The concentration of mineral solutes is much higher in the absorbing cells than outside in most cases. This increased concentration of minerals in the cells is called solute accumulation. Salt accumulation is a selective process. Absorption in such cases takes place against a concentration gradient. The theory of active solute absorption was put forward by Hoagland (1923 ) and others. According to this theory, the absorption of mineral salts takes place against a concentration gradient. ‘Me pumping up of salts is a continuous process. It causes accumulation of ions in cell. The rapid absorption of an anion retards the absorption of other anions. h promotes the absorption of a cation. Similarly rapid absorption of a cation inhibits the absorption of other cation. But it
increases the absorption. an anion. Active’ absorption requires a
continuous expenditure of energy. Nis energy is supplied by the adenosine triphosphate (ATP). ATP is produced in the respiratory activity of the root cells. Following factors promotes the active transport: I. Respiration is the essential conditions for the absorption of salts and
their accumulation. Therefore, this process is confined to growing and dividing cells. Meristematic cells and enlarging cells are the most active in absorbing ions. They lose the power of salt accumulation with stoppage Of growth. The capacity for salt accumulation decreases from backward of the tip of root.
- Lack of oxygen inhibits respiration. It also decreases the rate of absorption of ions.
- Adequate supply of respiratory material and favourable temperaturealso promote salt absorption.
4. Photosynthesis supply carbohydrates for respiration. Therefore, salt accumulation is also affected by the rate of photosynthesis.
Mechanism of Active absorption
Following mechanisms are involved in the active absorption of minerals:
I. Proton pump: Proton pump k present in the membrane of the root cells. It is an electrogenic pump. It uses ATP. Proton pump actively transport hydrogen ion of the cell. It also creates membrane potential in the cells. It set proton gradient across the membrane of cell. This
gradient enforces the minerals inside. •
2. Cotransport: The transport by which a substance indirectly drives the active transport of several other solutes is called con-transport. A substance that is pumped across a membrane has potential energy. It can leak back to the cell. Thus energy is released. Another specialized transport protein uses this energy and transport another inineral against concentration. Co-transport may be:
Antiprot: In this case. the two transported molecules moves in apposite direction.
(ii) Sytnport: In this ease. MO molecules move in the same direction.