Organisms need exchange

Living organisms need to exchange materials like oxygen, glucose, excretory products like urea, and heat with their environment.

This exchange occurs across plasma membranes.

Surface area to volume ratio affects exchange

The rate at which substances diffuse across an organism's outer surface depends on its surface area to volume ratio (SA:V).

How SA:V affects rate of diffusion:

1. High SA:V - These organisms have a large surface area relative to their volume so the diffusion of substances is fast.
2. Low SA:V - These organisms have a small surface area relative to their volume so the diffusion of substances is slower.

Generally, smaller organisms have a higher SA:V while larger organisms have a lower SA:V.

Calculating the surface area to volume ratio of a cube

To calculate the surface area (SA) of a cube:

SA = length x width x 6

To calculate the volume (V) of a cube:

V = length x width x depth

Multicellular organisms require specialised surfaces

In single-celled organisms, substances diffuse directly across the cell membrane.

This cannot happen in multicellular organisms because:

1. Cells are not in direct contact with the external environment.
2. Diffusion distances between cells and their environment are large.
3. Larger organisms have higher metabolic rates, so they need more oxygen and glucose.

To solve this problem, multicellular organisms have evolved specialised exchange surfaces.

Key features of specialised exchange surfaces

Exchange surfaces are specialised structures that allow materials to be transferred between cells and an organism’s surrounding environment. They have several features that help them to increase the rate of diffusion of materials.

These features include:

1. A large surface area - This provides a larger area across which substances can be exchanged.
2. Thin walls - These minimise the diffusion distance.
3. An extensive blood supply and/or ventilation - This maintains steep concentration gradients.
4. Being surrounded by selectively permeable plasma membranes - This controls what substances are exchanged.