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Absorption of Water by Plants


Absorption of Water by Plants

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									Absorption of Water by Plants

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

          Sub Topics
         Apoplast pathway
         Symplast pathway
         Mechanism of Water Absorption,Active Absorption,Passive Absorption

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.

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

Mechanism of Water Absorption
Water can be absorbed by two methods:Active absorption       and 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 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.

Most land-based plants obtain nutrients through soil. Soil can be sand, clay, wet or dry, or a combination of factors
depending on geographic location. Soil can also have different textures and different acidity levels. These
conditions determine the availability of nutrients and what types of plants can grow. For example, clay is a wet soil
that can hold more nutrients and water than sand. However, some plants do better in sandy conditions.

Macronutrients are nutrients like nitrogen and calcium which plants require in large quantities. Macronutrients
can be primary nutrients or secondary nutrients. Primary nutrients include nitrogen, phosphorus and potassium.
The secondary nutrients are calcium, magnesium and sulfur. These can be obtained through lime, or through the
decomposition of grass clippings and leaves.

Micronutrients^^Micronutrients, minor elements or trace elements, are minerals needed for plant growth
but in very small quantities. These include boron, copper, iron, chloride, manganese, molybdenum and zinc.
Fertilizers and recycled grass and leaves can provide these nutrients.

Many plants have roots which are densely spread through soil in order to obtain as much water and nutrition as
possible. A few plants do not have roots but only live in water or very wet areas. Although plants can obtain water
directly by osmosis, most of the water comes through the root system. A large amount if not all of the plant's
nutrition also comes in through the roots. Non-mineral nutrients like hydrogen, oxygen and carbon are obtained
through                                                                                             photosynthesis.

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