Diffusion and exchange surfaces – GCSE Combined Science Biology

Start here

Diffusion is how particles spread out. It is how you smell toast from another room, and it is how oxygen gets into your blood. The idea is simple. We will build it up in small steps.

It matters because every cell in your body needs to take in some substances and get rid of others. Diffusion does a lot of that work without using any energy.

By the end you will be able to define diffusion and say what makes it faster. You will calculate a surface area to volume ratio. You will also explain why lungs and other exchange surfaces are built the way they are.

What diffusion is

Particles in a liquid or a gas are always moving about at random. Diffusion is the spreading out of particles from a region of higher concentration to a region of lower concentration. Concentration just means how crowded the particles are.

Think of the smell of toast. It is strong in the kitchen and weak in the hall. More particles move from the crowded kitchen to the empty hall than the other way. This overall movement is called net movement.

Diffusion is passive. This means the cell does not need to use energy.

Diffusion in living things

Dissolved substances and gases can diffuse through cell membranes. Oxygen diffuses into cells, because the cells use it up in respiration. Carbon dioxide diffuses out of them.

Urea is a waste substance made in cells. It diffuses into the blood plasma, which is the liquid part of the blood. It is carried to the kidneys to be removed.

Only small molecules can diffuse through a membrane. Large molecules such as proteins cannot get through.

What changes the rate

Three things affect how fast diffusion happens. A steeper concentration gradient gives a faster rate. The gradient is the difference in concentration between two places.

A higher temperature gives particles more kinetic energy, which is movement energy. They move faster, so diffusion speeds up. A larger surface area gives more room for particles to cross.

A shorter distance to travel also helps. That is why exchange surfaces are very thin.

Surface area to volume ratio

Surface area is the total area of the outside of an object. Volume is the space inside it. A cube has six faces, so surface area = 6 × side × side. Volume = side × side × side.

Take a cube with sides of 2 cm. The surface area is 6 × 2 × 2 = 24 cm². The volume is 2 × 2 × 2 = 8 cm³. The ratio is 24 : 8, which is 3 : 1.

Always write the surface area first. Divide it by the volume.

Big and small organisms

Now try a cube with sides of 4 cm. The surface area is 96 cm² and the volume is 64 cm³. The ratio is 1.5 : 1. The bigger the object, the smaller the ratio.

Think of a cube of cheese. Cut it into small pieces and there is much more surface for the same amount of cheese.

A tiny organism has a large ratio, so diffusion across its surface is enough. A large organism has a small ratio. It needs special exchange surfaces, such as lungs, and a transport system such as the blood.

Adapted exchange surfaces

A good exchange surface has a large surface area. It is thin, so the distance is short. It also has a good blood supply. The blood carries substances away and keeps the gradient steep.

The lungs have many tiny air sacs called alveoli. Their walls are one cell thick, and breathing brings in fresh air. The small intestine has villi, which are tiny folds with thin walls and many capillaries.

Leaves are thin and flat. Root hairs give a large surface area. Fish gills have many thin filaments.

Recap

Diffusion is the net movement of particles from high to low concentration. It is passive.

It is faster with a steeper gradient, a higher temperature and a larger surface area.

Surface area to volume ratio = surface area ÷ volume. Large organisms have a smaller ratio, so they need exchange surfaces.

Good exchange surfaces are large, thin and well supplied with blood.