Osmosis is a specialized type of diffusion due to the selective permeability of cell membranes. Osmotic concentration is the volume of water held in a solution due to its movement across a membrane. The rate of diffusion and osmotic concentration are determined by particle motion, affected by size, shape, charge, and temperature. Selectively permeable membranes allow certain substances to pass through easily. Plasma membranes can prevent or allow passage of substances depending on the cell’s needs. Osmotic pressure refers to the tendency for water to enter a solution by osmosis.
The ability of cell membranes to be selectively permeable results in a specialized type of diffusion called osmosis. Osmotic concentration pertains to the volume of water held in the solution as a result of its movement across a membrane which is selective about what passes through. While water can move unhindered in either direction, the movement mostly occurs from where water molecules are most concentrated to where they are least concentrated. This difference in the concentration of some molecules in one region versus another is said to establish a concentration gradient. Solutes, which are molecules in solution, generally do not move unchecked beyond selective cell membranes.
Since diffusion can occur rapidly over a short distance, the rate of diffusion and osmotic concentration are usually determined by the motion of the particles. How fast or slow these particles move is affected by their shape and size, their electrical charges, and temperature. With an increase in temperature, the rate of diffusion also increases due to the increase in the speed of movement of the particles. Particles of different substances mixed together will diffuse separately. When particles are not added or taken away, a state of stability is reached, meaning that particles are distributed equally in both regions.
The degree of permeability of the membrane depends on the size of the substance, its electric charge and the composition of the membrane. A membrane is considered permeable to a substance if it allows it to pass through and impermeable when it does not allow it to pass through. Selectively permeable membranes allow certain substances to pass through them easily, but not others, such that cell membranes are more permeable to small molecules and fat-soluble molecules. Water molecules are an exception to this, as they can rapidly cross a liquid bilayer.
Determined by the external environment and the needs of the cell, plasma membranes can prevent the passage of a certain substance at certain times and allow its passage at other times. Alterations in cell volume and ion distribution can affect the osmotic concentration. By regulating osmotic concentration, a cell controls its volume and its internal environment, which can be significantly different outside of it.
The osmotic pressure of a solution refers to the tendency for water to enter that solution by osmosis. Solutions that have high concentrations of solutes will have low osmotic concentrations and higher osmotic pressures. Going the other way, a solution with a low solute concentration will have a high water concentration and a low osmotic pressure.
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