Mineral salt absorption

Mineral salt absorption

Plants absorb mineral salts from the soil through the roots.  The zones of elongation and the root hairs are responsible for the mineral salt absorption.  They are then translocated to various parts. The minerals are absorbed as ions.  Plants accumulate the ions against their concentration in the soil.  Plant cells, tissues or organs when placed in mineral solution show two phases in mineral absorption; initial and metabolic.

In the initial phase there is rapid uptake of ions into outer or free space of the cells.  This outer space consists of intercellular spaces and cell walls.  The ions absorbed in free space are freely exchangeable.

In the metabolic phase the ions pass into the inner space.  The inner space comprises cytoplasm and vacuole.  Here the ions are not freely exchangeable.

The entry of ions into outer space is passive absorption as it requires no energy.  Absorption of ions into inner space requires metabolic energy.  It is therefore an active absorption.

Passive mineral absorption:

When the absorption of solute or salt takes place without the expenditure of metabolic energy and salt enters into the plant cell by free diffusion it is termed as passive absorption.  It can be explained by the following theories:

Diffusion:

Transport of ions across the membrane occurs passively by simple diffusion and facilitated diffusion.

Simple diffusion:

It is also termed as non-mediated diffusion.  In this type of diffusion, ions while moving across the membranes do not associate themselves with the constituents of the membranes.  Single ion channels are present in the membranes.  They are transmembrane proteins which function as selective pores for the diffusion of ions.

Similarly there are aquaporins for the diffusion of water.  The channels known as aquaporins permit diffusion of substrate in both directions through the membranes.  The substances move only in the direction of lower concentration.  The channel proteins do not bind the molecules or ions to be transported.  No energy is required for keeping the pores open or maintaining solubility of matrix.

Passive diffusion operates through physical forces like chemical potential, electrochemical gradient, hydrostatic pressure, and diffusion pressure gradient.  Water, oxygen, carbon dioxide, and sodium are known to follow passive diffusion. 

Facilitated diffusion:

It is movement of solute molecules or ions from one side of the membranes to the other through the membrane protein.  Plasma membranes of both prokaryotic and eukaryotic cells as well as membranes of subcellular organelles contain proteins.  These membrane proteins function as transport proteins.

These transport proteins translocate the molecules or ions across the membranes.  This rapid protein-mediated diffusion is called facilitated diffusion.  The direction of transport in facilitated diffusion is from higher concentration to its lower concentration.

The rate of transport through channels is much higher than by transport proteins.

The process of facilitated diffusion involves the following steps:

1. Recognition of specific substrate by transport protein.
2. Translocation of solute across membrane.
3. Release of solute by transport protein, and
4. Recovery of transport protein to its original condition to accept another solute molecule.

There are three different mechanisms which control movement of one or two types of ions in one or both the directions simultaneously.  For example, uniport mechanism transports only a single molecule in one direction. Antiport mechanism transports two different molecules in opposite directions and symport mechanism transports two different molecules in same directions.

Ion exchange theory:

According to this theory, ions from the external solution exchange with the ions of similar charge adsorbed on the surface of the cell wall or membranes of the tissue.  The colloidal fraction of the soil has an important role in ion exchange.  These colloidal particles called micelles, possess negative charge and attract cations such as calcium, magnesium, potassium, ammonium, sodium etc.

Similarly, the root surface which assumes a negative charge carries many cations on its surface.  These cations may exchange with other cations present in the soil solution.  Thus, a new cation can be absorbed on the root surface.  The process of exchange between adsorbed ions in the solution is termed as ion exchange. Following two theories can explain the ion exchange mechanism.

1. Contact exchange theory.
2. Carbonic acid exchange theory. 

Contact exchange theory:

An ion which adsorbs electrostatically to a solid particle is not tightly bound.  But it oscillates within a small volume of space.  This volume is termed as oscillation volume.  Therefore, the cations and anions adsorbed to the surface of root cell membranes or clay particles oscillate in limited area.

Suppose, H+ ion  adsorbs on root cell surface and K+ ion on the clay particles and both oscillate.  They oscillate in such a way that the oscillation volume of H+ ion overlaps that of K+ ion.  As a result, transfer of H+ ion to clay particles and K+ ion to root surface takes place.  This phenomenon is termed as contact exchange theory. 

Carbonic acid exchange theory:

Respiration occurring in a root cell results in the production of carbon dioxide, which forms H2 CO3 when dissolved in water.  The carbonic acid dissociates into H+ ion and HCO3- ion.  These ions then may be exchanged for similar charged ions of the soil solution.

Donnan equilibrium:

This theory describes the effect of fixed or non-diffusible ions, which mostly accumulate on the inner surface of the outer membrane.  Outer membrane is impermeable to fixed anions.  However, the cell membrane is impermeable to cations and anions present in the external medium.

Normally, both cations and anions diffuse into cells in equal numbers to attain an equilibrium between the cell sap and external medium . Now, the equilibrium has to be electrically balanced, therefore, more cations from the external medium will be needed to electrically balance fixed anions present in the cell.  Thus, the cations will be greater in the internal solution than in the external medium.  This electrical balance or equlibibrium is termed as Donnan Equilibrium.

Mass flow:

According to Hylmo (1953-1955), the absorption of ion increases with increasing transpiration.  The ions move in a mass flow with water from the soil solution through the root and eventually to the shoot.  This uptake depends on transpiration pool.  An increase in transpiration will cause an increase in the absorption of ions.  An increase in water flow due to transpiration pull also increases the total uptake of ions by the roots.  This is a passive process and occurs as a result of transpiration pull. The process does not require any metabolic energy.

According to Lopushinsky (1960) increase in transpiration can increase the salt absorption. According to many other workers rate of transpiration affects the salt uptake.  Kramer (1956), Russel and Barber (1960) support the above theory.