Absorption of minerals are mainly absorbed by the roots which are in direct contact with the soil solution. Unlike the absorption of water, the minerals are absorbed over the entire surface of young root by the cells of epiblema. During absorption, the ions or molecules penetrate into the inner space of the living cells or tissues from the surrounding medium through a selective membrane. The movement of dissolved substances into and out of cells is also called transport or flux.
Various theories have been put forward from time to time to explain the mechanism of solute absorption. They are broadly categorized into two groups based on the involvement of metabolic energy in the process. Those which believe that metabolic energy is not involved in solute absorption have been grouped under passive mechanisms and those which believe that metabolism and metabolic energy is involved in the process have been placed under active mechanisms..(Absorption of mineral)
Though a broad distinction between passive and active absorption (or transport) is based on the involvement of metabolic energy in the process, several other distinctions between them a
(A) Passive Mineral Absorption:
The various modes of passive mineral absorption are as follows-.(Absorption of mineral)
1. Passive Diffusion.
The movement of ions or molecules from the region of its higher to lower chemical potential (concentration) along the concentration gradient is called diffusion. When the cells or tissues are placed in a solution having relatively higher chemical potential, the ions move into them by the process of free diffusion without the involvement of energy. The driving force in the process of such diffusion is chemical potential gradient..(Absorption of mineral)
|Passive Absorption (Passive Transport)||Active Absorption (Active Transport)|
|1.The driving force for passive absorption is physical (ie., non-me- nabolic) in origin||The driving force for active absorption is energy derived from metabolic processes|
|Energy-yielding metabolic processes are not involved in passive ab- sorption||Active absorption is not spontaneous and does not proceed towards an equilibrium|
|Passive absorption of ions and molecules is spontaneous and proceeds towards equilibrium.||The active absorption of a substance occurs across a protoplasmic membrane from its lower to higher chemical potential (concentration), Le., against concentration gradient|
|The active absorption of a substance occurs across a protoplasmic membrane from its lower to higher chemical potential (concentration), Le., against a concentration gradient||When energy-yielding metabolic processes are impaired active transport system is also inhibited|
|Passive transport occurs through the layer of protoplasm between the cell wall and the vacuole||.Active transport usually occurs across the protoplasmic membrane (ie., plasma membrane, tonoplast, etc.). It is often called a pump.|
2. Facilitated diffusion.
The passive absorption of solute mediated by a carrier is called facilitated diffusion. The cell membrane possesses certain permeases or some proteins which act as carriers or shuttles for a passive transport of ions or molecules across the membrane without involving the expenditure of energy. Facilitated diffusion occurs along the concentration gradient..(Absorption of mineral)
3. Mass Flow of ions.
Some workers are of the opinion that the ions move through roots to the shoot along with the mass flow of water. According to this theory, the mass flow of ions takes place due to transpiration pull..(Absorption of mineral)
4. Donnan Equilibrium.
Cells possess some fixed anions which mostly accumulate on the inner surface of the outer membrane. It results in intake of some extra cations from the surrounding medium to electrically balance the fixed anions. This electrical balance or equilibrium is called Donnan equilibrium,.(Absorption of mineral)
5. Ion exchange.
Mineral elements are absorbed by plants in the form of molecules, or as ions. Since the ions are charged (either +vely or -vely). they can not be absorbed alone without upsetting the electrostatic equilibrium between positively and negatively charged particles. When a positively charged ion (such as K+) is absorbed by a cell either a positively charged ion (such as H+) is displaced from the cell (ion exchange) or a negatively charged ion must accompany it.(Absorption of mineral)
The positively charged ions (eg, K+, Mg++, Ca++, etc.) of the external solution exchange with the hydrogen ion (H) adsorbed to the surface of cell wall or membrane of a tissue. Similarly anions exchange with free hydroxyl ions (OH). The ion exchange mechanism offers a greater opportunity for the absorption of ions from the external medium. The ion exchange mechanism could be explained by two widely accepted theories which are given below.(Absorption of mineral)
i) The contact exchange theory.
This theory is based on the ion exchange from one adsorbent to another without the participation of free electrolytes. An ion, which is adsorbed electrostatically to a solid particle, is not tightly bound but oscillates within a small volume of space. This is termed “oscillation volume.” Therefore, the cation or anions which are adsorbed on the surface of root cell membrane or clay particles, oscillate in a limited area.(Absorption of mineral)
Suppose H+ is adsorbed on the root cells surface and K+ is adsorbed on the clay particle and both oscillate in such a way that oscillation volume of H overlaps to that of K. This will result in the transfer of H+ to the clay particle and K+ to the root surface (Fig. 2.3). This phenomenon is called contact exchange..(Absorption of mineral)
ii) The carbonic acid exchange theory.
According to this theory the soil solution plays an important role by providing medium for ion ex- change, Respiration occurring in a root cell results in the production of carbon dioxide (CO2), which forms carbonic acid (H2CO3) when dissolved in water. The carbonic acid dissociates into hydrogen ions (H) and bicarbonate ions (HCO5). These ions may then be exchanged for similar charged ions of the soil solution.(Absorption of mineral)
(B) Active Mineral Absorption
The absorption (or transport) of solute linked with some metabolic reaction (or reactions) involving expenditure of energy is called active absorption or active transport. The molecules or ions, in active transport, cross a protoplasmic membrane (viz., plasma membrane, tonoplast, etc.) against the chemical potential (concentration) gradient.It is assumed that a specialized protoplasmic membrane constituent i.e., a carrier (also known as transporter, translocator and porter) is involved in solute transport across membranes. The exact nature of carrier molecule(s) is still not clearly known, but it is visualized that these are some transport enzymes (proteins) located here and there in the membrane extending from one surface to the other. .(Absorption of mineral)
he carrier molecules act as carriers of ions and molecules across the protoplasmic membrane in a turnover method. First of all the carrier molecule binds with the solute undergoing transport at the outer free space and forms a complex. The complex then moves across the membrane. This movement probably occurs due to diffusions across a membrane, rotation at 180° or by its conformational changes when bound to an ion or molecule. Finally the carrier solute complex breaks
apart in the inner space releasing the ion or molecule. These substances once taken into inner space cannot return back to outer space. The carrier molecule returns back to its original structure to outer space to pick up fresh ion or molecules. Singer (1974, 75) proposed a model of solute transport. According to it, the integral proteins, that occur in the form of bridges in the entire membrane, act as channel through which the substances are transported. The peripheral proteins bind specific ions or molecules and then get attached to integral proteins contain- ing hydrophilic pores. Finally a conformational change in the protein takes place that causes release of the solute across the memberan