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02 Mineral Nutrition

1. Introduction :
• Most of the minerals present in the soil can enter into plants through roots. More than 60 types of
elements are found in different plants. Which elements are found in plants, can be determined by
the method called Ash analysis.

2. Methods to Study the Mineral Requirements of Plants :

• In 1860 Julius Von Sachs (a prominent German botanist) demonstrated for the first time, that
plants could be grown to maturity in a defined nutrient solution in complete absence of soil.
• The technique of growing plants in a nutrient solution without soil is called as hydroponics.
• By this method, essential elements were identified and their functions and deficiency symptoms
were discovered.
• The nutrient solutions must be adequately aerated to obtain the optimum growth.
• Hydroponics has been successfully employed as a technique for the commercial production
of vegetables like, tomato, seedless cucumber and lettuce.

Funnel for
adding water Cotton Aerating tube
and nutrients

Nutrient solution

Nutrient solution
Pump

Diagram of a typical set-up for Hydroponic plant production. Plants are grown in
nutrient solution culture a tube or trough placed on a slight incline. A pump
circulates a nutrient solution from a reservoir to
the elevated end of the tube. The solution flows
down the tube and returns to the reservoir due to
gravity. Inset shows a plant whose roots are
continuously bathed in aerated nutrient solution.
The arrows indicates the direction of the flow.

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3. Essential Mineral Elements :
• About more than 60 types of elements are present in the plant body but only 17 elements are
considered as essential elements.
• C, H, O, N, K, S, Ca, Fe, Mg, P, Cu, Mn, B, Cl, Zn, Mo, Ni.
• Except these seventeen essential elements there are some beneficial elements such as Na, Si, Se,
Co.
• Among 17 essential elements C, H & O are obtained from the air and soil in the form of CO 2 & H2O
not in the form of ions, so these are called nonmineral nutrients or elements. Others are
mineral nutrients.

Criteria For Essentiality :

• According to Arnon-criteria for essentiality of minerals are :
(a) The element must be absolutely necessary for supporting normal growth and reproduction. In
the absence of the element the plants do not complete their life cycle or set the seeds.
(b) The requirement of the element must be specific and not replaceable by another element. In
other words, deficiency of any one element cannot be met by supplying some other element.
(c) The element must be directly involved in the metabolism of the plant.

(A) Classification of Nutrients :

On the basis of function :
• Essential elements can also be grouped into four broad categories on the basis of their diverse
functions. These categories are :
(a) Essential elements as components of biomolecules and hence structural elements of cells.
(e.g., carbon, hydrogen, oxygen and nitrogen)
(b) Essential elements that are components of energy - related chemical compounds in plants.
(e.g., magnesium in chlorophyll and phosphorous in ATP).
(c) Essential elements that activate or inhibit enzymes. (e.g., Mg, Zn, Mo etc.)
Mg2+ is an activator for both ribulose bisphosphate carboxylase oxygenase (RuBisCO) and
phosphoenol pyruvate carboxylase (PEPCase), both of which are critical enzymes in
photosynthetic carbon fixation; Zn2+ is an activator of alcohol dehydrogenase and Mo of
nitrogenase during nitrogen metabolism.
(d) Some essential elements can alter the osmotic potential of a cell. (e.g., K, Cl etc.)

(I) On the Basis of Quantity or Requirement :

• Arnon divided essential elements into two group on the basis of their requirement of plants –
(i) Major element/Macro nutrients :
• Concentration must be excess of 10 m mole kg–1 of dry matter.
C, H, O, N, P, K, S, Ca, Mg,
(ii) Minor element/Micro nutrients :
• Concentration required less than 10 m mole kg–1 of dry matter.
Fe, Cu, Zn, B, Cl, Mn, Mo, Ni
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(II) Role of Macro and Micro-Nutrients :
• Essential elements perform several functions. They participate in various metabolic processes in
the plant cells such as permeability of cell membrane, maintenance of osmotic concentration of cell
sap, electron transport systems, buffering action, enzymatic activity and act as major constituents of
macromolecules and co-enzymes. Various forms and functions of mineral elements are given below.

