- In order for any organism to function properly, it needs to exchange substances such as food molecules and waste products between itself and its environment
- This 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 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
- 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 the carbohydrate starch cannot (it’s too big)
- Examples of substances that enter cells by diffusion:
- Oxygen must diffuse into cells for aerobic respiration
- Carbon dioxide must diffuse into photosynthesising plant cells to be available for photosynthesis
- Examples of substances that leave cells by diffusion:
- Liver cells break down excess amino acids into the waste product urea which is highly toxic and must diffuse out of cells to be excreted by the kidneys
- Carbon dioxide produced by aerobic respiration must diffuse out of cells
- In both the transport of substances into and out of a cell by diffusion, the movement is driven by a concentration gradient existing between the cell and its immediate environment.
- If the direction of that concentration gradient changes, the movement of a substance by diffusion will also change
A photosynthesising leaf cell will likely have a lower carbon dioxide concentration on a bright day compared to its environment, as carbon dioxide is used in photosynthesis faster than it is produced in aerobic respiration.
At night, when the plant is not photosynthesising the carbon dioxide concentration inside the same leaf cell will be higher than the environment as the leaf cell is always respiring.