Active absorption of water
When the water is absorbed due to the activity of root itself, particularly root hairs, it is called active absorption. Active absorption of water may be osmotic or non osmotic.
Osmotic theory of active water absorption
Atkins (1916) and Priestly (1923) gave the osmotic theory of active water absorption. According to this theory, the root cells behave as an ideal osmotic pressure system. Water moves up from the soil solution to the root xylem along an increasing gradient of D.P.D .
The root hair and epidermal cells of the root contain the plasma membrane below the cell wall. The plasma membrane encloses inside it the cell sap. The cell sap contains solution of higher concentration than the soil water. A semi-permeable membrane separates the cell sap and soil water.
As the cell wall is permeable, it allows both the solute and solvent to pass through it. Water enters through the cell wall and comes in contact with plasma membrane.
It has been experimentally found that osmotic pressure of soil water is always less than 1 atm and osmotic pressure of cell sap is usually 2-8 atm.
If the osmotic pressure is more and turgor pressure is less in root hair cell, it will result in increased diffusion pressure deficit. High diffusion pressure deficit of cell sap causes endosmosis of water across the cytoplasm, plasma membrane and cell wall of root hair cell. Water enters into the root hairs as long as the diffusion pressure deficit of cell sap is greater.
Water moves from root hair cells to cortical cells along concentration gradient .Water finally reaches up to pericycle and endodermis. Since the xylem elements have no turgor pressure, their diffusion pressure deficit is higher than that of the surrounding cells. As a result of which water enters into the xylem.
This theory involves simplest movement of water, i.e., the water enters into living protoplasm of cells and then moves into the living protoplasm of another cell.
Fig: Active absorption of water
Rate of osmotic absorption of water:
A pressure called suction pressure (S.P) of the cell determines the rate of osmotic absorption of water. It is the net result of the difference between the osmotic pressure (O.P) and turgor pressure (T.P) of the cell.
S.P=O.P-T.P=Rate of Absorption of water
Here suction pressure determines rate of absorption of water by the cell . It depends upon the osmotic pressure (O.P) of the cell sap. As the osmotic pressure increases absorption of water also increases. As a result the volume of cell sap increases.
At this stage the cell develops turgor pressure (T.P) which acts against the osmotic pressure (O.P) . The turgor pressure developed by the cell tries to force some water out. Therefore the rate of absorption i.e. S.P depends upon the value of O.P-T.P.
Initially when there is no turgor pressure, suction pressure is equal to osmotic pressure (i.e., S.P=O.P). Gradually when more water accumulates in the cell sap the turgor pressure increases . The turgor pressure finally becomes equal to osmotic pressure, i.e., O.P=T.P.
At this stage the rate of absorption i.e., S.P is equal to zero . In other words in a fully turgid condition no absorption of water takes place in a cell.
Non osmotic theory of active absorption of water:
Thimann (1951) and kramer (1959) gave the non osmotic theory of active absorption of water . According to this theory water is absorbed against the osmotic gradient . Which requires some energy. The metabolic activity of the roots release the energy. So in this process energy supplies the active force for water uptake rather than osmotic pressure.
This view is also supported by some evidences. The respiratory process of roots release the metabolic energy required for the purpose. Inhibition of respiration causes reduction in the absorption of water . This indicates that energy is a major factor in the process.
When a cell is kept in a solution having equal concentration as that of the cell sap, diffusion of more water into the cell stops. This is as per the osmotic phenomenon. It has been observed that absorption of water takes place even if the concentration of cell sap in the root hair is lower than that of the soil water.
Normally under such conditions, the water molecules should diffuse out from root hairs into external solution. Due to process of exosmosis. But still, water is absorbed against the concentration gradient, i.e., from higher diffusion pressure deficit to lower diffusion pressure deficit. This type of absorption takes place at the expense of energy.
Thus, non osmotic absorption requires metabolic energy, which comes from the respiring cells of roots.
Evidences which show correlation between respiration and absorption:
Following are certain evidences which show a definite correlation between respiration and absorption:
Respiration and rate of absorption:
Factors which affect respiration also affect the rate of absorption. For example, respiratory inhibitors like potassium cyanide also inhibit the rate of absorption.
Wilting of plants in poorly aerated soil:
Respiration of root is closely dependent on the soil aeration. A well-aerated soil supplies required quantity of oxygen to the cortex and therefore, roots respire normally. In poorly aerated or water-logged soils, the external supply of oxygen is insufficient for the root respiration. Hence absorption by roots is inhibited in such soils and plant shows the signs of wilting.
Effects of auxins:
Like other metabolic activities, absorption also increases in the presence of auxins. Auxins enhance water absorption while respiratory inhibitors reduce the same.