Figure 1. 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.
Figure 2. Osmosis in a sugar solution.
Figure 3. Osmosis in plant cells.
Osmosis is 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 (see Figure 1). 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 of a sugar solution
Semipermeable membranes have small pores in them. While the pores allow small molecules through (1 in Figure 2), they also block larger molecules (2 in Figure 2).
If a semipermeable membrane is used to separate a sugar solution from a container of water (3), initially the concentration of water molecules in the solution will be lower than that in the pure water. Over time, water molecules will pass through the membrane, diffusing into the container, but as the pores are too small to allow the sugar solution to flow the other way, the level of the sugar solution rises.
Osmosis in plant cells
Osmosis can be studied quite easily in plant cells (see Figure 3). 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 osmoregulation and is found in many types of animal. If they did not get rid of the water they would burst because, unlike the case in plants, the cells have no cellulose walls.
Reverse osmosis is the movement of a liquid through a semipermeable membrane from a concentrated solution to a less concentrated solution. This movement is in the opposite direction to that in normal osmosis and can be achieved only by applying external pressure to the concentrated solution.
It is used in desalination plants to produce drinking water from seawater. The process is generally employed only on a small scale because the pressures involved are so high (up to 25 atmospheres).
Electro-osmosis is a flow of water caused when an electric current is passed through clay or other porous material, in a watery suspension. This phenomenon is caused by the formation of positive and negative ions, one type of which is bound to the clay while the other (in the water) moves toward the opposite electric pole.