Water Uptake

Defining Water Uptake

The process of water uptake refers to how plants absorb water from the soil through their roots.
The primary sites for water uptake are the root hair cells, which are long and thin extensions of root epidermal cells, providing a large surface area for absorption.

Mechanism of Water Uptake

Root hair cells take in water from the soil by the process of osmosis.
Osmosis is the movement of water molecules from a region of higher water potential (soil) to a region of lower water potential (inside the root hair cell) across a semi-permeable membrane.
The water potential inside the root hair cell is lower due to the presence of mineral salts and sugars.
The roots also use active transport to absorb necessary minerals from the soil. This process requires energy in the form of ATP.

Pathways of Water Movement in Roots

Water moves from the root hair cells to the centre of the root, where the xylem is located, through 2 pathways – symplastic and apoplastic.
The symplastic pathway involves water moving from one cell to another through the cytoplasm and via the plasmodesmata (channels connecting neighbouring cells).
The apoplastic pathway involves water movement through the cell walls and spaces between the cells, essentially bypassing the cell’s interior.

Cohesion-Tension Theory

The Cohesion-Tension Theory explains how water is pulled up from the roots to the top of the trees in the xylem vessels.
This theory involves Transpiration (the loss of water from the leaves), Cohesion (water molecules sticking together), and Tension (a negative pressure or suction).
As water evaporates from the leaves, it creates a pulling force that draws more water into the leaf. This pull is transmitted all the way down the xylem, thanks to cohesion between water molecules.
This process allows the xylem sap to move against gravity and reach all parts of the plant.

Endodermis and Casparian Strip

The endodermis is the last cell layer in the root before the xylem and acts as a checkpoint for water and minerals.
The Casparian strip, a band of suberin in the cell walls of endodermis cells, ensures that water and minerals must move through the endodermis cells to reach the xylem, allowing the plant to have control over what enters its vascular system.