Absorption of Solutes by Roots (With Diagram)

Let us make an in-depth study of the theories of selective absorption, ion exchange and active absorption, antagonism of salts or ions and synergism.

Theories of Selective Absorption:

We know that a perfectly true semipermeable membrane allows diffusion of only the solvent molecules and not the solutes.

It is clear then that if the plasma membrane of the root cells were a true semipermeable membrane, nothing but water could have entered from outside into the root cells and the continued existence of the plant and ulti­mately of animals would have been impossible.

But because the living plasma mem­brane, in contrast to the artificial semipermeable membrane is not perfect, it also allows some solutes to diffuse through it.

It must be clear, however, that in the diffusion of solute molecules through the plasma membrane, even if we assume that the absorption of solutes follows simple laws of diffusion, each one of the solute molecules will follow its own individual diffusion pressure gradient independently of diffusion of other solute molecules and water, of course, if the membrane is permeable to the solutes and solvent alike.

Thus it may happen, that at any particular time water may be diffusing into the cells from outside following its own concentration gradient, i.e., if the external solu­tion has a higher water potential than the cell sap and at the same time the solute molecules may be coming out of the cell or getting into the cells each according to its own chemical potential gradient, i.e., from higher concentration to lower concentration.

Suppose a plant cell with a cell sap osmotic concentration of 1M (composed of 0^.4M sucrose, 0.1M glucose, 0.2M KNO₃, (Q.3M MgSO₄) is im­mersed in a solution of 0^.8M concentration (com­posed of 0.5M sucrose, 0.1M fructose, 0.1M KNO₃ and 0.1M MgSO₄) separated by a membrane equally permeable to solutes and the solvent, water.

We shall disregard other factors such as ionisation of solutes, kind and amount of the electric charges on ions, etc. Water will enter into the cells from the external medium as the water potential is evidently higher outside than inside. Following the same principle sucrose molecules will go in, so will fructose whereas glucose, KNO₃ and MgSO₄ will come out of the cell into the external medium.

It has been frequently observed that plasma membrane of the living cells allows certain solute molecules to pass through it very slowly and at the same time other solute molecules comparatively rapidly though the membrane is apparently equally permeable to both the solute molecules, even if the concentration gradient is in the right direction.

Under another set of circumstances the reverse may be true. Frequently it is observed that the membrane is permeable to larger molecules of solutes but at the same time impermeable or only partially permeable to much smaller molecules (or ions).

This is what is known as selective absorption. Various theories have been proposed to explain the differential permeability of the plasma membrane.

For convenience, most of them are grouped under the following heads:

The Sieve or Ultra Filtration Theories:

These theories assume that the plasma membrane acts as a molecular sieve with intermolecular spaces which are very minute and only molecules below a certain size are supposed to diffuse through the membrane, while those exceeding the size limit are prevented from entering the cell.

According to later advocates of these theories, the spaces or pores are not fixed but are capable of constant change and adjustment.

The diameter of the pores is supposed to enlarge by aggregation of the particles of protoplasm into larger ones when large molecules of solutes are to enter the cell. Under another set of circumstances, the segregation of the same protoplasmic particles is supposed to make the sieve pores smaller, thus restricting the entry of large molecules while at the same time freely admitting the relatively smaller solute molecules.

The most serious objection to this theory seems to be that it is difficult to conceive this proposed sieve-like structure of the plasma membrane. Again it has been frequently observed that in certain plant cells, very large molecules of certain alkaloids enter through the plasma membrane easily and very rapidly while at the same time the entry of relatively smaller amino acid molecules are significantly restricted.

The Solution Theories:

According to these theories the plasma membrane is supposed to have definite solvent properties. Membranes are supposed to be permeable to only such substances or molecules which will dissolve in them while impermeable to others.

This also is known as lipid theory. The plasma membrane is supposed to be constituted chiefly of proteins and lipids and only those solutes, which readily dissolve in lipid matrix, will pass through the plasma membrane.

The serious objection to this theory is that all the inorganic solutes which for the most part are insoluble in lipid substances, enter through the plasma membrane freely and in large amounts.

The later advocates of this theory have attributed to the plasma membrane a mosaic structure consisting of an aqueous phase which permits the passage of water-soluble substances and a lipid-containing phase which facilitates the entry of fat-soluble materials.

