Substances may move into and out of cells across the cell membranes via ____. Diffusion is the spreading out of the ____ of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration. Some of the substances transported in and out of cells by diffusion are ____ and carbon dioxide in gas exchange, and of the waste product urea from cells into the blood plasma for excretion in the ____. Factors which affect the rate of diffusion are: • the difference in concentrations (concentration ____) • the ____ • the surface ____ of the membrane. A single-celled organism has a relatively large surface area to ____ ratio. This allows sufficient transport of molecules into and out of the cell to meet the needs of the organism.

Diffusion

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