Mechanism of salt absorption and active absorption

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1. Mechanism of salt absorption active absorption R a h e e

2. Mineral salt absorption: Mineral salts are absorbed from the soil solution in the form of ions. They are chiefly absorbed through the meristematic regions of roots near the tips . A net movement of mineral salt into cell takes place is termed as absorption of salt . There are two theories for absorption of mineral salts: 1. Passive absorption 2. Active absorption

3. ACTIVE ABSORPTION: “ The lipid protein membrane of a cell is largely permeable to the all free ions energy is used to transport of such free ions across the membrane and that is called active absorption. Characteristics: Active transport exhibit following characteristics: A. it is non-spontaneous and independent of solute coceratation B. it uses protein carriers driven by expenditure of chemical energy C . it does not proceed towards an equilibrium D. it is against a chemical protein gradient the driving force being metabolic energy

4. The major hypothesis that explain the mechanism of active transport of ions are: 1 . The Carrier Concept- transport by a protein 2 . Carrier mechanism involve ATP theory 3 . Cytochrome-pump theory

5. A . CARRIER CONCEPT THEORY: This concept was proposed by Vaden Honert in 1937 According to this theory the plasma membrane is impermeable to free ions. But some compound present in it acts as carrier and combines with ions to form carrier-ion-complex which can move across the membrane. On the inner surface of the membrane this complex breaks releasing ions into the cell while the carrier goes back to the outer surface to pick up fresh ions

6. Carrier concept:

7. Following observations strongly support the carrier concept of active absorption of min­eral salts: (i) Isotopic Exchange: Several times, it has been found that actively absorbed radioactive ion cannot diffuse back with other ions so its not permeable to free ions . (ii) Saturation Effects: Beyond a certain limit, increased concentration of salts does not bring an increase in the rate of mineral salt absorption because active sites on the carrier compound become saturated with ions . (iii) Specificity: Active sites on carrier compound may be specific which can bind only some specific ions. This also explains the selective and unequal absorption of ions.

8. There are two common hypotheses based on the carrier concept to explain the mechanism of active salt absorption. (1) Lundegardh’s Cytochrome Pump Theory : Lundegardh’s and Burstrom (1933) believed that there was a definite correlation between respiration and anion absorption. when a plant transferred from water to a salt the rate of respiration increases. This increase in rate of respiration over the normal res­piration has been called as salt respiration . Assumptions:   ( i) The mechanism of anion and cation absorption is different. (ii) Anions are absorbed through cytochrome chain by an active process. (iii) Cations are absorbed passively

9. According to this theory : (i ) Dehydrogenase reactions on inner side of the membrane give rise to protons (H + ) and electrons (e – ).   (ii) The electron travels over the cytochrome chain towards outside the membrane, so that the Fe of the cytochrome becomes reduced (Fe ++ ) on the outer surface and oxidized (Fe ++ ) on the inner surface . (iii) The electron thus released unites with H +  and oxygen to form water

10. Main defects of above theory are: I.it envisages active absorption of only anions II.it not explain selective uptake ions III. It has found that cations stimulate respiration

11. 2 ) Benet-Clark’s Protein-Lecithin Theory: In 1956 , Benet-Clark suggested that because the cell membranes chiefly consist of phos­pholipids and proteins and certain enzymes. According to this theory the phosphate group in the phosphatide is regarded as the active Centre binding the cations, and the basic choline group as the anion binding .

12.   Conclusion : In this mechanism of salt absorption many theories have been explained by different scientist but active absorption concept more supported than passive absorption of salt. So, it have been believed that one or both of these mechanism may be involved in the salt absorption. But more work has to be done to explain the mechanism in clear and correct manner.

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