(i) Nitrogen :- This is the mineral element required by plants in the greatest amount. It is absorbed
mainly as NO3– though some are also taken up as NO2– or NH4+. Nitrogen is required by all parts
of a plant, particularly the meristematic tissues and the metabolically active cells. Nitrogen is
one of the major constituents of proteins, nucleic acids, vitamins and hormones.
(ii) Phosphorus :- Phosphorus is absorbed by the plants from soil in the form of phosphate ions (either
as H2PO4– or HPO42–. Phosphorus is a constituent of cell membranes, certain proteins, all
nucleic acids and nucleotides, and is required for all phosphorylation reactions.
(iii) Potassium :- It is absorbed as potassium ion (K+). In plants, this is required in more abundant
quantities in the meristematic tissues, buds, leaves and root tips. Potassium helps to maintain
an anion-cation balance in cells and is involved in protein synthesis, opening and closing of
stomata, activation of enzymes and in the maintenance of the turgidity of cells.
(iv) Calcium :- Plant absorbs calcium from the soil in the form of calcium ions (Ca 2+). Calcium is
required by meristematic and differentiating tissues. During cell division it is used in the
synthesis of cell wall, particularly as calcium pectate in the middle lamella. It is also needed
during the formation of mitotic spindle. It accumulates in older leaves. It is involved in the normal
functioning of the cell membranes. It activates certain enzymes and plays an important role in
regulating metabolic activities.
(v) Magnesium :- It is absorbed by plants in the form of divalent Mg2+. It activates the enzymes of
respiration, photosynthesis and are involved in the synthesis of DNA and RNA. Magnesium is a
constituent of the ring structure of chlorophyll and helps to maintain the ribosome structure.
(vi) Sulphur :- Plants obtain sulphur in the form of sulphate (SO42–). Sulphur is present in two amino
acids – cysteine and methionine and is the main constituent of several coenzymes, vitamins
(thiamine, biotin, Coenzyme A) and ferredoxin.
(vii) Iron : Plants obtain iron in the form of ferric ions (Fe3+). It is required in larger amounts in
comparison to other micronutrients. It is an important constituent of proteins involved in the
transfer of electrons like ferredoxin and cytochromes. It is reversibly oxidised from Fe2+ to
Fe3+ during electron transfer. It activates catalase enzyme, and is essential for the formation of
chlorophyll.
(viii) Manganese : It is absorbed in the form of manganous ions (Mn2+). It activates many enzymes
involved in photosynthesis, respiration and nitrogen metabolism. The best-defined function
of manganese is in the splitting of water to liberate oxygen during photosynthesis.
(ix) Zinc : Plants obtain zinc as Zn2+ ions. It activates various enzymes, especially carboxylases. It is
also needed in the synthesis of auxin.
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(x) Copper : It is absorbed as cupric ions (Cu2+). It is essential for the overall metabolism in plants.
Like iron, it is associated with certain enzymes involved in redox reactions and is reversibly
oxidised from Cu+ to Cu2+.
(xi) Boron : It is absorbed as BO33– or B4O72–. Boron is required for uptake and utilisation of Ca2+,
membrane functioning, pollen germination, cell elongation, cell differentiation and
carbohydrate translocation.
(xii) Molybdenum : Plants obtain it in the form of molybdate ions (MoO22+). It is a component of several
enzymes, including nitrogenase and nitrate reductase both of which participate in nitrogen
metabolism.
(xiii) Chlorine :- It is absorbed in the form of chloride anion (Cl–). Along with Na+ and K+, it helps in
determining the solute concentration and the anion cation balance in cells. It is essential for
the water-splitting reaction in photosynthesis, a reaction that leads to oxygen evolution.

(B) Deficiency Symptoms of Essential Elements :

• Whenever the supply of an essential element becomes limited, plant growth is retarded. The
concentration of the essential element below which plant growth is retarded is termed as critical
concentration. The element is said to be deficient when present below the critical concentration.
• Since each element has one or more specific structural or functional role in plants, in the absence
of any particular element, plants show certain morphological changes. These morphological
changes are indicative of certain element deficiencies and are called deficiency symptoms. The
deficiency symptoms vary from element to element and they disappear when the deficient mineral
nutrient is provided to the plant. However, if deprivation continues, it may eventually lead to the
death of the plant.

(C) Mobility of Minerals :

• The parts of the plants that show the deficiency symptoms also depend on the mobility of the
element in the plant. For elements that are actively mobilised within the plants and exported
to young developing tissues, the deficiency symptoms tend to appear first in the older
tissues. For example, the deficiency symptoms of nitrogen, potassium, magnesium and
phosphorus are visible first in the senescent leaves. In the older leaves, biomolecules containing
these elements are broken down, making these elements available for mobilising to younger leaves.
• The deficiency symptoms tend to appear first in the young tissues whenever the elements
are relatively immobile and are not transported out of the mature organs, for example,
elements like sulphur and calcium are a part of the structural component of the cell and hence are
not easily released. This aspect of mineral nutrition of plants is of a great significance and
importance to agriculture and horticulture.
• The kind of deficiency symptoms shown in plants include chlorosis, necrosis, stunted plant growth,
premature fall of leaves and buds, and inhibition of cell division.
• Chlorosis is the loss of chlorophyll leading to yellowing in leaves. This symptom is caused by
the deficiency of elements N, K, Mg, S, Fe, Mn, Zn and Mo.
• Likewise, necrosis, or death of tissue, particularly leaf tissue, is due to the deficiency of Ca, Mg,
Cu, K.
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• Lack or low level of N, K, S, Mo causes an inhibition of cell division.
• Some elements like N, S, Mo delay in flowering if their concentration in plants is low.
• You can see from the above that the deficiency of any element can cause multiple symptoms and
that the same symptoms may be caused by the deficiency of one of several different elements.
Hence, to identify the deficient element, one has to study all the symptoms developed in all the
various parts of the plant and compare them with the available standard tables.
• We must also be aware that different plants also respond differently to the deficiency of the same
element.

