH)

Produced by the anterior lobe of the pituitary gland; it stimulates the

maturation of the Graafian follicle in the ovaries; stimulates the ovarian
tissue/wall to secrete oestrogen;

c. Oestrogen

Brings about/stimulates the healing and repair of the uterine wall; after
menstruation; stimulates the pituitary gland; to secrete luteinising hormone;

d. Progesterone

Secreted by the corpus luteum; it stimulates the thickening of the

endometrium/uterine wall; in preparation for implantation; inhibits secretion
of the Follicle Stimulating Hormone; therefore preventing further
development of the Graafian follicle;

32
10 a. What is secondary growth?

Type of growth that occurs due to cambium activity in woody plants/stems;

resulting in an increase in girth/width of plants;

b. Describe the process of secondary thickening in a woody stem

Facilitated by meristematic cells (cambium) located between the phloem and

the xylem (intervascular cambium); it divides radially to form cambium
tissues; with xylem forming the outer ring/to the inside;while the phloem
forming the outer ring/to the outside; division of the cambium ring; form a
secondary parenchyma; hence increases/forms the medullary rays; other
xylems (secondary xylem) are formed;hence pushes the phloem and
cambium ring outwards; this creates pressure on the outer cells;resulting in
stretching and eventual rupturing of epidermal cells;

A new band/volume of cambium cells are formed in the cortex beneath the
epidermis (cork cambium cells/phellogen); to replace these ruptured cells;
the phellogen cells divide on either side; where the inner cells become the
secondary cortex; while those produced on the outside become cork
cells;which are tightly packed; and become coated with an oily/waxy water-
proof material/suberin;

Further multiplication of cork cells; lead to formation of the bark; which forms
a protective layer (against water loss and damage by organisms); seasons
results into annual rings; some cork cells form a loose mass/lenticels that
allow gaseous exchange through the stem;

11. Discuss the role of the various hormones in plant growth and
development

Indole Acetic Acid/Auxins;

Influences/promotes cell division/elongation (in cambium causing secondary

thickening); induces tropisms;promotes fruit formation/parthenocarpy;
promotes formation of abscission layer/leaf fall; promotes cell differentiation
(of vascular tissue); causes apical dominance/inhibits lateral bud formation;
promotes growth of adventitious roots; in conjunction with cytokinins, it
induces callous tissue formation;
33
Gibberellins/Gibberellic acid;

Promotes cell division/elongation in dwarf plants; promotes parthenocarpy;

setting of fruit after fertilization initiating formation of fruits; formation of
side branches of stems/ends apical bud dormancy; inhibits adventitious root
growth; activates hydrolytic enzymes in germination/promotes germination
of seeds/breaks seed dormancy; affects leaf expansion and shape/retards
leaf abscission;

Cytokinins (e.g. Zeatin or Kinetin);

Promotes flowering in some plant species; breaks dormancy in some plant

species; promotes cell division in presence of auxins; stabilizes protein and
chlorophyll; promotes root formation on a shoot; low concentration
encourages leaf senescence/increases cell enlargement in leaves; stimulates
lateral bud formation;

Ethylene/Ethene;

Promotes ripening of fruits; induces thickening of stem/inhibits stem

elongation; promotes flower morphogenesis/formation or flowering in
pineapples; causes abscission of leaves/fruit/leaf fall;

Abscissic acid;

High concentration causes stomatal closure; inhibits stem elongation/growth;

inhibits sprouting of buds/induces bud dormancy; inhibits seed
germination/growth/causes/promotes seed dormancy; causes abscission of
leaves/fruits/leaf fall;

Traumatin;

Heals wounds by promoting callous formation;

Florigen;

Promotes flowering;

34
12. Discuss the adaptations of the male reproductive system of
humans

Consists of two oval-shaped testes; lying outside the abdominal cavity in a

special sac known as the scrotal sac/scrotum; for protection; the testes are
located outside the body to provide a relatively cooler environment/lower
temperature; suitable for sperm production; the inside of the testis is divided
into seminiferous tubules;there are three coiled and twisted tubules; having
rapidly/actively dividing cells that produce sperms;

Interstitial cells; found between these tubules produce the male sex
hormones/androgens (mainly testosterone); important in promoting the
development of secondary sexual characteristics; and maintaining
masculinity in males; the tubules join together to form the epididymis; which
are smaller ducts; that convey sperms out of the testes; they also form a
temporary storage area for sperms; the epididymis is connected to the
sperm duct/vas deferens; which has thick muscular walls; that contracts to
propel sperms to the urethra; the sperm duct is joined by a duct from the
seminal vesicle; a blindly ending sac; that produces an alkaline fluid
containing nutrients for the spermatozoa/sperm cells; to provide energy; at
the junction of the two sperm ducts (one from each testis) and urinary
bladder there is the prostate gland; that secretes an alkaline fluid that
neutralizes the acidic vaginal fluids; and also activates the sperms; by
addition ofenzymes and diluting the sperms; below the prostate gland is the
cowper’s gland; which secretes an alkaline fluid which neutralizes the
acidity caused by urine; along the urethra.

