In this article we will discuss about the Ion Absorption in Plants. After reading this article you will learn about: 1. Passive Uptake of Ion Absorption 2. Active Uptake of Ion Absorption (Metabolic).

Passive Uptake of Ion Absorption:

(i) Non-Mediated Passive Uptake:

Numerous investigators have demonstrated non-metabolic or passive uptake of ions due to the fact that when a plant cell or tissue is transferred from a low-salt concentration medium to a relatively high-salt concentration medium, there is an initial uptake of ions due to diffusion.

This initial rapid uptake is temperature independent and remains unaffected by the application of metabolic inhibitors. Passive absorption includes theories of diffusion, ion exchange, Donnan equilibrium, mass flow, etc.

(a) Diffusion:

If the above cell or tissue is again returned to a lower-salt solution, some of the absorbed ions will diffuse out into the external medium. This process of free diffusion of solutes follows the simple laws of diffusion. This free diffusion means free movement of ions in and out of the tissue.

The part of the tissue for free diffusion shows an equilibrium with the external medium. This part of a cell or tissue is referred to as outer space. The term apparent free space is also used to denote the apparent volume accommodating the freely diffused ions.

(b) Ion Exchange:

In the ion exchange mechanism the ions within the cells are exchanged for the ions of equivalent charge of the external solution. If a solution of a dissociable salt (A+B) is separated from distilled water by a membrane which is permeable to both ions diffusion will occur until the concentration of salt on both sides are equal. If the membrane is impermeable to either cation or anion, there will be no ion movement.

Now, if two solutions containing different salts (A1+B1 and A2+B2) are separated from one another by either a cation-permeable/anion-impermeable or cation-impermeable/anion- permeable membrane, one of the two ion species in each case is free to move across in exchange for an ion of same charge.

At equilibrium the ratio [A1+]/[A2+] (when cation is permeable) or [B1]/[B2] (when anion is permeable) is equal on the two sides (Fig. 5.2).

Ion-Exchange Mechansim

The total concentration of salts on either side is not affected by the exchanges of ions. Exchange involves equivalent electrical charge, so that two univalent ions are exchanged for one bivalent, three for one trivalent ion, and so on.

With the help of radioactive isotopes it is possible to calculate the rates of exchange of ions between the two solutions kept separated by an ion exchange membrane from measurements of changes in radioactivity with time on either side.

(c) Donnan Equilibrium:

The Donnan equilibrium theory accounts for the effect of fixed or non-diffusible ions and explains the co-operation of both electrical as well as diffusion phenomena. It is a complex ion-exchange system in which the membrane is impermeable to certain ions called fixed ions (Fig. 5.3).

Donnan Equilibrium

In both the cases represented in the figure X+ and Y are fixed ions and cannot move from right to left. Due to the presence of these fixed ions some extra ions are absorbed against the concentration gradient. In the first case where X+ is fixed, equal numbers of cations and anions from the left-hand side will diffuse across the membrane until an equilibrium is established. This equilibrium would also be electrically balanced.

However, additional anions are needed to balance the positive charges of the fixed cations on the right-hand side of the membrane. Therefore, the anion concentration would become greater on the right-hand side than it is on the left-hand side. Similarly, in the second case where Y is fixed, cation accumulation takes place at equilibrium.

Thus, the accumulation of ions against a concentration gradient can occur without the participation of metabolic energy until a Donnan equilibrium is reached.

Since the concentration of mobile ions are unequal on the two sides of the membrane in a Donnan system at equilibrium while the electrochemical potentials are unequal, it follows that there is an electrical potential difference between the two sides. This is sometimes called their Donnan membrane potential.

(d) Mass Flow of Ions:

Many workers believe that ions are absorbed by the roots along with the mass flow of water influenced by the transpiration stream. The theory states that, an increased transpiration rate causes an increase in absorption of ions. Lopushinsky (1964) working with de-topped tomato plants indirectly supported the concept that an increase in transpiration could increase the absorption of salts.

Some workers claim that transpiration pull indirectly affects ion absorption by continuous removal of ions after they have been released into the xylem ducts. Whatever may be the fact, mass flow mechanism may occur in the absence of metabolic energy.

(ii) Mediated Uptake:

All the experimental findings suggests the concept of mediated transport which states that the transport is accelerated due to the presence of carrier substances in the membrane, which interact with the transported ions or molecules. Ions form a complex with the carrier on the outer side of the membrane. This complex is broken down on the inner side.

The following reaction takes place inside the membrane:

C + S = CS where S = Substance and C = Carrier.

