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• Diffusion is the movement of particles from higher to lower concentrations.
• Diffusion happens naturally and so does not require energy.
• Substances like oxygen, carbon dioxide and glucose move in and out of cells by diffusion.

Diffusion is the overall movement of any substance from an area of higher concentration to an area of lower concentration. This can be within a cell or between them. Diffusion only happens in liquids and gases because their particles move randomly from place to place. It is an important process for living things; it is how substances move in and out of cells. It happens naturally and so does not require energy.

The particles collide with each other or with their container. This makes them change direction. Eventually, the particles are spread through the whole container. Diffusion happens on its own, without stirring, shaking or wafting.

Because diffusion is the movement of particles from a higher to lower concentration, we call this down the concentration gradient.

Diffusion doesn't just happen in cells. Look at the images below to see an example of how coffee particles diffuse in a cup of water.

Find out more about diffusion in gases and liquids

Air that is breathed in is around 21% oxygen, while air that is breathed out is only 16%. This means that some of the oxygen from the air has been absorbed into the body when it is breathed into the lungs.

Oxygen is transported to all cells in the body through the following process:

• Oxygen moves by diffusion from an area of higher concentration inside the lungs to an area of lower concentration in the blood. It is then carried away by red blood cells.
• The blood leaving the lungs now has a high concentration of oxygen.
• It travels round the circulatory system to the cells in tissues and organs.
• Here oxygen moves by diffusion from an area of higher concentration in the blood to an area of lower concentration in the cells.
• Cells are always lower in concentration of oxygen than the blood that arrives because the cells use oxygen in respiration.
• The blood now has a lower concentration of oxygen because it has diffused into other cells. It returns to the lungs to pick up more oxygen by diffusion and the cycle repeats.

Carbon dioxide is a waste product of respiration and moves by diffusion in the other direction. It moves from the cells into the blood and then from the blood into the lungs before it is breathed out. Unlike oxygen, carbon dioxide is not carried by red blood cells but dissolved in the blood plasma.

As well as oxygen, cells need glucose for respiration. This comes from food. Glucose is transported to all cells in the body by the following process:

• As food is digested, glucose moves by diffusion from an area of higher concentration in the small intestine to an area of lower concentration in the blood.
• The blood leaving the small intestine now has a high concentration of glucose.
• It travels round the circulatory system to the cells in tissues and organs.
• Here glucose moves by diffusion from an area of higher concentration in the blood to an area of lower concentration in the cells.
• Cells are always lower in concentration of glucose than the blood that arrives because the cells use it in respiration.
• The blood now has a lower concentration of glucose because it has diffused into the cells. The blood returns to the small intestines to pick up more glucose by diffusion and the cycle repeats.

The diffusion of water from an area of higher to lower concentration across a membrane is called osmosis. The expression 'across a membrane' often means in or out of a cell, which is surrounded by a membrane.