Factors Affecting the Rate of Diffusion

Three main factors affect the rate of diffusion:

  • TemperatureIncreasing the temperature also increases the rate of diffusion. This is because the particles have more kinetic energy, so they move faster
  • Concentration Gradient (the difference in concentration) – Increasing the concentration gradient will increase the rate of diffusion. For example, if there is a higher concentration of a substance outside a cell than inside it, then the particles of the substance will diffuse into the cell.
  • Surface AreaAs you increase a membrane’s surface area, the diffusion rate also increases. This is because the same volume of substance has a greater area to diffuse across.

Surface Area to Volume Ratio

Single-celled organisms, such as amoeba. have huge surface areas for their volume. In other words, they have high surface area to volume ratios. So, they can use diffusion to transport substances in and out of their cell.

Calculating the surface area to volume ratio

Let’s imagine an organism is shaped like a cube. The length of each side is 1 metre on the first cube and 2 metres on the second cube.

The surface area to volume ratio of cube 1 is 6:1 and with cube 2, the ratio falls to 3:1. So, as the size of the organism increases, the surface area to volume ratio decreases.

This creates a problem with multicellular organisms, as their surface area is not large enough for their volume of cells. Many cells are too far from the surface to diffuse oxygen into them.

Multicellular organisms solve this problem in two main ways:

  • They have unique gas exchange structures with very large surface areas
  • They have transport systems to carry substances around the body