The concept has been supported by the (1) radioactive ion exchange, (2) saturation effect, and (3) specificity.

(a) Radioactive Ion Exchange:

Using radioactive ions, Legget and Epstein (1956) observed that the ions failed to diffuse freely through the cell membrane. From this observation they pointed out that the movement of ions across the impermeable membrane might be accomplished by the intervention of carriers.

(b) Saturation Effect:

It has been observed that with the increase in salt concentration of the medium, the rates of ion absorption do not increase beyond limit. That is, a saturation point is reached. This is very analogous with the saturation effect found in enzyme catalysed reactions, suggesting that all active sites of the carriers are occupied by the ions.

At this point, ion transport is kept constant and cannot be made to proceed faster by an increase in salt concentration. So, the phenomenon of saturation effect indicates the presence of carriers.

Uniport, Symport and Antiport Translocation Systems

(c) Specificity:

Roots absorb ions selectively. Different ions are absorbed at different rates and show different levels of accumulation in the root tissue. This situation is analogous to enzyme-substrate activity. Epstein and Hagen (1952) have observed that the monovalent cations potassium, cesium and rubidium compete with each other for the same binding site.

There are different carriers for different cations and anions. They are also called transporters or perm-eases. They may be of different types such as uniport, symport and antiport. Transporters that carry only one substrate are called uniport systems.

On the other hand, the transporters may carry two ions or solutes simultaneously across a membrane, which are called co-transport systems. When two substrates move in opposite directions, the process is anti-port transport whereas, in symport system, two substrates are moved simultaneously in the same direction.

Mediated transport is classified into two categories depending on the thermodynamics of the system:

1. Passive-mediated transport, or facilitated diffusion, in which a specific molecule flows from high concentration to low concentration.

2. Active transport, in which a specific molecule is transported from low concentration to high concentration, that is, against its concentration gradient. Such an endergonic (energy requiring) process must be coupled to a sufficiently exergonic (energy generating) process to make it favourable (i.e., ΔG < 0).

(d) Passive Mediated Transport:

1. lonophores Facilitate Ion Diffusion:

One type of carrier molecule is an ionophore, an organic molecule.

Carrier ionophores increase the permeability of membranes to a particular ion by binding the ion, diffusing through the membrane, and releasing it on the other side. For net transport to occur, the un-complexed ionophore must then return to the original side of the membrane ready to repeat the process (Fig. 5.5). The ionic complexes of all carriers must therefore be soluble in nonpolar solvents.

Carrier Ionphores Transport Ions

Another type of ionophore called channel-forming ionophores, form solvent-filled, trans membrane channels or pores through which their selected ions can diffuse. Even small amounts of an ionophore greatly increase the permeability of a membrane toward a specific ion.

For example, a single molecule of carrier antibiotic valinomycin transports up to 104K+ ions per second across a membrane. Channel formers, such as the antibiotic gramicidin A, have an even greater ion throughput, over 107K+ ions per second.

Four fundamental classes of transport system are present at all membranes. These are — (1) carriers, (2) pumps, (3) ion channels arid (4) aquaporin’s.

2. Carriers:

Unlike pumps, carriers do not catalyse scalar reactions such as ATP hydrolysis.

In other words, the transport process does not involve chemical modification of any of the compounds bound to the carrier. The carriers catalyse only vectorial reactions, i.e., the movement of inorganic ions and simple organic solutes across membranes. The carriers exhibit Michaelis-Menten kinetics that indicate conformational changes during transport.

The array of ions and solutes trans-located by carriers is vast. The principal inorganic nutrients, including NH4+, NO3, Pi, K+, and SO42-, CI are all trans-located into cells by plasma membrane carriers.

The organic solutes like sugars, amino acids, and purine and pyrimidine bases are also trans-located into cells by carriers. The plasma membrane carriers are not only important for nutrient absorption from the soil but also play fundamental roles in the mobilization and storage of metabolites.

For example:

1. The sucrose carriers that are specifically responsible for loading of sucrose into the phloem.

2. Many species reduce NO3 in the leaves, and the resulting organic nitrogen compounds are loaded through the carriers into the phloem for transport to sink tissues.

3. During germination, hydrolysis of storage polymers yields sugars and amino acids that need carrier-facilitated mobilization to the growing embryo.

4. Tonoplast carriers catalyse sequestration (temporary storing) of Na+, Ca2+, Mg2+, and NO3 as well as sucrose and amino acids into the vacuoles.

5. The triose phosphate trans-locator in the chloroplast membrane exchanges equal amounts of triose phosphate (stromal) for Pi (cytosolic).

