Diffusion

b. Diffusion : Diffusion is the second step in a root hair's uptake of water, and it is defined generally as the net movement of ions, atoms or molecules of a substance from a region of their higher concentration to a region of their lower concentration, driven purely by the random kinetic energy of the particles themselves, with no membrane or living activity required. Diffusion continues until the concentration is equalised throughout — that is, until equilibrium is reached.

In the root hair, once water has been imbibed by the cell wall, it diffuses across that freely permeable wall and comes to lie at the interface between the wall and the plasma membrane, ready for the next step (osmosis) to carry it further in.

Diffusion generates what is called diffusion pressure (D.P.), a pressure that is directly proportional to the number of diffusing particles present; the diffusion pressure of pure water is always greater than the diffusion pressure of water in any solution (because dissolved solutes lower it). The difference between these two values — the diffusion pressure of pure solvent and the diffusion pressure of the same solvent in a solution — is called the Diffusion Pressure Deficit (DPD), also historically called Suction Pressure (SP), a term coined by B. S. Meyer in 1938. In everyday terms, DPD can be thought of as the 'thirst' of a cell — how strongly it draws in water from its surroundings. Because the water surrounding the cell wall generally has a higher diffusion pressure than the cell sap inside, water moves inward by diffusion. In today's terminology, DPD has been superseded by the concept of water potential (covered in section 6.5), but the underlying physics is the same. Beyond water absorption, diffusion is also biologically significant in mineral uptake, in helping water move upward against gravity, in the exchange of respiratory a …