The urethra; is a long tube running the length of the penis; used for
conduction and expulsion of urine; as well as passage of sperms during
copulation; the urethra follows the penis; that projects from the body at the
lower abdomen; it consists of a retractable skin known as the
prepuce/foreskin; that covers a swollen/bulbous end region of the penis
known as the glans; the glands excites the clitoris of the female as it brushes
on it during copulation; to stimulate ejaculation/orgasm; the penis is made
up of spongy erectile tissue; consisting ofnumerous small blood spaces,
muscle and blood vessels; the spongy tissue gets filled with blood; making
the penis to become erect during sexual stimulation excitement and activity;
enabling the penis to penetrate the vagina during coitus/copulation/sexual
intercourse; in order to deposit sperms in the vagina of the female;

35
13. State the economic importance of members of Monera and Fungi
and for each economic importance name a suitable or appropriate
organism

Fungi:

-Yeast is used in the production of alcohol; and raising dough in baking;

production of vitamins B2 and B12 and for beer brewing;

-Some mushrooms are used as food; Saprophytic fungi such as Mucor

spp/Rhizopus spp cause food spoilage;are also used in retting of natural
fibres such as flax; and in the curing of tea and tobacco;

-The fungi also decompose organic matter helping to clean the environment
and recycle materials.

-Saprophytic fungi are also used to make silage; compost; and digesting
sewage in sewage treatment plants;

-Penicillium spp is used for the manufacture of antibiotics;

-Fungi is used for making ghee and cheese in dairies;

-Parasitic fungi such as Phytophthora spp infest crops e.g. potatoes and
tomatoes and destroy them/cause blight;

-Some fungi such as Aspergillus spp produce food poisons and is also used in
the synthesis of enzyme amylase;

-Mycorrizae fungi enable trees e.g. pinus to absorb water and mineral salts
more efficiently in silviculture/manmade forests;

-Some fungi such as and Tinus spp cause diseases such as yeast infections
(Candidiasis;Athlete foot;) and;ringworms; respectively;

-Candida spp is used in making enriched food/single-celled proteins used in

animal feeds/silage;

-Gibberella spp is used in the synthesis of Gibberellins for plant growth; it

also causes poor crop growth through bolting of plants e.g. rice and wheat;

36
Bacteria:

-Some bacteria e.g. Vibrio spp cause disease to humans and other animals
(any one disease);

-Clostridium spp and Bacillus spp bacteria are used in retting of flax;

-Lactobacillus spp are important in the manufacture of yoghurt/silage; and

lactic acid;

-Some bacteria e.g. Streptomyces are used in making antibiotics

(Streptomycin);

-Organic acids such as acetic and ethanoic acids are made using Acetobacter
spp of bacteria;

-Some bacteria like the soil bacteria (Bacillus spp) make enzymes used in
detergents;

-Escherichia coli (E. coli)/colon bacteria contaminates drinking water and

may cause diseases;

-Bacteria such as Bacillus spp and are used in making hormones such as
insulin through genetic engineering/modification;

-All saprophytic bacteria such as E. coli/Bacillus spp/Pseudomonas spp are

used in the production of methane/biogas;

-Rhizobium spp of bacteria helps in nitrogen fixation making soils fertile;

37
 Form 4:essays
1 a. What is natural selection?

(Theory put forward by Charles Darwin)It explains that nature selects for
individuals that are well adapted to a particular environment; and against
those that are less adapted

b. Discuss three examples of natural selection in action

Melanic forms of peppered moths; in Europe, there are two forms of

peppered moths; white and black; before industrialization, the tree trunks
were white; therefore the white peppered moths were white; hence were
camouflaged; the black varieties were easily noticed and fed upon by
predatory birds; the white form therefore reproduced and increased in
number; during industrialization, the smoke from industries coated tree
trunks black; the black variety became camouflaged; reproduced and
increased in population; the white variety were easily noticed and fed upon
by predators; they reduced in population;

Resistance against drugs and antibiotics; where microorganisms are

continually exposed to a certain drug; their cells synthesise specific proteins;
which counter the drug; this ability to synthesize the protein is passed onto
the offspring;

Resistance to pesticides by insects; insects such as mosquitoes when

continually exposed to a particular pesticide; synthesize a specific protein
which make them resistant to the pesticide; this is then inherited by their
offspring;

2. Discuss Lamarck’s and Darwin’s theories of evolution

38
Larmarck’s theory states that when the environment demands the need or
use of a particular structure in the body; the body develops it in response;
for example giraffes used to have short necks; when all the grass was
exhausted, they started stretching their necks in search of leaves on trees;
therefore they developed long necks; which then were inherited by their
offspring;

However, when a structure is not continually used, it reduces in size and

becomes dysfunctional; this theory fails to explain how acquired
characteristics become inherited;

Darwin’s theory suggests that in nature there occur struggle for existence;
only those individuals with the desired adaptations survive; those poorly
adapted fail to compete; and become extinct;

