GCSE Biology - AQA

3.2.1 - Diffusion

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# Diffusion

Diffusion is one of the ways that substances move in and out of cells and move around within cells.

It is defined as follows:

Diffusion is the spreading out of the particles of a substance, resulting in a net movement from an area of higher concentration to an area of lower concentration (down the concentration gradient).

Diffusion can happen to any substance in a solution or any gas.

## Diffusion involves a spreading out of particles

The particles in a gas or a solution are able to move around freely. Therefore, they tend to spread out over time. If the particles start out all packed together in one area, they will gradually spread out until they are evenly distributed everywhere.

The particles in a gas or solution move around randomly. This causes them to spread out over time so that eventually they are evenly distributed everywhere.

## Diffusion results in a net movement from an area of higher concentration to an area of lower concentration

If one area initially has a higher concentration of a substance than another area, then there will be a net movement of particles from the area of higher concentration to the area of lower concentration (down the concentration gradient) until the concentrations in the two areas are equal.

The word 'net' means 'overall'. This refers to the fact that although individual particles may be moving in lots of different directions, the overall effect is a movement from the area of higher concentration to the area of lower concentration.

In other words, it is not that there are only particles moving from the area of higher concentration to the area of lower concentration. In fact, there are also particles moving back in the opposite direction. It is just that because one area has a higher concentration, there will be more particles at any moment in time moving away from that area than moving into it, resulting in an overall (net) movement of particles out of the area of higher concentration.

Similarly, once the concentration is the same everywhere, the particles do not stop moving. They continue moving around between areas. However, because the concentrations of all areas are equal, particles move in and out of each area at the same rate, so there is no net movement.

## Substances often diffuse across membranes

Many substances are able to move freely across membranes such as cell membranes and the membranes that surround some sub-cellular structures such as the nucleus.

Therefore, diffusion often takes place across membranes. If the area on one side of a membrane has a higher concentration of a substance than the area on the other side of the membrane, then the substance will diffuse across the membrane from the area of higher concentration to the area of lower concentration.

For example, urea is a waste product which is produced in the bodies of many animals. Inside the cells that produce it, the concentration of urea is very high. In the blood moving past those cells, the concentration of urea is much lower. Therefore urea diffuses out the cells and into the blood, passing through the cell membranes of the urea-producing cells.

Urea diffuses out through the cell membrane of the cells that produce it and into a blood vessel. This diffusion takes place because there is a concentration gradient between the cells and the blood vessel.

## Diffusion is important for gas exchange

Gas exchange is an important process for many organisms. In animals, gas exchange involves taking oxygen into the body for use in aerobic respiration, and removing carbon dioxide (a waste product of respiration) from the body.

Diffusion is an important part of the process by which these gases move. For example, oxygen diffuses out of the blood and into cells that need it, while carbon dioxide diffuses out of the cells that produce it and into the blood.

Oxygen diffuses down its concentration gradient from the blood vessel to the cells. Carbon dioxide diffuses down its concentration gradient from the cells to the blood vessel.

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What is diffusion?

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What types of substance can diffusion happen to?