Osmosis

All organic matter is made up of a complex system of cells. Although there are many types of cell, each with its own function and contents, there are some rules which govern all organic cells. One such rule is that of osmosis! It is right to use the term rule because it is not the actual process. The process it controls is water movement and it is a fundamental process to all organic life both plant and animal (and all other types).

First lets recap on cells. A cell is the unit of organic life. There are two main types; plant and animal. The plant cell has a outer membrane made of cellulose, a hard compound which allows water through. This surrounds the plasma membrane which is usually made of phospholipid. The inside contains cytoplasm, which is a kind of liquid medium in which organelle's are present (sort of microscopic reaction centres) and cell reactions occur. In the centre of the cell is a vacuole, which is a sort of air space (although it is not air, it contains some liquids, gases and nutrients) and is also separated by a membrane. (see fig below)

The animal cell is pretty much the same as the plant cell except it does not have a cellulose cell wall or vacuole. (see fig below) All other cell types are adapted versions of these two, for example; woody tissue is formed from a plant cell when the cell wall is ligninfied (a substance known as lignin is added to the outer membrane, it is even stronger than cellulose and impermeable to water!).

Cells can be found in three broad categories of saturation; fully plasmolised, flaccid or turgid. Fully plasmolised is when the level of water in the cell is so low that the cell can not recover. This state is not seen is nature as the plant will normally die before this state is reached. Flaccid cells are when the cell has lost so much water that the cell is beginning to shrink (this is not possible in plants as the cell wall is too strong, but the inner membrane will begin to pull away from the cell wall and the plant will wilt). Turgid is when the cell has plenty of water. (see fig below)

Water can move between cells (unligninified) freely as the cell walls are permeable. However its movement is controlled by a number of factors which are given names; solute potential(), Pressure potential (+ Matrix potential =0 normally)() and water potential().

Solute potential (): this refers to the amount of substance dissolved in the cytoplasm. This effects water movement because water always moves from a less concentrated medium to a more concentrated medium.

Pressure potential (): this refers to the pressure exerted by the cell walls. In a plant, the cell will fill with water until the inner membrane is pushing on the outer membrane to the same force as the wall is pushing back, they then are equal forces and cancel out. Therefore there is no more movement of water.

Water potential (): this is a figure worked out from the equation; water potential = solute potential + pressure potential (). The figure represents the tendency of a cell to give out water.

Osmotic potentials are actually simple maths but they are also very difficult because the values are always equal to or less than 0, i.e. negative. This is because as you increase the level of solute you decrease the tendency for the cell to give out water. The pressure potential is a positive value as it creates a force against incoming water. When the pressure potential is equal to the solute potential the value is 0 and there will be no movement of water. Water will always move from a less negative cell (nearer 0) to a more negative cell because, as I mentioned earlier, water always moves from a liquid with less solute (nearer 0) to one with more (-).

Try it out! Take a look at the cells below, each one has a water potential value in it, try and predict which cell water will move into! (Click it to find out if your right!)

If you still don't understand you can contact us by emailing or ask a scientist you know!