chapter: 1,2,3,4,5,6

Chapter 1: Cell structure & organisation

All living organisms are made of cells, and there are several types of cells,
some of which share some common features. Humans comprise eukaryotic
cells, which contain a nucleus and membrane-bound organelles. A
microscope can obtain a more detailed structure of cells called the
ultrastructure.

Eukaryotic & prokaryotic cells:

All living things are made up of cells, which can be either eukaryotic or
prokaryotic.

Main components of eukaryotic cells:

●​ Cell membrane
●​ Cytoplasm
●​ Nucleus containing DNA

Main components of prokaryotic cells:

●​ Cell wall
●​ Cell membrane
●​ Cytoplasm
●​ Singular circular strand of DNA ( small rings of DNA found in the
cytoplasm)

The above structures are examples of organelles—structures in a cell with

distinctive functions.

Animal & plant cells:

The subcellular structures inside cells all have a specific function.

Structures in animal and plant cells:

 ●​ Nucleus
○​ Contains DNA coding for a particular protein needed to build
new cells.
○​ Enclosed in a nuclear membrane.
●​ Cytoplasm
○​ Liquid substance in which chemical reactions occur.
○​ Contains enzymes (biological catalysts, i.e., proteins that speed
up the reaction rate).
○​ Organelles are found in it.
●​ Cell membrane
○​ Controls what enters and leaves the cell.
●​ Mitochondria
○​ Where aerobic respiration reactions occur, providing energy for
the cell.
●​ Ribosomes
○​ Where protein synthesis occurs.
○​ Found on a structure called the rough endoplasmic reticulum.

Structures in plant cells only:

●​ Chloroplasts
○​ Where photosynthesis takes place, providing food for the plant.
○​ Contains chlorophyll pigment (which makes it green), which
harvests the light needed for photosynthesis.
●​ Permanent vacuole
○​ Contains cell sap.
○​ Found within the cytoplasm.
○​ Improves cells’ rigidity.
●​ Cell wall
○​ Made from cellulose.
○​ Provides strength to the cell.

Bacterial cells are prokaryotic, so they do not share as many similarities in

the type of organelles as animal and plant cells do.

Structures in a bacterial cell:

 ●​ Cytoplasm
●​ Cell membrane
●​ Cell wall
○​ Made of a different compound (peptidoglycan).
●​ Single circular strands of DNA
○​ As they have no nucleus, they float in the cytoplasm.
●​ Plasmids
○​ Small rings of DNA.

Cell specialisation:
Cells specialise by undergoing differentiation, which involves the cell
gaining new sub-cellular structures to suit its role. Cells can either
differentiate once early on or have the ability to differentiate their whole life
(these are called stem cells). In animals, most cells only differentiate once,
but many cells retain the ability in plants.

Example of specialised cells in animals:

1.​ Sperm cells: specialised to carry the male’s DNA to the egg cell
(ovum) for successful reproduction.
●​ Streamlined head and long tail to aid swimming.
●​ Many mitochondria supply the energy to allow the cell to move.
●​ The acrosome (top of the head) has digestive enzymes that
break down the outer layers of the egg cell membrane.
2.​ Nerve cells: specialised to transmit electrical signals quickly from one
place to another.
●​ The axon is long, carrying the impulses along long distances.
●​ Having loads of extensions from the cell body (dendrites)
means branched connections can form with other nerve cells.
●​ The nerve endings have mitochondria that supply energy to
make special transmitter chemicals called neurotransmitters.
These allow the impulse to be passed from one cell to another.
3.​ Muscle cells: specialised to contract quickly to move bones or simply
to squeeze, therefore causing movement.
 ●​ Special proteins (myosin & actin) slide over each other, causing
the muscle to contract.
●​ Lots of mitochondria provide energy for contraction.
●​ They can store a chemical called glycogen that is used in
respiration by the mitochondria.

Examples of specialised cells in plants:

1.​ Root hair cells: specialised to take up water by osmosis and mineral
ions by active transport from the soil as they are found in the tips of
roots.
●​ Have a large surface area due to root hairs, meaning more
water can move in.
●​ The large permanent vacuole affects the speed of movement of
water from the soil to the cell.
●​ Mitochondria provide energy from respiration for the active
transport of mineral ions into the root hair cell.
2.​ Xylem cells: specialised to transport water and mineral ions up the
plant from the roots to the shoots.
●​ Upon formation, a chemical called lignin is deposited, which
causes the cells to die. They become hollow and are joined
end-to-end to form a continuous tube so water and mineral ions
can pass through.
●​ Lignin is deposited in spirals, which helps the cells withstand
the pressure from the movement of water.
3.​ Phloem cells: specialised to carry the products of photosynthesis to
all parts of the plant.
●​ Cell walls of each cell form structures called sieve plates when
they break down, allowing the movement of substances from
cell to cell.
●​ Despite losing many sub-cellular structures, the energy these
cells need to be alive is supplied by the mitochondria of the
companion cells.
Tissues & organs:
●​ A tissue is a group of specialised cells with similar structures and
functions. They can be made of more than one type of cell.
Examples include muscular tissue or epithelial tissue.
●​ Organs are formed from several different tissues, working together to
produce a specific function. An example is the stomach, which has
muscular tissue and epithelial tissue.
●​ Organs are organised into organ systems, which work together to
perform a certain function. The stomach is part of the digestive
system, along with organs such as the liver and small intestine.

Organ systems:
The human digestive system is an organ system, made up of organs
working together to perform a certain function. The food you eat is large
and insoluble and needs to be broken down to be in a form that can be
absorbed by cells.

It’s made up of the following organs:

1.​ Glands (salivary gland & the pancreas), which produce digestive
juices, contain enzymes that break down food.
2.​ The stomach produces hydrochloric acid to kill bacteria and to
provide the optimum pH for the protease enzymes to work.
3.​ The small intestine is where soluble molecules are absorbed into the
bloodstream.
4.​ The liver produces bile, which is stored in the gall bladder and helps
with the digestion of lipids.
5.​ The large intestine absorbs water from undigested food to produce
faeces. This passes out of your body through the rectum and anus.

Diffusion:
… The spreading out of the particles of any substance in solution, or
particles of a gas, results in a net movement from an area of higher
 concentration to an area of lower concentration. It’s a passive process, as
no energy is required.

Substances can move over cell membranes via diffusion, into and out of
the cell. The molecules must be small to move across—for example,
oxygen, glucose, amino acids, and water—but starch and proteins cannot.
Examples of where this takes place in the body:
●​ Oxygen moves through the membranes of structures in the lung
called alveoli into RBCs and is carried to cells across the body for
respiration. Carbon dioxide (a waste product of respiration) moves
from the RBCs into the lungs to be exhaled. These movements of
gases are called gas exchange.
●​ Urea (a waste product) moves from the liver cells into the blood
plasma to be transported to the kidney for excretion.

Factors affecting the rate of diffusion:

Factor Effect
The greater the difference in
Concentaration gradient concentration, the faster the rate.
(difference in gradient) Because more particles are randomly
moving down the gradient than are
moving against.
—---------------------------------------------
The greater the temperature, the
Temperature greater the movement of particles,
resulting in more collisions and thus a
faster rate.
—----------------------------------------------
The greater the surface area, the
Surface area of the membrane more space for particles to move
through, resulting in a faster rate.

Single-celled organisms can use diffusion to transport molecules into their

body from the air because they have a relatively large surface area to
volume ratio. Due to their low metabolic demands, diffusion across the
surface of the organism is sufficient to meet its needs.

In multicellular organisms, the surface area to volume ratio is small so they

cannot rely on diffusion alone. Instead, surfaces and organ systems have
several adaptations that allow molecules to be transported in and out of
cells.

Examples:
​ In the lungs, oxygen is transferred to the blood and carbon dioxide is
transferred to the lungs. This takes place across the surface of millions of
air sacs called alveoli, which are covered in tiny capillaries, which supply
blood.

In the small intestine, cells have projections called villi. Digested food
is absorbed over the membrane of these cells into the bloodstream.

The roots of plants are adapted to take up water and minerals. Roots
have root hair cells with large surface areas, which project into the soil.

In the leaves of the plant, there are many different tissues to aid with
gas exchange. Carbon dioxide diffuses through the stomata for
photosynthesis, while oxygen and water vapour move out through them.
The stomata are controlled by guard cells, which change their size based
on how much water the plant receives (the guard cells swell with lots of
water and make the stomata bigger).

Osmosis:
Water can move across cells by osmosis, the movement of water from a
less concentrated solution to a more concentrated one through a partially
permeable membrane. A dilute solution has a high concentration of water (
and therefore a high water potential). A concentrated solution has a low
concentration of water (and a low water potential). Water moves from a
dilute solution to a concentrated one because it moves from an area of high
water potential to a low water potential—down the concentration gradient. It
is passive (doesn’t require energy).

Osmosis in animals:
●​ If the external solution is more dilute (higher water potential), it will
move into animal cells, causing them to burst.
●​ If the external solution is concentrated (lower water potential), excess
water will leave the cell, causing it to become shrivelled.

Osmosis in plants:
●​ If the external solution is more dilute, water will move into the cell and
the vacuole, causing it to swell, resulting in pressure called turgor
(essential in keeping the leaves and stems of plants rigid).
●​ If the external solution is concentrated, water will move out of the cell,
and it will become soft. Eventually, the cell membrane will move away
from the cell wall and it will die.

Active transport:
Active transport is the movement of particles from an area of lower
concentration to an area of higher concentration—against their
concentration gradient. This is an active process that requires energy from
respiration.

In root hairs:
●​ They take up water and mineral ions from the soil.
●​ Mineral ions are usually in higher concentrations in the cells, meaning
diffusions cannot take place.
●​ This requires energy from respiration to work.
In the gut:
●​ Substances such as glucose and amino acids from food have to
move from the gut into the bloodstream.
●​ Sometimes there can be a lower concentration of sugar molecules in
the gut than in the blood, meaning diffusion cannot take place.
 ●​ Active transport is required to move the sugar into the blood against
its concentration gradient.

Chapter 2: Cell division & Differentiation

Chromosomes:
The nucleus contains genetic information.
●​ This is found in the form of chromosomes, which contain coils of
DNA.
●​ A gene is a short section of the DNA that codes for a protein and, as
a result, controls a characteristic; therefore, each chromosome
carries many genes.
●​ There are 23 pairs of chromosomes in each cell of the body, inherited
from each parent, resulting in 46 chromosomes in each cell.
●​ Sex cells (gametes) are the exception: each gamete cell has half the
number of chromosomes—23 chromosomes.

Mitosis and the cell cycle:

The cell cycle is a series of steps that the cell has to undergo to divide.
Mitosis is a step in this cycle.
1.​ (interphase): the cell grows, organelles (ribosomes,
mitochondria) grow and increase in number, the synthesis of
proteins occurs, DNA is replicated (forming the characteristic ‘X’
shape) and energy stores are increased.
2.​ (Mitosis): The chromosomes line up at the cell's equator, and
cell fibres pull each chromosome of the ‘X’ to either side of the
cell.
 3.​ (cytokinesis): two identical daughter cells form when the
cytoplasm and cell membrane divide.

Cell division by mitosis in multicellular organisms is important in their

growth and development and when replacing damaged cells. Mitosis is also
a vital part of asexual reproduction, as this type of reproduction only
involves one organism, so to produce offspring, it simply replicates its cell.