The Absorption and Chemical Reaction Theories:

The advocates of these theories proposed that particles of the protoplasm of the plasma membranes adsorb the molecules of solutes to which the membrane is permeable.

The stronger the adsorptive forces between the membrane particles and the solute molecules, the more readily the solute molecules will pass through the membrane. It is also possible that chemical reactions are involved in the transport of certain solute substances across the cell membrane.

According to the chemical reaction theory molecules are supposed to combine chemically with certain carrier substances on the outer boundary of the membrane forming an intermediate compound and it is in this bound form that the solute-molecule moves across the width of the membrane itself. 29 [one]

Once the inner boundary of the membrane that is lining the vacuolar cell sap is reached, a reverse chemical reaction is supposed to occur which releases the molecule from the membrane to the cell sap.

It is possible that once the entering solute molecules are dissolved, adsorbed or chemically combined with particles in the lining layer of the plasma membrane the penetration of the solutes across the membrane is facilitated by movement or kinetic activity of the membrane particles; each particle might be assumed to carry a load (as if on their back) of solute molecules across the membrane and once it is past the membrane, the load is freed from its carrier into the cell sap.

A more probable hypo­thesis of course will be that the dissolved, adsorbed or chemically bound penetrating solute molecules are not carried all the way across the membrane by the activity of one single particle.

Rather, the loads of solute molecules are transferred from one carrier particle to the other, as if in a ‘relay race’ as impacts occur between them, on the inward journey of the particle-solute combination across the membrane.

The Colloidal Theories:

These theories are based on the concept of protoplasm as an emulsion of oil and water and the differential distribution of water between the two liquid phases by phase inversion or a reversal of the continuous and discontinuous phase.

According to these theories, when the continuous phase of the emulsion constituting the plasma membrane consists of oil, the membrane becomes more permeable to pene­trating fat-soluble molecules.

With phase inversion taking place, the continuous phase becomes water; the membrane is now more readily and easily accessible to penetrating inorganic solute molecules. It must be admitted, however, that experimental evidences are as yet lacking to support this phase reversal idea of the protoplasmic emulsion in the cell membrane.

Ion Exchange and Active Absorption:

In the foregoing paragraphs we have assumed that both the electrolytes and non- electrolytes enter through the cell membranes as molecules. It is true that an electrolyte when dissolved in water is largely or entirely dissociated into ions but there is evidence which indicates that such dissociation is inhibited by the lipid constitution of the plasma membrane. Other investigators, however, believe that electrolytes penetrate the cell membrane only in the form of ions.

If penetration of solutes through the cell membrane is in the form of ions it is evi­dent that either an equivalent number of oppositely charged ions must enter the cell at the same time or there must be an exchange of ion of like charge with the cell if the electric potential across the membrane is to remain undisturbed.

Let us consider the case of a vegetative cell immersed in KNO₃ solution. There are several possibilities concerning the movement of dissociated K⁺ and NO^–₃ ions; K⁺ and NO^–₃, K⁺ and OH^– (H⁺ and OH^– obtained from dissociation of water) or an ionic exchange between the cell sap and the external solution might occur, an ion bearing a negative charge say HCO^–₃ moving out of the cell as penetration of NO^–₃ occurs or in the same manner, K+ ions move into the cells from the external solution as Na⁺ ions of the cell sap move out.

It has been observed that a rapidly penetrating anion seems to facilitate the en­trance into the cell of a cation. If two cations are entering a cell, the entry of one is re­tarded by the other.

Na⁺ ions always decrease the rate of entry of K⁺ ions. This is also true for anions; a fast moving NO^–₃ ion always retards the intake of anions such as SO₄⁼. In general, it is found that a rapidly penetrating ion retards the rate of absorp­tion of another ion of the same charge while increasing the rate of absorption of an ion of the opposite charge.

In all our previous discussions we have assumed that diffusion of inorganic solutes in the form of molecules or ions into cells takes place as a result of their greater concen­tration in the external medium than in cell sap, i.e., following the right concentration gradient. All this have been termed passive absorption.

The result of a number of investigations indicate, however, that the absorption of ions by plant cells is under many conditions a much more complicated process than simple diffusion.