There also occurs variation in nature; where organisms with desired

adaptations pass on their characteristics to offspring during reproduction;
those poorly adapted fail to reach maturity; and do not reproduce;
therefore,nature selects for individuals best suited to an environment; and
against those poorly adapted (natural selection); as there occurs survival of
the fittest;

3. Describe how the following vertebrae are adapted to their

functions

a. Atlas

Has a wide neural canal; to accommodate the large spinal cord at the
neck region;

Has large/broad wing-like cervical ribs; to increase the surface area for
attachment of the neck muscles;

Has facets on the anterior side; for articulation with the occipital
condyles of the skull; allowing up and down movement/nodding of the head;

Has posterior facets for articulation with the anterior facets of the
axis; forming a joint that allows sideways movement of the head;

b. Axis

39
Has a broad centrum; that projects to form the odontoid process; for
articulation with the neural canal of the atlas; a joint that allows turning of
the head;

Has a large and broad/flattened neural spine; and flat cervical ribs;
to increase the surface area for attachment of neck muscles;

c. Lumbar

Has many transverse processes; and additional projections

(metapophyses, hypapophyses,anapophyses); to offer a large surface area
for attachment of abdominal muscles;

Broad neural canal; to allow passage of the large spinal cord at the upper
abdominal area;

Large/thick centrum; to support the weight of the body; and withstand

strains/upthrust force due to movement;

d. Thoracic

Long/elongated neural spine; to offer a large surface area for attachment

of the large back muscles;

Have a large centrum and neural canal; to offer support to the thoracic
cage;

Has tubercular facet on the transverse processes; to articulate with

the tuberculum of the ribs; while the capitular demifacets on the centrum;
articulates with the capitula of the ribs;

Together with the ribs and the sternum form the thoracic/rib cage; for
protection of heart and lungs; and for breathing process;

4a. Why is locomotion necessary in higher animals?

Animals move in order to look for food; mates; escape danger/predators;

look for shelter/ suitable environmental conditions;

b. Explain how bony fish are adapted to their habitats

Have streamlined bodies; to reduce friction;

40
Body is covered with scales; which overlap backwards; to reduce friction;

Skin produces mucus; which covers the body making it slippery; reducing
friction;

Have swim bladder; which stores air; for buoyancy hence make the fish
float;

Myotomes/muscle blocks; that contract alternately; for forward thrust in

water;Lateral line system; on either side of the body which is sensitive to
pressure and water currents;

Possess fins; that are used for locomotion: tail/caudal fins; for propulsion;
dorsal; and anal fins; prevent rolling; pectoral fin; used for breaking/steering;
prevents yawing/side to side movement; controls pitching; pelvic fins; for
steering/pitching;

5. Describe how the various supportive tissues in plants adapt them

to their habitats

Sclerenchyma tissue;

Long, slender cells with tapering ends; with walls thickened with lignin;
provide support and protection to the more delicate tissues; and resistance
to storms and strong winds; main constituent of wood;

Xylem vessels;

longitudinally-elongated cells; with perforated end walls; with heavily

lignified walls; to increase rigidity and strength to the plant; a main
constituent of wood;

Tracheids;

mainly found in angiosperms; made up of long tapering dead cells; cell walls
are highly lignified;and pitted; cells lie in large overlapping groups; to offer
extra support;

Collenchyma tissue;

longitudinally elongated living cells; located beneath the epidermis and mid
rib of leaf veins; thickened at the corners by cellulose and pectin
compounds; to provide support in leaves, herbaceous plants and young
woody plants;
41
Parenchyma tissue;

large; spherical cells; with thin cellulose walls; forming the bulk of cortex and
pith of most plants; become tightly packed and rigid when turgid; to attain
and maintain an erect posture of plants; main support structures in
herbaceous stems/plants;

6.a. What is a reflex action?

Rapid and automatic; response to a particular stimulus;

b. Outline the activities that occur in the body when one touches a
hot object

When the hot object is touched, the pain receptors; in the skin of the finger
are stimulated;

Nerve impulses are initiated and transmitted through the sensory neurone;
to the grey matter; of the spinal cord to the brain; for interpretation; the
impulses are then transmitted through the relay neurone;via a synapse; the
impulses from the relay neurone are transmitted via the motor neurone;
throughanother synapse; to the effector; which are the biceps muscles of the
upper arm; making musclesto contract; straightening the arm; and the arm
is withdrawn from the hot object;

7. Describe the nitrogen cycle

This is the cycling of nitrogen and its compounds in nature;

Plants absorb nitrogen in form of nitrates and then assimilate it into plant
proteins; animals obtain this nitrogen in plant proteins through feeding on
plants;

When the animals die and decompose, they release the nitrogen in form of
ammonia to the soil; free atmospheric nitrogen is converted into nitrates
through a process known as nitrogen fixation; the process occurs in two
ways: Biological and non biological;

