State three ways in which root hairs are suited for the absorption of water from the soil
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Active absorption refers to the absorption of water by roots with the help of ATP, generated by the root respiration: as the root cells actively take part in the process, it is called active absorption. According to Renner, active absorption takes place in low transpiring and well-watered plants, and 4% of total water absorption is carried out in this process. The active absorption is carried out by two theories; active osmotic water absorption and Active non-osmotic water absorption. In this process energy is required.
Active osmotic water absorptionEdit
This theory was given by Pari (1910) and Priestley (1921). According to this theory, the root cells behave as an ideal osmotic pressure system through which water moves up from the soil solution to the root xylemalong an increasing gradient of D.P.D. (suction pressure, which is the real force for water absorption). If solute concentration is high and water potential is low in the root cells, water can enter from soil to root cells through endosmosis. Mineral nutrients are absorbed actively by the root cells due to utilisation of adenosine triphosphate (ATP). As a result, the concentration of ions (osmotica) in the xylem vessels is more in comparison to the soil water. A concentration gradient is established between the root and the soil water. Hence, the solute potential of xylem water is more in comparison to that of soil and correspondingly water potential is low than the soil water.If stated, water potential is comparatively positive in the soil water. This gradient of water potential causes endosmosis. The endosmosis of water continues till the water potential both in the root and soil becomes equal. It is the absorption of minerals that utilise metabolic energy, but not water absorption. Hence, absorption of water is indirectly an active process in a plant's life.Active transport is in an opposite direction to that of diffusion.[1]
Active non-osmotic water absorptionEdit
This theory was given by Thimann (1951) and Kramer (1959). According to the theory, sometimes water is absorbed against a concentration gradient. This requires expenditure of metabolic energy released from respiration of root cells. There is no direct evidence, but some scientists suggest involvement of energy from respiration. In conclusion, it is said that, the evidences supporting active absorption of water are themselves poor.[2]
Active osmotic water absorptionEdit
This theory was given by Pari (1910) and Priestley (1921). According to this theory, the root cells behave as an ideal osmotic pressure system through which water moves up from the soil solution to the root xylemalong an increasing gradient of D.P.D. (suction pressure, which is the real force for water absorption). If solute concentration is high and water potential is low in the root cells, water can enter from soil to root cells through endosmosis. Mineral nutrients are absorbed actively by the root cells due to utilisation of adenosine triphosphate (ATP). As a result, the concentration of ions (osmotica) in the xylem vessels is more in comparison to the soil water. A concentration gradient is established between the root and the soil water. Hence, the solute potential of xylem water is more in comparison to that of soil and correspondingly water potential is low than the soil water.If stated, water potential is comparatively positive in the soil water. This gradient of water potential causes endosmosis. The endosmosis of water continues till the water potential both in the root and soil becomes equal. It is the absorption of minerals that utilise metabolic energy, but not water absorption. Hence, absorption of water is indirectly an active process in a plant's life.Active transport is in an opposite direction to that of diffusion.[1]
Active non-osmotic water absorptionEdit
This theory was given by Thimann (1951) and Kramer (1959). According to the theory, sometimes water is absorbed against a concentration gradient. This requires expenditure of metabolic energy released from respiration of root cells. There is no direct evidence, but some scientists suggest involvement of energy from respiration. In conclusion, it is said that, the evidences supporting active absorption of water are themselves poor.[2]
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