Archive for the ‘Soil and Root Absorption’ Category
Active Absorption of Ions (Part Three) and Ionic Uptake by Roots
Active Absorption of Ions (Part Three)
ATPase Mediated Transport
ATPase can also generate proton gradient across the membrane. ATPase helps in hydrolysis of ATP. ATP is broken down into ADP and inorganic phosphate (Pi) with the addition of water. Water is spilt up into H+ and OH-. Then cations are transported activity in exchange for H+ in a direction opposite to H+. Anions follow passively, if anions are transported activity in exchange for OH-, cations show passive movement.
Active Absorption of Ions (Part Two)
Cytochrome Pump
This mechanism of ion transport is based on electrochemical gradient generated by electron transport. How such an electrochemical gradient is produced is explained by chemiosmotic hypothesis put forward by British biochemist P. Mitchel. According to Mitchell’s hypothesis, when hydrogen is removed from a substrate in respiration and carried along an electron transport chain, it is changed into two charged species:
Mechanism of Ionic Uptake – Active Absorption of Ions (Part One)
Active Absorption of Ions
The active transport of ions into the cells occurs at rates or in quantities which are against the laws of diffusion and electrochemical equilibrium. Such a transport uses energy derived from metabolism of the cell.
Mechanism of Ionic Uptake – Passive Absorption of Ions
Ions may be taken up by the plant cells by two methods:
- Passive Absorption.
- Active Absorption.
Passive Absorption of Ions
Movement of Solutes in the Cells and Ion Accumulation and Selective Uptake of Ions
Movement of Solutes in the Cells
As pointed out earlier, the soil solution diffuses freely into the cortical part of the apoplast. The ions present in the solution then move into the symplast. During this transport the ions have to pass through the plasmodesmata also.
Factors Affecting Rate of Water Absorption
The following factors affect the rate of absorption of water by the roots:
- Available Soil Water.
- Soil Temperature.
- Aeration of Soil.
- Concentration of the Soil Solution.
Mechanism of Water Absorption and Antagonism
Mechanism of Water Absorption
American physiologists, A. S. Crafts and T. C. Broyer proposed a mechanism of absorption of water by roots based on the apoplast – symplast concept. They suggested that soil solution, containing ions of salts dissolved in water, diffuses into the cortical apoplast. The ions are absorbed into the symplast by active absorption. They are then actively transported through the symplast from one cell to the other via plasmodesmata into the stele, where they are released into the stelar apoplast. The concentration of ions in the stelar apoplast becomes higher than that in the cortical apoplast. This sets up an osmotic gradient. Since water potential is higher in the cortex than in the stele, water diffuses across the endodermis into the stele. As a result of flow of water into the stele the hydrostatic pressure in the stele increases. This is the main cause of root pressure.
Entry of Water into Roots
Water diffuses directly from the soil into the free space of roots. Free space is that part of the root or tissue into which water reaches directly without any barrier. This space can not be measured exactly but its approximate measure is apparent free space (AFS). Its value can be estimated and expressed by the following equation:
Absorption of Water and Mineral Salts by Root
Absorption of water and mineral salts takes place through the much branched root system. A large part of water and mineral nutrients is absorbed by root. Most active absorption however, is known to occur at the root-hair zone, located a little behind the root tip. Root hairs increase the total absorptive area of root.
Composition of the Soil (Part Four)
Water and Soil Solution
Water is the largest component of the soil, next to mineral matter. In the soil water are dissolved many chemical compounds. These are formed from weathering of rock particles, decomposition of organic matter, activities of micro-organisms or reactions between roots of plants and soil constituents. Soil water, therefore, may rightly be spoken of as soil solution.