Biological fixation of nitrogen is done by nitrogen-fixing bacteria; which are

either free-living or symbiotic; symbiotic bacteria are of the genus
Rhizobium; and are found in root nodules of legumes (such as pea, clover
42
and alfalfa); the bacteria convert atmospheric nitrogen into ammonia; that is
used directly by the leguminous plants to form nitrogen containing organic
compounds (amino acids, nucleic acids, proteins);

When plants die, the nodules release ammonium compounds into the soil;
which are then converted to nitrites;by nitrifying bacteria of genus
Nitrosomonas and Nitrococcus (nitrite bacteria) and then to nitrates by

Nitrobacter (nitrate) bacteria; free-living micro-organisms that fix nitrogen

include putrefying/saprophytic bacteria; (such as Azobacter spp, Clostridium
and some algae such as Anabaena, Chlorella and Nostoc);

The organisms fix nitrogen into ammonia by break down of protein material
in dead organisms; the ammonia is converted to nitrites; then to nitrates;
However, denitrifying bacteria (e.g. Pseudomonas denitrificans and
Thiobacillus denitrificans); break down/reduce nitrates to nitrites,ammonium
compounds and even gaseous nitrogen; a process known as denitrification;
the process helps to release free nitrogen into the air for recycling;
nonbiological nitrogen fixing is carried out by lightning during
thunderstorms; the lightning energy, causes atmospheric nitrogen and
oxygen to combine forming oxides of nitrogen; which dissolve in rain water
to form nitrous acid/nitric acid; that is washed down into the soil; the nitric
acid formed reacts with other chemical compounds dissolved in soil water; to
form nitrates; the nitrates are then utilized by plants;

8. Discuss how the various tropisms adapt plants to their habitats

Phototropism;

growth curvature in response to direction of light; enables plant shoots to

grow and get light for maximum photosynthesis; allows for leaf mosaic;

Thigmotropism;

growth curvature in response to contact/hard surface; makes plants with

weak stems to get support on large plant/trees; this makes them to reach
and get light for maximum photosynthesis;

Geotropism;

growth curvature in response to gravity; enables plant roots to grow deep

into the soil for maximum support/anchorage;

43
Hydrotropism;

growth curvature in response to moisture/water; water is then used as a raw

material during photolysis stage of photosynthesis;

Chemotropism;

growth curvature in response to chemical concentration gradient; enables

pollen tubes to grow down the style and into the ovary for fertilization to
occur in plant flowers;

Thermotropism;

growth curvature in response to temperature changes; enables plants to

grow to where they can acquire optimum temperature for effective plant
process (e.g.sunflower orientates towards the direction of the sun);

9. Discuss the various evidences of organic evolution

Comparative anatomy/taxonomy;

Members of a phylum/group show similarities; organs have similar

structure/organs performing the same function such as the digestive system,
urinary system, vertebrate heart; homologous structures are structureswith
the same embryonic origin but have been modified to perform different and
specific functions; show a form of divergent evolution; e.g. the pentadactyl
limb in vertebrates which has been modified for racing;swimming and flight
or beaks of finches and birds; while analogous structures are those with
different embryonic origin but have been modified to perform the same
function e.g. wings of insects, bats and birds;eyes in octopuses and humans;
show a form of convergent evolution; Vestigial structures; have been
reduced in size and become functionless; in the course of evolution; e.g.
limbs in snakes, human hair and tail;

Cell biology/cytology;

Occurrence of similar organelles such as the mitochondria and the

endoplasmic reticula point to common ancestry;

Fossil records/Paleontology;

44
Remains of organisms preserved in naturally-occurring materials for many
years; fossil records show morphological changes of organisms over a long
period of time e.g. skull of humans and horse; they provide a direct evidence
of existence of organisms at a particular ecological era; however, since only
hard parts are preserved, no evidence is available for existence of soft-
bodied organisms; and there are many missing links; since remains are
accidentally preserved in rudimentary rocks and resins;

Comparative embryology;

Vertebrate embryos are morphologically similar during the early stages of

development; suggesting that the organisms had a common ancestry/origin
e.g. larvae of mollusks/annelids, embryos of chicken, humans, sheep; the
closer the semblance between embryos, the closer their ancestral
backgrounds;

Geographical distribution;

Present continents are thought to have been a large land mass joined
together; as a result of continental drift; isolation occurred bringing about
different patterns of evolution; where plants and animals from different
continents yet with common ancestry can no longer interbreed; because
they evolved into different species; examples of animals that moved to
different areas are the jaguars and Llamas in south America, lions in Africa,
Tigers in Asia, marsupials in Australia;

Comparative serology/physiology;

Semblance in blood components such as blood proteins, antigen-antibody

reactions, structure of haemoglobin in all vertebrates; reveal some
phylogenic relationship among organisms/show common ancestry;

10 Describe the structure and functions of the various parts of the

mammalian

Ear Pinna; is wide/funnel-shaped to collect/gather sound waves; and direct

them to the auditory canal into the ear;

45
Eardrum/tympanic membrane is thin and light; to convert sound waves into
vibrations;

Ear ossicles/maleus, incus and stapes are of high density; to magnify/amplify

sound waves;

