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?

Divide surface area (size length x side width x number of sides) by volume (length x width x depth).

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

Easier/ quicker to find/ more accurate because of 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 adaptions that facilitate exchange as SA:V reduces in larger organisms

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

   • Increases SA:V and overcomes (reduces) long diffusion distance.

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.

   • Maintains a concentration gradient for diffusion e.g. by ventilation/ good blood supply.