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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

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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…

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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…

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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…

Page 5

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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.


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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…

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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

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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…

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The tracheal system is an efficient method of gas exchange.

It does, however, have some limitations:

It relies mostly on diffusion to exchange gases between the environment and the
For diffusion to be effective, the diffusion pathway needs to be short
As a result this limits the size that…

Page 10

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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…


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