Exchange Surfaces

All living cells need a supply of oxygen and nutrients, then the removal of carbon dioxide. In smaller organisms, this exchange can occur over the surface of the body. They don't need any specialised exchange systems. In larger organisms with more than 2 layers of cells, exchange systems are needed.

3 main factors that affect the need for an exchange surface:

• Size
• SA : V ratio
• level of activity.

in small organisms, such as single-cell organisms, the cytoplasm is very close to the environment. Diffusion will supply enough oxygen and nutrients to keep the cell alive.

Multicellular organisms have several layers of cells. The dffusion pathway is longer.

SA : V ratio = the surface area of an organism divided by it's volume, expressed as a ratio.

Small organisms have a samll surface area and a small volume. The surface area is larger than the volume, so it has a large SA : V ratio. Their surface area is large enough to supply enough oxygen and nutrients.

Large organisms have a large surface area and a large volume. The volume is bigger than the surface area, so they have a small SA : V ratio.

Some animals change shape to increase their SA : V ratio.

Level of activity:

Some organisms are more active than others. Metabolic activity uses energy from food and requires oxygen to release the energy in aerobic respiration. The cells of an active organism need a good supply of oxygen and nutrients to supply energy for movement.

• A large SA to provide more space for molecules to pass through. This is often achieved by folding the walls and membrane involved. Eg) root hairs in plants.

• A thin barrier to reduce the diffusion distance - and that barrier must be prmeable to the substances being exchanged. Eg) in the alveoli of the lungs.

• A good blood supply. This can bring fresh supplies of molecules to one side, keeping the concentration high, or it may remove molecules from the demand side to keep the concentration low. This is important to maintain a steep concentration gradient so that diffusion can occur rapidly.