3.1 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 using 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 i.e. replace lost heat

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

1. change to body shape (e.g. long / thin)

• increases SA:V and overcomes (reduces) long diffusion distance / pathway

2. development of systems, such as 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 e.g. by ventilation / good blood supply