The hygroscopic water is held by colloidal soil particles, due to cohesive forces, and is absorbed by plants in a very
small quantity.
usually, water absorption occurs in plants through roots, which are often extensive and grow rapidly in the soil. The zone
of rapid absorption is characterized by the presence of root hairs.
root hairs develop mainly at the tip, just above the zone of elongation.
root hairs are thin-walled slender extensions of root epidermal cells that
increase the surface area of root and help in absorption of water from
the soil. a root hair is generally delicate and short lived. The cell wall of
root hair is composed of two distinct layers:
● The outer layer, which is composed of pectic substances, helps
to adhere soil particles.
● The inner layer is made up of cellulose.
The cell wall of the root hair is permeable to both solute and solvent molecules. It surrounds the plasma membrane.
each root hair has a central vacuole filled with osmotically active cell sap and a peripheral cytoplasm. for long distance
transport, plants have developed a mass or bulk flow system which operates through development of pressure
differences between the source and sink.
In mass or bulk flow, all the substances dissolved or suspended in solution travel at the same pace. Long distance bulk
movement of substances that occurs through conducting or vascular tissues of plants is called translocation.
There are two vascular tissues, xylem and phloem. Xylem translocation is mainly from roots to aerial parts. It passes
water with mineral salts, some organic nitrogen and hormones. Phloem translocates organic substances and inorganic
solutes, first from leaves to all other parts of the plant and storage organs. Storage organs re-export organic nutrients
to those parts which require the same, as newly formed leaves and fruits.
Translocation operates either due to positive hydrostatic pressure gradient, as in phloem, or a negative hydrostatic
pressure gradient as in xylem.
Mechanism of water absorption
Water absorption is of two types, passive (water is absorbed through the root) and active (water is absorbed by
the root).
The movement of water from the soil into the plant is due to the more negative water potential in the xylem than that
Table : Differences between active and passive absorption
Active absorption Passive absorption- The driving force is root pressure, and thus it is against
DPD gradient.
The driving force is transpiration, and thus proceeds
through DPD gradient.- It is at the expense of aTP. No aTP is needed.
- aTP is derived from cellular respiration, and thus, the rate
of respiration increases during absorption.
Since aTP is not required, rate of respiration remains
unaffected during absorption.- Oxygen is needed for respiration. Not affected by increase or decrease in the level of
oxygen. - The movement of water is against concentration
gradient.
Movement of water is in accordance to osmotic or
concentration gradient.- It usually occurs at night, when transpiration stops, due to
closure of stomata.
It occurs during the day, when transpiration is going
on, and stops at night, when transpiration stops.- bleeding and guttation are closely associated with active
absorption.
Transpiration is associated with passive absorption.