Question

How is water absorbed by root hairs in plants? Explain.

Answer 1

Hint: The root hair in a plant undergoes respiration to yield ATP (adenosine triphosphate). In plants, glucose breaks down into ATP by the process of cellular respiration.

Complete answer:
Root hairs are a tubular outgrowth of a trichoblast. In other words, they are lateral extensions of epidermal cells of the root. They don’t have chloroplasts (they don’t perform photosynthesis) and cuticles (as they hinder water absorption). They have large vacuoles with which they absorb the water quickly and transfer it to the next cell and the salt that is present inside their vacuole speeds up the process of water absorption. Soil particles trap water between them.

As the root hairs have extended structure they increase the total surface area for absorption of water. Root hair being permeable and hydrophilic absorbs the water from the spaces between the soil particles by active absorption and passive absorption.

Active absorption, the metabolic energy, that is, adenosine triphosphate resulting from respiration is used for absorption of water. This type of absorption occurs in low transpiring and well-watered plants and can be of two types: a) osmotic and b) non-osmotic.

The theory for active osmotic absorption was given by Atkins in 1916 and Priestley in 1923. According, to this theory the root cells behave as atypical osmotic pressure systems where the water is absorbed by the xylem present in the root in the presence of a concentration gradient.

The theory for non-osmotic active absorption was given by Thimann in 1951 and Kramer in 1959. According to them, the water from the soil is absorbed by the xylem present in the root against the concentration gradient.

In passive absorption, the absorption from soil to roots occurs in the absence of metabolic energy. It occurs in plants that respire rapidly. Passive absorption occurs due to transpiration (It is the process of movement of water in the plant, following its evaporation from aerial parts like a leaf, etc.). The transpiration force is then passed downwards to the root, in the elements of the xylem and the root hair acts as an organ and the absorption continues.

Note: The walls of root hair are thin and permeable to water which makes them hydrophilic and their large vacuole helps them to store the absorbed water. The cell wall of the root hair is thinly permeable with a semi-permeable plasma membrane. The minerals like chlorides, nitrates, and many more are transferred through root hairs.