ABSORPTION OF MINERAL NUTRIENTS
There are 113 or so different elements in this planet, of which fourteen or fifteen are absolutely required for the life processes without which plants exhibit diseased symptoms and ultimately die.† Such elements which are absolutely required for the normal growth and development of the plant body are called essential nutrients.† Among them, eight elements are required in sufficient quantities, others in small quantities; the former referred to as macronutrients and the latter as micronutrients or true elements.† Nevertheless, plants also contain elements other than the elements mentioned above, whose deficiency may not cause any diseased symptoms or death and such elements are named as non essential elements.† The macronutrients are carbon, hydrogen, nitrogen, phosphorus, potassium, calcium, sulphur, magnesium and iron.† The micronutrients are manganese, zinc, boron, copper, molybdenum and cobalt.† Non-essential elements are sodium, aluminum, silicon, chlorine, gallium, etc.
Most of the above said elements are found in soil solution either in the form of inorganic or organic salts or ions; which may exist in either in free state or bound to clay particles.† Whenever there is depletion of any free ions from the soil solution, respective ions are released from clay particles into the soil solution to maintain the equilibrium.† This is achieved by a process called ion exchange process.† This may be due to contact ion exchange mechanism or by carbonic acid ion exchange ion mechanism.
Plant roots are in contact with soil clay particulates which have colloidal dimensions, Root cells which are living, secrets hydrogen ions which are positively charged.† Such ions can easily displace cations like K+, Na+ ions that are bound to clay particulates.† Thus the cations are made available for the root system to absorb the required ion.† But the relative retentive capacity of the clay particles, though not absolutely fixed, is in the order of H+>Ca2+>Ng2+>K+>NH4+>Na+.† But hydrogen ions are capable of replacing any cation found on the clay particles.† Nonetheless it appears that hydrogen ions can replace any bound ions to clay particles but there is a preference of ions to be released.† As hydrogen ions have greater affinity, they can replace any of the above ions easily.
Roots continuously respire irrespective of day or night, and liberate significant quantities of CO2, which when dissolved in soil water produces carbonic acids.† Immediately they ionize into H+ and bicarbonate ions (HCO3)
The hydrogen ions thus formed are capable of exchanging with any bound cations on clay particles and make the bound cations available for the roots.
Though the above mechanisms were once proposed as theories, it is now clear that growing roots not only release CO2 but also secrete hydrogen ions. Thus the root does provide hydrogen ions for both carbonic acid ion and contact ion exchange processes.† Similarly adsorbed anions are also exchanged by anions like OH-- ions.
Aquatic plants do not need any special structures for the absorption of minerals, for the entire plant body acts as absorptive surface.† But terrestrial plants possess extensive root system with innumerable growing apices.† Short term radioactive isotope labeling experiments indicate that the meristematic regions of the root absorbs greater amount of ions than any other regions.† This is perhaps necessitated by their active metabolic state. †Though most of the minerals are absorbed by the growing meristems, minerals ultimately they find their way into xylem elements by active transport.† From the xylem elements they move upwards along with transpiration stream and get distributed to all other regions.
FACTORS THAT CONTROL ABSORPTION
Soil aeration:†† In most of the cases, living cells cannot survive without oxygen.† As the roots contain a large number of living cells, they require considerable amount of energy for their metabolic activities and growth.† So oxygen is absolutely essential for generating energy rich components by biological oxidative process.† As mineral absorption requires energy, poor soil aeration affects the ability of roots to absorb adequate quantities of minerals. Water logged soils or soils with higher content of clay have very little amount of air; under such conditions roots are subjected to anaerobic conditions and the absorption of minerals is drastically affected.† The inhibition of absorption of minerals due to the effect of respiratory poisons on roots clearly suggests that the absorption of minerals is an energy dependent process.
Soil temperature has a significant effect on roots metabolic activities and also it affects the mobility of ions in soil solution.† If the temperature of the soil is lowered, absorption of minerals will be drastically reduced; but with the increase in temperature, the rate of absorption also increases, but up to certain limits.† Drastic variations in the rate of absorption due to changes in the temperature suggest, the process is dependent on protein or enzymatic activity.