Oval window is smaller than eardrum; to magnify the sound waves; and
direct them to the inner ear;

Cochlea is long and coiled; to increase surface area; for attachment of

receptor cells/sensory hairs; cochlea has many sensory hairs; which receive
sound vibrations and generate impulses;

Liquid or fluid/endolymph in cochlea; transmit sound vibrations;

Auditory nerve; transmit impulses to the brain for interpretation;

Eustachian tube; link the mouth and middle ear to equalise pressure;
between middle and outer ear to prevent damage to delicate eardrum;

Round window; lose excess vibrations; to avoid continuous stimulation;

Semicircular canals; contain receptors for body balance and posture;

External auditory canal cells produce/secrete wax; to trap dust

particles/solid/micro-organisms that can damage eardrum;

11. Discuss the various ways employed by preys to avoid the

predators

-Some preys resemble inedible inanimate and animate objects; this is called
mimicry; e.g. walking stick insect resembles dry twigs of plants, some moths
look like bees or flowers of some plants; this prevents birds from easily
noticing and eating them;

-Many have the ability to run very fast; because of having muscular bodies;
and long legs; enabling them to escape predators e.g. antelopes, zebras;

-Some have a body colour that resembles the surrounding; which helps them
to camouflage or conceal in the background environment; e.g. zebras,
giraffes;

-Some graze in large herds; this enables them to fight off predators; e.g.
wildebeests and buffaloes;

46
-Some have evolved tough skin or coverings like shells; which can not be
broken by some predators e.g.snails, tortoises, armadillo;

-Production of foul smell e.g. in skunks; that discourages the predators;

-Confrontational display that can scare away the predator e.g. porcupine;

-Large eyes on both sides of the head give animals such as zebra a wide field
of vision; enabling them to keep track of their enemies from far; and take
precautions;

12 a. What is meant by the term symbiosis?

Nutritional association of two different organisms (2 plants or between an

animal and plant); for mutual benefit; the relationship enables the composite
organism to survive where neither can live on its own;

b. Describe five types of symbiotic relationships in a natural

ecosystem

Lichens; these are composite plants consisting of blue-green algae; within a

mycelial mass of a fungus;algal cells are provided with support, obtain water,
carbon (IV) oxide and minerals and protection from fungus; while the fungus
obtains oxygen and the carbohydrates made by algae; this enables the
plants to survive on hard bare rocks in high attitudes and polar regions;

Leguminous plants and nitrogen-fixing bacteria; the bacteria multiply and fix
nitrogen from air into nitrates for the benefit of the plant; bacteria are
protected and obtain nutrients from the plants;

Ruminants and bacteria; the rumen has bacteria that secrete cellulose; that
digests cellulose in the food/vegetation consumed by the animal to glucose
for the animal; while the bacteria get shelter and use part of digested food;

Mycorrhizal fungi and higher plants; the fungi found on forest trees gain
photosynthetic organic products made by the trees; while the trees get
nutrients/minerals absorbed by the fungus from the soil;

Tryconympha and termites; the former is a protozoan living in gut of

termites; and produce cellulose enzyme; that digests cellulose from the plant
into digestible products for the benefit of the termite; the termite on the
other hand provides shelter and protection; and absorbs some of the food for
its use;
47
13 a. Describe the adaptations of Schistosoma spp to their parasitic
mode of life

The parasite utilizes two hosts; the snail and humans; to increase chances of
transfer of the parasite from one place to another;

Have suckers for attachment to host walls; to prevent them from being
dislodged;

The parasite produces many larval forms (e.g. miracidia, cercariae and redia)
in snails; to increase chances of transmission and survival; as this feature
poses barriers/difficulties in efforts aimed at eradicating the parasite;

Cercariae larvae and eggs of the parasite have glands that secrete lytic
enzymes; which soften the tissues of humans/snails; to allow for penetration;

Chemical substances produced by the adult worm; protects the parasite from
the action of the hosts; defense mechanisms;

They exist as separate sexes; with the male carrying the female; this ensures
that eggs produced by the female are fertilized before being shed into the
blood stream;

b. Outline five measures that can be employed to prevent and

control the spread of the parasite

-Proper disposal of human waste; urine and faecal material should not be
disposed in water bodies to avoid contamination by the eggs or adult worms;

-Drainage of stagnant water pools and use of molluscides to kill the

intermediate hosts (snails);

-Avoid swimming/bathing in snail-infested water bodies;

-Wearing protective clothing such as gloves and gumboots when working or

walking in swampy areas;

-Personal hygiene that includes washing hands after visiting the toilet and
drinking of boiled or chemically treated water to kill the eggs and the larval
forms in the water;proper treatment of infected persons;

48
14. Describe the process of mitosis

Occurs in somatic/body cells; through five main stages/phases:

