3.3.1 Surface area to volume ratio

Question 1

Give four things that organisms need to exchange with their environment.

Answer 1

Cells need to…

• …take in oxygen (for aerobic respiration)
• …take in nutrients
• …excrete waste products (e.g. carbon dioxide, urea)
• …exchange heat

Question 2

Describe how body shape affects heat exchange.

Answer 2

• Animals with compact shapes have a small SA : V (minimising heat loss from their surface).
• Animals with a less compact shape have a larger SA : V (this increases heat loss from their surface).

Question 3

Explain why a small animal needs a relatively high metabolic rate compared to a large mammal.
[3 marks]

Answer 3

• A small mammal has a bigger surface area to volume ratio than a large mammal.
• This means that heat is lost more easily from a small mammal.
• So a smaller mammal needs a relatively high metabolic rate, in order to generate enough heat to maintain a constant body temperature.

Question 4

Why do multicellular organisms need Exchange Organs and Mass Transport Systems?

Answer 4

In multicellular organisms, diffusion across the outer membrane is too slow, due to:
- a large distance between the cells and the environment
- low surface area to volume ratio
Therefore, multicellular animals need specialised exchange organs and an efficient system to carry substances to and from their individual cells.

Question 5

Why is the diffusion rate quick in single-celled organisms?

Answer 5

Substances can diffuse directly into/out of the cell across the cell-surface membrane. The diffusion rate is quick because of the small distances the substances have to travel.