AQA Biology Revision
Chapter 10: Exchange and Transport Systems

Size and Surface Area

Organisms need to exchange substances with their environment

Every organism, whatever its size, needs toe exchange things with the environment.

1. Cells need to take in oxygen (for aerobic respiration) and nutrients.
2. They also need to excrete waste products like carbon dioxide and urea.
3. Most organisms need to stay at roughly the same temperature, so heat needs to be exchanged too.

How easy the exchange of substances is depends on the organism’s surface area to volume ratio.

Smaller animals have higher surface area : volume rations

A mouse has a bigger surface area to relative to its volume than a hippo. This can be hard to imagine, but you can prove it mathematically. Imagine these animals as cubes:

The hippo could be represented as a block measuring 2cm x 4cm x 4cm.

It’s volume is 2 x 4 x 4 = 32cm cubed.
It’s surface area is 2 x 4 x 4 = 32cm squared (top and bottom surfaces of cube) + 4 x 2 x 4 = 32cm squared (four sides of the cube)
Total surface area = 64cm squared.

So the hippo has a surface area : volume ration of 64 : 32 or 2 : 1

Compare this to a cube mouse measuring 1cm x 1cm x 1cm.
It’s volume is 1 x 1 x 1 = 1cm cubed.
It’s surface area is 6 x 1 x 1 = 6cm squared.
So the mouse has a surface area : volume ration of 6 : 1

Multicellular organisms need exchange organs and mass transport systems

An organism needs to supply everyone of ne of its cells with substances like glucose and oxygen (for respiration). It also needs to remove waste products from every cell to avoid damaging itself.

1. In single-celled organisms, these substances can diffuse directly into (or out of) the cell across the cell surface membrane. The diffusion rate is quick because of the small distances the substances have to travel.
2. In multicellular animals, diffusion across the outer membrane is too slow, for two reasons

Some cells...

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