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Ah, yes, osmosis, the one biology word everyone throws about to sound like a genius. But what actually is it? Well, read on to find out about osmosis, one of the most important processes that occurs across cell membranes.

Osmosis is a special type of diffusion that occurs with water molecules. The proper definition is the diffusion of water from a region of high water potential to a region of low water potential through a partially permeable membrane.

Let’s go through this bit by bit.

Diffusion is the net movement of particles across a membrane. Since particles are always in constant motion, this means that there are regions of higher and lower concentrations. Particles of a substance are more likely to move from high concentrations to low concentrations.

Water, as you know from my properties, is polar therefore substances (solutes) can dissolve in it. Water is a solvent so molecules cluster around the solute. When this happens, there are less free molecules to diffuse to other areas which lowers the concentration of water. But we don’t use the term concentration in osmosis ...

Water potential is the concentration of free water molecules and their tendency to diffuse to somewhere else. A higher water potential is due to more free water molecules. The units of water potential are kiloPascals (kPa) and pure water has a water potential (represented by psi) of zero. Lower water potentials are negative.

Water diffuses by osmosis from a region of high water potential to a region of low water potential through the concentration gradient. It is a passive process.

Water may move in or out of a cell due to its environment. You need to know what happens to animal and plant cells in different tonicities of solution.

In a plant cell, if the solution outside the cell is of a higher water potential, the solution is said to be hypotonic to the cell and therefore water moves in to the cell. The vacuole fills with water and the membrane pushes against the cell wall which stops it from bursting. The cell is turgid. 

When the solution outside the cell has a higher concentration of solute and therefore a lower water potential, the solution is said to be hypertonic to the cell. Water therefore moves out and causes the cytoplasm to shrink. If a lot of water leaves, the cytoplasm pulls away and becomes plasmolysed.

If a plant cell is placed in a solution of the same water potential, the solution is isotonic to the cell. Since the concentration of water is the same on each side, there is no net movement of water. The cell is not as turgid as it could be therefore it is called flaccid.

In a solution of higher water potential (hypotonic), animal cells will burst because they have no cell wall. In red blood cells, this is called haemolysis. If water leaves an animal cell by osmosis (in a hypertonic solution) it will shrink and become crenated.

Summary

The proper definition is the diffusion of water from a region of high water potential to a region of low water potential through a partially permeable membrane.

Diffusion is the net movement of particles across a membrane. Particles of a substance are more likely to move from high concentrations to low concentrations.

Water,is polar therefore substances (solutes) can dissolve in it. Molecules cluster around the solute. When this happens, there are less free molecules to diffuse to other areas which lowers the water potential

Water potential is the concentration of free water molecules and their tendency to diffuse to somewhere else. A higher water potential is due to more free water molecules. 

The units of water potential are kiloPascals (kPa) and pure water has a water potential (represented by psi) of zero. Lower water potentials are negative.

Water diffuses by osmosis from a region of high water potential to a region of low water potential through the concentration gradient. It is a passive process.

In a plant cell, if the solution outside the cell is of a higher water potential, the solution is said to be hypotonic to the cell and therefore water moves in to the cell. The vacuole fills with water and the membrane pushes against the cell wall which stops it from bursting. The cell is turgid.

When the solution outside the cell has a higher concentration of solute and therefore a lower water potential, the solution is said to be hypertonic to the cell. Water therefore moves out and causes the cytoplasm to shrink. If a lot of water leaves, the cytoplasm pulls away and becomes plasmolysed.

If a plant cell is placed in a solution of the same water potential, the solution is isotonic to the cell. Since the concentration of water is the same on each side, there is no net movement of water. The cell is flaccid.