B3 1.1 Osmosis

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Diffusion takes place when partuckes can spread freely from one place to another. However, the solutions inside cells are separated fro those outside by the cell membrane. This membrane does not let all types of particles through. Membranes which only let some types of particles through are called partially permeable. 


Partially permiable cell membranes let water move across them. Remember that a dilute solution of sugar contains a high concentration of water (the solvent). It has a low concentration of sugar (the solute). A concentrated sugar solution contains a relatively low concentration of water and a relatively high concentration of sugar.

The cytoplasm of a cell is made up of chemicals dissolved in water inside a partially permeable bag of cell membrane. The cytoplasm contrains a fairly concentrated solution of salts and sugars. Water moves from the high concentration of water molecuktes (in a concentrated solution) across the membrane of the cell.

This special type of diffusion, where only water moves across a partially permeable membrane, is called osmosis.

The concentration inside your cells needs to stay the same for them to work properly. However, the concentration of the solutions outside your cells may be very different to the concentration inside them. This can


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