It has been observed that the cells of fresh-water alga, Nitella accumulate K⁺ ions to a concentration of more than 1000 times greater than the concentration of K⁺ ion in the surrounding medium and in fact concentration of every ion analysed was far greater in the cell sap than in the water in which the alga grows.

The total concen­tration of electrolytes in the cell sap was about twenty-five times as great as in the external medium. In the marine alga Valonia similar, but not as striking, accumulation of ions takes place.

Thus the entrance of ions by plant cells is characterised by the feature that the living cell can accumulate ionic material or in other words they can continue to take up an ion even if the concentration level of this ion inside the cell sap in many times higher than the concentration of same ions in the external medium.

Apparently a very different mechanism from simple diffusion must be at work. This active absorp­tion since the penetration takes place against the gradient. This must not be confused with salt absorption in which no equilibrium is attained with the external solution because the penetrating solute is removed from the cell sap through utilisation in growth or by absorption or by precipitation.

While large amounts of salts may be held by a plant cell in this manner, an equilibrium is never attained in such cases between the cell sap and the external medium and they are not accumulated against the gradient.

The passive absorption of ions is, we know, essentially an exchange adsorption, i.e., absorption taking place mainly by diffusion on the surface of the cells whereas in active absorption, ion-building compounds and carriers are most probably involved.

The pas­sive absorption is primarily non-linear with time with equilibrium approaching within a very short time, even less than an hour. In active absorption, equilibrium is never really attained and the curve is more or less linear with time.

In active absorption, ions are essentially non-exchangeable whereas in osmotic uptake ions are readily exchange­able. The active absorption of ions or group of ions seems highly selective, hence highly specific with regard to competition for site and entry in contrast to osmotic uptake where there may be considerable indiscriminate competition among various ions with respect to entry.

Active absorption cannot be explained completely satisfactorily. The penetration of ions into the cells from the external medium or retention of the ions in the cell sap in greater concentration than in the external solution must require a continuous expendi­ture of energy and this energy is most possibly supplied by respiration.

Though the complex relationship between respiration and salt accumulation by plant cells is not as yet fully understood, there is no doubt, however, that the energy liberated in respiration is utilised in the accumulation and retention of ions.

The phenomenon of salt respiration or anion respiration and its relation with accumula­tion of ions in plant cells against a concentration gradient have been studied brilliantly by Lundegardh.

The increased respiration rates (salt respiration) in many tissues upon addition of salts over its normal respiration in distilled water (ground respiration) have been related to the uptake of anions only by plant cells, rather than the absorption of cations.

Thus Lundegardh bases his theory on the primary assumption that the absorp­tion of anions and absorption of cations are independent of each other to such an extent, that different mechanisms may be responsible for each, which assumption is, however, disputed by other workers, who prefer to attribute salt respiration phenomenon specifi­cally to the anion uptake.

The ratio of the moles of the extra oxygen consumed when tissues are immersed in salt solutions over ground respiration to that of moles of salts absorbed appears to ap­proach four as a limit.

Since each molecule of oxygen accepts four electrons and is equi­valent to oxidation of four molecules of ferrocytochrome c to ferricytochrome, evidences are strongly in favour of migration of cytochrome c in its two valence forms and the consequent entrance of anions of a salt into a plant cell from external medium.

If one anion is accumulated for each electron transferred by the cytochrome system, then apparently for each molecule of extra oxygen utilised in salt respiration, four anions of a salt should accumulate, assuming 100% efficiency. Actually with carrot tissue values close to four have been obtained.

It has actually been observed that cells which show markedly high respiration rates, such as those in the growing meristematic regions of roots and other similar tissues, also show considerable ion accumulating ability. It has been demonstrated in excised roots and tissues of potatoes that salt accumulation takes place only in presence of molecular O₂.

The respiration of the root cells is thus one of the essential factors for vigorous absorption of ions. This explains the favourable influence of the aeration of the soils and bubbling of air through water culture solutions for optimal salt absorption.

The digging of the soil in the neighbourhood of roots from time to time is a normal agricultural practice for supplying air to the root system for the normal respiration of the root cells and which certainly contribute largely to the maximum uptake of minerals from the soil.