Growth & Cell Differentiation

To become specialised and be suited to its rule, stem cells must undergo
differentiation to form specialised cells. This involves some of their genes
being switched on or off to produce different proteins, allowing the cell to
acquire different sub-cellular substances for it to carry out a specific
function.

In animals, growth occurs via cell division and differentiation. Cell

division occurs by mitosis, after which cells can differentiate into specialised
forms, specially adapted to their functions.
​ In animals, almost all cells differentiate at an early stage and then
lose this ability. Most specialised cells can make more of the same cell by
undergoing mitosis. Others, such as RBCs (which lose their nucleus),
cannot divide and are replaced by adult stem cells (which retain their ability
to undergo differentiation). In mature animals, cell division mostly only
happens to repair or replace damaged cells, as they undergo little growth.
In mature animals, cell division mostly happens to repair or replace
damaged cells, as they undergo little growth.

​ In plants, growth occurs through cell division and differentiation and

also through elongation. Plant cells can grow longer in a specific direction
by absorbing water into their vacuoles, and this is controlled by substances
called auxins.
​ In plants, many types of cells retain the ability to differentiate
throughout life. They only differentiate when they reach their final position in
the plant, but they still re-differentiate when it is moved to another position.

Stem Cells
A stem cell is an undifferentiated cell that can undergo division to produce
many more similar cells, some of which will differentiate to have different
functions.
​ Types of stem cells
1.​ Types of stem cells
○​ From when an egg and sperm cell fuse to form a zygote.
○​ They can differentiate into any type of cell in the body.
○​ Scientists can clone these cells (by culturing them) and
direct them to differentiate into almost any cell in the body.
○​ These could potentially be used to replace
insulin-producing cells in those suffering from diabetes,
new neural cells for diseases like Alzheimer’s or nerve
cells for those paralysed with spinal cord injuries.
2.​ Adult stem cells
○​ If found in bone marrow, they can form many types of
cells, including blood cells.
 3.​ Meristem in plants
○​ Found in root and shoot tips.
○​ They can differentiate into any type of plant—it may be
necessary if the parent plant has certain desirable
features (such as disease resistance) for research or to
save a rare plant from extension.

Therapeutic cloning involves an embryo being produced with the same

genes as the patient.
●​ The embryo produced could then be harvested to obtain the
embryonic stem cells.
●​ These could be grown into any cells the patient needed, such as new
tissues or organs.
●​ The advantage is that they would not be rejected as they would have
the same genetic make-up as the individual.

Benefits VS issues of research with stem cells:

Benefits Issues
Can be used to replace We do not understand the
damaged or diseased body process of differentiation, so it is
parts hard to control stem cells to
form the cells we desire
Unwanted embryos from fertility Removal of stem cells destroys
clinics could be used as they the embryo
would otherwise
Be discarded People may have religious or
ethical objections, as it is seen
as interference with the natural
process of reproduction
Research into the process of If the growing stem cells are
differentiation contaminated with a virus, an
infection can be transferred to
the individual
 Money and time could be better
spent in other areas of medicine

Cancer
Mitosis is just one part of the cell cycle, which is regulated by many
different genes to ensure that cells divide only when they need to and stop
when required

Cancer is caused as a result of changes in the DNA of cells that lead

to uncontrolled growth and division; this can result in the formation of
a tumour (a mass of cells)
○​ Usually, tumours form as a result of loss of control of the cell
cycle.
■​ Not all tumours are considered cancerous
■​ Benign tumours are growths of abnormal cells which are
contained in one area, usually within a membrane
■​ Crucially, benign tumours do not invade other parts of the
body
■​ Malignant tumour cells are cancers; the cells invade
neighbouring tissues and spread to different parts of the
body via the blood and lymphatic system, where they
form secondary tumours
■​ Malignant tumours are more likely to disrupt the
functioning of the organ they originate in (as they invade
healthy tissue) and the organs they spread to—this is why
they are dangerous and how they lead to death

Lifestyle Risk Factors & Cancer

●​ Anyone, at any age, can develop cancer, but increasing age and
many lifestyle factors are associated with an increased risk of having
cancer
 ●​ Treatments are constantly being developed, with targeted therapies
and immunotherapy helping to improve survival rates for many
different types of cancer
●​ Scientists have identified lifestyle risk factors for various types of
cancer:

Risk Types of cancer identified

with increased risk
Obesity—diet high in saturated fat Bowel, liver, and kidney
and sugars
Smoking—exposure to carcinogens in Lung, mouth, throat, and
cigarette smoke stomach
UV radiation—a type of ionising Skin
radiation
Viral infection— leads to disruption of Cervical (HPV); liver (hepatitis
the cell cycle and therefore B and C)
uncontrolled growth
●​ There are also genetic risk factors for many types of cancer;
inheriting faulty genes can make individuals more susceptible to
developing cancer
○​ Individuals with faulty mismatch repair (MMR) genes
responsible for proofing DNA are more likely to develop cancers
of the bowel and reproductive systems.
○​ Individuals with faulty BRCA genes are more likely to develop
breast and ovarian cancer than individuals with functioning
BRCA genes.

Chapter 3: Human biology—breathing

Exchanging material:
Single-celled organisms do not have a specialised transport system.
Substances can enter the cell by passive transport as the diffusion
distances are short. However, multicellular organisms require specialised
transport systems and gas exchange surfaces. This is because:
●​ Diffusion distance is greater
●​ Metabolic rate is higher
●​ The surface area to volume ratio is smaller
Therefore, large organisms have specialised gas exchange surfaces and a
mass transport system. In mammals, these include the alveoli and the
cardiovascular system.

Characteristics of a mass transport system:

●​ Vessels
●​ Directional movement
●​ Transport medium
●​ Maintenance of speed

Features of an efficient exchange surface:

●​ Having a large surface area.
●​ Being thin provides a shorter diffusion path.
●​ Having an efficient blood supply (in animals), which moves the
diffusing substances away, maintains a concentration gradient.
●​ Being ventilated (in animals) makes gas exchange more efficient by
maintaining a steep concentration gradient.

The respiratory system & gaseous exchange:

Key structures of the human respiratory system:
●​ Lungs: the main organs in the respiratory system, containing the
surfaces where gas exchange takes place.
●​ Ribs and intercostal muscles: the intercostal muscles are bound
between the ribs. Internal and external intercostal muscles work
antagonistically in pairs to expand and contract the rib cage during
breathing. The ribs protect the lungs and the heart from physical
damage.
●​ Larynx: contains the vocal cords.
 ●​ Trachea: connects the throat to the bronchi. C-shaped cartilage rings
are present to provide structural strength, keeping the trachea open
so the air can pass through.
●​ Bronchi: hollow tubes composed of cartilage rings that carry air from
the trachea to the lungs. The bronchi split into two tubes to enter the
left and right lung before branching further inside.
●​ Bronchioles: smaller tubes that branch off from the bronchi, leading
to the alveoli.
●​ Alveoli: where gas exchange occurs; compromising air sacs with a
capillary network. Oxygen from the air diffuses into the capillaries,
while waste CO₂ diffuses out by being exhaled.
●​ Diaphragm: a dome-shaped muscular division separating the thorax
(chest cavity) from the abdomen in mammals. It plays a major role in
breathing, as its contraction increases the volume of the thorax and
so inflates the lungs.

Ventilation: the act of moving air into and out of the lungs, allowing for
gaseous exchange.

Breathing in
●​ Internal intercostal muscles relax while external intercostal
muscles contract, pulling the ribs up and out while the
diaphragm flattens, pushing the abdominal muscles
downwards. The volume of the thorax (chest cavity) increases,
so air enters the lung rather than being sucked in. This is
because as the volume of the chest increases, there is a lower
concentration of air inside the lungs compared to outside, thus
air diffuses.
Breathing out
●​ The volume of the thorax decreases, increasing the pressure so
that air is forced out. This is passive, as it doesn’t require
muscle contraction except when forcibly breathed out, where
the internal intercostal muscles contract.
The majority of air in the atmosphere is composed of nitrogen, oxygen, and
carbon dioxide. Inhaled air is made up of more oxygen than exhaled air, as
oxygen is absorbed into the blood in the alveoli instead of being exhaled.
Oxygen is used in cells for respiration, and carbon dioxide is produced as a
waste product. This carbon dioxide is released from the blood at the alveoli
and diffuses out into the lungs before being exhaled; thus, there is more
carbon dioxide in exhaled air. Exhaled air also contains more water vapour
than inhaled air.

During physical activity, the rate and depth of breathing increase. When
exercise is carried out, muscles increase the rate of respiration to produce
energy for muscle contraction. Aerobic respiration requires oxygen; thus, a
greater amount of oxygen is demanded. In addition, a greater amount of
carbon dioxide is produced as a waste substance, which diffuses into the
blood. This increase in carbon dioxide in the blood is detected by the brain,
which causes the rate of breathing to speed up, allowing gas exchange to
happen more rapidly, expelling the carbon dioxide whilst taking in more
oxygen. The heart rate is also increased to pump substances around the
body more quickly in the blood.

Adaptations of exchange surfaces:

●​ Large surface area—allows more efficient diffusion. The alveoli allow
the lungs to have a huge surface area of 80–100 square metres.
●​ Thin surface—this means that there is a short diffusion distance,
thus exchange can occur more rapidly.
●​ Good blood supply—maintains concentration gradient by carrying
away substances that have diffused across already.
●​ Good ventilation with air—this means that waste gases can diffuse
out of the blood into the air in the lungs while oxygen diffuses into the
blood.
●​ Moist—allows gases to dissolve before diffusing across the
membrane.
The lungs are also adapted to protect from foreign pathogens and particles.
Goblet cells, found in the trachea and bronchi, are adapted to secrete
mucus into the respiratory tract. Foreign pathogens and particles stick to
this mucus, which is then moved upwards towards the throat by cilia
(hair-like projections from some cells). The mucus is then swallowed, and
pathogens are destroyed in the acidic conditions of the stomach.

Artificial breathing aids:

There are many different reasons why people sometimes struggle to
breathe and get enough oxygen into their lungs. For example:

●​ The tubes leading to the lungs may be very narrow so less air gets
through to them.
●​ The structure of the alveoli can break down. This results in a few big
air sacs that have a similar surface area for gas exchange to healthy
alveoli.
●​ Some people are paralysed in an accident or by disease so they can't
breathe.

There are many artificial aids for supporting or taking over breathing that
have saved countless lives. They work in two main ways: negative
pressure and positive pressure.

Negative pressure ventilators:

Polio is a disease that can leave people paralysed and unable to breathe.
To keep polio sufferers alive until their bodies recovered, an external
negative-pressure ventilator was developed. This was commonly known as
the iron lung. Nowadays we are all vaccinated against polio and it has
almost been wiped out worldwide.

The 'iron lung'—A patient is placed in a metal cylinder with their head
sticking out and a tight seal around their neck. Air is pumped out of the
chamber, lowering the pressure inside to form a vacuum. This causes the
chest wall to move up, increasing volume and decreasing pressure. Air
from outside is drawn into the lungs, but the vacuum automatically switches
off, causing the ribs to move down, lowering the volume and increasing
pressure inside the thorax, forcing air out of the lungs.

Positive pressure ventilators:

A positive pressure ventilator forces a carefully measured breath of air into
the lungs under positive pressure. Once the lungs have been inflated, the
air pressure stops. Then the lungs deflate as the ribs move down again,
forcing air out of the lungs.

It can be used in patients with many problems:

●​ It can be given using a simple face mask or a tube going into the
trachea. Doctors hold and squeeze positive-pressure ventilators in
emergency treatment; they’re simple and temporary but can save
lives.
●​ Full-scale positive-pressure ventilating machines can keep patients
alive during major surgery and help paralysed people survive for
years.