Some physiological deficiency diseases :

(i) Whiptail of cauliflower : caused by deficiency of Mo
(ii) Marsh spot of pea : caused by deficiency of Mn
(iii) Grey or speck spot of oat : caused by deficiency of Mn
(iv) Die back of Citrus : caused by deficiency of Cu
(v) Reclamation disease of cereals and legume crops : caused by deficiency of Cu
(vi) Khaira disease of paddy : caused by deficiency of Zn
(vii) Brown heart rot of beats : caused by deficiency of B
(viii) Stout axis : caused by deficiency of B

(D) Toxicity of Micronutrients :

• Any mineral ion (Micronutrient) concentration in plant tissue that reduce the dry weight of
tissues by about 10 percent is considered as toxic and this effect is called as toxicity.
Ex :- Excess of Mn (Manganese) cause appearance of brown spots surrounded by chlorotic veins
(prominent symptom) Mn competes with iron (Fe) and magnesium (Mg) for uptake and with Mg
for binding with enzymes. Mn also inhibits, calcium translocation into the shoot apex. So, the
excess of Mn may, in fact, induce deficiencies of Fe, Mg and Ca. Thus, what appears as symptoms
of manganese toxicity may actually be the deficiency symptoms of Fe, Mg & Ca.

4. Mechanism of Absorption of Elements :

• Soil is the main source of mineral salts. These mineral salts are absorbed by the by root hairs.
• The movement of mineral ions is usually called as flux. The inward movement inside the cell is
called influx and outward movement is efflux.

(A) The Mineral Absorption Occurs Mainly in Two Phases :

• In first phase, an initial rapid uptake of ions into the Free space or Outer space of the cell occurs,
the apoplast (intercellular spaces and cell wall), which is a passive process.
• In the second phase of uptake, the ions are taken in slowly into the inner space, the symplast of
the cell which is both active and passive process (mainly active process). The passive
movement of ions in symplast occurs through ion channels which are trans-membrane proteins
and act as selective pores. The active influx and efflux from symplast occurs with the help of pump
proteins and with expenditure of energy/ATP.
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Movement of minerals from its high conc. to low
conc. by transport protein (Passive process)

Cell wall
Initial phase
(Primary step) Apoplast
Symplast
Mineral
s

Soil
reservoir
Metabolic phase
(Secondary step) Root cells
Cell membrane
Movement of minerals from its low conc. to high
conc. by pump protein (Active process)

(B) Methods of Mineral Absorption :

(I) Passive absorption of Minerals : (Without expenditure of ATP)
(a) By diffusion : According to this method mineral ions may diffuse into root cells from the soil
solution.
(b) By mass flow : According to this method mineral ions absorption occurs with flow of water
under the influence of transpiration pull.
Why absorption is not completely passive?
(i) Minerals are present in the soil as charged particles (ions) which mostly cannot move across
cell membranes passively.
(ii) The concentration of minerals in the soil is usually lower than the concentration of minerals
in the roots.
(II) Active ion Absorption : (By expenditure of ATPs)
• Specific proteins in the membrane of root hairs actively pump ions from the soil into the cytoplasm
of the epidermal cells.
• Like all cells, endodermal cells of root have many transport proteins embedded in their plasma
membrane; they let some solutes cross the membrane, but not others. Transport proteins of
endodermal cells of root are control points, where a plant adjust the quantity and types of
solutes that reach the xylem. The root endodermis because of the layer of suberin has the
ability to actively transport ions in one direction only.

5. Translocation of Mineral Ions/Solutes :

• When ions reach to the xylem of root by active or passive absorption or by cumulative activity of
both then transport of these ions towards stem and all the parts of plant occurs due to transpiration
flow through xylem.
• Main storage regions or sinks for minerals elements are growing regions of the plants like
apices and lateral meristem, young leaves, developing flowers, fruits, seeds and storage
organs.
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• Unloading of mineral ions occurs at the fine vein endings through diffusion and actively uptake
by cells of sink.
• Analysis of xylem sap shows that some nitrogen is translocated in the form of inorganic ions
and most of it is translocated in the organic form i.e. amides and related compounds.
• Like this small amount of phosphorus and sulphur are also translocated in the form of organic
compounds. Except this, small amount of materials is also transferred between xylem and phloem.
So we cannot distinctly differentiate that xylem only translocate inorganic nutrients and
phloem translocate only organic materials, as was believed before.

6. Soil as Reservoir of Essential Elements :

• Majority of the nutrients that are essential for the growth and development of plants become
available to the roots due to weathering and breakdown of rocks. These processes enrich the soil
with dissolved ions and inorganic salts. Since they are derived from the rock minerals, their role
in plant nutrition is referred to as mineral nutrition.
• Soil consists of a wide variety of substances. Thus, soil play several roles :-
(i) Soil is reservoir of water for plants.
(ii) Soil supplies minerals and harbours nitrogen-fixing bacteria.
(iii) Soil supplies air to the roots.
(iv) Soil acts as a matrix that stabilises the plant.
(v) In terrestrial habitat soil is the site for decomposition (decomposition is a process to recycle
minerals from detritus).
Since deficiency of essential minerals affect the crop-yield, there is often a need for supplying them
through fertilisers. Both macro-nutrients (N, P, K, S, etc.) and micro-nutrients (Cu, Zn, Fe, Mn, etc.)
form components of fertilisers and are applied as per need.