The degree of ionization of minerals and other nutrients depends upon the hydrogen ion concentration of the soil solution.† For example, most of the phosphate ions, at alkaline pH exist either as bivalent H3PO4 ions or trivalent H3PO4 ions.† Such ions are not favored for absorption.† On the other hand neutral pH favors the absorption of monovalent ions.† So the soil pH has a significant effect not only on the rate but also the kind of ions uptake.† This property is also due to its effect on cellular components that are involved in absorption, which further suggests that proteins are involved in the ion uptake.† As the protein structure is very sensitive to pH, its function also changes if there is any change in the pH.† That is why the maintenance of proper soil pH is very important in agriculture.† Too acidic or too alkaline soil is virtually useless for cultivation.† Until and unless the soils are restored in terms of pH, such soils remain as wastelands.
Generally the concentration of minerals and its components found in soil solution is far below the levels of the same found in the cell sap.† It means that the absorption of ions takes place against concentration gradient.† The relative concentration of ions found in the cell sap and soil solution gives absorption ratio.
Ocean water contains relatively greater amount of salts than that of fresh waters.† The land plants which are adapted to grow in fresh water soils die in marine water, because the marine water is enriched with greater amount of metal ions. Physiologically dry for them.† But marine plant cells which have been adapted to such waters contain much more ionic contents than found in sea water.† Even here, the ions are absorbed against concentration gradient.
The rate of absorption of ions very and depends upon the concentration of the soil solution.† Normally, roots absorb greater amount of ions at a greater rate in dilute solutions than in a relatively high concentration solutions.† How exactly the dilution enhances the rapid uptake is not clear, but it is a fact.
Soil solution consists of a wide variety of ions in different concentrations.† While roots absorb inorganic nutrients, the ions of one kind present in the solution, either facilitate or interfere with the uptake of the other kind of ions.† This phenomenon is called ion antagonism.† On the other hand, a particular species of or ions enhance the uptake of another kind of ions.† Such a phenomenon is referred to as ion facilitated uptake.††††
Epstein has demonstrated that the absorption of K and Fe is antagonized by the presence of calcium and magnesium bivalent ions.† Similarly CaCl2 has been found to inhibit the uptake of Cu2 ions and save the plants from copper toxicity.† On the other hand, sodium chloride has been found to facilitate the uptake a wide variety of ions.
Such type of ion interactions leading to antagonistic or facilitated uptake is explained on the basis of carrier molecules.† Different ions have different carriers or transport proteins.† Because of the specific binding site, any ion that competes with the other ion for the same sites results in ion antagonism.† On the contrary, a particular ion binding to carrier molecules facilitates the binding of specific ion and enhance uptake of the said ion.† So the balanced inorganic nutrient is very important, otherwise roots absorb more of one kind of ions or the absorption f an essential ion may be prevented by the presence of another kind of ion.
1. Unequal absorption and specificity of ion:
If a mixture of different elements of equal molar concentration in the form of a buffered solution is provided to the root system, it absorbs some ions in greater amounts than other, the rest are absorbed in traces with variations. This indicates the unequal uptake and also specificity.† Certain cells, at a particular stage of development, absorb specific ions because they are required for their metabolism.† The specificity is demanded by the needs of cells or tissues.† In spite of it, the pH of the external solution remains more or less neutral.† This is certainly due to exchange of ions.† This can be demonstrated by placing a tomato plant with its roots intact in a dilute solution of NaCl.† After a period of time, certain ions like K and Ca2 are found in the external solution, which were not present before.† This phenomenon explains the exchange of ions between the external solution and internal sap. Another equally important aspect of unequal uptake or absorption of ions is the dilution effect, where greater the dilution of external solution greater is the rate of uptake.† This behavior is difficult to explain. Furthermore, the preferential uptake clearly suggests the role of specific carriers in the process of absorption of ions.
2. Salt Accumulation
Analysis of the concentration of specific nutrients present within the cell sap and the external solution reveals, that the relative concentration of specific components show greater concentration within the cells than in the external solution.† Use of radioactive isotopes as traces also supports the same view, where certain ions are accumulated or taken in against concentration gradient.† If the same is expressed in terms of chemical potential measured as milli volts.† In Nietellasí cellular cytoplasm, Na+ shows a chemical potential of 72 mV, potassium shows a difference of 40 MV and chorine +237mV.† The above observations clearly suggest that the concentration of Na is very high in the external solution.† In normal course, it should diffuse into the cell across the membrane by passive process, but to maintain chemical potential gradient and to prevent excess Na toxicity, Na ions are expelled out of the cell.† On the contrary, the movement of CI is an uphill journey, because the concentration of them is many folds higher inside the cell than outside.† The movement of ions inwards and outwards is referred to as ion flux and ion efflux respectively.