Respiration:
Respiration is a chemical reaction that happens in almost all cells in the
body to produce energy from nutrient molecules. This energy can be used
in a variety of processes, including:
●​ Muscle contraction.
●​ Protein synthesis.
●​ Send division.
●​ Active transport.
●​ Growth
●​ Nerve impulses.
●​ Maintaining a body temperature.

Aerobic respiration:
Aerobic respiration occurs in the presence of oxygen. Glucose is broken
down into carbon dioxide, water and energy with the help of oxygen. This
occurs in the cell mitochondria. Cells that require lots of energy, such as
muscle cells, therefore have high amounts of mitochondria.

Equations for aerobic respiration:

●​ Glucose + oxygen → carbon dioxide + water
●​ C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Anaerobic respiration:
Anaerobic respiration is a chemical reaction in cells that breaks down
nutrient molecules to release energy without using oxygen. It is less
efficient than aerobic respiration and produces less energy per glucose
molecule. It occurs in the cell cytoplasm and thus does not require
mitochondria.

Animal cells undergo anaerobic respiration during vigorous exercise as not

enough oxygen is delivered to muscles. In this reaction, glucose is broken
down to produce lactic acid as well as release energy. This lactic acid
builds up in muscles and causes muscle fatigue and aerobic respiration
also produces an oxygen debt. To repay this, the lactic acid must be
transported to the liver, where it is broken down into carbon dioxide and
water using oxygen. This is the reason why the breathing and heart rates
remain high after exercise.

Microorganisms such as yeast also undergo anaerobic respiration; yeast

breaks down anaerobically to form alcohol and carbon dioxide instead of
lactic acid.

Equations for anaerobic respiration in yeast:

●​ Glucose → alcohol + carbon dioxide
●​ C₆H1₂O₆ → 2C₂H₅OH + 2CO₂

Equations for anaerobic respiration in animal cells:

●​ Glucose → lactic acid
Chapter 4: Human biology—circulation
Large organisms require a circulatory system to supply substances like
oxygen and glucose to cells due to the limited surface area to volume ratio.
There are many common features of a circulatory system; some of these
are:
1.​ Suitable medium: in mammals, the transport medium is the blood. It
is water based so substances can easily dissolve into it.
2.​ Means of moving the medium: animals often have a pump known
as the heart to maintain pressure differences around the body.
3.​ Mechanism to control flow around the body: valves are used in
veins to prevent any backflow.
4.​ Close system of vessels: the circulatory system in most animals
and plants is closed and is branched to deliver substances to all parts
of the body.

The circulatory system in mammals is a closed double system, with the

heart pumping oxygenated blood to the lungs and supplying vital organs
and tissues.

Structure of the Human Heart and the Cardiac Cycle:

 ●​ 4 chambers: right and left atria, right and left ventricles.
●​ 4 main blood vessels:
○​ Pulmonary vein: connected to the left atrium and brings
oxygenated blood back from the lungs.
○​ Aorta: connected to the left ventricle and carries oxygenated
blood to all parts of the body except the lungs.
○​ Vena cava: connected to the right atrium and brings
deoxygenated blood back from the tissues except the lungs.
○​ Pulmonary artery: connected to the right ventricle and carries
deoxygenated blood to the lungs, where it is oxygenated and
the carbon dioxide is removed.

●​ Septum: muscle connective tissue that prevents oxygenated and

deoxygenated blood mixing.
●​ Coronary arteries: wrapped around the heart to supply the cardiac
muscle of the heart.
●​ Valves: make sure blood does not flow backwards.

Structure of blood vessels:

Arteries carry blood AWAY from the heart
●​ Layers of muscle in the walls make them strong. Also, contains
elastic fibres which allow them to stretch; this helps the vessels
withstand the high pressure created by the pumping of the heart
Veins carry blood TOWARDS the heart
●​ The lumen (the actual tube in which blood flows through) is wide to
allow the low-pressure blood flow to flow.
●​ They have valves to ensure the blood flows in the right direction.
Capillaries allow the blood to flow very close to cells to enable the
exchange of substances
●​ Capillary walls are one cell thick to create a short diffusion path
●​ Permeable walls so substance can move across
The process of circulation:

The process of circulation begins when deoxygenated blood from the body
returns to the right atrium of the heart via the superior and inferior vena
cava. From there, the blood is pushed through the tricuspid valve into the
right ventricle, which then contracts to send the blood through the
pulmonary valve into the pulmonary artery and toward the lungs. In the
lungs, carbon dioxide is exchanged for oxygen in the alveoli. The
oxygenated blood then travels back to the left atrium of the heart via the
pulmonary veins. From the left atrium, blood flows through the mitral valve
into the left ventricle, which contracts powerfully to pump blood through the
aortic valve into the aorta. The aorta distributes oxygen-rich blood to the
rest of the body through a network of arteries, and capillaries. At the
capillaries, oxygen and nutrients are delivered to the tissues, and waste
products like carbon dioxide are collected. The now deoxygenated blood
flows back through veins, eventually returning to the heart via the vena
cava, completing the cycle.

Coronary Heart Disease (CHD):

The coronary arteries supply blood to the heart and in CHD, these arteries
become blocked due to a buildup of fatty plaques (atherosclerosis). This
can cause ischaemia (lack of blood and oxygen), which can eventually lead
to muscle death and thus a heart attack.

Causes of CHD:
●​ Poor diet
○​ A diet rich in saturated fat increases cholesterol levels, which
can increase the chance of fatty plaques building up.
○​ High levels of salt can increase blood pressure, which damages
the blood vessels and increases the chances of fatty deposits
building up.
●​ Smoking
○​ Nicotine causes narrowing of blood vessels and increases
blood pressure, which can increase the chance of a blockage in
the coronary arteries.
 ●​ Stress
○​ Hormones produced in times of stress can increase blood
pressure, which can damage the vessel walls.

Transport in the blood:

Composition of blood:
Plasma
●​ The liquid that carries the components in the blood, e.g., cells,
platelets, amino acids, hormones, etc.
●​ Plasma is important for the transport of carbon dioxide,
digested food, urea, hormones and heat energy.
​ RBC
●​ Carry oxygen molecules from the lungs to all cells in the body.
●​ Contains haemoglobin, a red protein that combines with oxygen
to allow oxygen transport.
●​ No nucleus; to create more space for haemoglobin.
●​ Biconcave shape; to maximise surface area for oxygen to be
absorbed.
●​ Flexible, so they can fit through very narrow blood vessels.
​ WBC
●​ They are a part of the immune system, which is the body’s
defence against pathogens
●​ There are three types of WBCs:
1.​ Phagocytic WBCs
○​ A type of WBC that can undergo phagocytosis, where the
pathogen is engulfed, ingested, and killed.
○​ As phagocytosis can occur on any type of pathogen,
they’re considered to have a non-specific function.
2.​ Producing antibodies (lymphocytes)
○​ Each pathogen has an antigen on its surface. This
antigen has a specific structure complementary to an
antibody.
 ○​ Once antibodies begin to bind to the pathogen, the
pathogen starts clumping together, making it easier for
WBCs to find them.
○​ If the body is infected again with the same pathogen, the
specific complementary antibodies will be produced at a
faster rate. Therefore, the symptoms of the illness will not
be felt because the body is immune.
3.​ Producing antitoxins
○​ WBCs can bind to the toxins released by the pathogens
to neutralise them.
​ Platelets
●​ Platelets don’t have a nucleus and they are produced in the
bone marrow.
●​ When the skin is broken (i.e., there is a wound), platelets arrive
to stop the bleeding.
●​ A series of reactions (the clotting cascade) occur within the
blood plasma.
●​ Platelets release chemicals that cause proteins to form a mesh
of insoluble fibrin across the wound, trapping the RBCs and
thus forming a clot.
●​ The clot eventually develops into a scab to protect the wound
from the entry of bacteria.

The immune system & blood groups:

What is blood grouping?
●​ Blood grouping is the classification of blood into different blood groups
based on the presence or absence of blood group antigens on red
blood cells and antibodies in plasma.
Blood group system:
The ABO blood group system: It is the oldest and most important human
blood group system, which divides the blood into A, B, O, and AB types.
●​ The classification is based on the presence of either A antigen or B
antigen or none of the antigens in the membrane of RBCs and the
 presence of anti-A or anti-B or both anti-A and anti-B antibodies on the
blood plasma. The A and B antigens are glycoproteins found on the
membrane of RBCs. These glycoproteins are encoded by the ABO
gene.
●​ If the RBCs’ membrane contains A antigen and the plasma contains
anti-B antibodies, then the blood is classified as A-type blood.
●​ If the RBCs’ membrane contains B antigen and the plasma contains
anti-A antibodies, then the blood is classified as B-type blood.
●​ If the RBCs’ membrane contains both the A antigen and the B antigen
and the plasma contains none of the antibodies, then the blood is
classified as AB-type blood.
●​ If the RBCs’ membrane contains none of the antigens and the plasma
contains both the anti-A and the anti-B antibodies, then the blood is
classified as O-type blood.
●​ The A antigen and B antigen can violently react with anti-A and anti-B
antibodies, respectively, leading to clumping of RBCs and immediate
blood clot formation. This type of immune reaction causes a severe
haemolytic reaction and there is a very high chance that a person will
die if left untreated. Hence, ABO is considered one of the most
important blood group systems, and human blood is always first
classified according to the ABO type before transfusion or use for any
medical or research purpose.
Chapter 5: Human Biology—digestion
The digestive system is an organ system, made up of organs working
together to perform different functions. The food that people eat is large
and insoluble and needs to be broken down for it to be absorbed by cells.
It’s made up of:
1.​ Glands (salivary glands & the pancreas) produce digestive juices
containing enzymes that break down food.
2.​ The stomach produces hydrochloric acid to kill bacteria and provide
the optimum pH for the protease enzyme to work.
3.​ Duodenum is the first part of the small intestine and it’s responsible
for receiving partially digested food from the stomach and beginning
the absorption of nutrients.
4.​ The small intestine is where soluble molecules are absorbed into
the blood.
5.​ The liver produces bile, which is stored in the gall bladder and helps
with the digestion of lipids.
6.​ The large intestine absorbs water from undigested food to produce
faeces, which then pass out of the body through the rectum and
anus.
The digestion process:
The process of digestion begins in the mouth, where mechanical digestion
occurs. The teeth chew food into smaller pieces, breaking it down to
increase its surface area for enzyme action. Saliva, secreted by the salivary
glands, contains amylase, which begins the chemical digestion of starch
into maltose. The tongue shapes the food into a bolus, lubricated by saliva,
so it can be easily swallowed.

Next, the bolus travels down the oesophagus, a muscular tube connecting
the mouth to the stomach. Through wave-like contractions known as
peristalsis, the oesophagus pushes the bolus down without relying on
gravity.

In the stomach, food undergoes mechanical digestion through churning

actions and chemical digestion as protease enzymes start to break down
proteins. The stomach also produces hydrochloric acid, which kills bacteria
in the food and provides the acidic environment required for protease
enzymes to function effectively.

The partially digested food then enters the small intestine, beginning with
the duodenum, where enzymes from the pancreas continue digestion. The
pancreas secretes all three types of digestive enzymes—amylase,
protease, and lipase—into the duodenum, alongside an alkaline fluid to
neutralise the stomach's acidic contents. The small intestine is slightly
alkaline, with a pH of around 8, creating optimal conditions for enzyme
activity. In the ileum, the final section of the small intestine, digested food
molecules are absorbed into the bloodstream. The walls of the ileum are
lined with villi, tiny finger-like projections that increase the surface area for
efficient absorption.

