Should an ideal exchange surface be thick or thin?

An ideal exchange surface should be thin. A thin surface gives substances a short diffusion pathway, so oxygen, carbon dioxide, glucose and other molecules cross it quickly. A thick surface would make the distance longer and diffusion slower. Thinness works together with the other features of a good exchange surface: a large surface area and a steep concentration gradient, kept up by a blood supply and, for gases, ventilation.

Why an exchange surface should be thin

Most exchange happens by diffusion: particles spread from where they are more concentrated to where they are less concentrated. Diffusion is fast over tiny distances but very slow over long ones. Every extra layer of cells adds distance, so the thinner the barrier between, say, the air in a lung and the blood, the faster gas exchange happens.

That is why exchange surfaces in the body are often only one cell thick. The walls of the alveoli are one flattened cell thick, and so are the walls of the capillaries around them, so oxygen only has to cross two thin cells to reach the blood.

The features of an exchange surface

AQA GCSE Biology says the effectiveness of an exchange surface is increased by:

1. A large surface area, so more particles can cross at once.
2. A thin membrane, to provide a short diffusion path.
3. An efficient blood supply (in animals), which carries substances away and keeps the concentration gradient steep.
4. Ventilation (in animals, for gas exchange), which brings in fresh air or water and keeps the gradient steep on the other side.

At A-level you will also see that exchange surfaces are partially permeable, letting the right substances through, and that gas exchange surfaces are moist, because gases dissolve before they diffuse across.

Fick's law: putting it together

Many A-level textbooks sum up these features with Fick's law of diffusion:

Thickness is on the bottom, so the rate is inversely proportional to it. Double the thickness and the rate halves; halve it and the rate doubles. The graph shows this for a surface where everything else stays the same.

Here x is the thickness of the surface and y is the rate of diffusion, both compared with a surface of thickness 1. A surface half as thick (x = 0.5) exchanges twice as fast. This is also why diseases that thicken a surface are serious: in pulmonary fibrosis, scar tissue thickens the alveolar walls, so the diffusion pathway gets longer and less oxygen reaches the blood.

Surface area to volume ratio

Small organisms such as single-celled amoebae can rely on their outer surface, because they have a large surface area compared with their volume. As an organism gets bigger, its volume grows faster than its surface area:

Each time the side doubles, the ratio halves. A large animal has too little surface for its volume, and the middle of its body is too far from the outside for diffusion alone. So large organisms have specialised exchange surfaces, folded to give a huge area and kept thin, plus a transport system to carry substances to and from them. Try a surface area to volume calculation in the Cell Biology course, then press Mark.

Examples of exchange surfaces

In fish gills, water and blood flow in opposite directions. Because of this counter-current flow, blood always meets water holding more oxygen than it does, so oxygen keeps diffusing in all the way along each lamella.

Why exchange surfaces are not even thinner

A thin, moist surface loses water as easily as it takes in gases, and it is easily damaged. Land organisms balance gas exchange against water loss:

• Mammals keep their lungs deep inside the body, where the air is moist.
• Insects can close their spiracles, and have a waxy cuticle.
• Plants have a waxy cuticle on their leaves and can close their stomata.

So the ideal surface is as thin as possible while still being protected and kept moist.

Check your understanding.

Common mistakes

• Saying thick surfaces are stronger, so better. Strength matters less than speed of diffusion: thin is the answer.
• Writing "thin" without the reason. Say it gives a short diffusion pathway, which increases the rate of diffusion.
• Mixing up surface area and surface area to volume ratio. Large organisms have a large surface area but a small ratio.
• Forgetting the concentration gradient. Blood flow and ventilation matter as much as the surface itself.
• Saying cells are thin. Say the wall or lining is one cell thick, or the cells are flattened.

Practise with a short timed paper

Exchange surfaces come up in GCSE Biology and in the first year of A-level Biology. The daily GCSE Biology Higher and A-level Biology: Year 1 papers are short timed papers of 8 questions, with a new one every day.

Read next

Biology · Organisation and transportAlveoli adaptations: how the lungs exchange gasesAlveoli adaptations for GCSE Biology: huge surface area, thin walls, a rich blood supply and ventilation, and how each one speeds up diffusion.