Exchange Surfaces 7.1

Question 1

Why is diffusion alone enough to supply single-celled organisms?

Answer 1

They have fewer metabolic demands that can be met easier and they have a big SA: V ratio. They also are very small so the particles don’t have to travel very far to diffuse.

Question 2

Why can’t animals only use diffusion for gas exchange?

Answer 2

Animals are a lot bigger so our metabolic demands are also a lot higher. We need a lot more oxygen in order to respire and we release a lot more CO2. These gases also have to travel a long way to reach the cells in the centre of the body as animals are so big. The bigger the animal, the smaller the SA: V ratio which means diffusion cannot be used to exchange the gases fast enough.

Question 3

The volume of a sphere

Answer 3

4/3 pi r cubed

Question 4

The surface area of a sphere

Answer 4

4 pi r squared

Question 5

What do animals use instead of diffusion?

Answer 5

We use exchange surfaces that are specialised so that they can exchange the gases that they need to use and to release in an easier way.

Question 6

Features of exchange surfaces?

Answer 6

Increased surface area - bigger SA: V ratio
Thin layer - decreases distance travelled by substances as they diffuse.
Good blood supply - this supplies a good concentration gradient which will allow substances to diffuse faster
Ventilation - also maintains the concentration gradient and makes diffusion more efficient.