From there, the remaining material enters the large intestine, where water
is absorbed, and the undigested matter is compacted into faeces. The
faeces are stored in the rectum and eventually expelled through the anus.

The liver plays a crucial role by producing bile, which emulsifies fats,
breaking large droplets into smaller ones to aid the action of lipase. The
liver is also responsible for deamination, which removes amino acids no
longer needed, producing urea. The gall bladder stores bile until it is
needed in the duodenum.

Enzymes: biological catalysts ( a substance that increases the

rate of reaction without being used up).
●​ They are protein molecules and the shape of the enzyme is vital to its
function because each enzyme has its own uniquely shaped active
site where the substrate binds.
●​ Enzymes are present in many reactions so that they can be
controlled.
●​ They can both break up large and small molecules.

The lock and key hypothesis is a simplified explanation of how enzymes

work:
1.​ The shape of the substance is complementary to the shape of the
active site, so when they bond, it forms an enzyme-substrate
complex.
2.​ Once bound, the reaction takes place and the products are released
from the surface of the enzyme.
Enzymes require an optimum pH and temperature because they are
proteins.
●​ The optimum temperature is a range around 37℃
○​ The rate of reaction increases with an increase in temperature
up to this optimum, but above this temperature, it rapidly
decreases, and eventually, the reaction stops.
○​ When the temperature becomes too hot, the bonds in the
structure will break; this changes the shape of the active site, so
the substrate can no longer fit in.
○​ The enzyme is said to be denatured and can no longer work.
●​ The optimum pH for most enzymes is 7, but some that are produced
in acidic conditions have a lower optimum pH.
○​ If the pH is too high or too low, the forces that hold the amino
acid chains that make up the protein will be affected.
 ○​ This changes the shape of the active site, so the substrate can
no longer fit in.
○​ The enzyme is said to be denatured and can no longer work.

As molecules need to be broken down in the digestive system to be

absorbed into the bloodstream, enzymes are vital. They are released by
cells in many different places and they are specific to a certain type of
molecule.
1.​ Carbohydrases convert carbohydrates into simple sugars.
●​ Example: amylase breaks down starch into maltose.
○​ It’s produced in the salivary glands, pancreas, and small
intestine (most starch is digested by them).
2.​ Proteases convert proteins into amino acids.
●​ Example: pepsin (produced in the stomach); other forms can be
found in the pancreas and small intestine.
3.​ Lipases convert lipids into fatty acids and glycerol.
●​ Produced in the pancreas and small intestine.
Soluble glucose, amino acids, fatty acids, and glycerol pass into the
bloodstream to be carried to all the cells around the body. They are used to
build new carbohydrates, lipids, and proteins, with some glucose being
used in respiration.

Bile is produced in the liver, stored in the gall bladder and then released in
the small intestine. It has two roles:
1.​ It is alkaline to neutralise the hydrochloric acid that comes from the
stomach—the enzymes in the small intestine have a higher (more
alkaline) optimum pH than those in the stomach.
2.​ It breaks down the large drops of fat into smaller ones (emulsifies it).
The larger surface area allows lipase to chemically break down the
lipid into glycerol and fatty acids faster.
Chapter 6: Nervous coordination & behaviour

Responding to change:
●​ The nervous system:
○​ Communication is via electrical impulses
○​ The effects are short-lived; for example, causing muscle
contraction
○​ The response is localised
○​ Faster responses

The nervous system is composed of the central nervous system (CNS),

which is made up of the brain and spinal cord, and the peripheral nervous
system (PNS), which extends beyond the brain and spinal cord to the rest
of the organism.

The NS is made up of receptor cells that detect changes in the internal and
external environment known as stimuli. The sensory, motor, and relay
neurones of the CNS coordinate a response to the stimulus and decide
what to do, and the effector brings about the response. Effectors can be
muscles that contract or relax as a response or glands that secrete
chemicals such as hormones or enzymes.

Structure of neurones:
●​ Cell body (nucleus, cytoplasm, mitochondria, etc.)
●​ Extensions known as dendrites are involved in conducting impulses
towards the cell body.
●​ Axons that conduct impulses in the opposite direction of the cell body.

Types of neurones:
●​ Sensory neurones: transmit impulses from the receptors to the CNS.
●​ Relay neurones: located in the CNS, they transmit the electrical
impulse from sensory neurones to motor neurones.
●​ Motor neurones: transmit electrical signals from the CNS to the
effector organs.
Spinal reflex arc:
The reflex arc is an automatic response and its role is to protect the body
from harm. A reflect arc occurs as follows:
1.​ A potentially harmful stimulus is detected by receptors.
2.​ Sensory neurones take an impulse to the relay neurones in the CNS.
3.​ A response is coordinated in the spinal cord so the impulse does not
need to travel to the brain. This makes the response faster, thus more
likely to protect the body.
4.​ The impulse passes directly from the relay neurone to a motor
neurone, which carries the impulse to the effector organ.
5.​ The effector orga carries out the response, like moving away from the
stimulus.

Animal behaviour:
Components of behaviour
●​ Nature/Innate—Instinct or gene determines behaviour.
●​ Nurture/Learnt—Experience and learning influence behaviour.

Types of behaviour:
1.​ Habituation
○​ The decrease in an animal's behaviour in response to a stimulus
brought about by repeated exposure to the stimulus without
reinforcement. It is usually considered to be a form of learning
involving the elimination of behaviours that are not needed by
the animal.
2.​ Imparenting
○​ Imprinting is a learning process in which young animals
associate themselves with another organism, usually a parent
or large object. This can lead to social attachments and
difficulties in forming normal relationships with other species. In
natural situations, imprinting allows offspring to rapidly acquire
skills possessed by their parents, such as flying and hunting. It
can also affect an animal's future choice of a sexual partner.
However, it can have negative effects on later life.
3.​ Associative learning
○​ Associative learning simply means that an animal learns to
associate an event with a result.
i.​ Classic conditioning: involves pairing a neutral stimulus
with a reflexive response.
ii.​ Operant conditioning: involves reinforcing or punishing
voluntary behaviours to increase or decrease their
frequency.
chapter: 7,8,9, 10, 11, 12, 13, 14
Chapter 7: Homeostasis
...the maintenance of a constant internal environment. Mechanisms are in
place to keep optimum conditions, despite internal and external changes.
This is needed for enzyme action and all cell functions.

Homeostasis controls:
1.​ Blood glucose concentration
2.​ Body temperature
3.​ Water levels
4.​
1.​ Blood glucose concentration
The concentration of glucose in the blood needs to be kept within a certain
limit because cells use glucose for respiration. The pancreas controls this
concentration.
Eating foods that contain carbohydrates increases the glucose levels in the
blood.
●​ If glucose levels are too high, the pancreas produces the hormone
insulin.
●​ If insulin binds to cells in target organs (muscle & liver), causing:
→ 1) glucose to move from the blood into muscle cells for respiration.
→ 2) Excess glucose to be converted into glycogen stored in the liver.
●​ The blood glucose concentration is reduced.

Rigorous activity, e.g. exercise, uses glucose for respiration, and therefore
this is less in the blood.
●​ If glucose levels decrease, the pancreas produces the hormone
glucagon.
●​ Glucagon binds to the liver cells, causing glycogen to be broken
down into glucose.
●​ Glucose is released into the blood, increasing the blood glucose
concentration.
Diabetic diseases:
1.​ Type 1 diabetes: The pancreas cannot produce enough insulin.
●​ Blood glucose levels rise to a fatal amount.
●​ Glucose is discharged with urine.
●​ Individuals are always very thirsty.
●​ It’s treated with insulin injections at mealtime.
2.​ Type 2 diabetes: the body cells no longer respond to insulin.
●​ Blood glucose levels rise to a fatal amount.
●​ Obesity is a risk factor.
●​ Treatments include reducing the number of simple carbs in the
diet, losing weight, and increasing exercise.
●​
2.​ Control of body temperature
The thermoregulatory centre, which monitors and controls body
temperature, is found in the brain.
The human body temperature is 37.5 degrees Celsius. If it becomes
too high:
●​ Sweat is produced from sweat glands.
●​ Vasodilation means more blood flow closer to the skin’s
surface, resulting in increased energy transfer from the body.
If it decreases too much:
●​ Sweating stops.
●​ Skeletal muscles contract rapidly (shivering) to generate heat
from respiration.
●​ Hairs stand on the end to create an insulating layer, trapping
warmth.
●​
3.​ Maintaining Water balance/levels
Osmosis is the process by which water molecules move from a region of
high concentration to a low concentration.
●​ If the water concentration of the blood increases, then cells in the
body take up water.
Cells expand as more water is taken in, leading them to burst.
 ●​ If the water concentration of the blood decreases, then the cells lose
water and eventually shrink.
If body cells lose or gain too much water by osmosis, they do not function
properly.

Chapter 8: Pathogens & Diseases

A pathogen is an organism that causes disease. Pathogens include
bacteria and viruses. Organisms that harbour these pathogens are referred
to as hosts. Pathogens can be passed from one host and are thus called
transmissible diseases.

Pathogens are transmitted through:

●​ Direct contact: the pathogen can be passed from the host via the
transfer of blood and other bodily fluids.
●​ Indirectly: from contaminated surfaces, foods, animals, and air.

Defence against infections:

The body’s first line of defence includes:
●​ Mechanical barriers: nose hairs and skin.
●​ Chemical barrier: mucus, stomach acid, and tears.

Antibodies & Antigens:

Pathogens are detected by white blood cells and destroyed through an
immune response. Each pathogen has a specific antigen protein and
shape. Lymphocytes produce antibodies that bind to the antigen, creating
an antibody-antigen complex. The pathogen clumps together, making them
harmless. They can be killed directly or marked for destruction by
phagocytes.
Active immunity:
A defence mechanism against a pathogen by antibody production. It can be
gained after an infection or through a vaccination.

Infection:
After the pathogen has been killed, some of the lymphocytes remain as
memory cells. This means that if the same pathogen ever enters the body
again, the lymphocyte would recognise the antigens and be able to
produce new antibodies more quickly. Memory cells stay in the body for
years, giving long-term immunity.

Vaccination:
1.​ The patient is given an inactive or attenuated version of the pathogen
or its antigens.
2.​ The antigens evoke an immune response by lymphocytes, in which
antibodies are produced.
3.​ Memory cells are produced, giving long-term immunity.

Vaccinations can be used to control the spread of disease by providing

herd immunity. Where a large amount of the population is vaccinated and
are thus immune to the pathogen, so the disease cannot spread as there
are only a few people left to be infected. The few that cannot be
vaccinated, for example, due to medical reasons, are therefore protected
against the disease.

Passive immunity:
Passive immunity is a short-term defence against a pathogen and can be
gained through acquiring antibodies from another individual. One example
of passive immunity is antibodies being passed to a baby through the
mother's milk; thus, babies need to be breastfed to reduce the risk of
diseases. It can also be gained through injections of antibodies from a
donor.
Passive immunity is short-term as no memory cells are produced.

Drugs:
A drug is a substance that, when taken into the body, affects the chemical
reactions that take place. There are a variety of different drugs that treat
different diseases.

