Gas Exchange

Cellular respiration, breathing, and gas exchange — not the same thing

Term	Meaning
Cellular respiration	The chemical process inside cells that releases energy from food
Breathing (ventilation)	The physical movement of air into and out of the lungs
Gas exchange	The diffusion of oxygen and carbon dioxide between an organism and its environment

What makes a good gas exchange surface?

Large surface area — maximises the area available for diffusion.
Thin — short diffusion distance for gases to cross.
Moist — gases must dissolve before they can diffuse across a membrane.
Well ventilated — maintains a steep concentration gradient.
Connected to a transport system — quickly carries gases to and from the rest of the body.

A simple organism like a flatworm (Planaria) has a large surface area relative to its volume, so gases can diffuse directly across its whole body surface — no special gas exchange organ is needed. Larger, more complex organisms need dedicated gas exchange organs to compensate for their smaller surface area to volume ratio.

Different organisms, different solutions

Explore how mammals, bony fish, insects, earthworms, and dicotyledonous plants each meet their gas exchange requirements in the Explore tab.

Answer in your exercise book.

Question 1 · (3 marks)

Distinguish between cellular respiration, breathing, and gas exchange.

Question 2 · (5 marks)

List FIVE requirements of an efficient gas exchange organ.

Question 3 · (2 marks)

Explain why a flatworm (Planaria) doesn't need a special gas exchange organ.

Question 4 · (4 marks)

Name the gas exchange structures used by a bony fish and an insect, and briefly describe each.

Question 5 · (2 marks)

Name the structures in a leaf through which a plant exchanges gases.

Question 6 · (6 marks)

The table below gives the surface area and volume of three cube-shaped model organisms of increasing size:

Cube side length (mm)	Surface area (mm²)	Volume (mm³)
1	6	1
5	150	125
10	600	1000

(a) Calculate the surface area to volume (SA:V) ratio for each cube (surface area ÷ volume). Show your working. (3 marks)
(b) Rank the three cubes from LARGEST to SMALLEST SA:V ratio. (1 mark)
(c) Using your answers, explain why small organisms can rely on diffusion across their body surface for gas exchange, while larger organisms need a specialised gas exchange organ. (2 marks)

Cellular respiration, breathing, and gas exchange — not the same thing

What makes a good gas exchange surface?

Different organisms, different solutions

Gas Exchange Across Organisms