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Introduction |
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The study of world of life is as old as man itself. All living organisms are made up of a few elements which are repeated in each of them. The basic unit of life - cell, is a structural and functional entity of life. |
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Absorption and Movement of Water |
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The movement of the molecules from the region of higher concentration to the region of lower concentration is known as diffusion. |
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Plasmolysis |
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If a plant cell is placed in a highly concentrated sugar or salt solution (i.e., hypertonic solution), water from the cell sap flows out due to exosmosis through the plasma membrane outside the cell. The loss of water from the cell sap causes contraction or shrinkage of the protoplast. Since the cell wall is firm and less elastic, it cannot keep pace with the contraction of plasma membrane. Ultimately the protoplasm separates from the cell wall and contracts or shrinks to a spherical shape. This condition is called plasmolysis. |
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Permeability |
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Permeable membrane allows free movement of water and solutes in and out of the cell. |
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Water Potential |
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Water potential is regarded as the tendency of water to leave a system. It is often used while explaining the direction in which water flows from one cell to another or from one part of the plant to another e.g., from soil to root, from root to leaves, from leaf to air or from soil to air. |
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Imbibition |
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Imbibition is the phenomenon of adsorption of water by the solid particles of a substance without forming a solution. |
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Availability of Water in the Soil |
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Soil is the major source of water for plants. The plants absorb water through root hairs from the soil. The total water content present in the soil is called holard. Out of this, the water which can be absorbed by plants is chresard and remaining is called echard. |
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Absorption of Water by Plants |
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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. |
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Passage of Water in Root or Pathway of Water in Root |
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The entry of water into the root hair dilutes the cell sap. Thus water molecules in root hair increases as compared to adjacent cortical cells. |
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Theories for Ascent of Sap |
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According to vital theories upward movement of water takes place due to activity of living cells of plant bordering xylem. |
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Guttation |
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The loss of extra water in liquid drops from margins of leaves of herbaceous plants when root pressure is high and transpiration is low. |
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Transpiration |
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Transpiration is the loss of water in the form of water vapour from the aerial parts of the plant. |
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Internal Plant Factors |
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Variations in leaf structure to reduce transpiration. |
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Role of Stomata in Transpiration |
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Since most of the water (90%) is lost through stomata, plants regulate the degree of stomatal opening to reduce the water loss. |
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Mechanism of Stomatal Action |
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The mechanism namely the opening and closing of stomata depends upon the turgor pressure in the guard cells. When the guard cells are turgid, the stoma opens and when the guard cells lose water, stoma closes. |
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Summary |
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Cell is the structural and functional unit of the organism. Water is the principal constituent of plants and is essential for the maintenance of life, growth and development. |
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Points to Remember |
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The direction in which water flows from one cell to another cell depends on the water potential. |
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Some Interesting Features |
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Plants wilt when turgor pressure inside the cells of such tissues go down below zero. |
