Surface area to volume ratio

Describe the relationship between the size and structure of an organism and

its surface area to volume ratio (SA:V)

● As size increases, SA:V tends to decrease

● More thin / flat / folded / elongated structures increase SA:V

How is SA:V calculated? Use an example

Suggest an advantage of calculating SA:mass for organisms instead of SA:V

Easier / quicker to find / more accurate because irregular shapes

What is metabolic rate? Suggest how it can be measured

● Metabolic rate = amount of energy used up by an organism within a given period of time

● Often measured by oxygen uptake → as used in aerobic respiration to make ATP for energy release

Explain the relationship between SA:V and metabolic rate

As SA:V increases (smaller organisms), metabolic rate increases because:

● Rate of heat loss per unit body mass increases

● So organisms need a higher rate of respiration

● To release enough heat to maintain a constant body temperature ie. replace lost heat

Explain the adaptations that facilitate exchange as SA:V reduces in larger

organisms

1. Changes to body shape (eg. long / thin)

○ Increases SA:V and overcomes (reduces) long diffusion distance / pathway

2. Development of systems, such as a specialised surface / organ for gaseous exchange e.g. lungs:

○ Increases (internal) SA:V and overcomes (reduces) long diffusion distance / pathway

○ Maintain a concentration gradient for diffusion eg. by ventilation / good blood supply

Exam insight: common mistakes ❌