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Friday, July 30, 2010

absorption of water by plant roots animation

Plants absorb water through the entire surface - roots, stems and leaves. However, mainly the water is absorbed by roots. The area of young roots where most absorption takes place is the root hair zone. The root hairs are delicate structures which get continuously replaced by new ones at an average rate of 100 millions per day. The root hairs lack cuticle and provide a large surface area. They are extensions of the epidermal cells. They have sticky walls by which they adhere tightly to soil particles. As the root hairs are extremely thin and large in number, they provide enormous surface area for absorption. They take in water from the intervening spaces mainly by osmosis.
Water in the roots move by two pathways. They can be classified as1) Apoplast pathway2) Symplast pathway

Apoplast pathway

In this pathway the movement of water occurs exclusively through cell wall without the involvement of any membranes. Majority of the amount of water goes through the apoplast pathway. The cortex of the root does not oppose such movement of the water.
three pathways of water movement

Symplast pathway

Here the movement of water molecules is from cell to cell through the plasmodesmata. The plasmodesmata forms a network of cytoplasm of all cells.
The Casparian strip separates the cortex and the endodermis. It is composed of a wax like substance called suberin, which blocks water and solute molecules through the cell wall of the endodermis. Now the water is forced to go through the cell membranes of different cells leading to a transmembrane pathway.
areas of root Involved in absorption and translocation of water
Areas of Root Involved in Absorption and Translocation of Water

Mechanism of Water Absorption

Water can be absorbed by two methods:
  • Active absorption
  • Passive absorption

Active Absorption

Water is absorbed due to activities going on in roots. Absorption of water occurs with the help of energy in the form of ATP, which is released due to metabolic activities of root cells such as respiration. Absorption takes place against concentration gradient - even when the concentration of cell sap is lower than that of soil water.

Passive Absorption

Passive absorption is by osmosis. Passive absorption takes place along the concentration gradient - when the concentration of cell sap is higher than that of soil water. Water is absorbed when transpiration rate is high or soil is dry. Due to high transpiration rate, water deficit is created in transpiring cells. Rapid transpiration removes water and reduces turgor pressure in living cells of root. The suction force thus developed is transmitted to root xylem. It pulls water from surrounding root cells to make up water deficit.

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