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Saturday, 12 January 2019

Absorption of Water and Mineral Salts

 Uptake of soil water

 Root hairs absorb water. These hairs grow in a zone just behind the root tip. each hair is only a projection of part of a single cell on the piliferous layer of the root. Large number of toot hairs provide a vast absorbing surface.

     Root hairs absorb water from the soil by osmosis. The cell sap of the root hair cell is a solution of higher concentration than the soil solution. The cell membrane of the toot hair acts as selectively permeable membrane. So water enters the sap of the root cell from the soil. This extra water diluted the sap of the root hair cell so that it becomes less concentrated than the sap of neighboring cell in the cortex. Thus, water passes into the neighbouring cell by osmosis, and dilutes its sap. This process is repeated from cell to cell until the water eventually reached the xylem vessels in the centre of the root. some water also passes through the cell walls and cytoplasm by diffusion and capillary, to reach the xylem vessels.

 Uptake of mineral salts

 Plants also absorb mineral salts through root hairs. The concentration of ions in the cell sap of the toot cells is high, while that in the soil solution is low. Here, simple diffusion cannot explain the uptake of mineral ions into the root cells. Active transport is involved. The energy needed in this process is obtained by the oxidative break down of glucose in the root cells.

     To absorb mineral salts efficiently, roots must grow in well aerated soil. Once inside the root cells, the mineral ions move across to the xylem, and then upwards and through out the plant in the moving sao of the xylem vessels.  
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