Factors Affecting the Rate of Diffusion

The rate of diffusion is influenced by several factors. Three main factors are:

  • 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 two organisms shaped like cubes. The length of each side is 1 metre for the first cube and 2 metres for the second.

A comparative illustration displaying two green cubes of different sizes and their respective calculations for surface area, volume, and the surface area to volume ratio.On the left side:A cube is depicted with each side measuring 1 metre. Below the cube, calculations are provided:Surface Area (SA): For one side of the cube, the surface area is calculated as 1 x 1, equalling 1 square metre. Since a cube has 6 sides, the total surface area of the cube is 1 x 6, equalling 6 square metres.Volume (V): The volume is computed as length x width x height, which equates to 1 x 1 x 1, resulting in a volume of 1 cubic metre. The surface area to volume ratio (SA:V) is 6On the right side:Another cube is shown, but this one has dimensions of 2 metres for its length (l), width (w), and height (h). The calculations provided are:Surface Area (SA): For one side of this cube, the surface area is 2 x 2, equalling 4 square metres. Given that there are 6 sides to a cube, the total surface area is 4 x 6, which is 24 square metres.Volume (V): The volume is worked out as length x width x height, giving 2 x 2 x 2, which totals 8 cubic metres.The surface area to volume ratio (SA:V) is 3

For the first cube, the surface area to volume ratio is 6:1, while for the second cube, the ratio decreases to 3:1. So, as the size of the organism increases, the surface area to volume ratio decreases.

This poses a problem for multicellular organisms whose surface area is insufficient for their cell volume. Many cells are positioned too far from the surface for oxygen to diffuse into them effectively

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

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