Ion Uptake

CHAPTER: 9 (A)

Ion Uptake

(Mechanism of Absorption of Minerals Salt)

Ion Uptake:

(Mechanism of Mineral Nutrients Absorption)

•      In plants, mineral uptake is the process in which minerals enter the cellular material, typically following the same pathway as water.

•      The most normal entrance portal for mineral uptake is through plant roots.

•      Some mineral ions diffuse in-between the cells.

•      In contrast to water, some minerals are actively taken up by plant cells.

•      Mineral nutrient concentration in roots may be 10,000 times more than in surrounding soil.

•      During transport throughout a plant, minerals can exit xylem and enter cells that require them.

•      Mineral ions cross plasma membranes by a chemiosmotic mechanism.

•      Plants absorb minerals in ionic form: nitrate (NO₃⁻), phosphate (HPO₄⁻) and potassium ions (K⁺); all have difficulty crossing a charged plasma membrane.

•      It has long been known plants expend energy to actively take up and concentrate mineral ions.

•      Proton pump hydrolyzes ATP to transport H⁺ ions out of cell; this sets up an electrochemical gradient that causes positive ions to flow into cells.

•      Negative ions are carried across the plasma membrane in conjunction with H⁺ ions as H⁺ ions diffuse down their concentration gradient.

Mechanism of Mineral Salt Absorption:

•      Formerly it was assumed that inorganic salt were passively carried away into the plant with the absorption of water.

•      It was also suggested that the salt could diffuse into the root tissue along concentration gradients.

•      Thus, the early worker took into consideration only the physical aspects of the absorption of salts and neglected the role of metabolic energy in the process.

•      Recently, it is clearly shown that mineral salt absorption is predominantly on active process rather than a passive process.

Ø  Basically, plant absorbs minerals from the soil through the roots in two ways:

            1. Passive Absorption

            2. Active Absorption

1. Passive Absorption:

•      In passive absorption of mineral nutrients , the minerals move into the cell or tissue along their potential gradient without the expenditure of energy.

•      This is a purely physical phenomenon and takes place in many ways:

1. Diffusion
2. Ion exchange Theory
3. Donnan Equilibrium
4. Mass Flow Theory

a.         Diffusion:

•      It has been observed that an initial passive or non- metabolic uptake of ions takes place when a plant cell or tissue is kept in a medium of relatively high salt concentration.

•      This method follows simple laws of diffusion.

•      In this method uptake of ions takes place against a concentration gradient but without the participation of metabolic energy.

•      This type of passive absorption of salts is possible either by Ion exchange mechanism or by establishment of Donnam Equilibria.

2. Ion exchange Theory:

•      In ionic exchange mechanism, anions or cations from within the cells are exchange for anions or cations of equivalent charge of the external solution in which the tissue is immersed.

•      The phenomenon has been experimentally confirmed in excised barely roots in which radioactive K⁺ ions exchange take place with non-radioactive  K⁺ ions.

•      A similar exchange mechanism operates between soil solution and clay micelles.

•      The ions get accumulated against a concentration gradient without the participation of metabolism energy because cations and anions of the external medium get exchanged with H⁺ and OH^- ions, which always remain absorbed on the surface of the membrane .

•      H⁺ and OH^- ions are readily available from water.

•      The process of ionic exchange has been explained by two theories:

            a.         Contact Exchange Theory

            b.         Carbonic acid Exchange Theory

 

1. Contact Exchange Theory:

•      According to this theory, ions need not to dissolved in the medium of water before these reach the roots.

•      On the other hand, roots are in close proximity to clay particles in the soil

•      Cations are found adsorbed on the clay colloids.

•      However, these cations can be released if ions of equivalent charge are made available.

•      H⁺ ions are released from the roots.

•      These positively charged ions are directly exchanged with the cations of clay particles.

2. Carbonic acid Exchange Theory:

•      It is assumed that the ions first dissolve in the soil solution.

•      CO₂  released during the respiration during the respiration of root dissolve in soil water forming carbonic acid (H₂CO₃ ).

•      Carbonic acid being a weak acid dissociates into H⁺ ions and HCO₃^-  ions.

•      H⁺ ions reach the clay particles and release other cations such as K⁺ ions from clay by the exchange process.

•      The released cations  goes into the soil solution.

•      From the medium of soil solution, cations  reach the root surface.

•      The ion exchange does not require the use of metabolic energy and is , therefore, a passive physical process.

3. Donnan Equilibrium:

•      According to this theory there are certain pre-existing fixed or non difusable ions inside the cells which cannot diffuse outside through membrane.

•      However the cell membrane is permeable to both anions and cations present in the external medium.

•      If the cell consisting of fixed ions (either Cation or anion), is immersed in an external salt solution then the movement of equal number of anions and cations take place until product of anion and cation in the internal solution becomes equal to the product of anions and cations in the external solution. This equilibrium is known as Donnan Equlibrium.

So, according to Donnan Equlibrium , the movement of ions inside and outside takes place until the ratio of cations inside to the cations outside equals to the anions outside to the anions inside.

•      Suppose a cell has potassium salt of a large indifusable organic anion (A^-). It is placed in a solution of KBr. K⁺ is present already in the cell, therefore the external K⁺ can not directly diffuse inwardly, however Br^- will enter the cell due to its diffusion gradient. Since the internal (A^-) does not diffuse out to balance the ionic equilibrium of the external solution the externally situated K⁺ ions also passes inwardly to maintain electrical or Donnan equilibrium. The result will be increased K⁺ and decreased anion Br^- concentration in the interior as compared to external solution.

4. Mass Flow Theory:

•      A number of workers have observed increased salt uptake in rapidly transpiring plants .

•      Transpiration causes a mass flow of water which can also drive the salt along with it.