6. A carrier in the inner mitochondrial membrane executes the stoichiometric exchange of matrix ATP for cytosolic ADP, maintaining the substrate-product ratio at values favourable for mitochondrial ATP synthesis.

Like the enzymes the carriers are specific for their substrates. Even they are stereo-specific, i.e., specific for D- and L-forms. The acidic, basic and neutral amino acids are transported by distinct plasma membrane carriers. Thus a wide range of carrier types are found in all membranes.

Most of these plant carriers are energized by coupling to the proton motive force (pmf). They behave as H+ – symporters or –anti-porters. Molecular identification of carriers defines them as members of the major facilitator super family. The carriers can be identified genetically.

The most widely applicable and successful approach applied to plant carriers has been yeast complementation. Yeast transport mutants that are defective in growth on the solute of interest are transformed with cDNA from a plant library.

Transformed yeast colonies that are able to grow on the particular solute must accordingly contain functional copies of the relevant plant transport system, which can then be sequenced from the vector. In this way plasma membrane carrier systems for sugars, amino acids, inorganic cations (e.g., K+), and inorganic anions (e.g., SO42-) have been identified in plants.

All the plant carriers identified so far are hydrophobic and contain only single subunit with molecular masses 40 – 50 kDa. The structure of the carrier shows 12 trans membrane spans, with the most extensive hydrophilic loop appearing between trans membrane spans 6 and 7.

Carrier in a Membrane

The motifs present in the proteins are separated by the large hydrophilic loops, indicating that an ancient progenitor of these carrier systems underwent a gene duplication and fusion. Sequence analysis and the general membrane distribution, suggest to place these plant plasma membrane carriers into a large and diverse group of transport systems known as the major facilitator superfamily (MFS).

This group includes both H+ -coupled sugar transport systems from bacteria and uniporters, carriers from animal cells. Expression of such carriers in particular cell types gives clues to cell function.

3. Ion Channels:

Ion channels are ubiquitous in plant membranes and are studied by electrophysiological techniques. Ion fluxes through channels are driven solely by electrochemical potential differences. Ion flow through channels is passive. Thus, in contrast to pumps or ion-coupled carrier activity, the direction of flow of a particular ion through a channel is dictated by the electrochemical potential gradient for that ion.

The ion channels exhibit ionic substrate selectivity. Most classes of ion channels in plants are of two types — (1) cation channels and (2) anion channels. Cation channels are further subdivided into K+ -selective and Ca2 +-selective channels. Most plasma membrane anion channels allow a wide range of anions, including CI, NO3, and organic acids. Other anion channels in the tonoplast select specifically for malate.

Ion fluxes through channels are monitored as electrical currents. The selectivity of ion channels is due to the specific binding sites located within the channel pores. In some cases, these ion binding sites have been identified at a molecular level.

Ion channels are gated, often by voltage or ligands, through changes in open state probability. The channels are tightly controlled by conformational shifts between permeable (open, O) and non-permeable (closed, C) states.

This alteration between O and C states, known as gating, is represented by the following reaction:

Channelc ↔ ChannelQ

In all types of channels gating is controlled by membrane voltage, a ligand, or both. When a gating factor activates a channel, the equilibrium of the above reaction shifts from left to right. Voltage-dependent K+ channels at the plasma membrane allow controlled K+ uptake and loss. Time-dependent inward and outward currents are carried by separate classes of ion channel.

The channels carrying these currents are called rectifiers. Like valves, rectifying channels carry current in one direction but not the other. For these reasons, the channels are known as K + inward rectifiers and K+ outward rectifiers.

Inward rectifiers take up K+ ions not only from the soil but also from the apoplast surrounding most cells. The resulting K+ accumulation contributes to cell turgor. Inward rectifiers have been cloned by complementation of yeast mutants defective in K+ uptake.

Plant inward rectifier subunits are products of a multi-gene family, members of which show tissue-specific expression. One member, KAT 1, is expressed selectively in guard cells, whereas another AKT1 is expressed in roots and hydathodes.

Structure of Inward Rectifying K+ Channel AKT1

Several structural features define plant inward rectifying channels as members of the Shaker family, a superfamily of voltage-gated K +, Ca2+, and Na+ channels.

The fourth trans membrane helix (S4 domain) of AKT 1 exhibits a regular pattern of positively charged lysine or arginine residues that tend to project from one side of the helix. This region of the protein forms the voltage sensor, which is involved in opening the channel in response to permissive voltage.

