Why do cells multiply?
 

Cells multiply in order for the organism to grow, develop, repair and for the organism to produce offspring.

What limits the size of a cell and forces it to divide rather than keep getting larger is the ratio of surface area to volume of the cell.

When a sphere's radius,diameter or circumference doubles, its surface area increases fourfold, and its volume eightfold (area of a sphere is 4πR^2, volume is 4/3πR^3).

This means eight times as much metabolic activity, which requires the exchange of nutrients and waste through the cell membrane, but only four times as much surface area and cell membrane to do it with.

Soon, the needs of the cytoplasm swamp the ability of the cell membrane. This sets an upper limit on the cell size.

If the cell divides, the same amount of volume now has two cell surfaces, or twice the surface area with which to interchange substances with its environment.

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