Interphase/Resting stage;

intense internal activities occur in the cell at this stage in preparation for the
division; the activities include;replication of each chromosome to multiply
genetic material to retain chromosomal number in daughter
cells;chromosomes appear as a diffuse tangle of threads (chromatin);
synthesis of new cellular organelles; build-up of energy stores (ATP) to drive
the entire cell division process;

Prophase;

chromosomes become visible; as they shorten and thicken appearing as

discrete strands (chromatids) lying parallel to each other; in animal cells,
centrioles separate and move to opposite ends (poles) of the cell; they
radiate from each of the ends forming spindle fibres; nuclear membrane
begins tobreakdown; nucleolus disappear;

Metaphase;

Chromosomes migrate/move to the centre of the cell; and align themselves

along the equatorial plane of the spindle; they get attached to the
chromosomes, by their centromeres; nuclear membrane breaks down and
disappears; spindle fibres lengthen; and attach to the centrioles at both
poles forming asters;

Anaphase;

chromatids separate at the centromere; shortening of the spindle fibres

occurs; resulting in the chromatids migrating to opposite poles of the cell;
spindle apparatus begins to disappear;

Telophase;

final stage where chromatids reach the poles; become densely packed
together and uncoil; a nuclear membrane forms around each mass/set of
chromatids (now referred to as chromosomes); cytoplasm divides into two
(cytokinesis); in animal cells, the cytoplasm divides by constriction of the cell
membrane; while in plant cells, a cell plate forms within the cytoplasm and

49
grows to separate the cell into two; spindle fibres disappear within the
cytoplasm; and nucleoli reappear in the nuclei; of the two daughter cells
formed at the end of telophase;

15. Discuss the various mechanisms that hinder self-pollination and

self fertilization in plants

Protandry and protogyny;

these are mechanisms where either the male or female parts of the
reproductive organs ripen at different times in some flowers; Protandry is a
case where stamens ripen earlier; and anthers release their pollen grains
before the stigma is mature; while protogyny refers to a case where the
stigma matures earlier; and hence becomes ready to receive pollen grains
before the anthers are ready/ripe to shed the pollen grains; common in
plants of the grass family;

Self-sterility or incompatibility;

is a case where pollen grains cannot germinate on stigma of the same

plant;but only germinate on a different plant of the same species; hindering
self-pollination;

Heterostyly;

condition of having different arrangements of style and stigma; for instance

flowers could have shorter stamens than pistils; hence becomes impossible
for the pollen to land, germinate and fertilise the ovules of the same flower;
pistils on some flowers could also be shorter than the stamens therefore
other mechanisms that hinder self-pollination are utilized;

Dioecius and monoecius plants;

dioecius plants have reproductive parts located separately on different

plants of the same species; discouraging self-pollination; while monoecius
plants have the parts located at different parts of the same plant body;
encouraging cross-pollination;

50
16 How are seeds and fruits of plants adapted to their mode of
dispersal?

Water;

Fruit mesocarp/seed testa has air spaces; thus light/buoyant to float; carried
away by water;

Fruits/seeds protected from soaking by waterproof pericarp/testa;

Animal;

Have hooks for attachment to animals; thus carried to other places;

Fruits are brightly coloured;succulent/fleshy; aromatic/scented, to attract

animals; which feed on them;

The seed coats/hard seeds are resistant to digestive enzymes; thus are
unaffected; seeds dropped away from parent in faeces/droppings;

Wind;

Have hairs/wing-like structures/floss/extensions; which increase surface

area/for buoyancy; making it easy to be blown away;

Fruits/seeds are light due to small size; therefore easily carried away by
wind;

Censor mechanism;perforated/open/split/capsule; usually loosely attached to

the stalk/long stalk; is swayed by wind; scattering seeds;

Selfdispersal/Explosive mechanism;

Tension/pressure is created inside a dry pod; pod opens (violently) along

lines of weaknesses; the two halves curl outwards; scattering the seeds;

17 a. Distinguish between mutations, mutants and mutagens

Mutations are sudden, spontaneous and permanent changes; in an

individual’s genetic material;

Mutants are individuals who develop and exhibit unusual characteristics that
were not previously present in the population; due to mutations;

Mutagens are factors in the environment;that cause mutations to occur;

51
b. Give two causes of mutations

Irradiations such as gamma rays and ultra violet rays; chemical substances
such as mustard gas and other heavy metals (mercury, lead, asbestos);
sudden extreme (high or low) temperatures;

c. Describe the causes and effects of chromosomal mutations

Deletion;

refers to the absence of a portion of a chromosome; it results from breakage

and falling off of a portion of a chromosome; leading to loss of a group of
genes that may have a disastrous effect on the development of an organism;

Inversion;

refers to reversal of normal sequence of genes in portion of a chromosome;

occurs when a middle portion of a chromosome breaks, turns or rotates
(inverts) through 180o and joins up again; this does not change the genetic
constitution of the organism; but may bring into close proximity genes whose
combined effects to an organism produce a beneficial effect to an organism;
or cause disadvantages to the organism;

