​Definition

Osmosis is the movement of WATER molecules across a selectively permeable membrane from an area of high concentration (of water molecules) to an area of lower concentration.

Osmosis can only happen when there is a selectively PERMEABLE membrane.

On the left side of the beaker, the concentration of water molecules is lower as there are sugar molecules taking up some of the space. On the right it is pure water.

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​Example of osmosis

​Because of this, the water molecules are going to move from the right side to the left due to osmosis. This means that it will reach an equilibrium (an equal concentration of water molecules on each side).

Osmosis in cells

Osmosis is a very important process in cells. The cell membrane in both plant cells and animal cells is selectively permeable. This means that only certain substances can pass through.

Plant cells have a cell wall around the outside of the cell. This is fully permeable - it has large holes in it that allow both water and solutes though. It's cell membrane however is selectively permeable.

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​Plant Cells

​This is where the cell loses water due to osmosis and the cytoplasm no longer pushes against the cell wall. The cell loses volume.

​​Flaccid

​Eventually if the cell continues to lose water it shrinks so much that the membrane and the cytoplasm split away from the cell wall. Gaps appear between the wall and the membrane.

​​Plasmolysed

​This is where the cell absorbs water by osmosis and swells up so that the cytoplasm pushes against the cell wall. The pressure inside the cell has increased.

​​Turgid