Absorption of water

Fig: Absorption mechanism of water in plants

a) Plants absorb water from soil. The water is utilized for various types of activities in plants.

b) Plants absorb the water through the entire surface. i.e. stem, leaves, roots but mainly from the roots.

c) In the aquatic plant and lower plants (algae) absorb water through the entire external surface.

d) Maximum water uptake found in the root hair zone.

e) Water in the soil is found in various forms. i.e. gravitational water, field capacity water, hygroscopic water, chemically combined water, rainwater, and capillary water.

                                     Absorption of water occurs by two methods.

A) Passive absorption:

                     The absorption of water not due to the activity of root cells but due to another process is called passive absorption. Passive absorption mainly occurs due to transpiration. During transpiration, water evaporates out from the aerial parts of the plant body through the stomata of the leaf. Removal of water through stomata creates tension into the stomatal cavity which reaches to the xylem of leaves. From the xylem of the leaf, tension is transmitted to the xylem of roots through the xylem of the stem. From the Xylem of the roots, tension goes into root hair cells through the endodermis and cortex. Due to this tension, water can be easily absorbed by root hair cells.

B) Active absorption:

                     According to this theory, water is absorbed as a result of the activity of roots. Here, shoot doesn’t play any role. Active  absorption occurs in two ways:-

1) Osmotic active absorption: This theory was proposed by Atkins(1916) and Priestly (1922). According to this theory, absorption of water occurs due to the osmotic concentration of root hair.

                         The absorption of water which occurs due to the osmotic concentration of root hair is called osmotic active absorption. Root hair cells consist of cytoplasm which considers a concentrated solution. In soil, there is the presence of water or water along with minerals which behave as weak solutions. Water enters into the root hair cells from the soil by osmosis because the cytoplasm of root hair cells shows high osmotic concentration.

2) Non-Osmotic absorption: The absorption of water against the concentration gradient is called non-osmotic absorption. In this case, water enters into the root hair cells from a mineral solution by utilizing the energy of root hair cells. It takes place during respiration.