Diffusion · Year 12
In Year 12 you get fluent with: surface area to volume ratio and exchange surfaces.
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
- 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
A cube of agar jelly has sides of 2 cm. Find its surface area to volume ratio.
Answer: 3 : 1
Example 2
Three cubes of agar jelly have sides of 1 cm, 3 cm, 5 cm. Which has the largest surface area to volume ratio?
Answer: the cube with 1 cm sides
Example 3
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.
Answer: (a) the alveoli in the lungs, (b) the gill filaments, and the villi in the small intestine
Example 4
Explain three ways the alveoli are adapted for gas exchange.
Answer: a huge surface area; walls only one cell thick (short distance); a good blood supply and breathing keep a steep concentration gradient
Example 5
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?
Answer: it becomes 4 times as fast
Example 6
Explain why a single-celled amoeba does not need a gas exchange system but a mouse does.
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