Antibiotics:
Antibiotic drugs are used to treat bacterial infections. Some antibiotics kill
bacteria by destroying their cell wall, leading to the cell bursting, while
others inhibit the growth of the bacteria. Viruses cannot be killed by
antibiotics as they do not grow and reproduce in the same way as bacteria
and do not have the same structure.

Some bacterial strains become resistant to antibiotics as the result of

natural selection:
1.​ A mutation occurs in a bacterial cell that makes it resistant to the
antibiotic.
2.​ When the antibiotic is administered, the cell is not killed, whereas
cells that have not become resistant are.
3.​ The resistant cell can therefore survive and reproduce, producing
more resistant bacteria.

Resistance to antibiotics results in antibiotic-resistant bacterial infections in

hospitals, such as MRSA. It is therefore important to try and slow the
development of resistant bacterial strains. This can be done by only using
antibiotics for serious infections and always completing the full course of
antibiotics to make sure that all the bacteria are killed.
Chapter 9: Plants as Organisms

Photosynthesis:
Photosynthesis is a reaction in which light energy is converted to chemical
energy in the form of glucose. Oxygen is a waste product of this reaction
and is released into the atmosphere.

Photosynthesis occurs in the chloroplasts, which are adapted for

photosynthesis in the following way:
●​ They contain the photosynthetic pigment chlorophyll, which absorbs
light energy and converts it to chemical energy.

The rate of photosynthesis:

The rate of photosynthesis is determined by limiting factors such as carbon
dioxide concentration, light intensity, light wavelength, and temperature.
The rate of photosynthesis increases as these factors increase; however, at
a high light intensity and temperature, the leaves can be damaged and
enzymes denature, thus the rate is slowed. These factors can be controlled
when growing crops to maximise efficiency and yield. This can be done by
growing crops in greenhouses.

Exchange in plants:
Adaptations of Gas Exchange Surfaces:

●​ Effective exchange surfaces in organisms have:

○​ A large surface area
○​ Short diffusion distance
○​ Concentration gradient (maintained)

To carry out photosynthesis, plants must have an adequate supply of

carbon dioxide. Leaves have evolved adaptations to aid the uptake of
carbon dioxide
 ●​ Structure of a leaf:
○​ Waterproof cuticle
○​ Upper epidermis: a layer of tightly packed cells
○​ Palisade mesophyll layer: a layer of elongated cells containing
chloroplasts
○​ Spongy mesophyll layer: a layer of cells that contains an
extensive network of air spaces
○​ Stomata: pores (usually) on the underside of the leaf which
allow air to enter
○​ Guard cells: pairs of cells that control the opening and closing
of the stomata
○​ Lower epidermis: a layer of tightly packed cells
●​ Mechanism:
○​ When the guard cells are turgid (full of water), the stoma
remains open, allowing air to enter the leaf.
○​ The air spaces within the spongy mesophyll layer allow carbon
dioxide to diffuse into cells rapidly.
○​ The carbon dioxide is quickly used in photosynthesis by cells
containing chloroplasts, maintaining the concentration gradient.
○​ No active ventilation is required, as the plant tissues' thinness
and stomata's presence help create a short diffusion pathway.

Transport in plants:

1.​ Osmosis: The movement of water molecules from an area of high

water concentration to an area of low water concentration.
2.​ Diffusion: The movement of molecules (gas or liquid) from an area of
high concentration to an area of low concentration.
3.​ Active transport: The movement of molecules or ions across a cell
membrane from an area of low concentration to an area of high
concentration.
4.​
There are two transport systems in a plant:
One that transports water and minerals from the root to all other parts.
These vessels are called the xylem. Another vessel transports sugars
(sucrose) and amino acids made in leaves to all other plant parts. These
are called phloem vessels.

Xylem adaptations:

●​ Surrounded by thick walls with lignin.

●​ They are hollow, with no cell contents, providing more room to
transport water.
●​ Xylem cells are joined end-to-end with no cross walls to form a long,
continuous tube for water to pass through.

Water uptake:

Water is taken up by the root hair cells via osmosis before entering the
xylem vessel. At the leaves, it diffuses into mesophyll cells, where it is used
in a metabolic reaction called photosynthesis.

The root hairs increase the surface area of the cell. This increases the rate
of osmosis into the root, which maximises the rate of water uptake. The
rate of ion uptake is also increased by the active transport. They also have
a thin wall, so the diffusion distance is shortened.

Transpiration:

When the plant opens its stomata to let in carbon dioxide, water on the
surface of the cells of the spongy mesophyll and palisade mesophyll
evaporates and diffuses out of the leaf. This process is called transpiration.

Translocation:

Photosynthesis produces glucose in the green parts of plants, which are

often leaves. This is then converted into sucrose. The sucrose is
transported around the plant in phloem vessels. It needs to be able to
reach all cells in the plant so that the sucrose can be converted back into
glucose for respiration. The movement of sucrose and other substances
like amino acids around a plant is called translocation.

Chapter 10: Genetics, Inheritance &

Reproduction
Chromosomes, genes, and proteins:
Keywords
●​ Chromosome: a thread-like structure of DNA that carries genetic
information in the form of genes.
●​ Gene: a length of the DNA that codes for a particular protein.
●​ Allele: one or several different versions of a gene.
●​ Diploid nucleus: nuclei that contain a full set of chromosomes (23
pairs→ 46).
●​ Haploid nucleus: nuclei that only contain half the number of
chromosomes. These are egg and sperm cells that fuse during
fertilisation to produce a diploid cell.

Mitosis:
Mitosis is a form of cell division. During mitosis, nuclear division of a parent
cell occurs, producing two genetically identical daughter cells. Mitosis is
used to create new cells in the body to repair and replace old or damaged
tissues, as well as allowing growth of the organism and playing a role in
asexual reproduction. Mitosis involves the splitting of chromosomes into
their two halves, each of which is known as a chromatid.

Meiosis:
Meiosis is used to make four genetically unique daughter cells and is used
in the production of gametes. During meiosis, the chromosome number is
halved, and a diploid cell divides to produce four haploid cells. As each
gamete produced is genetically unique, each of the offspring will also be
unique. This is beneficial for species as it produces genetic variation.
DNA structure:
DNA stands for deoxyribonucleic acid. It’s a coiled double helix and is a
polymer that contains instructions for the body.
●​ DNA is made up of many small parts called nucleotides. Each is
made up of one sugar, one phosphate, and one of the four organic
bases.
●​ The four types of organic bases are A, C, G, and T. (Adenine,
Cytosine, Guanine, Thymine)
●​ Each base is connected to another base in the other strand of the
DNA.
●​ A connects to T, and C connects to G.
●​ Each group of three bases codes for an amino acid that are joined
together to make a protein.

Monohybrid Inheritance:
Keywords
●​ Inheritance: the transmission of genetic information from generation
to generation.
●​ Genotype: the genetic makeup of an organism, consisting of all
alleles present.
●​ Phenotype: the observable features of an organism as a result of the
expression of particular alleles of the gene.
●​ Homozygous: an organism containing two identical alleles of a
particular gene.
●​ Heterozygous: an organism containing two different alleles of a
particular gene.

An offspring inherits characteristics from both parents and two sets of

genes are inherited, one from each. If both parents pass down the same
allele for a particular trait, e.g., both pass down the allele for blue eyes, the
offspring will have two identical alleles for this trait, which is referred to as
homozygous. If two separate alleles are passed down, e.g., the mother has
blue eyes and the father has brown eyes, the offspring will have two
different alleles for the gene, which is called heterozygous.
Alleles can be dominant and recessive. A dominant allele is always
expressed if present, whereas the recessive allele is only expressed in the
absence of the dominant allele.

Inherited Disorder:
Genetic disorders are caused by inheriting certain alleles.
Polydactyly: having extra fingers or toes
●​ Caused by a dominant allele.

Cystic fibrosis: this is a disorder of the cell membranes, resulting in thick

mucus in the airways and pancreas.
●​ Caused by a recessive allele.
●​ Both parents need either to be carriers or one has CF themselves
and the other is a carrier.

Variations:
The phenotype an organism has depends on two things:
1.​ Genotype: the genes it inherited
●​ Genes are passed on from the parent in sex cells.
●​ The combining of the genes from both parents creates genetic
variation.
●​ Only identical twins have the same genotype.
●​ There is a lot of genetic variation in the population.
2.​ Environmental: the place it lives in
●​ The conditions the organism grows and develops in also affect
its appearance.
●​ Examples include scars in animals or smaller and yellow leaves
in plants.

Sometimes characteristics can result from a combination of genetics and

the environment, such as weight. Weight depends on the food you eat, but
also how quickly your body breaks it down and how much it stores as fat
depends on your genes.
 Punnett square diagrams:
A single gene cross looks at the probability of the offspring of two parents
having certain genotypes and phenotypes. This is done using the allele the
two parents have for a gene and a Punnett square diagram.

Uppercase letters are used to represent dominant characteristics.

Lowercase letters represent recessive characteristics.

E.g., 1) Crossing two heterozygous green and yellow pea plants:

G= green (dominant)
g= yellow (recessive)

Parent one → G g
Parent two ↓
G GG Gg
g Gg gg
The outcomes are GG, Gg, Gg and gg. As G is dominant, there is a 75%
chance that the offspring will display this allele in the phenotype and be
green. There is a 25% chance that the offspring will be yellow; therefore,
the ratio is 3:1.

E.g., 2) Crossing a homozygous recessive (yellow) pea plant with a

heterozygous pea plant:
G= green (dominant)
g= yellow (recessive)

Parent one → g g
Parent two ↓
G Gg Gg
g gg gg
The outcomes are Gg, Gg, gg and gg. There is a 50% chance of the
offspring being green or yellow; therefore, the ratio is 1:1.

Pedigrees:
A pedigree diagram is used to see the pattern of inheritance of a trait in
different generations of a family.
●​ Males are represented by a square and females are represented by a
circle.
●​ Affected individuals are filled in, and unaffected individuals are
unfilled.
●​ Horizontal lines link males and females, which are mates.
●​ Vertical lines link couples to their offspring.

In the pedigree above, every generation has affected individuals.

There are four females and one male affected. The rest of the
members are unaffected.

If a trait appears in every generation, it is most likely a dominant trait.

The pedigree above shows a dominant trait.

If a trait skips generations, it tends to be a recessive trait. Individuals

can carry a recessive allele without expressing the trait themselves.
They can still pass it on to their offspring. If the offspring inherit the
recessive allele from both parents, the offspring will be affected. This
 is represented in pedigrees as unaffected parents having affected
offspring.

Chapter 12: Evolution, adaptations &

interdependence

Theories of evolution:
1)​ Jean-Baptiste Lamarck’s Theory of inheritance of acquired
characteristics:
●​ Changes that occurred during the lifetime of an organism were
passed onto offspring.
●​ If an individual continually repeated an action, the
characteristics that allowed it to do this would develop further.
●​ For example, if a giraffe stretched to leaves on a tree high up,
its neck would become longer, allowing it to do that more easily.
This trait would then be passed on to its offspring.
Lamarck’s theory was proven wrong when it was understood that changes
caused by the environment were not passed on in the sex cells.

2)​ Charles Darwin’s theory of evolution by natural selection:

●​ Variation exists within species as a result of mutations in DNA.
●​ Organisms with traits most suited to the environment are more
likely to survive to reproductive age and breed
successfully—called survival of the fittest.
●​ The beneficial traits are then passed on to the next generation.
●​ Over many generations, the frequency of alleles for these
advantageous traits increases within the population.
Darwin’s theory was supported by genetics as it provided a mechanism for
beneficial traits caused by mutations to be passed on. Fossil evidence
showed how developments in organisms arose slowly.

