This membrane separates pure water, on the left, from a solution of sucrose and water, on the right. The membrane is semipermeable: water can cross it, but sucrose is too large for the membrane’s pores. Water molecules will move from a solution with a lower concentration of solutes to one with a higher concentration of solutes. A solution with fewer solutes has more water molecules that are free to move about and follow their concentration gradient across a membrane. Thus there will be net movement of water across this membrane from left to right. This diffusion of water across a membrane is called osmosis. Osmosis is of great importance in the water balance of all cells.

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