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Lecture No: 8

NUTRIENTS - MECHANISM OF UPTAKE

and PHYSIOLOGICAL ROLE

Dr. T. Sivakumar,Assistant Professor (Crop Physiology)

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Plant root structure

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Young root in cross section

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Longitudinal section of a typical root

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Light micrograph of a mature root

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How Mineral nutrients are absorbed?

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Diagram for Passive absorption

Apoplast-Symplast concept given by E. Munch.

The interconnecting walls (Endodermal and Exodermal walls with

Casparian strips and water filled xylem elements (tracheids and

vessels) is Apoplastand rest of living part is Symplast.

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Movement through endodermis(endo=inside, dermis=skin/layer)

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Factors affecting absorption of

mineral salts from soil

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MechanismPreviously it was thought that absorption of mineral

salts takes place along with water absorption. But it is nowunderstood that mineral salt absorption and waterabsorption are two different processes.

Mineral salts are absorbed from the soil solution inthe form of ions. They are chiefly absorbed through themeristematic regions of the roots near the tips.

Plasma membrane of the root cells is not permeableto all the ions. It is selectivity permeable. All the ions of thesame salt are not absorbed at equal rate but leads unequalabsorption of ions.

First step in the absorption of mineral salts is the

process of Ion exchange which does not requiremetabolic energy.

The further processes of the absorption of mineralsalts may be of two types.

1.Passive - does not require metabolic energy.

2. Active - requires metabolic energy

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1. Passive absorption

When the concentration of mineral saltsis higher in the outer solution than in the cellsap of the root cells, the mineral salts areabsorbed according to the concentration

gradient by simple process of diffusion.This is called as passive absorption

because it does not require expenditure ofmetabolic energy.

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Ion exchangeThe ions adsorbed on the surface of the plasma

membrane of the root cells may be exchanged with the ions

of same sign ions from external solution. Two theories toexplain the mechanism of ion exchange.

1. Contact exchange theory - the ions adsorbed on thesurface of root cells and clay particles are not held tightly

but oscillate within small volume of space. The ionsadsorbed on clay particle may be exchanged with the ionsadsorbed on root surface directly without first beingdissolved in soil solution.

2. Carbonic acid exchange theory - CO2 released duringrespiration of root cells combines with water to formcarbonic acid (H2CO3). Carbonic acid dissociates into H

+and an anion HCO3

-in soil solution. These H+ions may beexchanged for cations adsorbed on the clay particles. Thus,the soil solution plays an important rolein carbonic acidexchange theory.

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2. Active absorptionThe accumulation of mineral salts against to

concentration gradient is an active process which involves

the expenditure of metabolic energy through respiration.The active absorption of mineral salts involves theoperation of a carrier compound present in the plasmamembrane of the cells.

The carrier concept theory

According to this theory, the plasma membrane isimpermeable to free ions. But some compounds present in itacts as carrier and combines with ions to form carrier- ion-complex which can move across the membrane. On theinner side of the membrane this complex leaves releasingions into the cell, while the carrier goes back to the outersurface to pick up fresh ions. There are two hypothesesbased on the carrier concept to explain the mechanism ofactive salt absorption. Although they are not universallyaccepted.

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How carrier works?

The term carrier implies that proteins pick up solutes and move

across the membrane i.e. Reversible structural change of Protein

facilitate solute transfer

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1. Lundegardhs cytochrome pump theoryLundegardh (1954) proposed cytochrome pump theory which isbased on the following assumptions.

1.The mechanism of anion and cation absorption is different.2.Anions are absorbed through cytochrome chain by an activeprocess.

3.Cations are absorbed passively.

According to this theory Dehydrogenase reactions oninner side of the membrane give rise to protons (H+) andelectrons (e-). The electrons travels over the cytochrome chaintowards outside the membrane, so that the Fe of thecytochrome becomes reduced (Fe++) on the outer surface andoxidized (Fe+++) on the inner surface. On the outer surface, thereduced cytochrome is oxidized by oxygen releasing theelectron (e-) and taking an anion (A-). The electron thusreleased unites with H+ and oxygen to form water

The anion (A-) travels over the cytochrome chain towards inside.On the inner surface the oxidized cytochrome becomes reducedby taking an electron produced through the dehydrogenasereactions and the anion (A) is released. As the result of anion

absorption, a cation (M) moves passively from outside to insideto balance the anion.

