Biology » Introducing the Cell » Cell Structure And Function



When the concentration of solutes in solution is low, the water concentration is high, and we say there is a high water potential. Osmosis is the movement of water from a region of higher water potential to a region of lower water potential across a semi-permeable membrane that separates the two regions. Movement of water always occurs down a concentration gradient, i.e from higher water potential (dilute solution) to lower potential (concentrated solution).

Osmosis is a passive process and does not require any input of energy. Cell membranes allow molecules of water to pass through, but they do not allow molecules of most dissolved substances, e.g. salt and sugar, to pass through. As water enters the cell via osmosis, it creates a pressure known as osmotic pressure.


Image credit: Siyavula

In biological systems, osmosis is vital to plant and animal cell survival. The figure below demonstrates how osmosis affects red blood cells when they are placed in three different solutions with different concentrations.


Image credit: Siyavula

Hypertonic (concentrated)IsotonicHypotonic (dilute)
The medium is concentrated with a lower water potential than inside the cell, therefore the cell will lose water by osmosis.The water concentration inside and outside the cell is equal and there will be no nett water movement across the cell membrane. (Water will continue to move across the membrane, but water will enter and leave the cell at the same rate.)The medium has a higher water potential (more dilute) than the cell and water will move into the cell via osmosis, and could eventuality cause the cell to burst.

Plant cells use osmosis to absorb water from the soil and transport it to the leaves. Osmosis in the kidneys keeps the water and salt levels in the body and blood at the correct levels.

Investigation: Predicting the Direction of Osmosis


To predict the direction of osmosis


  • 1 x 500 ml beaker
  • 1 x large potato
  • potato peeler/scalpel
  • 2 x pins
  • concentrated sucrose/sugar solution. To obtain this, add 100 g of sugar to 200 ml of water.


  1. Peel off the skin of a large sized potato with a scalpel/potato peeler.
  2. Cut its one end to make the base flat.
  3. Make a hollow cavity in the potato almost to the bottom of the potato.
  4. Add the concentrated sugar solution into the cavity of the potato, filling it about half way. Mark the level by inserting a pin at the level of the sugar solution (insert the pin at an angle into the cavity at the level) (See (a) part of the figure below).
  5. Carefully place the potato in the beaker containing water.
  6. Observe what happens to the level of the sugar solution in the potato.
  7. After 15 to 20 minutes, mark the level by inserting the second pin at the level of the sugar solution (insert as the first pin) (See (b) part of the figure below).


Image credit: Siyavula


  1. What do you observe happening to the level of the solution inside the potato?
  2. What conclusion can you draw based on your observation?
  3. What conditions were met in this experiment that makes this type of transport different to diffusion?

Video: Osmosis

The Amoeba Sisters video below discusses osmosis with real life examples. The terms hypertonic, hypotonic, and isotonic are explored throughout the video.

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