3.3.1 - Surface area to volume ratio

Question 1

Describe the relationship between the size and structure of an organism and its surface area to volume ratio (SA:V)

Answer 1

● As size increases, SA:V tends to decrease
● More thin / flat / folded / elongated structures increase SA:V

Question 2

How is SA:V calculated?

Answer 2

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

Question 3

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

Answer 3

Easier / quicker to find / more accurate because irregular shapes

Question 4

What is metabolic rate? Suggest how it can be measured

Answer 4

● 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

Question 5

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

Answer 5

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

Question 6

• Model A: oxygen uptake through system developed to the outside of the body
  > eg: fish gills
• Model B: oxygen uptake through system developed the inside of the body
  > eg: lungs
• Suggest 2 reasons how the environmental conditions have resulted in adaptations of systems using Model A rather than Model B.

Answer 6

1) - Water has low(er) oxygen partial pressure/concentration (than air);
- So (system on outside) gives large surface area (in contact with water
- So (system on outside) reduces diffusion distance (between water and blood);

2) - Water is dense(r) (than air)
- So water supports the systems/gills;