8.1–8.3Exchange surfaces and gas exchange
Every organism has to move substances in and out. Oxygen and dissolved food molecules have to get in; carbon dioxide and urea have to get out; water and mineral ions move in both directions. In a single-celled organism that happens by diffusion straight across the cell surface, and no other machinery is needed. In anything larger it does not work, and the reason is a ratio.
As an organism gets bigger, its volume increases faster than its surface area. Every cell inside still has to be supplied, so the demand grows with the volume — but the surface available to supply it grows more slowly. The surface area to volume ratio falls, and past a certain size the outside of the organism simply cannot take in enough oxygen for the cells inside, and the distance to the innermost cells is too great for diffusion to cover in a useful time.
That is why multicellular organisms need specialised exchange surfaces and a transport system to carry substances between those surfaces and the cells.
Worked example
An organism is modelled as a cube of side 4 cm. Calculate its surface area to volume ratio, and compare it with a cube of side 1 cm.
- Surface area of the 4 cm cube = 6 × (4 × 4) = 96 cm².
- Volume of the 4 cm cube = 4 × 4 × 4 = 64 cm³.
- Ratio = 96 : 64, which simplifies to 1.5 : 1.
- For the 1 cm cube: surface area = 6 cm², volume = 1 cm³, so the ratio is 6 : 1.
Answer: 1.5 : 1, against 6 : 1 for the small cube — the larger organism has four times less surface for each unit of volume.
An effective exchange surface has the same four features whatever organism it belongs to, and a question asking you to explain an adaptation wants the feature linked to what it achieves:
• a large surface area, so many molecules can cross at once • a thin membrane, giving a short diffusion distance • in animals, a good blood supply, which carries substances away and keeps the concentration gradient steep • for a gas exchange surface, ventilation, which keeps the gradient steep on the other side too.
The alveoli are the example to know. There are millions of them in each lung, which gives an enormous total surface area in a small space. The wall of an alveolus is a single cell thick, and so is the wall of the capillary wrapped around it, so oxygen crosses only two cells to reach the blood. Each alveolus is surrounded by a dense capillary network, and blood flowing through it constantly carries oxygen away and brings carbon dioxide, maintaining the gradient in both directions. Breathing refreshes the air inside, so the oxygen concentration there stays high. The surface is also moist, because gases dissolve before they diffuse.
Oxygen therefore diffuses from the air in the alveolus, where its concentration is high, into the blood in the capillary, where it is low. Carbon dioxide diffuses the other way.
Common mistake
"The alveoli have a large surface area" on its own is a description, not an explanation. The mark is for the consequence — a large surface area means more molecules can diffuse across in the same time.
8.4B–8.5BWhat controls the rate of diffusion
Biology only — not on the Combined Science specification
Three things control how fast a substance diffuses across an exchange surface, and they are the same three whether the surface is an alveolus, a root hair or a gill:
• the surface area — a larger area means more places to cross at once • the concentration difference across the surface — a steeper gradient means a faster net movement • the thickness of the membrane — a thinner barrier means a shorter distance to travel.
Fick's law puts all three into one relationship:
rate of diffusion is proportional to (surface area × concentration difference) ÷ thickness of the membrane
Read it as three separate statements and it is much easier to use. Surface area is on the top, so doubling the surface area doubles the rate. Concentration difference is on the top, so doubling it doubles the rate. Thickness is on the bottom, so halving it doubles the rate — and doubling it halves the rate.
Questions give you a change in one, two or all three factors and ask what happens to the rate. Work through the factors one at a time and multiply the effects together.
Worked example
A gas exchange surface is damaged. Its surface area falls to half, and the walls thicken so that the diffusion distance doubles. The concentration difference is unchanged. What happens to the rate of diffusion?
- Halving the surface area multiplies the rate by 1/2.
- Doubling the thickness multiplies the rate by 1/2, because thickness is on the bottom.
- The concentration difference is unchanged, so it multiplies the rate by 1.
- Combine them: 1/2 × 1/2 × 1 = 1/4.
Answer: The rate falls to a quarter of what it was.
Common mistake
Fick's law is a proportionality, not an equation you can put numbers into and get a rate out of — there are no units on the answer. It tells you how the rate changes, so answer in multiples: twice as fast, a quarter of the rate.
Before you practise
- Volume grows faster than surface area, so the surface area to volume ratio falls as an organism gets larger.
- Calculate the ratio by working out the surface area and the volume separately, then simplifying.
- An effective exchange surface is large, thin, well supplied with blood, and ventilated if it exchanges gases.
- Each alveolus is one cell thick and wrapped in capillaries, and breathing keeps the concentration gradient steep.
- Rate of diffusion rises with surface area and with the concentration difference, and falls as the membrane gets thicker.
- Fick's law is a proportionality: work out what each factor multiplies the rate by, then multiply them together.