WATER UPTAKE FROM THE SOIL
Soil particles surround plant roots. The outermost layer of cells (the epidermis) contains root hair cells which increase the surface area of the root. These cells absorb minerals by active transport using ATP as energy. These absorbed minerals make the water potential in the cell cytoplasm reduce. This makes the water potential in the cell lower than in the soil and so water will move by osmosis down the water potential gradient across the plasma membrane.
MOVEMENT ACROSS THE ROOT
The movement of water across the endodermis is driven by an active process. The endodermis is the layer of cells surrounding the xylem. It is also known as the starch sheath as it contains granules of starch. Granules of starch is a sign that energy is being used.
The endodermis consists of special cells which contain a waterproof strip in some of its walls. This is called the Casparian strip. It blocks the apoplast pathway and forces water through the symplast pathway.
The endodermis cells move minerals by active transport from the cortex to the xylem. This decreases the water potential in the xylem. As a result, water moves from the cortex to the xylem through the endodermal cells by osmosis.
This reduces water potential in the cells just outside the endodermis. This, combines with water entering the root hair cells, creates a water potential gradient across the whole cortex. Therefore water moves along the symplast pathway from the root hair cells, across the cortex and into the xylem.
THE ROLE OF THE CASPARIAN STRIP
- blocks the apoplast pathway between the cortex and the xylem
- ensures water and nitrate ions pass into the cell cytoplasm through cell membranes