Translocation;

attachment of a portion of a chromosome to a non-homologous

chromosome; occurs when a chromosome breaks and the portion joins
another non-homologous chromosome; this may lead to serious
consequences, even death depending on what genes are missing;

Duplication;

situation where a set of genes is represented twice in a chromosome; a part

of a chromatid formed during cell division may replicate further to form an

52
extra piece; which may attach onto the same or another chromatid; resulting
to traits controlled by some genes being excessively expressed;

Non-disjunction;

this is failure of a pair of homologous chromosomes to separate during the

first stage of meiosis; resulting in one of the daughter cells formed after
division of the cell having two of one kind of a chromosome; while the other
cell has less or none; diseases or disorders known as syndromes are known
to result from this aberration e.g. Down’s Syndrome
(Mongolism),Turner Syndrome, Klinefelter Syndrome;

Polyploidy;

this is the presence of more than two sets of chromosomes in a cell; occurs
due to a failure of a cell to divide after the first stage of meiosis or after the
chromosomes have replicated in mitosis; common in plants than animals; in
plants, it causes some improvements such as resistance to drought, certain
diseases and pests, improved yields and early maturity;

18. How is the mammalian eye adapted to its functions?

Sclera/sclerotic layer; white fibrous layer; made up of thick connective tissue;

protects the eye; maintains shape of eyeball;

Cornea; transparent; disc-shaped layer; that allows light to enter the eye;
refracts light towards the retina;

Conjunctiva; delicate membrane; lining the inside of the eyelid; protects the
cornea/eye;

Eyelids and eye lashes; thin muscle with hairs; protects the cornea/eye from
mechanical/chemical damage/protects the eye from entry of foreign
particles; protects retina from bright light;

Choroid; dark pigmented and membranous layer; that prevents light

reflection within the eye/absorbs light; to prevent distortion of the image;
has blood vessels; that nourish eye/retina/supply oxygen/remove carbon (IV)
oxide and wastes; extends to form the ciliary body and iris;

53
Ciliary muscles; have elastic muscles that contract and relax; to alter
shape/curvature of lens during accommodation;

Ciliary body; thickened front edge of the choroids layer; that produces
aqueous humour;

Suspensory ligaments; made up of elastic connective tissue whose

contraction and relaxation helps to adjust the shape of lens during
accommodation/holds lens in position;

Lens; transparent; biconvex; balloon-like; it refracts light rays/focus light

onto the retina;

Vitreous humour; nourishes cornea/lens; refraction of light; maintains eyeball

shape;

Iris; thin circular ring; with circular and radial muscles; it gives eye
colour/absorbs light; controls the amount of light entering the eye/adjusts
size of pupil;

Pupil; an aperture through which light enters the eye;

Retina; has photoreceptor cells/rods/cones for image formation; generates

impulses to the brain for interpretation;

Fovea/Yellow spot; with only cones; for high visual acuity/most sensitive part
of the retina;

Blind spot; point where nerve fibres emerge from the optic nerve/where optic
nerve leaves eye/point where nerve fibres and blood vessels enter the eye;

Optic nerve; transmits impulses to the brain;

Muscles; inferior and superior oblique muscles; move eye from left to right;
superior and inferior rectus muscles; move the eye up and down; external
and internal rectus muscles steady the eye in its up and down movement;

Tear/Lachrymal glands; secrete a watery and saline fluid containing

lysozymes/lytic enzymes/is antiseptic (tears); that moisten the conjunctiva
and cornea; washes away dust and other foreign objects; kills
microorganisms entering the eye;

19. Explain how the process of evolution may result to the formation
of a new Species.
54
For a new species to be formed, a population of organisms must become
completely isolated or separated from others; over long periods of time so
that any new variations that arise will not therefore flow to the other
population; there are various isolation mechanisms:

Geographical isolation; this is due to physical barriers such as

oceans/seas/deserts;

Ecological isolation; a barrier resulting from the occupation of different types

of habitats from the original type; it may be due to isolation for reasons of
feeding/predation/breeding; as well as environmental changes such as
climate and vegetation; which may result in a population living in different
habitats; to become ecologically separated from one another;

Behavioural isolation; alteration of behaviour proceeding mating; which

include courtship behaviour/lack of attraction between males and females in
different populations; due to production of different chemicals or
pheromones or colouration/songs;

Reproduction isolation; a barrier to successful mating between individuals of

a population; due to structural differences in reproductive organs; as well as
failure in fertilization/incompatibility;

Genetic isolation; even if fertilization takes place; the zygote may be

inferior/fails to develop; however if the zygote develops, the offspring may
be inferior or infertile/sterile;

19. Discuss the structure and functions of the various muscle

tissues found in humans

Smooth/Visceral Muscle; consists of spindle-shaped cells; made up of long

filaments or myofibrils; the cells lack cross striations and sarcolemma; they
are uninucleate/with one cell; they contract and fatigue slowly; to bring
about contraction and relaxation of the walls of blood vessels,

Urino-genital tract and the gut; which aids in blood flow, urine and sperm
flow and peristalsis of food respectively;