There was lots of controversy surrounding his ideas for many reasons:
 I.​ It contradicted the idea that God was the creator of all species on
earth.
II.​ There was not enough evidence at the time, as few studies had been
done on how organisms change over time.
III.​ The mechanisms of inheritance and variation were not known at the
time.

Speciation:
Alfred Wallace developed the theory of speciation alongside Darwin. He
proposed that new species develop through the selection of different
alleles. This increases the genetic variation until the new population cannot
breed with those in the old population to produce fertile offspring.

The process of speciation:

1.​ Variation exists within a population as a result of genetic mutations.
2.​ Alleles that provide a survival advantage are selected through natural
selection.
3.​ Populations of species can become isolated, for example, through
physical berries such as a rock fall, preventing them from breeding
together.
4.​ Different alleles may be advantageous in the new environment,
leading to them being selected.
5.​ Over time, the selection of different alleles will increase the genetic
variation between the two populations.
6.​ When they are no longer able to breed together to produce fertile
offspring, a new species has formed.

Example: Speciation at Mount Bosavi

In 2009, an expedition to Mount Bosavi in Papua New Guinea discovered
over 40 new species, including the giant Bosavi woolly rat, which is larger
than native rodents.

I.​ Explain how these two species of rodent could have developed from
the same common ancestor.
isolation:
●​ When the crater collapsed, any rodents living there
will have been isolated from the other rodents on
the island.
Different condition:
●​ The conditions in the crater will have been different
to the conditions outside. There are fewer large
predators and the climate might be different.
Evolution by natural selection:
●​ The larger rats will have been more able to survive
and reproduce in the environment within the crater.
They will have passed on the alleles for these
characteristics to their offspring.
Speciation:
●​ Eventually, the rats will have become so different
to the rodents outside the crater, that they could
no longer interbreed to produce fertile offspring.

Adapting for survival:

An individual is part of a species but lives in its habitat within a population.
Many different populations interact in the same habitat, creating a
community. The populations are often dependent on each other. An
ecosystem is the interaction of a community with non-living (abiotic) parts
of the environment. Organisms are adapted to the conditions of the
environment.

Organisms that need the same resources compete for it.

●​ There can be competition within a species or between different
species.
●​ Plants may compete for light, space, water and mineral ions.
●​ Animals may compete for territory, food, water, and mating partners.

Interdependence describes how organisms in a community depend on

other organisms for vital services.
 ●​ These include food, shelter, and reproduction (pollination, seed
dispersal); e.g., birds take shelter in trees, and flowers are pollinated
with the help of bees.
●​ The removal or addition of a species to the community can affect the
populations of others greatly, as it changes prey or predator numbers.
●​ A stable community is one where all the biotic (living) and abiotic
(non-living) factors are in balance.
○​ As a result, the population sizes remain roughly constant.
○​ When they are lost, it is very difficult to replace them.
○​ Examples include tropical rainforests, oak woodlands, and coral
reefs.

Abiotic factors that can affect a community:

1.​ Light intensity
●​ Light is required for photosynthesis.
●​ The rate of photosynthesis affects the rate at which the plant
grows.
●​ Plants can be food sources or shelter for many organisms.
2.​ Temperature
●​ Temperature affects the rate of photosynthesis.
3.​ Moisture levels
●​ Both plants and animals need water to survive.
4.​ Soil pH & mineral content
●​ Soil pH affects the rate of decay and therefore how fast mineral
ions return to soil ( which are then taken up by other plants).
●​ Different species of plants thrive at different nutrient
concentration levels.
5.​ Wind intensity & direction
●​ Wind affects the rate of transpiration (movement of water from
roots to leaves) in plants.
●​ Transpiration affects the temperature of the plant and the rate of
photosynthesis because it transports water and mineral ions to
the leaves.
6.​ Carbon dioxide levels
●​ CO2 affects the rate of photosynthesis in plants.
 ●​ It also affects the distribution of organisms, as some thrive in
high C02 environments.
7.​ Oxygen levels for aquatic animals
●​ Levels in water vary greatly, unlike oxygen levels in air.
●​ Most fish need a high concentration of oxygen to survive.

Biotic factors that can affect a community:

1.​ Food availability
●​ More food means organisms can breed more successfully and
therefore the population can increase in numbers.
2.​ New predators
3.​ New pathogens
●​ When a new pathogen arises, the population has no resistance
to it so they can be wiped out quickly.
4.​ Competition
●​ If one species is better adapted to the environment than
another, then it will outcompete it until the numbers of the lesser
adapted species are insufficient to breed.

Adaptations:
Organisms have adaptations that allow them to survive in the conditions
where they live.
1.​ Structural: shape or colour of a part of an organism, e.g.,
●​ Sharp teeth of a carnivore to tear meat apart.
●​ Camouflage, such as the tan/brown of a lionessess coat, to
prevent prey from spotting her.
●​ Species in cold environments may have a thick layer of fat for
insulation.
2.​ Behavioural: the way an organism behaves, e.g.,
●​ Individuals may play dead to avoid predators.
●​ Basking in the sun to absorb heat.
●​ Courting behaviours to attract mates.
3.​ Functional: involved in processes such as reproduction and
metabolism.
 ●​ Late implantation of embryos.
●​ Conservation of water through producing little sweat.

Extremophiles live in environments that have extreme conditions. These

include high temperatures, pressure, or salt concentrations. An example is
bacteria, which live in deep sea vents where the pressure is very high.

Examples of adaptations for different scenarios:

1.​ Cold climates: smaller surface area to volume ratio to reduce heat
loss, lots of insulation (blubber, fur coat).
2.​ Dry climates: adaptations to kidneys so they can retain lots of water,
produce very concentrated urine, be active in the early morning and
evenings when it’s cooler, rest in shady areas, and have a larger
surface area to ratio to increase heat loss.
3.​ Examples of plant adaptations: are curled leaves to reduce water
loss; extensive root systems to take in as much water as possible;
waxy cuticles to stop water from evaporating; and water-storing tissue
in stems.

Chapter 13: Ecology

Pyramids of biomass:
Feeding relations are shown by food chains.
1.​ They begin with a producer.
●​ These are always photosynthetic organisms.
●​ Through photosynthesis they make glucose
●​ Glucose is used to make other biological molecules in the plant,
which make up biomass.
2.​ Producers are eaten by primary consumers—energy is transferred
through organisms in an ecosystem when one is eaten by another.
3.​ Primary consumers are eaten by secondary consumers—the animals
eaten are called the prey and the consumers that kill and eat them
are predators.
4.​ Secondary consumers are eaten by tertiary consumers.
Pyramids of biomass show the relative biomass at each trophic level.
●​ It shows the relative weights of material at each level.
●​ There is less biomass as you move up the trophic levels.
●​ Not all food consumed by an animal is converted into biomass; this
means the biomass of the organism in the level above will always be
higher, as not all the organisms can be consumed and converted into
biomass.
Producers transfer about 1% of the incident energy from light for
photosynthesis, as not all the light lands on the green (photosynthesising)
parts of the plant.

Only approximately 10% of the biomass of each trophic level is transferred

to the next.
●​ Not all biomass can be eaten
○​ Carnivores cannot generally eat bones, hooves, claws, and
teeth.
●​ Not all of the biomass eaten is converted into the biomass of the
animal eating it.
○​ Lots of glucose is used in respiration, which produces the waste
product CO2.
○​ Urea is a waste substance that is released in urine.
○​ Biomass consumed can be lost as faeces.
■​ Herbivores do not have all the enzymes to digest all the
material they eat, so it is ingested instead.

Efficiency of biomass transfers: (biomass transferred to next level/biomass

available at the previous level) x 100.

Because less biomass is transferred each time, it is common to find fewer

animals in the higher trophic levels.
Chapter 14: Human Population & pollution

How materials are cycled:

The carbon cycle:
●​ CO₂ is removed from the air in photosynthesis by green plants and
algae—they use carbon to make carbohydrates, proteins, and fats.
They are eaten and the carbon moves up the food chain.
●​ CO₂ is retired to the air when plants, algae, and animals respire.
Decomposers (a group of microorganisms that break down dead
organisms and waste) respire while they return mineral ions to the
soil.
●​ CO₂ is returned to the air when wood and fossil fuels are burnt
(combustion) as they contain carbon from photosynthesis.

The water cycle:

●​ The sun’s energy causes water to evaporate from the sea and lakes,
forming water vapour.
●​ Water vapour is also formed as a result of transpiration in plants.
●​ Water vapour rises, then condenses to form clouds.
●​ Precipitation (rain, snow, or hail) returns water to the land, which runs
into lakes to provide water for plants and animals.
●​ This then runs into the sea, and the cycle begins again.

Decomposition:

Several factors affect the rate of decomposition:

1.​ Temperature: chemical reactions generally work faster in warmer

conditions, but if it’s too hot, the enzymes can denature and stop
decomposition.
2.​ Water: microorganisms grow faster in water-rich conditions, as water
is needed for respiration. Water also makes food easier to digest.
3.​ Availability of oxygen: most decomposers respire aerobically.

Compost:
 ●​ When biological material decays, it produces compost.
●​ It’s used by gardeners and farmers as a natural fertiliser.
●​ To do this, they have to provide optimum conditions for decay.
○​ If more oxygen is available, they respire aerobically, producing
heat.
○​ The increased temperature increases the rate of decay so the
compost is made quicker.

Methane gas:

●​ Microorganisms decompose waste anaerobically to produce methane

gas, which can be burnt as a fuel.
●​ Biogas generators are used to produce methane
○​ It requires a constant temperature (30 °C) so the
microorganisms keep repairing.
○​ It cannot be stored as a liquid, so needs to be used
immediately.

Impact of environmental change: environmental changes affect the

distribution of species in an ecosystem.

●​ Temperature: climate change may lead to insects migrating to places

in the world that are becoming hotter.
●​ Water availability: populations will migrate to find water.
●​ Atmospheric gas composition: certain pollutants can affect the
distribution of organisms.

Biodiversity: the variety of different species of organisms on Earth within an

ecosystem.

High biodiversity means the ecosystem will be stable.

●​ Biodiversity means that species are less dependent on each other for
things such as food and shelter.

Many human activities are harming biodiversity. The future of humans on

Earth depends on maintaining biodiversity. The impact of our activities is
getting bigger as the population is increasing and as more resources are
being used, more waste is being produced.

Deforestation: the cutting down of a large number of trees in the same area
to use the land for something else. It happens in tropical areas to:

●​ Provide land for cattle and rice fields.

●​ To grow crops for biofuels, which are used to produce energy

Problems with deforestation:

1.​ As trees contain carbon, burning them results in more CO2 being
released into the environment, which contributes to global warming.
Following deforestation, microorganisms decompose the dead
vegetation, producing CO2 as they respire.
2.​ Trees take in CO2 when they photosynthesise, so fewer trees means
less CO2 is taken.
3.​ The number of habitats is reduced, decreasing biodiversity.

Global warming refers to the fact that the temperature around the world is
increasing because we are producing more greenhouse gases (C02,
methane, water vapour, etc.), resulting in more heat being absorbed and
reflected to the earth, heating it.

The consequences of temperature are:

●​ Melting of ice glaciers, reducing habitats

●​ Rising sea levels are reducing habitats as low-lying areas will be
flooded with water.
●​ Temperature and rainfall levels will affect migration and therefore the
distribution of different species, as they will no longer be able to
survive where they live.
●​ Organisms will become extinct as their habitats are lost, reducing
biodiversity.

