Diffusion
Transport across cell membranes
- In order for any organism to function properly, it needs to move substances in and out of cells
- Exchange of substances occurs across the cell membrane
- There are three transport processes that living organisms use for exchange: diffusion, osmosis and active transport
Diffusion is the movement of particles from higher to lower concentration
- Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement of particles from an area of higher concentration to an area of lower concentration
- This means:
- Particles in a solution or a gas are always moving about randomly
- If there happen to be more particles in one area/region, then there tends to be an overall (net) movement of particles from this region of higher concentration to a region where there are fewer particles (an area of lower concentration)
Diffusion of perfume particles through a room is one of the simplest examples of diffusion
- Diffusion is an entirely passive process; the movement of particles in a fluid is dependent on how much kinetic energy they have
- The higher the temperature, the more kinetic energy particles have and therefore the faster they can move by diffusion
Diffusion across cell membranes
- All cells are surrounded by a cell membrane which separates the inside of the cell from its outside environment
- Substances have to be able to diffuse through the cell membrane to enter or exit the cell unaided (there are other ways for substances to get in)
- Not all substances are able to diffuse across the cell membrane which is why it is described as being partially permeable; some substances are able to enter or leave the cell whilst others aren’t
- For example, oxygen and carbon dioxide can diffuse across the cell membrane during gas exchange whereas complex carbohydrates like starch cannot (it’s too big)
The cell membrane acts as a barrier which substances need to move across to get into or out of a cell
Examples of diffusion in living organisms
- You will need to learn examples of substances that organisms obtain by diffusion
Examples of Diffusion in Living Organisms Table
Examiner Tip
Remember that diffusion is a passive process, so when it occurs in a living organism, the cells of that organism do not provide the particles involved with energy to diffuse. The particles that are moving about randomly have their own kinetic energy.