3. Saturation Effect:†
If a root system is provided with an excess amount of specific ions, initially ions are taken up at a greater rate but later the rate of uptake remains steady and constant.† This observation further suggests that for a given ion there is a fixed number of specific carrier sites; if all are loaded with their respective ions, the rate of uptake can not be increased until and unless the number of carriers is increased.
4. Metabolic Energy:
Energy is required for all metabolic processes.† To demonstrate whether metabolic energy is required for ion uptake or not, it is possible to test it by providing respiratory poisons like KCN, DNP, rotenone, etc, to the root system engaged in the absorption of minerals.† As soon as the inhibitors are added, the uptake of ions drastically reduces, which suggests that energy is absolutely required for the absorption of ions.
5. Apparent free space:
If a plant, with its root system intact, is provided with a known amount of radioactive ions like 35SO4 or 32P for about 30 minutes, it is possible to determine the total amount of ions taken up by the root system by measuring the amount of radioactivity left in the exogenously provided solution.†
Then if such a plant is transferred to water significant amount of radioactivity re appears in the water which can be easily quantified.† The difference between the total amount of radioactivity absorbed during the first incubation and the total radioactivity that re appears in water is actually the amount of radioactivity taken into the root cells.† The radioactivity that diffuses out into water is the amount of radioactive ions that taken into the free spaces found in the root cells.† Thus, the free spaces are free for diffusion of ions and such spaces are called apparent free spaces.† In roots such spaces are found in the intercellular regions and cell walls.† In fact estimations show that the total AFS found is substantial in comparison to the total volume of the root.† The movement of ions or water in AFS is a passive process and concentration dependent.
6. Donnanís free space:
A physiologist by name Donnan proposed that all cells contain the some negatively charged macromolecules in the outer region of the cytoplasm.† The space occupied by such molecules is called Donnanís space.† In order to neutralize such charges, ion pairs like potassium and chloride enter into the cell in equal amounts.† Some of the K+ ions get adsorbed onto the negatively charged Donnaís molecules.† This leads to greater negative charge within the cell.† So, to neutralize it, more of K+ ions enter.† Thus greater amount of K ions are absorbed. This process is believed to be passive.† Furthermore, such spaces are assumed to be found within the peripheral region of the cytoplasm.
Some people have estimated that Donnanís molecules are supposed to be ureic acid molecules.† In reality, most of the organic macromolecules found in the cytoplasm do show both charges.† Then, where does the Donnanís space is found in the cell?† It is difficult to explain ion uptake by Donnanís mechanism.
From time to time, various theories have been proposed to explain the mechanism of ion uptake or absorption.† Salt respiration theory by Lundergardh, cation ladder theory by Middleton and Russell, lecithin carrier hypothesis by Bennet and Clark and others have made attempts to explain the mechanism, but all of them fail to explain experimental observations.††††† Though the said theories have no relevance to what we known today, these theories have been retained in many textbooks as historical landmarks in understanding the mechanism.
As evidenced by experimental observations, the overall process is not a single mechanism, but it is a combined action of multiple events.† Without going into the details of various events, a comprehensive account has been given in this presentation.†† The present explanation is based on the evidences obtained by using radioactive tracers, respiratory inhibitors, ionophores and mutants in plants, animals and microbes.† The entire process of ion absorption takes place in two phases i.e. first passive phase and the second active phase.
It is yo be noted that plant absorb not just cat ions but also anions too.
When the intact root system is provided with an exogenous supply of mineral nutrients, to begin with, ions diffuse into the apparent free spaces found within the cell wall and the intercellular spaces found in the tissue.† The rate of diffusion depends upon the steepness of the ionic gradient between the external solution and the solution found in AFS.
