Diffusion · Grade 10
In Grade 10 you learn: 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
- 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.
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.
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.
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?
Answer: 3 mm per minute
Example 5
Which process moves substances in this example: the smell of perfume spreading across a room?
Answer: diffusion
Example 6
A cube of agar jelly has sides of 2 cm. Find its surface area to volume ratio.
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?
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.
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.
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?
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.
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