Mechanisms of water absorption in plants : Active and Passive Transport - Practice Questions & MCQ

• The terms ‘active absorption’ and ‘passive absorption’ were given by Renner.
• These concepts were defined by Kramer. 
• Passive absorption of water occurs due to the negative force generated in the shoot system.
• Due to the high rate of transpiration, the leaves lose water at a high rate. 
• The loss of water increases the osmotic pressure of the mesophyll cells of the leaves.
• Their water potential is set at low as well.
• They start drawing water from the adjacent cells.
• This results in the continuous build up of tension.
• The cells of the leaves draw water from the xylem elements of the midrib which in turn draw water from the xylem elements of the stem.
• The xylem elements of the stem ultimately draws water from the roots, where water potential is high.
• Hence, roots only serve as a pathway for water absorption. These are not actively involved in the process.
• Since roots are working as passive organs, this and there is no expenditure of energy in the absorption of water, this mechanism of water absorption through roots is called passive absorption.
• Roots absorb water due to tension created by transpiration called the transpiration pull.
• Passive absorption accounts for 98% of total water uptake.  
• This is the most rapid method of water absorption.

• In the active absorption of water, roots play an active role.
• Roots not only serve as the path for water absorption but also develop pressure for water absorption. This is a positive force.
• The mechanism of active water absorption can be explained by two theories:
  • Osmotic theory
  • Non-osmotic theory

Osmotic theory of active water absorption:

• It was proposed by Atkins
• Soil water is a diluted solution.
• It is hypotonic w.r.t the cell sap of the root hair cells.
• Therefore, the root hair cells absorb water through endosmosis.
• There is no expenditure of metabolic energy.
• The absorbed water moves according to the DPD, that is, from low DPD to high DPD.

Non-osmotic theory of passive water absorption:

• It was proposed by Bennet and Clark.
• It requires expenditure of metabolic energy.
• Roots absorb minerals through active absorption against their concentration gradient.
• This creates a hypertonic cell sap of the root hair cells.
• Thus, water moves according to the concentration gradient into the root hair cells.