†In fact the fine micro spaces found in the cell walls never act as an impediment for the free diffusion of ions, instead the charged cellulose material greatly facilitates rapid diffusion of ion into the cell wall spaces.† Thus ions diffuse through the AFS of the root system and cross through the cortical cells and outer xylem elements through the passage cells.† In xylem cells, the concentration of ions is always low because of continuous upward movement of sap by transpiration pull.† So the initial uptake is very rapid and a passive process.† Moreover this entire event is a kind of mass movement of ions along the concentration gradient till they reach the plasma membrane.
It is not just mineral uptake is facilitated by protein transporter, even watr is carried by their own protein carriers called aquaporins.† They have transcellular domains, at one surface they have H2O binding site, when H2O binds, it changes its structure, in such a way water is released at the other side.
Once the AFS are filled with mineral nutrients by free diffusion, the entry of ions across the plasma membrane into the cellular cytoplasm is highly regulated, in the sense the uptake is selective and energy dependent. Though some ions, because of local destabilization of membrane potential or due to membrane leakiness diffuse across the membrane along the concentration gradient, but the entry of majority of ions is selective, facilitated and dependent on metabolic energy.† Probably that might be the reason as to why most of the ions absorbed by the root system are found in meristematic cells because they are actively metabolic and produce more of ATPs than any other cells.
Recent electron microscopic studies of fractured and intact plasma membrane surfaces of plant, animal and bacterial origin reveal that the membranes contain an array of globular proteins of different sizes and dimensions scanning the entire cross-section or a part of the membranes.† Some of them are aggregated in one site and others are randomly distributed.† However the position of such granules in the membranes is never constant because of the fluid nature of the membrane.† A large number of proteins with their 3-D globular structures act as carrier proteins or transport proteins.† Such carrier proteins have high affinity towards specific ions; because they do contain specific binding site or sites.
In the past 10-15 years or so a quite a number of carrier proteins have been isolated and purified to homogeneity.† The 3-D structure of a few of these proteins has been determined.† Some of the common examples for carrier proteins are K, Na-ATPase pumps, hydrogen protein pumps, calcium transporting protein called Calmodulin, phosphate carrier proteins, etc.† The above mentioned proteins have been isolated from different sources and their structures as well as kinetic properties have been studied in detail.
Interestingly enough, the discovery of some cyclic polypeptides like valinomycin, nistatin, cyclosporin, etc, from many lower fungi has given a new dimension to the carrier mediated ion or nutrient uptake.† Such compounds are called as ĎIonophoresí, because when they are added to the medium, they penetrate into the membrane easily and create a holes, which permit the entry of only one specific kind of ions.† For example, valinomycin greatly facilitates the uptake of K+ ions only.† Similarly nistatin allows specific anions.† It is now speculated that the plasma membranes contain such specific ion transporting complexes and they are responsible for the movement of certain ions; under certain circumstances.† Ionophores role can be used as a generalized process of ion uptake, but they too play a significant role in the entry of specific ions in some species.
Using mutation as the tool molecular biologists have discovered that there are specific genes for specific carrier proteins which transport a specific substance.† And such carrier proteins have been detected in the plasma membrane of the cell.† Inorganic ions, monosaccharide, amino acids and many such components including proteins and nucleic acids have been found to be transported across the membrane facilitated by specific membrane proteins.† Some of the receptor proteins have been found to carry the stimulants across the membranes.
Most of the experimental evidences suggest that ion uptake is carrier mediated and the carriers are proteins.† Only proteins can explain certain properties like ion antagonism, specificity of ion uptake, saturation effect, kinetic properties requirement of metabolic energy, etc.† The carrier proteins exhibit 3-D shape.† They have specific binding sites for the ions or substrates and no other ion can bind to such sites.† After binding the proteins undergo rapid conformational change in their 3-D structure and exhibit mobility in the lipid core of the membrane.†† For its active transformations and transport they require metabolic energy.† The activity of carrier proteins can be regulated by effectors or affecters, similar to that of allosteric enzymes.† The transport activity of these proteins can be inhibited by competitive or non competitive inhibitors. Another important feature of carrier proteins is their ability to transport molecules against concentration gradient by active process.
Peter Mitchell a Nobel laureate has proposed three possible mechanisms by which carrier proteins operate in the transportation of ions or other nutrients, namely, uniport, antiport and symport mechanisms.† In fact, in the same cell all the system are found and all of them †operate.
