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Diffusion, Osmosis and Osmotic Pressure

Diffusion is the net movement of molecules because of a difference in the chemical potential or concentration between two regions. Molecules move faster when the difference/gradient is larger. Molecules tend to move to the region where their chemical potential or concentration is lower.


Osmosis is the diffusion/movement of water through a cell membrane. Since water molecules are small, they have a very high permeability through cell membranes. When there is a high concentration of solute, there’s a low concentration of solvent (ie. water). This means the chemical potential of water is low. On the other hand, a low concentration of solute means a high concentration of solvent. This leads to a high chemical potential of water.


When concentrations of solute and solvent are equal on both inside and outside the cell membrane, there is no movement of water molecules. The net movement of water is zero. The extracellular solution and cytoplasm is said to be isotonic. If the concentration of solute is higher in the extracellular solution compared to the cytoplasm, the net movement of water is out of the cell. The extracellular solution is said to be hypertonic. If the concentration of solute is lower in the extracellular solution compared to the cytoplasm, the net movement of water is into the cell. The extracellular solution is said to be hypotonic.


Since cell membranes have a limited surface area, they cannot stretch easily or as much compared to a balloon. As a result, the addition of water can only increase the volume of the cell by a little. The addition of more water increases the pressure inside the cell as well. Eventually, the pressure will increase enough to prevent more water from entering the cell. When this happens, the chemical potentials are equal and the pressure is called osmotic pressure. When the osmotic pressure is greater than the maximum pressure the membrane can withstand, the membrane can burst and have its contents disperse out. When this happens, the cell is said to have lysed.


This article was written for you by Samantha, one of the tutors with SchoolTutoring Academy.

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