A probable connection between respiration and absorption of ions into the cells from soil may be that CO₂ released in cellular respiration combines with water of the cell sap to form H₂CO₃. This H₂CO₃ is dissociated into H⁺ and HCO^–₃ ions. This H⁺ ions and HCO^–₃ ions in the cell sap can be exchanged with other cations and anions of the external soil solution, thus facilitating rapid absorption of K⁺, Ca⁺⁺, Mg⁺⁺ and anions such as NO^–₃, PO₄^— , etc., which are essential for normal growth of plants.

Recent researches seem to indicate, however, that ions which are actively absorbed enter essentially irreversibly into the vacuoles of the root cells and these ions are not; therefore, free to move to the aerial parts. This would seem to indicate that only osmotically absorbed ions are available for movement through the plant and these ions occur only in the cytoplasmic outer space of the root cells.

Antagonism of Salts or Ions:

Antagonism means enmity and so the term antagonism of salts suggests that there is enmity or better active opposition between salts or to be more precise, between diff­erent ions of the same electric charge.

The first observation of antagonism of salts was made with eggs of Fundulus (a marine organism). It was observed that the development of the eggs was greatly inhibited if immediately after fertilisation, they were placed in a solution of NaCl having the same concentration as this salt has in sea water.

If, however, small quantities of the salt of a divalent metal were added—salts of calcium, strontium or magnesium—development proceeded normally. It was the first indication that solutions of single salts were much more toxic in their action than solutions composed of two or more salts.

Thus a number of salts when present together in solution appears to be able to antagonise the toxic effects of each other; such solutions are called balanced solutions.

If a slice of freshly cut beet root is placed in a solution of NaCl of a particular conc. after thoroughly washing it to remove all the colour from the cut surface, the red colour gradually diffuses into the NaCl solution. If the slice of beet root is properly washed in pure water, no diffusion of colour from the tissue to the outside is observed.

It shows that the effect of NaCl on the beet root tissue is to increase the permeability of the cell walls to the red pigment of the vacuole which diffuses out into the solution. If, however, CaCl₂ is added to the solution of NaCl in proper proportion, no outward diffusion of colour could be detected. Evidently the presence of Ca ions in some way counteracts the usual effects of Na ions on the permeability.

It was later seen that the cause of diffusion of red colour from beet root tissue out wards is that the single salt solution of NaCl increases the permeability of the plasma membrane to a point that is injurious to it whereas in a solution of CaCl₂ only, the reverse is true—the permeability of the plasma membrane is reduced to the point of injury.

If, however, these two salts, NaCl and CaCl₂ are mixed in proper proportion, their com­bined effect on the plasma membrane will neutralise the toxic effects of each other and the permeability of the membrane may remain unaltered.

Other examples of antagonism of salts can be cited. In an experiment with roots of lupins, the roots elongated only about 3.5 mm per day in a very dilute solution of CuCl₂. When, however, proportionately sufficient CaCl₂ was mixed with CuCl₂ and the roots allowed to grow in the mixture, the increase in length per day was about 11 mm.

The antagonism between Cu and Ca ions was sufficient to greatly reduce the toxicity of Cu ions alone. In another experiment with wheat roots the rate of growth was about six times or more when the roots were allowed to grow in a mixture of different salt solutions like NaCl, MgCl₂ and KCl combined in proper proportions compared to their growth in individual single salt solutions.

However, these effects are complicated by the rela­tive beneficial effects of Ca⁺⁺, Mg⁺⁺ and K⁺ ions for nutrition and the growth effects observed may not be entirely due to antagonism.

We know that in mixture of salt solutions, antagonism disappears for the toxic effect of one salt or ion is antagonised by the presence of other. Such solutions where the antagonism has been neutralised are, as stated before, balanced solutions.

Sea water is typically balanced for in addition to NaCl, the sea water also contains small amounts of CaCl₂ and other salts. Ordinary soil water and artificial sand and water culture solutions are really balanced solutions.

The various salts present are in themselves toxic to the well- being of the plant, but when mixed together in a solution, they are harmless and appear to antagonise each other’s toxic effects. The mineral solution which is certainly a mix­ture of various salts in the soil in which the roots grow must also be in a balanced state for the minerals in the soil are readily available to the roots.