Maintaining biodiversity:
 Positive human interaction w/ Negative human interaction w/
ecosystems ecosystems
●​ Maintaining rainforests, ensuring ●​ The production of greenhouse
habitats are not destroyed. gases leading to global
warming.
●​ Reducing water pollution and ●​ Producing sulphur dioxide in
monitoring the changes over factories, which leads to acid
time. rain, affects habitats.
●​ Preserving areas of scientific ●​ Chemicals used in framing leak
interest by stopping humans into the environment.
from going there.
●​ Replanting hedgerows and ●​ Clearing land to build on,
woodlands to provide habitats reducing the number of habitats.
that were previously destroyed.

To reduce our negative impact on ecosystems, programs have been put in

place to maintain biodiversity.

1.​ Breeding programs.

2.​ Protection of rare habitats.
3.​ Reintroduction of hedgerows and field margins around land where
only one type of crop is grown.
4.​ Reduction of deforestation and carbon dioxide production.
5.​ Recycling rather than dumping waste in landfills.
questions
Questions:

Homeostasis
Explain the role of the pancreas in blood glucose regulation.​
The pancreas regulates blood glucose levels by producing two hormones: insulin and glucagon.
When blood glucose levels are high, the pancreas secretes insulin, which prompts cells,
particularly in the liver and muscles, to take in glucose from the blood, storing it as glycogen.
When blood glucose levels are low, the pancreas releases glucagon, which stimulates the liver
to convert stored glycogen back into glucose and release it into the bloodstream.

Describe the effects of vasodilation on body temperature.​

Vasodilation is the widening of blood vessels near the skin's surface, which increases blood flow
to those areas. This allows more heat to escape from the body, thereby cooling it down. It is a
response to overheating and helps maintain a stable internal temperature.

How does shivering help to increase body temperature?​

Shivering is the rapid contraction and relaxation of skeletal muscles. This process generates
heat through increased cellular respiration, which helps raise body temperature when it drops
below the normal range.

What is the significance of maintaining a constant internal environment in the human

body?​
Maintaining a constant internal environment, or homeostasis, is crucial for optimal enzyme
activity and cell function. Enzymes, which catalyse biochemical reactions, work best within a
narrow temperature and pH range. Homeostasis ensures conditions like temperature, pH, and
glucose levels remain stable, supporting processes like respiration, nutrient absorption, and
waste removal.

Explain the process of negative feedback in blood glucose regulation.​

Negative feedback maintains stable blood glucose levels. When blood glucose rises after
eating, the pancreas releases insulin, reducing glucose by converting it to glycogen in the liver.
If blood glucose falls, the pancreas releases glucagon, which stimulates glycogen breakdown to
release glucose. This regulation prevents extreme fluctuations.

Why do Type 2 diabetes patients often need to change their diet?​

Type 2 diabetes patients must adjust their diet to control blood glucose levels. Consuming fewer
simple carbohydrates reduces glucose spikes, while eating foods with a low glycaemic index
can stabilise levels. A balanced diet, combined with weight management and exercise, can help
cells become more sensitive to insulin.
Pathogens & Diseases
What are the differences between bacterial and viral pathogens?​
Bacteria are single-celled organisms that can reproduce independently, while viruses are
non-living entities that require a host cell to reproduce. Bacterial infections can often be treated
with antibiotics, while viral infections typically rely on antiviral medications or vaccines for
prevention. Bacteria can live in a variety of environments, while viruses are specific to the cells
they infect.

Describe the role of white blood cells in the immune response.​

White blood cells are crucial to the immune system. Phagocytes engulf and digest pathogens,
while lymphocytes produce antibodies specific to pathogens' antigens. Some lymphocytes also
create memory cells, which provide long-term immunity by quickly recognising and responding
to previously encountered pathogens.

Why are memory cells important in the immune system?​

Memory cells are a type of lymphocyte that remain in the body after an infection has been
cleared. They enable the immune system to recognise a pathogen more rapidly and produce a
faster and stronger immune response upon subsequent exposures, providing long-term
immunity.

How does the body prevent pathogens from entering through the respiratory system?​
The respiratory system has several defences to prevent pathogen entry. The mucus in the
respiratory tract traps pathogens, while cilia on the cells lining the airways move mucus towards
the throat for removal. Additionally, the nasal hairs filter larger particles, and the cough reflex
expels irritants.

What role does herd immunity play in disease prevention?​

Herd immunity occurs when a significant portion of a population is immune to a disease,
typically through vaccination. This reduces the spread of the pathogen, indirectly protecting
those who cannot be vaccinated due to medical reasons, as there are fewer hosts for the
disease to infect.

Describe how antibiotics work and why they are ineffective against viruses.​
Antibiotics target specific bacterial structures or functions, such as cell wall synthesis or protein
production, leading to bacterial death or inhibition. Viruses lacking a cell structure and hijacking
host cells for replication are unaffected by antibiotics necessitating antiviral treatments for viral
infections.

What measures can be taken to reduce antibiotic resistance?​

To combat antibiotic resistance, it is essential to use antibiotics only when necessary, follow
prescribed dosages, and complete the full course. Reducing the use of antibiotics in livestock,
improving infection control in healthcare settings, and encouraging the development of new
antibiotics are also critical measures.
Photosynthesis
What factors can affect the rate of photosynthesis?​
The rate of photosynthesis can be affected by several factors: light intensity, carbon dioxide
concentration, temperature, and the wavelength of light. Adequate water supply and chlorophyll
levels are also important. If these factors are too low or too high, they can limit or slow down the
photosynthetic process.

Explain the role of the stomata in gas exchange for photosynthesis.​

Stomata are small openings on the underside of leaves that regulate gas exchange. They open
to allow carbon dioxide to enter the leaf, which is necessary for photosynthesis. Oxygen, a
by-product of photosynthesis, exits the leaf through the stomata. Stomata can also be closed to
prevent water loss when conditions are dry.

Why is oxygen considered a waste product of photosynthesis?​

Oxygen is considered a waste product of photosynthesis because it is not needed for the plant’s
metabolic processes during photosynthesis. It is released into the atmosphere as a by-product
of splitting water molecules to provide electrons for the light-dependent reactions.

Why is light intensity considered a limiting factor in photosynthesis?​

Light intensity is a limiting factor because it provides the energy needed for the light-dependent
reactions of photosynthesis. As light intensity increases, the rate of photosynthesis also rises,
up to a point where other factors (like carbon dioxide and temperature) become limiting. In low
light, less ATP and NADPH are produced, slowing the conversion of carbon dioxide to glucose.

How do guard cells regulate gas exchange and water loss in plants?​
Guard cells control the opening and closing of stomata. When turgid, they open the stomata,
allowing carbon dioxide in for photosynthesis and oxygen out. When dehydrated, guard cells
lose water and become flaccid, closing the stomata to reduce water loss through transpiration,
balancing gas exchange with water conservation.

Explain how greenhouses can increase the rate of photosynthesis.​

Greenhouses optimise photosynthesis by controlling environmental conditions. Temperature,
light intensity, and carbon dioxide levels can be adjusted to enhance the rate of photosynthesis.
For example, artificial lighting extends light exposure, while heating maintains optimal
temperatures for enzyme activity involved in the photosynthetic process.

What is the role of chlorophyll in photosynthesis?​

Chlorophyll is a pigment located in the chloroplasts of plant cells. It absorbs light energy,
primarily from the blue and red wavelengths, and converts it into chemical energy during the
light-dependent reactions of photosynthesis. This energy is used to produce ATP and NADPH,
which power the synthesis of glucose.
Genetics, Inheritance & Reproduction
Describe the difference between mitosis and meiosis.​
Mitosis is a type of cell division that results in two genetically identical daughter cells, each with
a full set of chromosomes (diploid). It is used for growth, repair, and asexual reproduction.
Meiosis, on the other hand, produces four genetically unique daughter cells with half the number
of chromosomes (haploid). Meiosis is crucial for sexual reproduction, creating genetic diversity
in offspring.

How can genetic variation occur during reproduction?​

Genetic variation arises through the process of meiosis, where homologous chromosomes
randomly assort and crossing over of genetic material occurs between them. This results in
unique combinations of genes in gametes. Fertilisation further increases variation by combining
genetic material from two parents.

What is the significance of a Punnett square in genetics?​

A Punnett square is a diagram used to predict the possible genetic outcomes of a cross
between two organisms. It helps determine the probability of an offspring inheriting particular
alleles, showing the potential genotypes and phenotypes based on parental genotypes.

What are the consequences of mutations in DNA?​

Mutations are changes in the DNA sequence that can lead to altered proteins. Some mutations
are beneficial and drive evolution, while others are harmful, leading to genetic disorders or
diseases like cancer. If a mutation occurs in a gamete, it can be passed to offspring, affecting
future generations.

Why is meiosis important for genetic diversity?​

Meiosis creates genetic diversity through two main processes: independent assortment and
crossing over. During independent assortment, chromosomes are randomly distributed to
gametes. Crossing over involves exchanging genetic material between homologous
chromosomes, leading to unique combinations in gametes. This diversity is crucial for
adaptation and evolution.

What information can a pedigree chart provide?​

A pedigree chart helps identify dominant and recessive patterns, carrier status, and the
likelihood of passing genetic conditions to offspring. Pedigrees are valuable for genetic
counselling and understanding the risks of inherited diseases.
units: 2,5,14
Chapter 2: Cell division & Differentiation

Chromosomes:
The nucleus contains genetic information.
●​ This is found in the form of chromosomes, which contain coils of
DNA.
●​ A gene is a short section of the DNA that codes for a protein and, as
a result, controls a characteristic; therefore, each chromosome
carries many genes.
●​ There are 23 pairs of chromosomes in each cell of the body, inherited
from each parent, resulting in 46 chromosomes in each cell.
●​ Sex cells (gametes) are the exception: each gamete cell has half the
number of chromosomes—23 chromosomes.

Mitosis and the cell cycle:

The cell cycle is a series of steps that the cell has to undergo to divide.
Mitosis is a step in this cycle.
1.​ (interphase): the cell grows, organelles (ribosomes,
mitochondria) grow and increase in number, the synthesis of
proteins occurs, DNA is replicated (forming the characteristic ‘X’
shape) and energy stores are increased.
2.​ (Mitosis): The chromosomes line up at the cell's equator, and
cell fibres pull each chromosome of the ‘X’ to either side of the
cell.
 3.​ (cytokinesis): two identical daughter cells form when the
cytoplasm and cell membrane divide.

Cell division by mitosis in multicellular organisms is important in their

growth and development and when replacing damaged cells. Mitosis is also
a vital part of asexual reproduction, as this type of reproduction only
involves one organism, so to produce offspring, it simply replicates its cell.

Growth & Cell Differentiation

To become specialised and be suited to its rule, stem cells must undergo
differentiation to form specialised cells. This involves some of their genes
being switched on or off to produce different proteins, allowing the cell to
acquire different sub-cellular substances for it to carry out a specific
function.

In animals, growth occurs via cell division and differentiation. Cell

division occurs by mitosis, after which cells can differentiate into specialised
forms, specially adapted to their functions.
​ In animals, almost all cells differentiate at an early stage and then
lose this ability. Most specialised cells can make more of the same cell by
undergoing mitosis. Others, such as RBCs (which lose their nucleus),
cannot divide and are replaced by adult stem cells (which retain their ability
to undergo differentiation). In mature animals, cell division mostly only
happens to repair or replace damaged cells, as they undergo little growth.
In mature animals, cell division mostly happens to repair or replace
damaged cells, as they undergo little growth.