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2. BennetClarks protein Lecithin Theory

In 1856, BennetClark suggested that because the cellmembranes chiefly consist of phospholipids and proteins andcertain enzymes seem to be located on them, the carrier couldbe a protein associated with the phosphatide called as lecithin.He also assumed the presence of different phosphatides tocorrespond with the number of known competitive groups ofcations and anions.

According to this theory Phosphate group in thephosphatide is regarded as the active centre binding the cationsand the basic choline group as the anion binding centre. Theions are liberated on the inner surface of the membrane bydecomposition of lecithin by the enzyme lecithinase. The

regeneration of the carrier lecithin form phosphatidic acid andcholine takes place in the presence of the enzyme cholineacetylase and choline esterase and ATP. The latter acts as asource of energy.

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3. Donnans Equilibrium theory

The accumulation of ions inside the cells withoutinvolving expenditure of the metabolic energy can be explainedto some extent by Donnansequilibrium theory.

According to this theory there are certain pre existingions inside the cell which cannot diffuse outside throughmembrane. Such ions are called as in diffusible or fixed ions.However, the membrane is permeable to both anions and

cations of the outer solutions.Suppose there are certain fixed anions in the cell which isin contact with outer solution containing anions and cations.Normally equal number of anions and cations would havediffused into the cell through an electrical potential to balanceeach other, but to balance the fixed anions more cations will

diffuse into the cell. This equilibrium is known as Donnansequilibrium. In this particular case, there would be anaccumulation of cations inside the cell.

If however, there are fixed cations inside the cell, theDonnans equilibrium will result in the accumulation of anionsinside the cell.

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Specific roles of essential mineral elements

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A. Primary MacronutrientsNitrogen

Nitrogen is important constituent of proteins, nucleic acids,porphyries (chlorophylls & cytochromes) alkaloids, somevitamins, coenzymes etc. Thus N plays very importantrole in metabolism, growth, reproduction and heredity.

Phosphorus

It is important constituent of nucleic acids, phospholipids,coenzymes NADP, NADPH

2and ATP. It plays important role in

protein synthesis through nucleic acids and ATP.

It plays important role in energy transfer reactions of cellmetabolism through coenzymes NAD, NADP and ATP

e.g. Photosynthesis, respiration and fat metabolism etc.

Potassium

Although potassium is not a constituent of important organiccompound in the cell, it is essential for the process of respirationand photosynthesis. It acts as an activator of many enzymesinvolved in carbohydrate metabolism and protein synthesis

It regulates stomatal movement and water balance

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B. Secondary MacronutrientsCalcium

It is important constituent of cell wall

It is essential in the formation of cell membranesIt helps to stabilize the structure of chromosome

It may be an activation of may enzymes

Magnesium

It is very important constituent of chlorophylls

It acts as activation of many enzymes in nucleic acid synthesisand carbohydrate metabolism

It plays important role in binding ribosomal particles duringprotein synthesis.

Sulphur

It is important constituent of some amino acids (cystine,

cysteine and methionine) with which other amino acids formthe protein

S helps to stabilizethe protein structure

It is also important constituent of vitamin i.e. biotin, thiamineand coenzyme A

Sulpho hydryl groups are necessary for the activity of many

enzymes.

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MicronutrientsIron

Important constituent of iron porphyrin proteins like

cytochromes, peroxidase, catalases, etc.It is essential for chlorophyll synthesis

It is very important constituent of ferredoxin which playsimportant role in photochemical reaction in photosynthesis andin biological nitrogen fixation.

Zinc

It is involved in the biosynthesis of growth hormone auxin(indole 3 acetic acid) and acts activator of many enzymes likecarbonic anhydrase and alcohol dehydrogenase, etc.

Manganese

It is an activator of many respiratory and nitrite reductaseenzymes and necessary for the evolution of oxygen(photolysis) during photosynthesis

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Micronutrients Conti

CopperIt is an important constituent of plastocyanin (copper containingprotein) and constituent of several oxidizing enzymes.

Boron

Boron facilitates the translocation of sugars by forming sugarborate complex.

It involves in cell differentiation and development since boron isessential for DNA synthesis

Also involves in fertilization, hormone metabolism etc.

Molybdenum

It is constituent of the enzyme nitrate reductase and thus playsan important role in nitrogen metabolism

It is essential for flower formation and fruit set.

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"A man looking the top

of hill at its very base

will not reach hills top