• Hydroponics has been successfully employed as a technique for the commercial production of
vegetables like, tomato, seedless cucumber and lettuce.
• Phosphorus is a constituent of cell membranes, certain proteins, all nucleic acids and nucleotides,
and is required for all phosphorylation reactions.
• Sulphur is present in two amino acids – cysteine and methionine.
• The best defined function of manganese is in the splitting of water to liberate oxygen during
photosynthesis.
• For elements that are actively mobilised within the plants and exported to young developing
tissues, the deficiency symptoms tend to appear first in the older tissues.
• The deficiency symptoms tend to appear first in the young tissues whenever the elements are
relatively immobile and are not transported out of the mature organs.
• Transport proteins of endodermal cells of root are control points, where a plant adjust the
quantity and types of solutes that reach the xylem.
• Analysis of xylem sap shows that some nitrogen is translocated in the form of inorganic ions and
most of it is translocated in the organic form i.e. amides and related compounds.
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INTRODUCTION, METHODS TO STUDY THE MINERAL
BEGINNER’S BOX-1 REQUIREMENTS OF PLANTS, MECHANISM OF ABSORPTION
OF ELEMENTS, TRANSLOCATION OF MINERAL IONS/ SOLUTES,
SOIL AS RESERVOIR OF ESSENTIAL ELEMENTS

1. The technique of growing plants in a nutrient solution, in absence of soil is called as :-

(1) Exudation (2) Tissue culture (3) Hydroponics (4) Soil culture
2. Plants can be grown in
(1) Soil containing essential nutrients
(2) Water containing essential nutrients
(3) Either water or soil containing essential nutrients
(4) Water or soil without essential nutrients.
3. Minimum concentration of an essential element below which plant growth is retarded, is termed as:
(1) Optimum concentration (2) Maximum concentration
(3) Critical concentration (4) Toxic concentration
4. Deficiency of which of the following elements cause delay in flowering in plants?
(1) Fe, Mn, Mo (2) N, S, Mo (3) Ca, Mg, K (4) N, K, S
5. Select the incorrectly matched pair :-
(1) Magnesium (Mg) – Formation of mitotic spindle
(2) Iron (Fe) – Formation of chlorophyll
(3) Chlorine (Cl) – Anion-cation balance in the cell
(4) Sulphur (S) – Component of vitamins
6. Select non-mineral element from the following:
(1) Carbon (2) Hydrogen (3) Both (1) and (2) (4) Nitrogen
7. How many elements are considered as “Essential elements”?
(1) 10 (2) 9 (3) 18 (4) 17
8. Which of the following is a macronutrient?
(1) Molybdenum (2) Zinc (3) Manganese (4) Magnesium
9. Which element is related with auxin biosynthesis?
(1) Nitrate (2) Copper (3) Manganese (4) Zinc
10. Binding of ribosomal subunits requires :-
(1) Manganese (2) Magnesium (3) Calcium (4) All of the above
11. Chlorophyll structure contains :-
(1) Calcium (2) Phosphorous
(3) Magnesium ion (4) Non-ionic Magnesium
12. Cofactors required by water splitting enzyme are :-
(1) Calcium (2) Manganese (3) Chlorine (4) All of these
13. Sulphur is found in :-
(1) Thiamine (2) Biotin (3) Coenzyme A (4) All of the above
14. Which of the following act as check point for absorption of minerals ?
(1) Hypodermis (2) Cortex (3) Endodermis (4) Pericycle
15. Root endodermis act as check point which controls:-
(1) Qualitative absorption of minerals (2) Quantitative absorption of minerals
(3) Both (1) and (2) (4) Cell Division
16. Casparian Strips are found in :-
(1) Hypodermis (2) Cortex (3) Pericycle (4) Endodermis
17. At the level of cell wall and plasma membrane, absorption of minerals are respectively:-
(1) Active and passive (2) Passive and active
(3) Only passive (4) Only active

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• C, H, O, N, P and S are called protoplasmic elements as they are main constituent of organic part
of protoplasm.
• C, H and O are called frame work elements as they are main constituent of proteins,
carbohydrates and fats.
• N, P and K are called critical elements as they are required by plants in high doses. To fulfil this
great demand soil mostly faces a shortage of such elements. Therefore, farmers frequently supply
them in the form of fertilisers to obtain optimum crop yield.
• Plants grown in moistened air with nutrients is called Aeroponics.
• About 98 percent of the mass of every living organism is composed of six elements including
carbon, hydrogen, oxygen, nitrogen, calcium and phosphorus.
• Nitrogen is required in maximum quantity among all the essential mineral elements.
• Among micronutrients, Fe is required in maximum quantity and Mo is required in minimum
quantity.
• Some plants store nutrients e.g. (1) Peach plant - Strontium
(2) Astragalus & Neptunia plant - Selenium.
(3) Equisetum and mustard plant - Gold.

