Absorption of Minerals
The mechanism by which plants absorb mineral nutrients from the soil involve both passive absorption and active transport. Passive absorption is the process by which substances pass across membranes according to concentration gradients; substances move from a place of higher concentration to that of a lower concentration. Active transport on the other hand, involves the movement of substances against a concentration gradient using metabolic energy.
Passive Absorption of Minerals
Absorption of salts takes place through the root system which is in intimate contact with the soil colloids or soil solution. When a plant cell or tissue is transferred from a medium of low salt concentration to a medium of relatively high salt concentration, the tissue absorbs ions rapidly at the initial stages. This is followed by a slow steady uptake of mineral ions. The initial rapid uptake of ions is not affected by factors such as temperature and metabolic inhibitors showing clearly that the process does not utilize metabolic energy. When the plant tissue mentioned above, is transferred to a medium of low salt concentration, some of the ions taken up earlier will diffuse out into the external medium. This free diffusion of ions through biological membranes, according to the concentrations gradients, is passive diffusion. It is one of the major mechanisms of absorption of minerals by plants.
Apart from passive diffusion of mineral ions, there are other mechanisms by which the plant root systems absorb minerals without using metabolic energy. One of such mechanisms is ion exchange and through the establishment of Donnan equilibrium. Ions absorbed into the surfaces of the cell walls or membranes of a tissue may be exchanged with ions from the external solution in which the tissue is immersed. For example, potassium ions (K+) may be exchanged for hydrogen ion (H+) absorbed to the surface of the cell membrane. In the same way anions could exchange with hydroxyl ions. The ion exchange mechanism allows for a greater absorption of ions from the external medium than could normally be possible by the process of free diffusion.
Mineral ions can also move through roots with the mass flow of water. It has been shown that an increase in transpiration causes an increase in the absorption of ions; however, the effect of transpiration on ion absorption could be a direct effect or an indirect effect. In short, it can be said that if the concentration of minerals is higher in the soil water, the salts are carried up into the xylem elements, along with water, through the mechanism of passive absorption.
Most of the time, plant cells and their organelles require a much higher concentration of minerals and ions than what may be available in water surrounding soil particles. To meet this requirement of plant cells, minerals and ions are constantly absorbed against their concentration gradients using metabolic energy. The absorption of minerals using respiratory energy is called active absorption. Active absorption is influenced by factors such as temperature, oxygen concentration or by respiratory poisons. Mineral uptake by roots shows a decline under conditions of low temperature, low oxygen concentration or by the inhibition of respiratory processes by inhibitors or poisons. All these results demonstrate that the process of active absorption of minerals by root system is dependent on the utilisation of metabolic energy. In has been found that the major amount of mineral uptake in plants takes place through active absorption.
Various mechanisms have been proposed to explain the process of active absorption. In many instances, the transport of materials across the membrane is facilitated by the presence of carrier protein molecules on the cell membrane.
Ions are known to accumulate in cells, i.e. they may move (e.g. into the inner space) against concentration or ecp(electrochemical potential) gradients. This movement requires additional energy. The additional energy is derived directly or indirectly through metabolism. Various evidences indicate that active ion uptake is probably carried out by carrier mechanism for both the influx and efflux of ions, and have indicated the existence of ionic pumps. There has been a lot of speculation on the possible nature of the carriers.
Ion Traffic into the Root
Mineral nutrients absorbed by the root are carried to the xylem. This takes place by two pathways called apoplast and symplast.
The apoplastic pathway, essentially, involves diffusion and bulk flow of water from cell to cell through spaces between cell wall polysaccharides. The ions entering the cell wall of epidermis move across cell wall of cortex, cytoplasm of endodermis, cell walls of pericycle, and finally, accumulate in xylem vessels.
In symplast pathway, ions entering the cytoplasm off epidermis move across the cytoplasm, cortex, endodermis of pericycle through plasmodesmata, and finally, to xylem vessels.