osmosis

The principle of osmosis. The osmotic pressure difference across a semipermeable membrane separating two solutions of different concentrations causes transport of the solvent through the membrane tending to equalize the concentrations of the two solutions. The process continues until an equilibrium state is achieved, either equal concentrations, or, in this case, balance between the osmotic pressure difference and the hydrostatic pressure excess of the solution in the inverted thistle funnel. The hydrostatic pressure difference thus provides a measure of the osmotic pressure difference.

Osmosis in plant cells

The diffusion of a solvent, such as water, through a semipermeable membrane that separates two solutions of different concentration, the movement being from the more dilute to the more concentrated solution, owing to the thermodynamic tendency to equalize the concentrations. The liquid flow may be opposed by applying pressure to the more concentrated solution: the pressure required to reduce the flow to zero from a pure solvent to a given solution is known as the osmotic pressure of the solution.

Osmosis was studied by Thomas Graham, who coined the term (1858); in 1886 Van't Hoff showed that, for dilute solutions (obeying Henry's law), the osmotic pressure varies with temperature and concentration as if the solute were a gas occupying the volume of the solution. This enables molecular weights to be calculated from osmotic pressure measurements, and degrees of ionic dissociation to be estimated. Osmosis is important in dialysis and in water transport in living tissues.

Osmosis in plant cells

Osmosis can be studied quite easily in plant cells. The cells are surrounded by a stiff outer wall which is lined with cytoplasm. Inside the cytoplasm is a space (vacuole) full of liquid (sap). The pressure of the sap presses the cytoplasm against the cell wall just like air in a leather football presses the bladder against the case. The cytoplasm is selectively semipermeable and lets water and some dissolved salts in. If the cell is put into a strong solution, water will pass out of the cell which will then lose its turgor and the cytoplasm will shrink away from the cell wall. If the cell is then put into pure water, water will pass into the sap and push the cytoplasm against the wall again. If the cell sap is very concentrated it may absorb enough water to burst the cell wall. Normally the cell wall is strong enough to overcome the osmotic force.

All water-living animals, unless they have impermeable skins, must have some method of getting rid of the excess water that they absorb. The process is called osmo-regulation and is found in many types of animal. If they did not get rid of the water they would burst for the cells have no cellulose wall like plants have,

Related categories

• PHYSICAL CHEMISTRY
• PROPERTIES OF MATTER

Also on this site:

Encyclopedia of Alternative Energy & Sustainable Living
Encyclopedia of History

BACK TO TOP