2.15 understand the factors that affect the rate of movement of substances into and out of cells, to include the effects of surface area to volume ratio, temperature and concentration gradient

Diffusion and osmosis occur because molecules have kinetic energy. The molecules constantly bounce off each other all the time, gradually spreading out. Eventually there will be an even mixture of molecules, which is called an equilibrium. Diffusion can be affected by;

Changed in kinetic energy

Increased temperature and stirring the medium both increase kinetic energy which will increase the rate of diffusion/osmosis. This is because the molecules will have more energy meaning the molecules will collide with the cell membrane more often making movement through it more likely.

Alternatively, a decrease in temperature will decrease the rate of diffusion/osmosis as it will decrease the amount of kinetic energy in the molecules.


Surface area for diffusion and the surface area to volume ratio

With a larger surface area, molecules have more surface through which to diffuse, this increases the rate of movement which consequently increases the rate of diffusion/osmosis.


The size of the concentration gradient

The concentration gradient is the difference between the concentration inside and outside the cell. The bigger the difference is the more opportunity molecules have of diffusing.


NOTE: The thickness / distance molecules have to diffuse can also affect the rate of movement of substances. For example, diffusion through several cells will take longer than diffusion through a membrane 1 cell thick.