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Substances may move into and out of cells across the cell membranes via diffusion. 

Diffusion is the spreading out of the particles of any substance in solution, or particles of agas, resulting in a net mvoement from an area of higher concentration to an area of lower concentration.

Some of the substances transported in and out of cells bu diffusion are:

- Oxygen

- Carbon dioxide 

- Urea (waste product from cells into the blood plasma for excretion in the kidney)

Single celled organisms have a relatively large surface area to volume ratio. This allows sufficient transport of molecules into and out of the cell to meet the needs of the organism.

Factors affecting Diffusion

The factors which affect the rate of diffusion are:

- The difference in concentrations (concentration gradient)

- The temperature

- The surface area of the membrane

Diffusion in Action

In multicellular organisms, surfaces and organ systems are specialised for exchanging materials. This is to allow sufficient molecules to be transported into and out of cells for the organsism's needs. The effectiveness of an exchange surface is increased by:

- Having a large surface area

- A membrane that is thin, to provide a short diffusion path

- Having an efficient blood supply (in animals)

- Being ventilated (in animals, for gaseous exchange)

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