observation Lundegardh ( and ) proposed cytochrome pump theory which is based on the following assumptions: i. The mechanism of anion and cation absorption are different. ii. Anions are absorbed through cytochrome chain by an active process, cations are absorbed passively.
iii. An oxygen gradient responsible for oxidation at the outer surface of the membrane and reduction at the inner surface. According to this theory, the enzyme dehydrogenase on inner surface is responsible for the formation of protons (H + ) and electrons (e – ). As electrons pass outward through electron transport chain there is a corresponding inward passage of anions.
Anions are picked up by oxidized cytochrome - - . . PM . .
Donnan equilibrium Within the cell, some of the ions never diffuse out through the membrane. They are trapped within the cell and are called fixed ions. But they must be balanced by the ions of opposite charge. Assuming that a concentration of fixed anions is present inside the membrane, more cations would be absorbed in addition to the normal exchange to maintain the equilibrium.
Therefore, the cation concentration would be greater in the internal than in the external solution. This electrical balance or equilibrium controlled by electrical as well as diffusion phenomenon is known as the Donnan equilibrium . Summary There are two types of transports namely short and long distance in plants to translocate sap and solutes. Based on energy requirement, the transport may either be passive or active.
The process of diffusion, facilitated diffusion, imbibition and osmosis are driven by concentration gradient like a ball rolling down to a slope and hence, no energy is needed. The water absorbed (either active or passive) from the soil by root hairs must reach the xylem for further transportation. There are three possible routes to reach the xylem from root hairs. They are i) apoplast ii) symplast and/or iii) transmembrane.
Various theories explain the path of sap in the xylem and Dixon’s Cohesion-tension theory is the most accepted one. Transpiration is mostly carried out by stomata, which has guard cells. The general mechanism of stomatal