•      According to this theory, the movement of ions occur through the root along with the mass flow of water under the influence of transpiartion.

•      This theory was forwarded by Kramer (1956), Russl and Batber (1960).

•      Though some amount of ions may possibly enter the roots by the mass flow, it is difficult to accept this proposition in the light of our present knowledge.

2. Active Absorption:

•      The uptake of mineral ions against concentration gradient is called active absorption.

•      Such movement of minerals requires investment of energy by the absorbing cells.

•      It has been found that the cell sap plants accumulates large quantities of mineral salt ions against the concentration gradient.

•      This led to believe that absorption and accumulation of mineral salts against concentration gradient is an active process which involves the expenditure of metabolic energy.

•      It has been also accepted that in the active absorption of minerals salts there is involvement of a carrier compound present in the plasma membrane of the cell.

Ø  The active absorption of minerals can be explained by the following theories:

1. Carrier concept Hypothesis
2. Cytochrome Pump Hypothesis
3. Bennet-Clark’s Protein- Lecithin Hypothesis

1. Carrier concept Hypothesis:

•      The substance to be transported initially bounds to a specific site on the carrier protein.

•      This requirement for binding allows carriers to be highly selective for a particular substrate to be transported .

•      Binding causes a conformational change in the protein which exposes the substance to the solution on the other side of the membrane.

•      This carrier mediated transport is active also involving the movement of molecules or ions against concentration gradient with the involvement of metabolic energy.

•      It is assumed that on the outer face of the membrane the ion to be transported combines with the carriers, forming ion carrier complex. This complex then moves across the membrane and reaches the inner face. It is assumed that while the membrane is impermeable to ions, it is permeable to ion-carrier complex. Finally, the ion-carrier complex is broken down on the inner face of the membrane and the ion is released from the membrane to the protoplasm.

•      This theory was first proposed by P. Fefter (1990) and Von den Honest (1937).

2. Cytochrome Pump Hypothesis:

Ø  This theory was proposed by Lundegardh and Burstrom (1933), who believed that there is a definite correlation between respiration and anion absorption.

Ø  According to them, the absorption of anion was accompanied by an increase in the rate of respiration.

Ø  The transport of anions takes place through cytochrome system, so the cytochrome acts as carrier.

Ø  The part of respiration which is solely due to anion absorption is termed as salt respiration.

Ø  The main assumptions of this theory are:

1. Mechanism of anion and cation absorption is different.
2. Anions are absorbed through cytochrome chain by an active process.
3. Dehydrogenase reactions on inner side of membrane give rise to protons (H⁺)   ions and electrons (e^-).
4. The election travels over the cytochrome chain towards outside the membrane so that the "Fe" of the cytochrome become reduced to Fe++ on the outer surface and “Fe” is oxdised to (Fe+++) on the inner surface.
5. On the outer surface, the reduced cytochrome is further oxidised by oxygen releasing the electron and taking an anion (A^- )
6. The electron thus released unites with H+ and oxygen to form water.
7. The anion (A^-) travels over the cytochrome chain towards inside surface.
8.  At the inner surface the oxdised cytochrome become reduced by taking an electron produced through the dehydrogenase reactions and the anion (A^-) released.
9. As a result, a cation (M+) moves passively from outside to inside to balance the Anion (A^-  ).

Evidence in Support to this Theory:

•      Stimulation of respiration by salt uptake is assumed to be due to the increased rate of electron flow which is facilitated by the inward movement of anions in exchange for electrons.

•      The similarities in the action of oxidase inhibitors on salt accumulation and respiration implies a common role for cytochrome system in both the processes.

•      The spectrophotometric changes show rapid oxidation of reduced respiratory components when salts are added to living wheat roots in distilled water.

Demerits of Lundegardh's theory:

1. It does not explain selective absorption of ions.
2. It principally concerned with active absorption of only anions, but cations passively absorbed and transported.
3. It fails to explain uptake of ions under anaerobic conditions.
4. It also fails to explain the absorption of anions in the membrane like tonoplast, plasma membrane where the enzymes of electron transport chain are not present.

3.   Bennet-Clark’s Protein- Lecithin Hypothesis:

Ø  This theory was suggested by Bennet Clarke (1956).

Ø   According to his theory -

1. Carrier molecule composed of protein with the phospholipids called as lecithin.
2. There are different phospholipid groups present in membrane correspond with the number of competitive group of cations and anions.
3. Phosphate group in the phosphatide is regarded as active centers of carrier.
4. The ions are liberated on the inner surface of the membrane by decomposition of the lecithin by the enzyme lecithinase.
5. The regeneration of the carrier lecithin from phosphatide acid and choline takes place in presence of enzymes viz. choline acetylase and choline esterase and ATP.
6. ATP acts as the energy source for active transport.

Factors affecting absorption of mineral salts:

Ø  Some of the important factors affecting absorption of mineral salts by plants are as follows:

(i) Temperature: The absorption of inorganic salts increases with an increase in temperature, but is confined to a very narrow range.

(ii) Oxygen: It greatly influences active absorption of salts.

(iii) Light: Light enhance transpiration and effects photosynthesis, so exhibits it influence in salt absorption.

(iv) Hydrogen ion concentration: The decrease in the pH of soil solution accelerates the absorption of anions and increase in pH favours the absorption of cations.

(v) Presence of other ions: The rate of absorption of one type of ion is influenced by the presence of other ions in the medium. For example, rate of absorption of K⁺ decreases if Ca⁺⁺, Mg⁺⁺ etc. are present in the medium. In other words the presence of Ca++ antagonises the absorption of k+. This phenomenon is called ion-antagonism.

(vi) Growth: Active cell division, elogation and developmental processes promote absorption of salt.