The uptake of inorganic ions across the *semipermeable membranes
of plants. If the absorption is against a diffusion gradient, as is often the case across the tonoplast, then *active transport
is involved. In addition, the uptake is often selective. For example, in sea water the concentration of sodium ions is around forty times higher than that of potassium ions. However within the cells of marine algae the potassium concentration is often about six times greater than that of sodium.