It has been observed that the greatest amount of growth in plants takes place in a balanced solution of a mixture of different salts, when the concentration of one of the salts is slightly in excess of half the total concentration of the mixture.

Several explanations have been offered to account for the phenomenon of salt antagonism. It is known that antagonism is only effective as long as the plasma mem­brane remains intact; so the seat of antagonism must be there.

It is apparent that any theory which accounts for the antagonism of salts must explain:

(1) Why both NaCl and CaCl₂ are toxic individually?

(2) Why then when mixed in proper proportions their toxicity is diminished?

(3) Why NaCl causes an in­crease in the permeability of the plasma membrane and CaCl₂, a decrease?

(4) Why the reduction in permeability caused by CaCl₂ is later followed by an increase and

(5) Why toxicity disappears in sea water?

One explanation of salt antagonism is provided by the prevalent concept of proto­plasm of the plasma membrane being a system of emulsion of water and oil. This system is reversible, but for the normal functioning of protoplasm the system must be at the transition stage, that is neither an emulsion of oil-in-water nor water-in-oil but somewhat intermediate between the two.

When a single salt solution, e.g., NaCl is present in the medium, it produces a pure protoplasmic emulsion of oil-in-water while in CaCl₂, the system is reversed, producing an emulsion of water-in-oil. In both the cases, the normal functioning of the plasma membrane is hampered.

If, however, both NaCl and CaCl₂ are present in proper proportions, a condition of equilibrium is obtained for the proto­plasm of the plasma membrane, where the effect due to one is counterbalanced by the opposite influence due to other, producing the desired system at the transition stage essential for the normal functioning of the protoplasm.

As a result, the permeability of the plasma membrane is supposed to remain unaltered in a mixture of two or more salts.

Another hypothesis bases its idea on several primary assumptions such as:

(1) The normal charge upon the particles comprising the plasma membrane is negative;

(2) Salts may be classified in respect to their action upon the plasma membrane, as positive or negative. Negative salts, such as NaCl (or alkalis) cause the particles on the plasma mem­brane to separate and thus increasing permeability while positive salts, such as CaCl₂ (or acids) have the opposite effect during the first few minutes of exposure, because it causes the particles on the plasma membrane to approach each other and hence the per meability is decreased;

(3) Antagonism exists between salts which produce dissimilar electric effects upon the protoplasm.

If to a given protoplasmic surface of the plasma membrane a molecule of NaCl is added, the positive charge of sodium will neutralise a negative protoplasmic charge on the particles comprising it. The substituted CI^– unit, being smaller than the protoplas­mic negative charge on the particles, has, it is supposed, a more concentrated surface charge and as a result there will be a readjustment of the relative positions of the particles on the protoplasm at a little greater distance than before.

The total effect is loosening up of the mesh or a significant increase in the permeability of the plasma membrane. When all the negative charges on the particles of protoplasm are neutralised and substituted by negative Cl^– units, the permeability is at a maximum.

When a salt like CaCl₂ is allowed to come in contact with the protoplasmic sur­face of the plasma membrane alone, its first effect (for the first few minutes) will be to cause the particles on the protoplasmic surface to approach each other and the per­meability is decreased.

As the negative charges on the protoplasm are later replaced by the negative charges of Cl^– units, the negative charges again repel each other and the permeability is increased considerably as in the case of NaCl—only difference is that, increase in permeability with CaCl₂ occurs later, and not immediately, as is true for NaCl.

In sea water or in other balanced solutions, toxicity disappears because of the balance existing between the various ions which compose it.

It is interesting to note that of the six normal plant elements taken in quantity from the soil solution by the roots, three occur in the cation with five positive charges (K+, Ca⁺⁺, Mg++) while three occur in the anion with six negative charges (NO₈^–, SO₄^— and PO₄^— . This is really too nearly balanced to be a mere coincidence.

Synergism:

Combined activity of agencies, e.g., chemicals, hormones, which separately in­fluence a certain process in the same direction, such that the effect produced is greater than the sum of the effects of each agency acting alone, is referred to as synergism.

The term may also be used for combined activity such that effect is either the sum of the separate effects (summation), or greater than the sum of the separate effects (potentiation), it does not matter which, i.e., they (agencies) are not antagonistic to each other.