Tag: transport in plants

The Pressure flow hypothesis often regarded as mass flow hypothesis explains best about the movement of Plant sap through phloem .The concept was proposed by E.Munch . The transpiration of water from leaves and other part of plant body produces negative pressure inside transport vessels causing root to absorb large amount of water along with ions to maintain the continuity of column of water . 

The mineral salts are absorbed into the symplasm, i.e., cytoplasm of root cells. There are special specific mineral transporters present in the plasma membrane of cells, which absorb the minerals from soil solution by transporting them across the membrane from soil solution to the cytoplasm of root cells. This way minerals can be absorbed irrespective of or against the concentration gradient of minerals. The minerals are present in the soil solution at very low concentrations and are usually absorbed against the concentration gradient with the help of specific carriers.

The roots respire continuously giving out carbon dioxide into the rhizosphere. Carbon dioxide combines with water and forms carbonic acid. Carbonic acid dissociates into  H$^+$ and HCO$ _3$$^-$. Because of high respiratory activity, a zone of carbonic acid is developed in root tips. The H$^+$ ions are exchanged with cations adsorbed on the clay micelle. The cations then come into the soil and diffuse onto the root surface. This is the carbonic acid exchange theory. Thus the correct answer is option C.

Soil respiration refers to the production of carbon dioxide when soil organisms respire. They include roots and other microbiota. Roots continuously respire and liberate significant quantities of CO2. The CO2, when dissolved in soil water, produces carbonic acid.  Immediately they ionize into H+ and bicarbonate ions. Thus during respiration in plant root carbonic acid is formed. Thus the correct answer is option C.

According to the theory of ions put forth by Jenny and Overstreet, ions are transferred from soil to root or vice versa without passing into solution. The ions are electrostatically adsorbed to the surface of root cells and are held tightly. These ions oscillate within a small volume of space called oscillation volume. When the oscillation volume of two ions of same charge overlap one ion is exchanged for the other. This is called contact exchange. Thus the transfer of ions from soil to root takes place without passing into solution. Thus the correct answer is option D.

Plant root cells contain relatively large quantities of organic cations and anions that do not diffuse through cell membranes. These organic ions could act as non-diffusible ions and may bind inorganic ions by electrostatic attraction. Thus ions are absorbed to the surface of roots electrostatically.