​ In plants, growth occurs through cell division and differentiation and

also through elongation. Plant cells can grow longer in a specific direction
by absorbing water into their vacuoles, and this is controlled by substances
called auxins.
​ In plants, many types of cells retain the ability to differentiate
throughout life. They only differentiate when they reach their final position in
the plant, but they still re-differentiate when it is moved to another position.

Stem Cells
A stem cell is an undifferentiated cell that can undergo division to produce
many more similar cells, some of which will differentiate to have different
functions.
​ Types of stem cells
1.​ Types of stem cells
○​ From when an egg and sperm cell fuse to form a zygote.
○​ They can differentiate into any type of cell in the body.
○​ Scientists can clone these cells (by culturing them) and
direct them to differentiate into almost any cell in the body.
○​ These could potentially be used to replace
insulin-producing cells in those suffering from diabetes,
new neural cells for diseases like Alzheimer’s or nerve
cells for those paralysed with spinal cord injuries.
2.​ Adult stem cells
○​ If found in bone marrow, they can form many types of
cells, including blood cells.
 3.​ Meristem in plants
○​ Found in root and shoot tips.
○​ They can differentiate into any type of plant—it may be
necessary if the parent plant has certain desirable
features (such as disease resistance) for research or to
save a rare plant from extension.

Therapeutic cloning involves an embryo being produced with the same

genes as the patient.
●​ The embryo produced could then be harvested to obtain the
embryonic stem cells.
●​ These could be grown into any cells the patient needed, such as new
tissues or organs.
●​ The advantage is that they would not be rejected as they would have
the same genetic make-up as the individual.

Benefits VS issues of research with stem cells:

Benefits Issues
Can be used to replace We do not understand the
damaged or diseased body process of differentiation, so it is
parts hard to control stem cells to
form the cells we desire
Unwanted embryos from fertility Removal of stem cells destroys
clinics could be used as they the embryo
would otherwise
Be discarded People may have religious or
ethical objections, as it is seen
as interference with the natural
process of reproduction
Research into the process of If the growing stem cells are
differentiation contaminated with a virus, an
infection can be transferred to
the individual
 Money and time could be better
spent in other areas of medicine

Cancer
Mitosis is just one part of the cell cycle, which is regulated by many
different genes to ensure that cells divide only when they need to and stop
when required

Cancer is caused as a result of changes in the DNA of cells that lead

to uncontrolled growth and division; this can result in the formation of
a tumour (a mass of cells)
○​ Usually, tumours form as a result of loss of control of the cell
cycle.
■​ Not all tumours are considered cancerous
■​ Benign tumours are growths of abnormal cells which are
contained in one area, usually within a membrane
■​ Crucially, benign tumours do not invade other parts of the
body
■​ Malignant tumour cells are cancers; the cells invade
neighbouring tissues and spread to different parts of the
body via the blood and lymphatic system, where they
form secondary tumours
■​ Malignant tumours are more likely to disrupt the
functioning of the organ they originate in (as they invade
healthy tissue) and the organs they spread to—this is why
they are dangerous and how they lead to death

Lifestyle Risk Factors & Cancer

●​ Anyone, at any age, can develop cancer, but increasing age and
many lifestyle factors are associated with an increased risk of having
cancer
 ●​ Treatments are constantly being developed, with targeted therapies
and immunotherapy helping to improve survival rates for many
different types of cancer
●​ Scientists have identified lifestyle risk factors for various types of
cancer:

Risk Types of cancer identified

with increased risk
Obesity—diet high in saturated fat Bowel, liver, and kidney
and sugars
Smoking—exposure to carcinogens in Lung, mouth, throat, and
cigarette smoke stomach
UV radiation—a type of ionising Skin
radiation
Viral infection— leads to disruption of Cervical (HPV); liver (hepatitis
the cell cycle and therefore B and C)
uncontrolled growth
●​ There are also genetic risk factors for many types of cancer;
inheriting faulty genes can make individuals more susceptible to
developing cancer
○​ Individuals with faulty mismatch repair (MMR) genes
responsible for proofing DNA are more likely to develop cancers
of the bowel and reproductive systems.
○​ Individuals with faulty BRCA genes are more likely to develop
breast and ovarian cancer than individuals with functioning
BRCA genes.

Chapter 5: Human Biology—digestion

The digestive system is an organ system, made up of organs working
together to perform different functions. The food that people eat is large
and insoluble and needs to be broken down for it to be absorbed by cells.
It’s made up of:
1.​ Glands (salivary glands & the pancreas) produce digestive juices
containing enzymes that break down food.
2.​ The stomach produces hydrochloric acid to kill bacteria and provide
the optimum pH for the protease enzyme to work.
3.​ Duodenum is the first part of the small intestine and it’s responsible
for receiving partially digested food from the stomach and beginning
the absorption of nutrients.
4.​ The small intestine is where soluble molecules are absorbed into
the blood.
5.​ The liver produces bile, which is stored in the gall bladder and helps
with the digestion of lipids.
6.​ The large intestine absorbs water from undigested food to produce
faeces, which then pass out of the body through the rectum and
anus.

The digestion process:

The process of digestion begins in the mouth, where mechanical digestion
occurs. The teeth chew food into smaller pieces, breaking it down to
increase its surface area for enzyme action. Saliva, secreted by the salivary
glands, contains amylase, which begins the chemical digestion of starch
into maltose. The tongue shapes the food into a bolus, lubricated by saliva,
so it can be easily swallowed.

Next, the bolus travels down the oesophagus, a muscular tube connecting
the mouth to the stomach. Through wave-like contractions known as
peristalsis, the oesophagus pushes the bolus down without relying on
gravity.

In the stomach, food undergoes mechanical digestion through churning

actions and chemical digestion as protease enzymes start to break down
proteins. The stomach also produces hydrochloric acid, which kills bacteria
in the food and provides the acidic environment required for protease
enzymes to function effectively.
The partially digested food then enters the small intestine, beginning with
the duodenum, where enzymes from the pancreas continue digestion. The
pancreas secretes all three types of digestive enzymes—amylase,
protease, and lipase—into the duodenum, alongside an alkaline fluid to
neutralise the stomach's acidic contents. The small intestine is slightly
alkaline, with a pH of around 8, creating optimal conditions for enzyme
activity. In the ileum, the final section of the small intestine, digested food
molecules are absorbed into the bloodstream. The walls of the ileum are
lined with villi, tiny finger-like projections that increase the surface area for
efficient absorption.

From there, the remaining material enters the large intestine, where water
is absorbed, and the undigested matter is compacted into faeces. The
faeces are stored in the rectum and eventually expelled through the anus.

The liver plays a crucial role by producing bile, which emulsifies fats,
breaking large droplets into smaller ones to aid the action of lipase. The
liver is also responsible for deamination, which removes amino acids no
longer needed, producing urea. The gall bladder stores bile until it is
needed in the duodenum.

Enzymes: biological catalysts ( a substance that increases the

rate of reaction without being used up).
●​ They are protein molecules and the shape of the enzyme is vital to its
function because each enzyme has its own uniquely shaped active
site where the substrate binds.
●​ Enzymes are present in many reactions so that they can be
controlled.
●​ They can both break up large and small molecules.

The lock and key hypothesis is a simplified explanation of how enzymes

work:
 1.​ The shape of the substance is complementary to the shape of the
active site, so when they bond, it forms an enzyme-substrate
complex.
2.​ Once bound, the reaction takes place and the products are released
from the surface of the enzyme.
Enzymes require an optimum pH and temperature because they are
proteins.
●​ The optimum temperature is a range around 37℃
○​ The rate of reaction increases with an increase in temperature
up to this optimum, but above this temperature, it rapidly
decreases and eventuall,y the reaction stops.
○​ When the temperature becomes too hot, the bonds in the
structure will break; this changes the shape of the active site, so
the substrate can no longer fit in.
○​ The enzyme is said to be denatured and can no longer work.
●​ The optimum pH for most enzymes is 7, but some that are produced
in acidic conditions have a lower optimum pH.
○​ If the pH is too high or too low, the forces that hold the amino
acid chains that make up the protein will be affected.
○​ This changes the shape of the active site, so the substrate can
no longer fit in.
○​ The enzyme is said to be denatured and can no longer work.

As molecules need to be broken down in the digestive system to be

absorbed into the bloodstream, enzymes are vital. They are released by
cells in many different places and they are specific to a certain type of
molecule.
1.​ Carbohydrases convert carbohydrates into simple sugars.
●​ Example: amylase breaks down starch into maltose.
○​ It’s produced in the salivary glands, pancreas, and small
intestine (most starch is digested by them).
2.​ Proteases convert proteins into amino acids.
●​ Example: pepsin (produced in the stomach); other forms can be
found in the pancreas and small intestine.
3.​ Lipases convert lipids into fatty acids and glycerol.
 ●​ Produced in the pancreas and small intestine.
Soluble glucose, amino acids, fatty acids, and glycerol pass into the
bloodstream to be carried to all the cells around the body. They are used to
build new carbohydrates, lipids, and proteins, with some glucose being
used in respiration.

Bile is produced in the liver, stored in the gall bladder and then released in
the small intestine. It has two roles:
1.​ It is alkaline to neutralise the hydrochloric acid that comes from the
stomach—the enzymes in the small intestine have a higher (more
alkaline) optimum pH than those in the stomach.
2.​ It breaks down the large drops of fat into smaller ones (emulsifies it).
The larger surface area allows lipase to chemically break down the
lipid into glycerol and fatty acids faster.

Chapter 14: Human Population & pollution

How materials are cycled:

The carbon cycle:
●​ CO₂ is removed from the air in photosynthesis by green plants and
algae—they use carbon to make carbohydrates, proteins, and fats.
They are eaten and the carbon moves up the food chain.
●​ CO₂ is retired to the air when plants, algae, and animals respire.
Decomposers (a group of microorganisms that break down dead
organisms and waste) respire while they return mineral ions to the
soil.
●​ CO₂ is returned to the air when wood and fossil fuels are burnt
(combustion) as they contain carbon from photosynthesis.

The water cycle:

●​ The sun’s energy causes water to evapourate from the sea and
lakes, forming water vapour.
 ●​ Water vapour is also formed as a result of transpiration in plants.
●​ Water vapour rises, then condenses to form clouds.
●​ Precipitation (rain, snow, or hail) returns water to the land, which runs
into lakes to provide water for plants and animals.
●​ This then runs into the sea, and the cycle begins again.

Decomposition:

Several factors affect the rate of decomposition:

1.​ Temperature: chemical reactions generally work faster in warmer

conditions, but if it’s too hot, the enzymes can denature and stop
decomposition.
2.​ Water: microorganisms grow faster in water-rich conditions, as water
is needed for respiration. Water also makes food easier to digest.
3.​ Availability of oxygen: most decomposers respire aerobically.

Compost:

●​ When biological material decays, it produces compost.

●​ It’s used by gardeners and farmers as a natural fertiliser.
●​ To do this, they have to provide optimum conditions for decay.
○​ If more oxygen is available, they respire aerobically, producing
heat.
○​ The increased temperature increases the rate of decay so the
compost is made quicker.

Methane das:

●​ Microorganisms decompose waste anaerobically to produce methane

gas, which can be burnt as a fuel.
●​ Biogas generators are used to produce methane
○​ It requires a constant temperature (30 °C) so the
microorganisms keep repairing.
○​ It cannot be stored as a liquid, so needs to be used
immediately.
Tab 5