Skeletal/Striated muscles; made up of long cylindrical cells; with long

myofibrils running parallel to each other; the cells have cross
striations/stripes; are multinucleated; they form bundles of long fibres
55
attached to bones by tendons; they contract and fatigue rapidly; to bring
about movement of bones; (on the body) they contain contractile protein
myosin and actin;

Cardiac/Heart muscle; this is the muscle of the heart; is made up of short

cylindrical cells; with parallel myofibrils; the ends of each all are thickened
into intercalated discs; that connect adjacent cells; the myofibrils have cross
striations; each cell is uninucleate; the myofibrils contract without fatigue;

20.Describe the adaptations of the nervous system to its functions

The central nervous system consists of the brain; and the spinal cord; and
nerve fibres; that serve the sensory organs ; and the effector organs and
glands; the brain is a collection of millions/billions/109 neurones; that form
the biggest ganglion;

It is highly convoluted; to provide a large surface area for impulse reception,

processing and transmission;

The brain and the spinal cord are protected by the meninges; the brain and
the spinal cord have spaces/canals and ventricles; filled with a cerebrospinal
fluid; which acts as a bridge/supply medium for oxygen and nutrients; and
the removal of metabolic waste;

The brain has centres for the storage; retrieval and processing of impulses;
the cerebrum processes and stores information; the cerebellum; sends
impulses to joints and muscles; to correct balance; the medulla oblongata
sends impulses to the cardiovascular; and breathing/ventilation systems; to
regulate them;

The brain has the hypothalamus that secretes a neurosecretion to influence

a pituitary gland that secretes hormones; involved in reproduction; and
homeostatic functions; the hypothalamus; detects changes in temperature;
and osmotic pressure; and sends impulses to relevant effector organs for
their regulation; the thalamus; receives majority of the impulses and
channels them to the relevant areas of the brain;

Both the brain and the spinal cord have regions of the grey matter; that
enable very rapid processing/transmission of impulses;

56
The nervous system has neurones (relay/intermediate, motor and sensory);
that transmit impulses at a very rapid note/speed (100 ms-1) to and from the
central nervous system to effect suitable responses;

There exists in the central nervous system an electrochemical

gradient/concentration gradient; that allows for the generation of electrical
impulses;

They have numerous mitochondria; for generating energy for the function of
the sodium pump; which enables polarization and repolarisation; during
impulse transmission and refractory/recovery periods;

The spinal cord has no integration/association functions and is therefore

suited for reflex actions; needed in emergencies; the spinal cord is long; and
connects nerve fibres of the peripheral nerves with the brain for storage of
information; the spinal cord has a dorsal root for sensory fibres/neurones;
and a ventral root; for motor neurones/fibres;

21. Describe the defects that affect the mammalian eye and how
they could be corrected

Short-sightedness (myopia);

A condition where light rays from a distant object are focused in front of the
retina; while those from a near object are clearly focused on the retina; it is
caused by an abnormally elongated eyeball; or too much refractive power of
the eye lens;

-It is corrected by wearing concave/diverging lenses; which help to diverge

light rays; or reduce the refractive power of the eye before they reach the
eye lens;

Long-sightedness (Hypermetropia);

light rays from a near object are not focused by the time they reach the
retina; or may be focused behind the retina; while the rays from a distant
object are sharply focused; the defect is caused by an eyeball that is too
short; or a weak lens system (distance between lens and the retina is short);

-Corrected by wearing a convex/converging lens; which refracts light rays

before reaching the eye lens;this enhances refraction resulting in rays being
sharply focused onto the retina;

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Astigmatism;

rays from an object are brought to focus on different planes; due to unequal
curvature of the cornea/lens; causing unequal refraction of light entering the
eye; this defect is corrected by wearing special cylindrical lens in front of the
eye; the lens corrects the focus in the defective planes;

Colour-blindness;

a genetic defect; in which an animal is unable to distinguish between colours

particularly within the red-green spectrum; the retina lacks cones; pigments
that respond to colour vision;

Squintedness;

an eye defect in which extrinsic muscles of the eye; that controls the turning
of the eyeball do not co-ordinate accordingly on stimulation; it affects the
paired rectus muscles that move the eyeball up and down; and the lateral
rectus muscles that move the eyeball left to right; the eyeballs therefore
face different directions;making focusing and accommodation difficult to
achieve; corrected by specialized surgery;

Old sight (Presbyopia);

caused by old age; when supplies of nutrients and oxygen to the lens is far
much reduced; hence the cells of the lens die; elasticity is reduced; and
hence cannot change shape; and becomes fixed into a shape that is not
suitable for distant vision; managed by use of reading glasses that have
converging lenses; to give the eyes an extra power to manage close work;

Cataracts;

associated with old age; but may also be caused by an eye injury due to a
blow; or complications of diabetes mellitus; the eye lens become cloudy;
blocking transmission of light rays; protein fibres become denatured; and
clump together making the lens opaque; corrected by surgery; to replace the
defective lens with a normal one from a donor; or use of artificial lens;

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