AQA biology unit 3

a)Dissolved substances move by diffusion and

by active transport.

b) Water often moves across boundaries by osmosis.

Osmosis is the diffusion of water from a dilute to a

more concentrated solution through a partially

permeable membrane that allows the passage of

water molecules.

c) Differences in the concentrations of the solutions

inside and outside a cell cause water to move

into or out of the cell by osmosis.

a) The lungs are in the upper part of the body (thorax),

protected by the ribcage and separated from the

lower part of the body (abdomen) by the diaphragm.

b) The breathing system takes air into and out of the

body so that oxygen from the air can diffuse into the

bloodstream and carbon dioxide can diffuse out of

the bloodstream into the air.

c) To make air move into the lungs the ribcage moves

out and up and the diaphragm becomes flatter. These

changes are reversed to make air move out of the 

lungs. The movement of air into and out of the lungs

is known as ventilation.

a) In plants:

■ carbon dioxide enters leaves by diffusion

■ most of the water and mineral ions are

absorbed by roots.

b) The surface area of the roots is increased by root

hairs and the surface area of leaves is increased

by the flattened shape and internal air spaces.

c) Plants have stomata to obtain carbon dioxide from

the atmosphere and to remove oxygen produced

in photosynthesis.

The circulatory system transports substances

around the body.

b) The heart is an organ and pumps blood around

the body. Much of the wall of the heart is made

from muscle tissue.

c) There are four main chambers (left and right atria

and ventricles) of the heart.

a) Blood is a tissue and consists of a fluid called

plasma in which red blood cells, white blood cells,

and platelets are suspended.

b) Blood plasma transports:

■ carbon dioxide from the organs to the lungs

■ soluble products of digestion from the small

intestine to other organs 

■ urea from the liver to the kidneys.

a) Flowering plants have separate transport systems:

■ xylem tissue transports water and mineral ions

from the roots to the stem and leaves

■ the movement of water from the roots through

the xylem and out of the leaves is called the

transpiration stream

■ phloem tissue carries dissolved sugars from

the leaves to the rest of the plant, including the

growing regions and the storage organs.

Humans need to remove waste products from their bodies to keep their internal environment relatively constant.

People whose kidneys do not function properly may die because toxic substances accumulate in their blood. Their

lives can be saved by using dialysis machines or having a healthy kidney transplanted. Water and ion content,

body temperature and blood glucose levels must be kept within very narrow ranges.

a) Waste products that have to be removed from the

body include:

■ carbon dioxide, produced by respiration and

removed via the lungs when we breathe out

■ urea, produced in the liver by the breakdown of

amino acids and removed by the kidneys in the

urine, which is temporarily stored in the bladder.

b) If the water or ion content of the body is wrong, too

much water may move into or out of the cells and

damage them. Water and ions enter the body

when we eat and drink.

a) Sweating helps to cool the body. More water is lost

when it is hot, and more water has to be taken as

drink or in food to balance this loss.

b) Body temperature is monitored and controlled by the

thermoregulatory centre in the brain. This centre has

receptors sensitive to the temperature of the blood

flowing through the brain.

c) Also temperature receptors in the skin send impulses

to the thermoregulatory centre, giving information

about skin temperature.

a) The blood glucose concentration of the body is

monitored and controlled by the pancreas. The

pancreas produces the hormone insulin, which

allows the glucose to move from the blood into

the cells.

b) A second hormone, glucagon, is produced in

the pancreas when blood glucose levels fall. 

This causes glycogen to be converted into

glucose and be released into the blood.

a) Rapid growth in the human population and an

increase in the standard of living means that

increasingly more waste is produced. Unless

waste is properly handled, more pollution will

be caused.

b) Waste may pollute:

■ water, with sewage, fertiliser or toxic chemicals

a) Large-scale deforestation in tropical areas, for timber

and to provide land for agriculture, has:

■ increased the release of carbon dioxide into the

atmosphere (because of burning and the

activities of microorganisms)

■ reduced the rate at which carbon dioxide is

removed from the atmosphere and ‘locked up’

for many years as wood.

b) Deforestation leads to reduction in biodiversity. 

a) Levels of carbon dioxide and methane in the

atmosphere are increasing and contribute to

‘global warming’. An increase in the Earth’s

temperature of only a few degrees Celsius:

■ may cause big changes in the Earth’s climate

■ may cause a rise in sea level

■ may reduce biodiversity

■ may cause changes in migration patterns, eg in birds

■ may result in changes in the distribution of species.

a) At each stage in a food chain, less material and less

energy are contained in the biomass of the organisms.

