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Osmosis is the net movement of water molecules through a semi-permeable membrane down a water potential gradient.

Plants use osmosis to take in water through their root hair cells. It is a key part of the transpiration stream.


Osmosis and animal cellsEdit

If a cell produces water during a chemical reaction, the cytoplasm becomes more dilute ( HIGHER WATER CONCENTRATION ) and so water will move out of the cell by osmosis.

If the cell uses up water during a chemical reaction, the cytoplasm will become more concentrated ( LOWER WATER CONCENTRATION ) and so water will move into the cell by osmosis so that there is no difference in concentration between the two solutions- equilibrium has been reached.

Osmosis may be problematic if:

- the solution outside of the cell ( the tissue fluid which contains the dissolved substances needed by the cells ) is more dilute than the cytoplasm. Water will move into the cell by osmosis and will swell and may even burst. **Too much water --> swell up and burst **

- if solution outside of the cell is more concentrated than the cytoplasm water will move by osmosis from the area of higher water concentration ( inside the cell) to the area of lower water concentration ( outside of the cell). The cytoplasm will become too concentrated and will the cell will shrivel up. The process of cell shrinkage can be referred to as CRENATION. ** Too little water --> cytoplasm too concentrated so shrivels up and shrinks - crenation.**


Osmosis always occurs from a high water potential to a low water potential. It particles diffuse in the opposite direction it is called ACTIVE TRANSPORT.

Osmosis and plant cellsEdit

When the cell is in a hypotonic solution (less solute, so higher water potential) --> cytoplasm pushes hard against cell wall --> turgid. When it's placed in an isotonic solution (same water potential) --> no net movement, cytoplasm pushes against cell wall

In a hypertonic solution (lower water potential) --> water leaves cell by osmosis --> cell membrane pulls away from cell wall + vacuole and cytoplasm shrink --> plasomolysis


- Need diagram

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