In this case, the activated carrier proteins pick a specific nutrient at one surface and translocate across and unload the same on the other side.† After unloading, the carrier proteins revert back to its position by conformational change.† Many bacterial and animal cells have ATPase dependent carrier proteins which transport many nutrients like glucose by uniport mechanism.† There are many such carrier proteins which transport nutrients passively but along with the concentration gradient.† Such a process is called facilitated transport but it is passive.† Such uniport mechanism has been observed with respect to the transport of many monosaccharides, amino acids, etc.† A very good example for this is calcium binding protein.
This process involves coupled transport where one specific ion is transported in and the other is transported out by the same transporter.† The antiport mechanism is very well exhibited by Na/K ATPase pump found in the plant cells like Nietella and in animal cells like erythrocytes.† The internal concentration of K+ ions is many folds higher than the concentration of K+ ions found in the extracellular fluid.††† Cells have to maintain such high concentrations of K+ ions intracellularly, because they are required for various metabolic activities and for the maintenance of turgidity of the cells.† On the contrary, the concentration of Na+ ions is always higher in the
external fluid than inside the cell.† Hence Na ion often leaks in or taken in by other processes.† As excess sodium ions inside the cell is toxic and the same are expelled out.† Na/K-ATPase pump does this by antiport mechanism.
Transport of this kind involves the binding of two different molecules or ions to two active sites of the same carrier protein.† By virtue of the binding of both the ions the carrier complex rotates by conformational change.† There by the said ions are released simultaneously at the opposite side of
the membrane.† Such symport type of transportation has been recorded in a variety of organisms.† But the best example for symport mechanism is the transport of Na+ and glucose found in animal and bacterial cells.
In this case, the carrier protein is a complex macromolecule and possesses two sites, one for Na ion and the other for glucose.† While the binding sites of the protein are facing the external fluid, both Na+ and glucose bind to their specific sites.† The binding is facilitated in the sense the binding of one molecule promotes the binding of the other.† However, as soon as Na and glucose bind to the protein, it undergoes conformational change and opens at the inner surface and the Na+ and glucose are released into the cytoplasm almost at the same time.† The release of molecules makes the protein revert to its original shape and position.† The Na+ ions that enter into the cell are expelled by Na/K ATPase pump, i.e. by antiport mechanism.
Pinocytosis is not just restricted to animal cells only, it is also found in plants and perform the functions similar to that of animal cells.† This process is an energy dependent process and involves the uptake of nutrients in bulk.
Plasma membrane on its outer surface possesses binding sites for ions or nutrients in the form of clusters in specific areas.† Throughout the surface one finds such groups of binding sites.† Each of these clusters may contain only one kind of binding sites or different kinds.† Once the ions bind to their specific sties, the inner surface of the plasma membrane in such regions, coated pits gets activated.† Involvement of Clathrin coated and †the activity of microfilaments that are associated on the face pull the membrane inside by contractile activity.† Thus the inwardly puckered membrane with all the solutes and some water is drawn inwards and ultimately pinched off as solute loaded vesicles.† Later these vesicles are transported intracellularly or they may break down into smaller vesicles.† The contractile activity which is responsible of Pinocytosis is an energy dependent process.
Absorbed mineral salts and other components like cytokinins, and others absorbed by roots, ultimately reach the xylem elements found in the root system i.e xylem vessels.† Most of the minerals are absorbed by meristems than root hairs. The minerals then are transported to the vascular system of
young and developing xylem elements or they can be transported into mature vascular elements . The xylem elements have xylem parenchyma, which are living and supporting the process of translocation of minerals into upward conducting system.† Once the elements are loaded into xylem cells, they are transported upwards along with the transpiration stream and thus they reach the aerial parts of the plant body where the nutrients; first diffuse into apparent free spaces then they are absorbed into living cells by active/facilitated transport.† While the minerals are translocated all along the length of the stem, considerable amount of them diffuse laterally and reach the cortical cells.†
In spite of the process of absorption of mineral salts by root system is highly specific and energy dependent, the upward movement is entirely due to passive process for the dissolved components just move along with the water column due to transpiration pull.† Yet one cannot rule out the ability of livings cells to do such† active transportation for dead xylem† are supported by living xylem parenchyma.† Even Xylem parenchyma tissue a living tissue has roles in such mineral uptake and transportation.† Whichever factor that affects the transpiration pull also affects the translocation of mineral salts in the plant body.