Diffusion

Diffusion is the net movement of particles from where they are more concentrated to where they are less concentrated, because they move about at random. It needs no energy. Oxygen and carbon dioxide diffuse in and out of cells this way. Diffusion is faster with a steeper concentration gradient, a higher temperature and a bigger surface area, which is why large organisms need exchange surfaces such as the alveoli.

Key facts

  • Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. It needs no energy. Oxygen and carbon dioxide diffuse in the lungs, and urea diffuses from cells into the blood.
  • Diffusion is faster with a steeper concentration gradient, a higher temperature and a larger surface area of membrane.
  • For a cube of side L, surface area = 6L² and volume = L³. Small organisms have a large surface area to volume ratio, so diffusion across their surface is enough; large organisms need exchange surfaces and transport systems.
  • A good exchange surface has a large surface area, a thin membrane (a short diffusion path), a good blood supply and, in animals, ventilation. Examples are the alveoli, the villi, fish gills and leaves.

Worked examples

Example 1

Oxygen moves from the air in the lungs into the blood. Explain how, and why it moves in that direction.

  1. Diffusion is the spreading of particles from where they are more concentrated to where they are less concentrated.
  2. The air breathed in has more oxygen than the blood arriving at the lungs.
  3. So oxygen diffuses into the blood. No energy from the cell is needed.

Answer: by diffusion: oxygen is more concentrated in the air in the lungs than in the blood, so it spreads from high to low concentration

Example 2

Give three things that make diffusion faster.

  1. A steeper concentration gradient means more particles move one way than the other.
  2. At a higher temperature the particles move faster.
  3. A bigger surface area, or a thinner surface, lets more particles cross at once.

Answer: a bigger difference in concentration, a higher temperature, and a larger surface area (or a shorter distance)

Example 3

A drop of purple dye is put in a beaker of still water. After an hour the water is evenly purple. Explain this using particles.

  1. Particles in a liquid move about randomly all the time.
  2. At first there are many dye particles in one place, so more move out of it than into it.
  3. This net movement goes on until the dye is evenly spread: then diffusion stops.

Answer: the dye particles move about randomly, so more spread from where they are concentrated to where they are not, until they are evenly mixed

Example 4

A dye spreads 12 mm through agar jelly in 4 minutes. What is its mean rate of diffusion?

  1. Divide the distance by the time. 12 ÷ 4 = 3
  2. So the dye diffuses 3 mm per minute.

Answer: 3 mm per minute

Example 5

Which process moves substances in this example: the smell of perfume spreading across a room?

  1. The particles move from a higher to a lower concentration, with no energy needed.
  2. So the answer is: diffusion.

Answer: diffusion

Example 6

A cube of agar jelly has sides of 2 cm. Find its surface area to volume ratio.

  1. Surface area: 6 faces, each 2 × 2 = 4 cm². 6 × 4 = 24 cm²
  2. Volume: 2 × 2 × 2. 8 cm³
  3. Divide the surface area by the volume. 24 ÷ 8 = 3
  4. So the ratio is 3 : 1.

Answer: 3 : 1

Example 7

Three cubes of agar jelly have sides of 1 cm, 3 cm, 5 cm. Which has the largest surface area to volume ratio?

  1. For a cube the ratio is 6L² ÷ L³ = 6 ÷ L.
  2. The smallest cube has the largest ratio: 6 ÷ 1 = 6.
  3. This is why small organisms can rely on diffusion across their surface, but large ones need exchange surfaces and transport systems.

Answer: the cube with 1 cm sides

Example 8

Name the exchange surface where oxygen is taken in by (a) a human, (b) a fish, and where digested food is absorbed in a human.

  1. In the lungs, gas exchange happens across millions of tiny air sacs: the alveoli.
  2. A fish takes oxygen from water across the thin gill filaments.
  3. Digested food is absorbed through villi, tiny finger-like folds of the small intestine.

Answer: (a) the alveoli in the lungs, (b) the gill filaments, and the villi in the small intestine

Example 9

Explain three ways the alveoli are adapted for gas exchange.

  1. There are millions of alveoli, giving a very large surface area.
  2. Their walls and the capillary walls are each one cell thick, so gases diffuse a short distance.
  3. Blood flowing away and breathing in fresh air keep the concentration gradient steep.

Answer: a huge surface area; walls only one cell thick (short distance); a good blood supply and breathing keep a steep concentration gradient

Example 10

The rate of diffusion is proportional to surface area × difference in concentration ÷ thickness of the membrane. If the surface area doubles and the thickness halves, what happens to the rate?

  1. Doubling the surface area doubles the rate. × 2
  2. Halving the thickness doubles it again. × 2
  3. Together: 2 × 2 = 4, so 4 times as fast

Answer: it becomes 4 times as fast

Example 11

Explain why a single-celled amoeba does not need a gas exchange system but a mouse does.

  1. A tiny organism has a large surface area compared with its volume, and every part is close to the surface.
  2. A bigger animal has a smaller surface area to volume ratio, and its inner cells are far from the surface.
  3. So it needs an exchange surface (lungs) and a transport system (blood) to supply its cells.

Answer: the amoeba has a large surface area to volume ratio, so diffusion through its surface is enough; a mouse has a small ratio and long distances, so it needs lungs