Mechanism or mode of water absorption by the plants

It is taking place in two ways :

1. Active Absorption of water

In this process, the root cells play an active role in the absorption of water and metabolic energy released through respiration is consumed. Active absorption may be of two kinds.

a. Osmotic absorption: When water is absorbed from the soil into the xylem of the roots along the osmotic gradient.

b. Non-osmotic absorption: When water is absorbed against the osmotic gradient.

1a. Active Osmotic Absorption of Water

The first step in the osmotic absorption of water is the imbibition of soil water by the hydrophilic cell walls of root hairs. Osmotic pressure (OP) of the cell sap of root hairs is usually higher than the OP of the soil water. Therefore, the DPD and the suction pressure in the root hairs become higher and water from the cell walls enters into them through the plasma membrane (semi-permeable) by osmotic diffusion. As a result, the OP, suction pressure, and DPD of root hairs now lower, while their turgor pressure is increased



Now, the cortical cells adjacent to root hairs have higher OP, suction pressure, and DPD in comparison
to the root hairs. Therefore, water is drawn into the adjacent cortical cells from the roothairs by osmotic
diffusion (Fig.3).
In the same way, the water reaches the innermost cortical cells and the endodermis by the cell to cell
osmotic diffusion gradually.
Osmotic diffusion of water into endodermis takes place through special thin-walled passage cells because the other endodermis cells have Casparian strips on their walls which are impervious to water.

Water from endodermal cells is drawn into the cells of the pericycle by osmotic diffusion which now becomes rigid and their suction pressure is decreased.
In the last step, water is drawn into the xylem from turgid pericycle cells. It is because, in the absence of turgor pressure of the xylem vessels (which are non-elastic), the suction pressure of xylem vessels becomes higher than the suction pressure of the cells of the pericycle.
When the water enters into the xylem from the pericycle, the pressure is developed in the xylem of roots, which can raise the water to a certain height in the xylem. This pressure is called Root Pressure.
1.b Active Non- Osmotic Absorption of water
Sometimes, it has been observed that absorption of water takes place even when the Op of the soil water is higher than the OP of the cell sap. This type of absorption, which is non-osmotic, and against the gradient requires the expenditure of metabolic energy probably through respiration.
2. Passive Absorption of Water
It is mainly due to transpiration; the root cells do not play an active role and they remain passive.
This theory explains that some forces are responsible for the absorption of water into the roots, which are actually governed by other cells. The governing force originates in the cells of transpiring shoots rather than in the root itself. These forces develop due to transpiration.

Passive absorption of water takes place when the rate of transpiration is usually high. Rapid evaporation of water from the leaves during transpiration creates tension in water in the xylem of the leaves. The tension is transmitted to water in the xylem of roots through the xylem of the stem and the water rises upward to reach the transpiring surfaces. As a result, soil water enters into the cortical cells through root hairs to reach the xylem
of the roots to maintain the regular supply of water. The force for this entry of water is created in leaves due to rapid transpiration and hence, the root cells remain passive during this process.


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