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Osmosis Essay Gcse


Water can move across cell membranes because of osmosis. For osmosis to happen you need:

  • two solutions with different concentrations
  • a partially permeable membrane to separate them

Partially permeable membranes let some substances pass through them, but not others. The animation shows an example of osmosis.

Osmosis is the movement of water from a less concentrated solution to a more concentrated solution through a partially permeable membrane.

This is shown in the animation above. Eventually the level on the more concentrated side of the membrane rises, while the one on the less concentrated side falls. When the concentration is the same on both sides of the membrane, the movement of water molecules will be the same in both directions. At this point, the net exchange of water is zero and there is no further change in the liquid levels.

Osmosis is important to plants. They gain water by osmosis through their roots. Water moves into plant cells by osmosis, making them turgid [turgid: Having turgor; enlarged and swollen with water ] or stiff so that they are able to hold the plant upright.

Now try a Test Bite.

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Osmosisosmosis: the movement of water molecules across a semi permeable membrane down a concentration gradient is a type of diffusion that involves the random movement of water molecules [molecule: A molecule is a collection of two or more atoms held together by chemical bonds. It is the smallest part of a substance that displays the properties of the substance. ]. It is the movement of water across a partially-permeablepartially permeable: Allowing some particles to pass through but not others membrane, from an area of high water concentration to an area of low water concentration.


A partially-permeable membrane is a membrane that allows small particles such as water molecules through it, but not larger particles such as sugar molecules and ions from salts.

Water concentration

A dilute solution has a high water concentration, while a concentrated solution has a low water concentration. For example, when salt is dissolved in water:

  • A little dissolved salt produces a dilute solution with a high water concentration
  • A lot of dissolved salt produces a concentrated solution with a low water concentration.


The animation models how osmosis works.

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