Question

Passive absorption occurs due to
A. Tension in root
B. Tension in xylem sap
C. ATP
D. None of the above

Answer 1

Hint: The passive absorption is the movement of minerals into the cell or tissue without the expenditure of energy.

Complete answer:
This is a physical phenomenon. It is the most prevalent method of water absorption. In this process, the force concerned with this type of absorption emanates from the aerial parts of the plants, especially leaves, and causes tension in the xylem sap. The tension occurring in the xylem network of vascular plants due to the loss of water.

Additional information
- Passive absorption occurs due to the tension created in the xylem cells of the leaves by rapid transpiration or evaporation.
- The tension is transmitted to the xylem roots through the xylem of the stem and water rises upward to reach the transpiring surface.
- This process will result in the entering of soil water into cortical cells through root hairs.
- It occurs when the soil moisture level is high either at night or when transpiration is low during the day.
- When transpiration is high, xylem sap is usually under tension, rather than under pressure, due to transpiration pull.
- A tension of up to 100 atmospheres has been reported in the xylem sap by Mac Dougal (1936) with the cohesive force of sap can be as low as 45 atmospheres.

So, the correct answer is ‘Tension in xylem sap’.

Note: The opposite of passive absorption is active absorption. Due to the activity of roots and mainly the root hairs the active absorption takes place. It is due to both osmotic and nonosmotic mechanisms. In this absorption, the osmotic process involves diffusion pressure deficit (DPD) of the cells. It is seen in the case of root pressure, bleeding, and guttation.