AS Biology Gas Exchange

AimsUnderstand the relationship between the size

of an organism and its surface area:volume.Understand the relationship between the

surface area:volume and the need for a specialised gas exchange surface

Objectives by the end of the lesson you should be able toExplain the process of diffusionExplain why small unicellular organisms do

not need exchange surfaces whilst larger multicellular organisms do

Explain the relationship between size, surface area to volume ratio and rate of diffusion in a model cell.

Gas exchange surfaces

This is a photomicrograph of an Amoeba

Amoeba is an aquatic unicellular organism about 1mm in diameter

The exchange surface is the cell membrane

HydraHydra are

multicellular aquatic organisms about 10-13mm long

They carry out gas exchange through their body surface

Earthworms Are terrestrial

organisms although mainly subterranean

About 80-120mm long

They have a damp body surface and so need to live in damp earth

The whole body surface is the gas exchange surface

RabbitsRabbits are

mammalsThey vary in size but

are approximately 300mm long

Terrestrial organisms with an internal specialised gas exchange surface-the lungs

Small or large organismsSmall and unicellular organisms do not need

a specialised gas exchange systemTheir body surface is large enough to supply

all the oxygen they needLarger, multicellular organisms have a larger

surface area but they also have a larger volume

The size of surface is not enough to supply the needs of the volume so special exchange surfaces like lungs or gills are needed

Surface area to volume ratioThe size of an organisms surface divided by

the size of its volume gives the surface area to volume ratio

The larger the organism, the smaller the surface area to volume ratio