AQA A Level Biology
Topic 3 Organisms
exchange substances
with their environment
Model answer notes by @biologywitholivia (updated for 2025)
Topic Understand Memorise Practise
3.1 Surface area to volume ratio
3.2 Gas exchange
3.3 Digestion and absorption
3.4.1 Mass transport in animals
Required practical 5
3.4.2 Mass transport in plants
These notes are for PERSONAL USE ONLY. Redistribution,
reproduction, or sale of any portion of this material is prohibited.
For enquiries, please email
,More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2025 Biology with Olivia
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
Divide surface area by volume
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
2
, ❌
More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2025 Biology with Olivia
Exam insight: common mistakes
Mistake Explanation
“As size increases, SA:V increases.” As size increases, SA:V decreases because although both volume and
surface area increase, volume increases faster than surface area.
“Respiration produces heat / energy.” Energy can’t be produced, only released.
3.2 Gas exchange
Explain how the body surface of a single-celled organism is adapted for gas
exchange
● Thin, flat shape and large surface area to volume ratio
● Short diffusion distance to all parts of cell → rapid diffusion eg. of O2 / CO2
Describe the tracheal system of an insect
1. Spiracles = pores on surface that can open / close to allow diffusion
2. Tracheae = large tubes full of air that allow diffusion
3. Tracheoles = smaller branches from tracheae, permeable to allow gas exchange with cells
Explain how an insect’s tracheal system is adapted for gas exchange
● Tracheoles have thin walls
○ So short diffusion distance to cells
● High numbers of highly branched tracheoles
○ So short diffusion distance to cells
○ So large surface area
● Tracheae provide tubes full of air
○ So fast diffusion
● Contraction of abdominal muscles (abdominal
pumping) changes pressure in body, causing air to
move in / out
○ Maintains concentration gradient for diffusion
● Fluid in end of tracheoles drawn into tissues by
osmosis during exercise (lactate produced in
anaerobic respiration lowers ψ of cells)
○ As fluid is removed, air fills tracheoles
○ So rate of diffusion to gas exchange surface
increases as diffusion is faster through air
3
Topic 3 Organisms
exchange substances
with their environment
Model answer notes by @biologywitholivia (updated for 2025)
Topic Understand Memorise Practise
3.1 Surface area to volume ratio
3.2 Gas exchange
3.3 Digestion and absorption
3.4.1 Mass transport in animals
Required practical 5
3.4.2 Mass transport in plants
These notes are for PERSONAL USE ONLY. Redistribution,
reproduction, or sale of any portion of this material is prohibited.
For enquiries, please email
,More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2025 Biology with Olivia
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
Divide surface area by volume
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
2
, ❌
More GCSE & A Level Biology resources available at stan.store/biologywitholivia Copyright © 2025 Biology with Olivia
Exam insight: common mistakes
Mistake Explanation
“As size increases, SA:V increases.” As size increases, SA:V decreases because although both volume and
surface area increase, volume increases faster than surface area.
“Respiration produces heat / energy.” Energy can’t be produced, only released.
3.2 Gas exchange
Explain how the body surface of a single-celled organism is adapted for gas
exchange
● Thin, flat shape and large surface area to volume ratio
● Short diffusion distance to all parts of cell → rapid diffusion eg. of O2 / CO2
Describe the tracheal system of an insect
1. Spiracles = pores on surface that can open / close to allow diffusion
2. Tracheae = large tubes full of air that allow diffusion
3. Tracheoles = smaller branches from tracheae, permeable to allow gas exchange with cells
Explain how an insect’s tracheal system is adapted for gas exchange
● Tracheoles have thin walls
○ So short diffusion distance to cells
● High numbers of highly branched tracheoles
○ So short diffusion distance to cells
○ So large surface area
● Tracheae provide tubes full of air
○ So fast diffusion
● Contraction of abdominal muscles (abdominal
pumping) changes pressure in body, causing air to
move in / out
○ Maintains concentration gradient for diffusion
● Fluid in end of tracheoles drawn into tissues by
osmosis during exercise (lactate produced in
anaerobic respiration lowers ψ of cells)
○ As fluid is removed, air fills tracheoles
○ So rate of diffusion to gas exchange surface
increases as diffusion is faster through air
3
