What Is Osmosis?

Osmosis can be considered as a specialised form of diffusion and is the means by which, in plant systems, water (the solvent) moves across a partly-permeable membrane (the plasma membrane of the cell) in an attempt to equalise solution concentrations on each side of that membrane.

A solution consists of the solvent (water) plus the solute (e.g. sugar or salts).

As you can see, only water can diffuse through the partly permeable membrane because the pores are too small to allow any solute through. Hence, to equalise concentrations on each side of the membrane, water passes from the side with the highest proportion of water (the most dilute, or 'hypotonic' side) to that with the least proportion of water (the most concentrated, or 'hypertonic' side). Consequently, the volume of this hypertonic side increases.

If allowed to continue unhindered, each side of the membrane will eventually become equal in concentration, or 'isotonic'. However, this rarely happens in plants, since the cell wall exerts a backward pressure called the 'turgor pressure'. Once the turgor pressure equals the osmotic pressure, an equilibrium is reached and water stops crossing the membrane.

The key thing to remember with osmosis is that it involves the passage of WATER only, from a relatively dilute (greater proportion of water) solution to a more concentrated one, across a partly permeable membrane (plasma membrane of the cell in plants, but could also be cellophane or pig's bladder in experiment).

In plant root cells, the surrounding soil solution usually contains a relatively dilute concentration of mineral salts to that of the cytoplasm of the root cells (i.e. it is hypotonic to the root cells), so water enters the root by osmosis. I have often heard it said that feed enters plant roots by osmosis or that it moves around the plant by osmosis. This is untrue. Remember, only WATER MOVEMENT is involved.