AQA AS Biology Unit 2: Exchange between Organisms and their Environment

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Exchange between Organisms and
their Environment
All living organisms take in gases from their environment and return gases to it.
This exchange of gases between an organism and its environment takes place by diffusion.
The part of the organism across which the exchange takes place is called the gas
exchange surface.
Once absorbed, materials must be rapidly distributed to the cells that require them and
the waste products returned to the exchange surface for removal.
The size and metabolic rate of an organism will affect the amount of each material
that needs to be exchanged.
This will influence the type of exchange surface and transport system that has evolved
to meet the requirements of each organism.
Things which need to be interchanged between an organism and its environment:
Respiratory gases Oxygen
Carbon dioxide
Nutrients Glucose
Fatty acids
Amino acids
Excretory products Urea
Carbon dioxide
This exchange can take place in two ways
Passively No energy is required by diffusion and Osmosis
Actively Energy is required by active transport

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Surface Area to Volume Ratio
Exchange takes place at the surface of an organism, but the materials absorbed are
used by the cells that mostly make up its volume.
For exchange to be effective:
The surface area of the organism must be large compared with its volume
Small organisms have a surface area that is large enough, compare with their volume, to
allow efficient gas exchange across their body surface.…read more

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Features of Specialised Exchange Surfaces
To allow effective transfer of materials across them by diffusion or active transport,
exchange surfaces show the following characteristics:
Large surface area to volume ratio Increases rate of exchange
Thin So diffusion distance is short and therefore
materials cross the exchange surface rapidly
Partially permeable Allows selected materials to cross without
Movement of environmental medium For example, air, to maintain a diffusion
Movement of the internal medium For example, blood, to maintain a diffusion
Diffusion is proportional…read more

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Length of edge of a Surface area of Volume of cube Ratio of surface area
cube /cm whole cube /cm2 /cm3 to volume
1 1x6=6 1x1=1 6/1 = 6.0
2 4 x 6 = 24 2x2x2=8 24/8 = 3.0
3 9 x 6 = 54 3 x 3 x 3 = 27 54/27 = 2.0
4 16 x 6 = 96 4 x 4 x 4 = 64 96/64 = 1.…read more

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Gas Exchange in Single-celled Organisms
Single-celled organisms are small and therefore have a large surface area to volume ratio.
This means they can exchange gases quickly directly through their cell
Oxygen is absorbed by diffusion across their body surface, which is covered only by a cell-
surface membrane.
In the same way, carbon dioxide from respiration diffuses out across their body surface.…read more

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Gas Exchange in Insects
Most insects are terrestrial ­ they live on land.
The problem for all terrestrial organisms is that water easily evaporates from the surface of
their bodies and they can become dehydrated.
They therefore need to conserve water.
However, efficient gas exchange requires:
- Thin surface area
- Permeable surface
- Large surface area
These features conflict with the need to conserve water.…read more

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Respiratory gases move in and out of the tracheal system in two ways:
Along a diffusion gradient
When cells are respiring, oxygen is used up and so its concentration towards the end of the
tracheoles falls.
This creates a diffusion gradient that causes gaseous oxygen to diffuse from the
atmosphere along the trachea and tracheoles to the cells.
Carbon dioxide is produced by cells during respiration.
This creates a diffusion gradient in the opposite direction.…read more

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Gases enter and leave the trachea through tiny pores, called spiracles, on the body surface.
The spiracles may be opened and closed by a valve.
Open Water can evaporate from the insect
Allows gas exchange
Closed Prevents water loss
When the insect is at rest, water diffuses out of its cells into the ends of the tracheoles.
This mean oxygen has to diffuse through water to reach the cells, which is a slow process.…read more

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The tracheal system is an efficient method of gas exchange.…read more

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Gas Exchange in Fish
Fish have a waterproof, and therefore a gas-tight outer covering.
Being relatively large they also have a small surface area to volume ratio.
Their body surface is therefore not adequate to supply and remove their
respiratory gases and so, like insects and humans, they have developed a specialised gas
exchange surface: the gills.
The structure of the gills
The gills are located within the body of the fish, behind the head.
They are made up of gill filaments.…read more


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