7. Metabolism of Nitrogen :
(A) Introduction :
1. Nitrogen is an essential element in all living organisms. It is the constituent of amino acids,
proteins, hormones, chlorophylls and many of the vitamins.
2. Nitrogen is present in the atmosphere abundantly in N2 form, but eukaryotes and many
prokaryotes can't uptake nitrogen directly from the atmosphere. The nitrogen enter into the soil
from atmosphere by fixation process, then from the soil, plants absorb it and from plants it moves
to animals and fulfil the requirements of all living beings.
3. Plants competes with microbes for the limited nitrogen that is available in the soil, so
nitrogen is a limiting nutrient for both natural and agricultural ecosystem.

(B) Nitrogen (N2) Cycle :

Atmospheric N2

Biological N2 Industrial N2 Electrical N2 Denitrification

Fixation Fixation Fixation

Nitrification
NH3
Soil N Pool

(Ammonification) (Uptake)

Decaying biomass Plant biomass

Animal biomass
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Steps of N2 Cycle :
1. N2 fixation
2. Uptake and assimilation of nitrogen by plants
3. Ammonification
4. Nitrification
5. Denitrification

(C) Nitrogen Fixation :

• Process of conversion of atmospheric N2 into nitrogenous compounds (NH3, NO3–) is called
nitrogen fixation.
• Nitrogen fixation is of two types :
(i) Abiological / Physicochemical
(ii) Biological / Diazotrophy

(i) Abiological Nitrogen Fixation :

• Abiological Nitrogen Fixation / Physicochemical N2 fixation is further divided into two types:
(a) Atmospheric or electrical
(b) Industrial

(a) Atmospheric or electrical : In nature, lightening and ultraviolet radiation provide enough energy
to convert nitrogen to nitrogen oxides (NO, NO2, N2O).
(i) 
N2 + O2 ⎯⎯ → 2 NO
(ii) 2NO + O2 ⎯→ 2NO2
(iii) 3NO2 + H2O (rain water) ⎯→ 2HNO3 + NO
4NO2 + O2 +2H2O (rain water) ⎯→ 4HNO3
(iv) HNO3 + NH3 ⎯→ NH4NO3
2HNO3 + CaO ⎯→ Ca(NO3)2 (Absorbed by plants) + H2O

(b) Industrial N2 fixation : In the presence of high pressure, temperature and catalysts nitrogen(N2)
and hydrogen combines to form ammonia (NH3). This ammonia is used in the formation of chemical
fertilizers.

(ii) Biological Nitrogen Fixation :

Conversion of atmospheric nitrogen (N2) into inorganic nitrogenous compounds like – NH3 by living
organisms is called biological nitrogen fixation or Diazo trophy. Only CERTAIN PROKARYOTIC SPECIES are
capable of fixing nitrogen.
Nitrogen Fixing organisms (Diazotrophs) –

(a) Free living or non–symbiotic :

• Eubacteria :– Azotobacter, Beijernickia, (both aerobic) and Rhodospirillum (anaerobic).
• Cyanobacteria (Blue green algae) – Nostoc, Anabaena.

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(b) Symbiotic :
Symbiotic Diazotrophs or N2 fixers
Micro-organism Symbiotic Host
structure
1. Eubacteria
(i) Rhizobium & Root nodules Leguminous plants
Bradyrhizobium
(ii) Azorhizobium Stem nodules Leguminous plant – Sesbania
2. Actinomycetes
(Filamentous bacteria)
Frankia Root nodules Non-leguminous angiosperms
e.g. Alnus, Casuarina.
3. Cyanobacteria
(i) Anabaena azollae – Pteridophyte : Azolla (in leaves)
(ii) A. cyacadae – Gymnosperm : Cycas (in coralloid roots)
(iii) Nostoc – Bryophyte : Anthoceros (in thallus)
Angiosperm : Gunnera : (in stem)

• Both Rhizobium & Frankia live freely in the soil but fix nitrogen only when in symbiotic
association with host plant.
• Enzyme nitrogenase catalyses the conversion of atmospheric N2 to NH3. It possess two units unit-
Ist is Mo-Fe protein & unit-IInd is Fe-S protein.
• Nitrogenase is extremely sensitive to oxygen. So to protect it from oxygen, leguminous nodules
contains an O2 scavanger called leghaemoglobin (LegHb) which combines with O2 to form
oxyleghaemoglobin (LegHbO2)
• Leghaemoglobin is pink or red in colour (Globin part synthesised by plant and haem part
synthesised by bacteria).
• Many cyanobacteria capable of fixing nitrogen are filamentous and contain thick walled cells called
heterocyst. These are the sites of nitrogen fixation.
• Heterocyst lacks oxygen evolving photosystem II thus do not evolve O2 and protect
nitrogenase.