Due to hyperpolarization of the membrane, the S4 helix is thought to screw out of the membrane slightly. This conformational transition opens the “gate” that controls ion flow through a separate part of the protein.

Voltage Sensing S2 Domain Screws

The voltage-insensitive cation channels are major pathways for Na+ uptake across the plasma membrane and for salt release into the xylem. Such channels give plants tolerance to salinity. These channels are partially blocked by external Ca2+.

Active Uptake of Ion Absorption (Metabolic):

Active uptake of ions is one of the most important features of life processes. It is accomplished through the coupling of diffusion fluxes to the exergonic reactions that take place in the bulk of the membrane. The transfer of ions occurs at the expense of the free energy liberated in chemical reactions. As a rule, this is the energy of hydrolysis of ATP.

It has been observed that both anions and cations are accumulated by plants against concentration gradients to a great extent, that cannot be explained by any known electrochemical mechanism, such as ion exchange, Donnan equilibrium, etc.

It has also been observed that the rate and amount of absorption of salts is directly related to the expenditure of metabolic energy. Hober (1945), reported that fresh water alga Nitella, absorbed potassium ions to a concentration 1000 times greater than the concentration of K+ ions in the surrounding medium.

This type of absorption where the concentration of ions is much higher within the cells than in the external solution is called accumulation.

The extent to which the concentration becomes greater internally than externally is called the accumulation ratio. Thermodynamic laws show that free diffusion and other passive mechanisms not involving expenditure of metabolic energy could not be responsible for such great accumulation.

Many studies indicate that solute transport into cells is strongly dependent upon metabolic energy. Ion accumulation is inhibited when the metabolic activity of the plant is inhibited by low temperatures, low oxygen tension, metabolic inhibitors, and so on. Many workers like Steward (1932), Hopkins (1956) and others have found a parallel curve for salt accumulation and respiration as a function of oxygen tension.

With the decrease in oxygen content of the medium, ion accumulation decreases and ultimately stops completely. Robertson and Turner (1945) and Lundegardh (1955) have reported that metabolic inhibitors like azides and cyanides inhibit ion accumulation with the inhibition of respiration.

An additional evidence in support of the existence of an active process of ion uptake is provided by the phenomenon of salt induced respiration. Lundegardh and Burstrom (1933) observed that the rate of respiration increases when a plant or tissue is transferred from water to a salt solution. The amount by which respiration is increased over the normal rate has been called salt respiration or anion respiration.

(i) Mechanism of Active Uptake:

Now, it is necessary to evaluate the concentration effect and the electro potential effect to understand whether the ion absorption takes place under the influence of metabolic energy.

The electrochemical potential difference across the membrane is designated as Δµ and expressed by the following modified Nernst equation:

Δµ = Δ (RT InC) + Δ (zFE) [‘z’ is the algebraic valency]

where Δ (RT InC) = chemical potential difference due to concentration effect

Δ (zFE) = electro potential difference

R = universal gas constant

T = absolute temperature

C = ion concentration inside and outside the membrane

F = Faraday’s constant

E = electro potential in volts on each side of the membrane.

The above equation can be simplified as follows:

Log Ci/C0 = -zFΔE/2.3RT

where Ci = inside ion concentration

C0 = outside ion concentration

Ci/C0 is the predicated ratio at equilibrium, when µ = 0. If the actual-ratio is greater than the predicted ratio, the cell is performing work to absorb ions using energy. This is called active absorption. Passive absorption occurs when the actual ratio is equal to or lesser than the predicated value.

The active ion transport may be (1) primary and (2) secondary.

(ii) Primary Active Transport:

The primary active transport is coupled directly to a source of energy other than electrochemical potential gradient, such as ATP hydrolysis, an oxidation reduction reaction, etc. The membrane proteins that carry out primary active transport are called pumps. Most pumps transport ions. Ion pumps are further characterized as either electro genic or electro neutral.

In general, electro genic transport refers to ion transport involving the net movement of charge across the membrane. In contrast, electro neutral transport, as the name implies, involve no net movement of charge. For example, the Na +/K+ -ATPase of animal cells pumps three Na+ ions out of every two K+ ions in, resulting in a net outward movement of one positive charge.

(iii) Secondary Active Transport:

Secondary active transport uses the energy stored in electrochemical-potential gradient. Protons are extruded from the cytosol by electro genic H+ ATPase operating in the plasma membrane and tonoplast. Consequently, a membrane potential and a pH gradient are created at the expense of ATP hydrolysis.

This gradient of electrochemical potential for H+, the proton motive force represents stored free energy in the form of H+ gradient.