Question

Describe the mechanism for absorption of water.

Answer 1

1. A mechanism for absorption of water: In plants, water is absorbed mainly by two processes: Passive absorption and Active absorption
2. Passive absorption:
   1. About 98% of the total water absorbed in plants occurs passively.
   2. In passive absorption, living cells of the root do not play an important role in water absorption.
   3. The driving force is transpiration pull and it thus proceeds through the DPD gradient.
   4. There is no expenditure of energy (ATP) as water moves in accordance with the concentration gradient. Hence, it is passive absorption.
   5. Passive absorption occurs during day time when transpiration is in progress. It stops at night when transpiration stops.
   6. Rapid transpiration creates tension in the xylem vessel due to negative water potential. This tension is transmitted to xylem in the roots. Consequently, water is pulled upwards passively.
   7. During passive absorption, no ATP is utilized. Thus, the rate of respiration is not affected.
3. Active absorption:
   1. In this water is absorbed due to the activity of roots.
   2. Root cells play an active role in the absorption of water.
   3. The driving force is the root pressure developed, in the living cells of the root.
   4. Active absorption occurs usually at night when transpiration stops due to closure of stomata.
   5. As water absorption is against the DPD gradient, there is an expenditure of ATP (energy) generated through the respiratory activity of cells.