(c) Mechanism of Biological N2 fixation :

• Nitrogenase enzyme reduces N2 by the addition of hydrogen atoms.
• The three bonds between two nitrogen atoms N  N or dinitrogen are broken step by step &
ammonia (NH3) is formed by reduction of N  N.
• N2 fixation requires 3 components :
(i) A strong reducing agent - NADPH2/FADH2/NADH2 – from photosynthesis & respiration.
(ii) ATP – from respiration. (In symbiotic fixation from the respiration of host cells).
(iii) Nitrogenase enzyme.
(iv) Genes (nod, nif, fix) - Nod gene present in both plant and bacterium while nif (nitrogenase
inducing factor) and fix present only in bacterium.
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Product
Substrate [ammonia (NH3)]
[nitrogen gas (N2)] H
H
N N
H N
N H H
H
H
H H
N Reduction H N Reduction H N Reduction H N Release
N H N H N H N of products
H H
H
Enzyme Binding of +2H +2H +2H Free nitrogenase
(nitrogenase) can bind another
molecule of N2

N  N + 8e– + 8H+ + 16 ATP ⎯→ 2NH3 + H2 + 16 ADP + 16Pi

(D) Rhizobium - Legume Symbiosis :

• Rhizobium is a free-living, gram negative non sporulating, aerobic and motile rod-shaped
bacterium. Rhizobia are more prominent in the rhizosphere of leguminous plants.
Nodule formation :
• This is a necessity during nitrogen fixation by bacterial symbiosis only.
• Nodule formation involves a sequence of multiple interaction between Rhizobium and roots of the
host plant. Principal stages in the nodule formation are summarised as follows :-
(i) Rhizobia multiply and colonise the surroundings of roots and get attached to epidermal and root
hair cells.
(ii) The root-hairs curl and the bacteria invade the root-hair.
(iii) An infection thread is produced carrying the bacteria into the cortex of the root, where they
initiate the nodule formation in the cortex and pericycle of the root.
(iv) Then the bacteria are released from the thread into the cells which leads to the differentiation of
specialised nitrogen fixing cells.
(v) The nodule thus formed, establishes a direct vascular connection with the host for exchange
of nutrients.

Cortex Cells
Plant releasing
specific chemicals
Chemotactic
Movement

Root hair

• Plant releases specific chemicals in • Bacteria attach to root hairs.

surrounding soil, which attracts rhizobium • This Attachment involves the interaction between
bacteria and bacteria start moving towards specific proteins present on root hairs and specific
roots. (Chemotactic movement) proteins & polysaccharides of bacteria.

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• The root-hairs curl and the bacteria invade the root-hair.
• An infection thread is produced carrying the bacteria into the cortex of the root.
Infection Plasma membrane of
thread root hair cell

Curled root Dissolved

Rhizobium hair Cell wall
Bacteria

Xylem
1. Release of Bacteria
from infection
thread.
2. Now bacteria are
called bacteroids
(Rod shape bacteria
surrounded by
membrane)
3. Specialized
nitrogen
fixing cells

Soil particles
Hook

Root hair
Infection
Bacteria thread
containing
Bacteria bacteria Mature nodule
Inner cortex and
pericycle cells under division

(E) Uptake and Assimilation of Nitrogen by Plants (Fate of Ammonia) :

+
• At physiological pH, the ammonia is protonated to form NH4 (ammonium) ion. While most of the
plants can assimilate nitrate as well as ammonium ions. Ammonium ions are quite toxic to plants
and hence cannot accumulate in them. Let us now see how the NH4+ is used to synthesise amino
acids in plants.
• There are two main ways in which this can take place :
(i) Reductive Amination : NH4+ reacts with –ketoglutaric acid to form an amino acid glutamic acid.
This process known as Reductive amination.
 – Ketoglutaric acid + NH4+ + NADPH + H+ Glutamate + H2O + NADP+
Glutamate
dehydrogenase
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(ii) Transamination : Transfer of amino group from one amino acid to the keto group of a keto acid is
known as transamination. This is a process of formation of other amino acids mainly from glutamic
acid and catalysed by transaminase enzyme.

H H

R1 C COO− + R1 C COO− + R2 C COO−

NH3+ O
NH3+
Amino Amino - acceptor
donor
Glutamic acid + OAA Aspartic acid + –ketoglutaric acid.
Glutamic acid + Pyruvic acid Alanine + -ketoglutaric acid.

(F) Transportation of Assimilated N2 :

• In plants transportation of assimilated N2 through xylem occurs mainly in form of amides
(Glutamine and Asparagine), especially in leguminous plants.
• Amides are more stable than amino acids and possess high nitrogen to carbon ratio (2N to 5C in
glutamine, while glutamic acid possess 1N to 5C).
• Formation of amides from amino acids by the addition of amino group, (The hydroxyl part of
acid replaced by NH2 radicle) is called Catalytic amidation.
• In addition, along with the transpiration stream the nodules of some plants (e.g., soyabean) export
the fixed nitrogen as ureides. These compounds also have a particularly high nitrogen to carbon
ratio.

