Plants do not use all the water they absorb. Most of it is lost by transpiration. Why then plants absorb more water than they need?
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Plant roots play an important role in absorbing water and nutrients needed for plant growth. In order for water and soluble ions to enter roots they must first come in contact with the root surface. Contact is achieved by direct contact as roots grow into the soil, mass flow of ions in soil water as transpiration of moisture out of the leaves pulls a minute stream of water from the soil, and diffusion within the water from locations of higher nutrient ion concentration like soil humus and mineral particles to areas of lower concentration near the roots.
Once in contact with roots, the ions can passively move into a free space between cells of the root epidermis and cortex cells. This movement is done through diffusion and surface electrostatic ion exchange. The cell surfaces of the epidermis and cortex are largely negative in charge and effectively exchange positively charges ions (cations), such as potassium (K+), calcium (Ca+2) and magnesium (Mg+2). In order to maintain ion charge stability, roots will release hydrogen (H+) ions off the cell surfaces into solution while adsorbing the cations noted above. The ions move towards the center of the roots passively until they come to a selective layer called the endodermis.
Movement past the impermeable endodermis layer requires that energy from metabolism be used to selectively move the ions across the membrane through tiny pores. Energy is needed because the osmotic concentration within plant cells is greater than soil solution. Cells outside the endodermis, such as root hairs can also actively absorb nutrient ions into themselves and pass the ions from cell to cell where the cell membranes contact each other. Movement is towards the endodermis and then towards the root and stem conductive organs called xylem that act as tiny pipes moving water and nutrient ions up from roots towards leaves.
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Once in contact with roots, the ions can passively move into a free space between cells of the root epidermis and cortex cells. This movement is done through diffusion and surface electrostatic ion exchange. The cell surfaces of the epidermis and cortex are largely negative in charge and effectively exchange positively charges ions (cations), such as potassium (K+), calcium (Ca+2) and magnesium (Mg+2). In order to maintain ion charge stability, roots will release hydrogen (H+) ions off the cell surfaces into solution while adsorbing the cations noted above. The ions move towards the center of the roots passively until they come to a selective layer called the endodermis.
Movement past the impermeable endodermis layer requires that energy from metabolism be used to selectively move the ions across the membrane through tiny pores. Energy is needed because the osmotic concentration within plant cells is greater than soil solution. Cells outside the endodermis, such as root hairs can also actively absorb nutrient ions into themselves and pass the ions from cell to cell where the cell membranes contact each other. Movement is towards the endodermis and then towards the root and stem conductive organs called xylem that act as tiny pipes moving water and nutrient ions up from roots towards leaves.
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Plants absorb minerals and nutrients from the soil in liquid form through Osmosis so they have to absorb much water even to take up small quantities of these nutrients.The excess of water is of no use for the plant so it gets rid of it through transpiration, also evaporation of this excess water from the leaves has a cooling effect on the plant and protects it from scorching in summer that is why in summer there is relatively more transpiration than in winter . this will help you
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