This means that the efficiency of food production can be

improved by reducing the number of stages in food chains.

b) The efficiency of food production can also be

improved by restricting energy loss from food

animals by limiting their movement and by

controlling the temperature of their surroundings.

d) If the core body temperature is too high:

■ blood vessels supplying the skin capillaries

dilate so that more blood flows through the

capillaries and more heat is lost

■ sweat glands release more sweat which

cools the body as it evaporates.

e) If the core body temperature is too low:

■ blood vessels supplying the skin capillaries

constrict to reduce the flow of blood through

the capillaries

■ muscles may ‘shiver’ – their contraction

needs respiration, which releases some

energy to warm the body

d) Most soft drinks contain water, sugar and ions.

e) Sports drinks contain sugars to replace the sugar

used in energy release during the activity. They

also contain water and ions to replace the water

and ions lost during sweating. 

f) If water and ions are not replaced, the ion / water

balance of the body is disturbed and the cells do

not work as efficiently.

c) A healthy kidney produces urine by:

■ first filtering the blood

■ reabsorbing all the sugar

■ reabsorbing the dissolved ions needed by the body

■ reabsorbing as much water as the body needs

■ releasing urea, excess ions and water as urine.

d) People who suffer from kidney failure may be treated

either by using a kidney dialysis machine or by having

a healthy kidney transplanted.

e) Treatment by dialysis restores the concentrations of

dissolved substances in the blood to normal levels

and has to be carried out at regular intervals.

c) Red blood cells transport oxygen from the lungs

to the organs. Red blood cells have no nucleus.

They are packed with a red pigment called

haemoglobin. In the lungs haemoglobin

combines with oxygen to form oxyhaemoglobin.

In other organs oxyhaemoglobin splits up into

haemoglobin and oxygen.

d) White blood cells have a nucleus. They form part of

the body’s defence system against microorganisms.

e) Platelets are small fragments of cells. They have no

nucleus. Platelets help blood to clot at the site of

a wound.

f) In a dialysis machine a person’s blood flows between

partially permeable membranes. The dialysis fluid

contains the same concentration of useful substances

as the blood. This ensures that glucose and useful

mineral ions are not lost. Urea passes out from the

blood into the dialysis fluid. 

g) In kidney transplants a diseased kidney is replaced

with a healthy one from a donor. However, the

donor kidney may be rejected by the immune

system unless precautions are taken.

d) Plants mainly lose water vapour from their leaves.

Most of the loss of water vapour takes place

through the stomata. 

■ Evaporation is more rapid in hot, dry and

windy conditions.

■ If plants lose water faster than it is replaced

by the roots, the stomata can close to prevent wilting.

e) The size of stomata is controlled by guard cells,

which surround them.

d) Blood enters the atria of the heart. The atria contract

and force blood into the ventricles. The ventricles

contract and force blood out of the heart. Valves in

the heart ensure that blood flows in the correct

direction. Blood flows from the heart to the organs

through arteries and returns through veins. There are

two separate circulation systems, one for the lungs

and one for all other organs of the body.

e) Arteries have thick walls containing muscle and

elastic fibres. Veins have thinner walls and often

have valves to prevent back-flow of blood.

f) If arteries begin to narrow and restrict blood flow

stents are used to keep them open.

g) In the organs, blood flows through very narrow,

thin-walled blood vessels called capillaries. Substances

needed by the cells in body tissues pass out of the

blood, and substances produced by the cells pass

into the blood, through the walls of the capillaries.

h) Antigens are proteins on the surface of cells. The

recipient’s antibodies may attack the antigens on

the donor organ as they do not recognise them as

part of the recipient’s body.

i) To prevent rejection of the transplanted kidney:

■ a donor kidney with a ‘tissue-type’ similar to

that of the recipient is used

■ the recipient is treated with drugs that suppress

the immune system.

■ air, with smoke and gases such as sulfur dioxide,

which contributes to acid rain

■ land, with toxic chemicals such as pesticides and

herbicides, which may be washed from the land

into waterways.

c) Humans reduce the amount of land available for other

animals and plants by building, quarrying, farming

and dumping waste.

c) Deforestation has occurred so that:

■ crops can be grown from which biofuels, based

on ethanol, can be produced

■ there can be increases in cattle and in rice fields to

provide more food. These organisms produce

methane and this has led to increases in methane

in the atmosphere.

d) The destruction of peat bogs and other areas of

peat releases carbon dioxide into the atmosphere

b) Carbon dioxide can be sequestered in oceans, lakes

and ponds and this is an important factor in removing

carbon dioxide from the atmosphere.

c) Biofuels can be made from natural products by

fermentation. Biogas, mainly methane, can be produced

by anaerobic fermentation of a wide range of plant

products or waste material containing carbohydrates.

c) Fish stocks in the oceans are declining. It is important

to maintain fish stocks at a level where breeding

continues or certain species may disappear altogether

in some areas. Net size and fishing quotas play an

important role in conservation of fish stocks. 

d) The fungus Fusarium is useful for producing

mycoprotein, a protein-rich food suitable for

vegetarians. The fungus is grown on glucose

syrup, in aerobic condition.