8. Ammonification :
• After the death of plants and animals the protein (organic nitrogen) present in dead biomass,
degraded or decomposed by some bacteria, in the form of ammonia, the process is called
ammonification & the soil bacteria used in process are called ammonifying bacteria.
• This breakdown is both anaerobic as well as aerobic. Anaerobic breakdown of protein is called
putrefaction while aerobic breakdown is called decay.
e.g. Bacillus vulgaris, Bacillus ramosus, Bacillus mycoides.
• The ammonia produced by ammonification is divided into two fractions –
(i) Some amount volatilises
(ii) Most of the amount is converted into nitrate in soil. (Nitrification)

9. Nitrification :
• Oxidation of ammonia into nitrates by nitrifying bacteria (Chemoautotrophs) is called
nitrification. During this process energy is released with the help of which, bacteria synthesise
their own food.
Nitrosomonas / Nitrococcus
(i) 2NH3 + 3O2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ → 2NO2– + 2H2O + 2H+ + Energy
Ammonia Nitrite ion
– Nitrobacter –
(ii) 2NO2 + O2 ⎯⎯⎯⎯⎯→ 2NO3 + Energy
Nitrate ion
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➢ Nitrate reduction :
• The nitrate thus formed is absorbed by plants and transported to the leaves.
NO3– ⎯⎯⎯⎯⎯⎯⎯ – ⎯⎯⎯⎯⎯⎯⎯ → NH3
Nitrite reductase
(Cytoplasm ) → NO2
Nitrate reductase
( Plastid )

• In leaves this nitrate is reduced to form ammonia that finally forms the amine groups of amino acid.
• The process of nitrate reduction into ammonia is called assimilatory nitrate reduction. It is
catalysed by enzyme nitrate reductase and nitrite reductase.

10. Denitrification :
• Some amount of nitrate present in the soil is also reduced to nitrogen (N2) by the process of
Denitrification. NO3– ⎯→ NO2– ⎯→ N2
• The soil bacteria involved in this process are called denitrifying bacteria.
e.g. Pseudomonas and Thiobacillus.

• Plants competes with microbes for the limited nitrogen that is available in the soil, so nitrogen is
a limiting nutrient for both natural and agricultural ecosystem.
• Both Rhizobium & Frankia live freely in the soil but fix nitrogen only when in symbiotic
association with host plant.
• The nodules of some plants (e.g., soyabean-Glycine max) export the fixed nitrogen as ureides.
• Oxidation of ammonia into nitrates by nitrifying bacteria (Chemoautotrophs) is called
nitrification.
• Some amount of nitrate present in the soil is also reduced to nitrogen (N 2) by the process of
Denitrification. Ex. Pseudomonas and Thiobacillus.

BEGINNER’S BOX-2 METABOLISM OF NITROGEN

1. The bacterium _________ belonging to group Actinomycetes, produces N 2-fixing nodules on the
roots of non-leguminous plants (e.g. Alnus)
(1) Frankia (2) Rhizobium (3) Rhodospirillum (4) Azotobacter
2. The process of transfer of amino group from one amino acid to the keto group of a keto acid is
called as _________ .
(1) oxidative amination (2) reductive amination
(3) transamination (4) deamination
3. Leg-haemoglobin is required in the process of :-
(1) respiration (2) photosynthesis
(3) fatty acid synthesis (4) N2 fixation
4. Which one is the correct equation of nitrogen fixation ?
(1) N2 + 8e– + 8H+ + 8ATP → NH3 + H2 + 16ADP + 16Pi
(2) N2 + 8e– + 8H+ + 16 ATP → 2NH3 + H2 + 16ADP + 16Pi
(3) 2NH3 + 4O2 → 2H+ + 2H2O + 2NO3−
(4) 2NH3 + 3O2 → 2NO2− + 2H+ + 2N2O
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5. Nitrifying bacteria :-
(1) oxidize ammonia to nitrates
(2) convert free nitrogen to nitrogen compounds
(2) convert proteins into ammonia
(4) reduce nitrates to free nitrogen.
6. Which of the following is correct for Heterocyst ?
(1) Involved in nitrogen fixation (2) Involved in oxygen evolution
(3) Lack PSII (4) Both (1) and (3)
7. Which statement is correct for nitrogen fixing bacteria:-
(1) Frankia is filamentous bacteria which form nodules
(2) Frankia is unicellular bacteria which form nodules
(3) Frankia is filamentous bacteria which does not form nodules
(4) Frankia is filamentous bacteria which is associated with legumes.
8. Nitrogenase enzyme is composed of -
(1) Molybdenum (2) Iron (3) Sulphur (4) All of the above
9. Nitrification involves which of the following bacteria :-
(1) Chemo-autotrophs (2) Photo-autotrophs
(3) Heterotrophs (4) All of these
10. Which type of conversion is not involved in denitrification?
(1) Nitrate to nitrite (2) Nitrite to nitrogen
(3) Nitrogen to ammonia (4) Both (1) and (2)
11. During nitrification, Ammonia to nitrite conversion is done by:-
(1) Nitrococcus (2) Nitrosomonas (3) Nitrobacter (4) Both (1) and (2)
12. How many H and ATP are required for synthesis of one NH3 molecule?
+

(1) 8H+ and 4ATP (2) 4H+ and 8ATP (3) 4H+ and 4ATP (4) 8H+ and ATP

BEGINNER’S BOX ANSWERS KEY

Que. 1 2 3 4 5 6 7 8 9 10
Ans. 3 3 3 2 1 3 4 4 4 2
BEGINNER'S BOX-1
Que. 11 12 13 14 15 16 17
Ans. 4 4 4 3 3 4 2

Que. 1 2 3 4 5 6 7 8 9 10
Ans. 1 3 4 2 1 4 1 4 1 3
BEGINNER'S BOX-2
Que. 11 12
Ans. 4 2

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• Hydroponics is the technique of growing plants in a nutrient solution. This concept, for the first
time demonstrated by German Botanist Julius Von Sach.
• By hydroponics method, essential elements were identified and their deficiency symptoms
can be discovered.
• Hydroponics has been successfully employed for commercial production of tomato, seedless
cucumber and lettuce.

