3.1 Surface area to volume ratio

Question 1

What is the relationship between the size of organism and its SA:V?

Answer 1

• Smaller organisms tend to have a higher SA:V than larger organisms
• Eg/ hippo vs mouse

Question 2

What is the relationship between SA:V and metabolic rate?

Answer 2

• Rate of heat loss/heat lost per unit body mass increases as SA:V increases
• i.e. more heat lost per unit body mass in smaller animals with a high SA:V
• So they need a higher metabolic rate/faster respiration
• To generate enough heat to maintain a constant body temperature (i.e. replace lost heat)

Question 3

What are the adaptations to facilitate exchange as this ratio reduces in larger organisms include?

Answer 3

• Larger organisms need a specialised surface / organ for gaseous exchange e.g. lungs
• Because they have a smaller SA:V and a long diffusion pathway (and skin is waterproof / gas
  tight)
• As well as having a high demand for oxygen and to remove carbon dioxide

Question 4

What are the features of a specialised exchange surfaces?

Answer 4

• A large surface area relative to the volume of the organism which increases the rate of exchange
• Very thin so that the diffusion distance is short and therefore materials cross the exchange surface rapidly
• Selectively permeable to allow selected materials to cross
• Movement of the environmental medium
• A transport system to ensure the movement of the internal medium

Question 5

How do organisms maximise SA:Vol ratio?

Answer 5

1. Cells divide - more cells = more efficient. Allows for cell differentiation, specialised functions
2. Cells compartmentalise - use membranes to carry out metabolic processes
3. Organelles are made up of membranes - maximising surface area for reaction
4. Organs fold up to maximise SA:Vol ratio - making absorption of food more efficient