Active Absorption

Active mineral absorption moves ions against their own concentration gradient, and, because that is thermodynamically uphill, it must be powered by metabolic energy - a requirement demonstrated by the fact that plant vacuolar sap accumulates both anions and cations against their gradients in ways that no purely passive mechanism can account for. The general explanation for how this happens is the carrier concept, proposed by Van den Honert in 1937: because the cell membrane is largely impermeable to free ions on their own, specialised carrier molecules embedded in the membrane instead bind an ion at the outer surface, forming a carrier-ion complex that is able to cross the membrane; once inside, the complex breaks apart, releasing the ion into the cell while the now-empty carrier returns to the outer surface to pick up another ion and repeat the cycle. Two specific theories elaborate on this general carrier idea. Lundegardh's cytochrome pump theory (Lundegardh and Burstrom, 1933; further developed 1950 and 1954) grew out of the observation that transferring a plant from water into a salt solution measurably raises its respiration rate - an effect called anion or salt respiration - and proposes that anion and cation absorption use fundamentally different mechanisms: dehydrogenase enzymes on the inner membrane surface generate protons and electrons, the electrons pass outward through a chain of cytochrome carriers (cycling between the Fe2+ and Fe3+ states) that ultimately reduces oxygen to water at the outer surface, and this outward electron flow is directly coupled to an inward passage of anions, which are picked up by the oxidised cytochrome at each step; cations, in this scheme, are assumed to move passively inward, simply following the electrical gradient that the anion accumulation creates. The theory does have acknowledged weaknesses - cations, too, are found to induce respiration, contrary to what the theory predicts; it cannot explain how ions are absorbed selectively; and it really only accounts for anion absorption in detail, leaving cation uptake comparatively unexplained. Bennet-Clark's protein-lecithin theory (1956) offers an alternative carrier mechanism, proposing that the mobile carrier is a protein associated with a phosphatide called lecithin; because lecithin is amphoteric (able to act as either an acid or a base), it can bind …