ELEMENTS IN PLANTS

Beneficial elements Essential elements

Na, Si, Co, Se
Macro-nutrients Micro-nutrients
Required in excess of Required in amount less than
10 m mole kg–1 of dry matter. 10 m mole kg–1 of dry matter.
C, H, O, N, P, K, S, Ca & Mg. Fe, Cu, Zn, Mn, Mo, Cl, B & Ni.

Non-minerals

Nitrogen – Absorbed in the form of NO3– (Nitrate), NO2– (Nitrite) and NH4+ (Ammonium)
(Mainly NO3–).
Phosphorus – Absorbed in the form of H2PO4– or HPO4–2. Phosphorus is a constituent of certain proteins
and all nucleic acids.
Potassium – Required in more abundant quantities in meristematic tissues. Helps in opening and
closing of stomata, and maintenance of the turgidity of cells.
Calcium – In the middle lamella as calcium pectate and formation of mitotic spindle. Involved in the
normal functioning of the cell membranes.
Magnesium– Constituent of chlorophyll structure and maintain the ribosome structure.
Sulphur – Present in amino acids – Cysteine and methionine. Constituent of thiamine, biotin,
Coenzyme A and Ferredoxin.
Iron – Absorption in the form of Fe+++. Part of ferredoxin and Cytochromes (Fe++ Fe+++). Not the
constituent of chlorophyll but essential for the formation of chlorophyll.
Manganese – Involved in splitting of water (photolysis) in photosynthesis.
Zinc – Needed in the synthesis of auxin.
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Copper – Like iron, involved in redox reaction (Cu+ Cu++) as a part of plastocyanin (PC), cytochrome a
and a3.
Boron – Absorbed as BO3–3 or B4O7–2. Required for pollen germination and carbohydrate translocation.
Molybdenum – Absorbed in the form of MoO2+2 (molybdate ion). Participate in nitrogen metabolism
as the part of enzymes. (Nitrogenase and Nitrate reductase.)
Chlorine – Involved in water splitting in photosynthesis (water splitting involved - Mn, Cl and Ca but main
role is of Mn).
• Essential Mineral element required by plants in the greatest amount – Nitrogen.
• Among micronutrients Iron is required in greatest amount.
• Actively mobile elements – N, K, Mg (Deficiency symptoms first in senescent leaves).
• Immobile Elements – Ca, S (deficiency symptoms first in young tissues)

DEFICIENCY SYMPTOMS

Chlorosis Necrosis Inhibition of cell division Delay in flowering

(loss of chlorophyll Death of tissues N, K, S, Mo. N, S, Mo.
leading to yellowing particularly leaf tissues
in leaves) Ca, Mg, Cu, K.
N, K, Mg, S, Fe,
Mn, Zn, Mo.

ABSORPTION OF MINERAL IONS

First Phase Second phase

(Passive & rapid) (Active (Mainly) or Passive & Slow)
Soil → Apoplast / Outer space of root cells Apoplast → Symplast
(Cell wall & Intercellular spaces) (from outer space of cell to cytoplasm)

Crossing the cell membrane

It can be

Passive Active (Mainly)

(According to concentration gradient) (Against the concentration gradient)
By ion-channels By Pump Protiens

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Atmospheric N2

Biological Industrial Electrical Denitrification

N2 fixation N2 fixation N2 fixation In soil
Nitrification
• Substrate – nitrogen Nitrosomonas Nitrobacter Pseudomonas, Thiobacillus
NH3 Nitrococcus NO2– NO–3
• Enzyme – nitrogenase
• Active process Soil 'N' Pool
I. Reductive amination
used of (ATP)
• Requires low Glutamic acid
concentration of (Ammonification) (Uptake and (Amino Acid)
oxygen In soil Assimilation)
II. Transamination
Glutamic Acid Other
• Decomposition of Amino Acids
organic nitrogen Decaying biomass Plant biomass
(Protein, Amides,
Amino acid)
• Carried by Bacillus Animal biomass
ramosus, Bacillus
mycoides, Bacillus The nitrogen cycle showing relationship between the
vulgaris etc. three main nitrogen pools - atmosphere, soil and biomass
NCERT XI Page No. 201, Figure, No. 12.3

Free Living N2 Fixers

• Aerobic bacteria - Azotobacter and Beijerinckia
• Anaerobic bacteria - Rhodospirillum
• Cyanobacteria - Anabaena and Nostoc

Symbiotic N2 Fixers
• Rhizobium (in alfalfa, sweet clover, sweet pea, lentils, garden pea, broad
bean, clover beans, etc.)
• Frankia (Alnus, Casuarina)

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