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MINERAL NUTRITION
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.
ESSENTIAL 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 CO2 & 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 :
The element must be absolutely necessary for supporting normal growth and reproduction. In the ab-
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(a)
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The requirement of the element must be specific and not replaceable by another element. In other
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(b)
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sence of the element the plants do not complete their life cycle or set the seeds.
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words, deficiency of any one element cannot be met by supplying some other element.
The element must be directly involved in the metabolism of the plant.
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Methods to study the mineral requirements of plants–
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In 1860 Julius Von Sachs (a prominent german botanist) demonstrated for the first time, that plants
could be grown to maturty 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.
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Funnel for
adding water
and nutrients
Cotton
Aerating tube
Nutrient solution
Nutrient solution
Pump
Diagram of a typical set-up for
nutrient solution culture
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Hydroponic plant production. Plants are grown in 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.
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. Theses
categories are :
(i)
Essential elements as components of biomolecules and hence structural elements of cells (e.g., carbon,
hydrogen, oxygen and nitrogen)
(ii)
Essential elements that are components of energy - related chemical compounds in plants
(e.g., magnesium in chlorophyll and phosphorous in ATP).
(iii)
Essential elements that activate or inhibit enzymes. (e.g., Mg, Zn, Mo etc.)
• Mg2+ is an activator for both ribulose bisphosphate carboxylaseoxygenase (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.
(iv)
Some essential elements can alter the osmotic potential of a cell. (e.g., K, Cl etc.)
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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,
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Minor element/Micro nutrients :
*
Concentration required less than 10 m mole kg–1of dry matter.
*
Fe, Cu, Zn, B, Cl, Mn, Mo, Ni
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.
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.
2.
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.
3.
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.
4.
Calcium : Plant absorbs calcium from the soil in the form of calcium ions (Ca2+). 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.
5.
Magnesium : It is absorbed by plants in the form of divalent Mg 2+. 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.
6.
Sulphur : Plants obtain sulphur in the form of sulphate (SO4–2). 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.
7.
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.
8.
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.
9.
Zinc : Plants obtain zinc as Zn2+ ions. It activates various enzymes, especially carboxylases. It is also needed
in the synthesis of auxin.
10.
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+.
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11.
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.
12.
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.
13.
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.
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.
Mobility of minerals : The parts of the plants that show the deficiency symptoms also depend on the
21
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.
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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.
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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.
•
Likewise, necrosis, or death of tissue, particularly leaf tissue, is due to the deficiency of Ca, Mg, Cu, K.
•
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
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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.
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Toxicity of micronutrients :-
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Any mineral ion (Micronutrient) concentration in plant tissue that reduce the dry weight of tissues
by about 10 percent is considerd 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.
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.
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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
21
which is both active and passive process (mainly active process). The passive movement of ions in
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symplast occurs through ion channels which are trans-membrane proteins and act as selective pores. The active
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influx and efflux from symplast occurs with the help of pump proteins and with expenditure of energy/ATP.
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Apoplast
Symplast
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Minerals
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Initial phase
(Primary step)
Movement of minerals from its high conc. to low conc. by
transport protein (Passive process)
Cell wall
Soil reservoir
Metabolic phase
(Secondary step)
Cell membrane
Root cells
Movement of minerals from its low conc. to high conc. by
pump protein (Active process)
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Methods of Mineral Absorption –
(A) Passive
(A)
(B)
Active
Passive absorption of Minerals : (Without expenditure of ATP)
(1)
By diffusion : According to this method mineral ions may diffuse into root cells from the soil solution.
(2)
By mass flow : According to this method mineral ions absorption occurs with flow of water under the
influence of transpiration pull.
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Why absorption is not completely passive ?
(1)
Minerals are present in the soil as charged particles (ions) which mostly can not move across cell membranes
passively.
(2)
The concentration of minerals in the soil is usually lower than the concentration of minerals in the roots.
(B)
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.
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.
Unloding 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
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Like this small amount of phosphorus and sulphur are also translocated in the form of organic compounds. Except
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most of it is translocated in the organic form i.e. amides and related compounds.
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this, small amount of materials are also transfered between xylem and phloem. So we cannot distinctly
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materials, as was believed before.
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differentiate that xylem only translocate inorganic nutrients and phloem translocate only organic
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Soil as reservoir of essential elements :
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.
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Majority of the nutrients that are essential for the growth and development of plants become available to the
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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
(3) Chlorine (Cl) – Anion-cation balance in the cell
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(2) Iron (Fe) – Formation of chlorophyll
(4) Sulphur (S) – Component of vitamins
SOME EXTRA POINTS
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
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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,
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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 musturd plant - Gold.
METABOLISM OF NITROGEN
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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 fullfil 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.
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Nitrogen (N2) Cycle :
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Atmospheric N2
Electrical
N2 fixation
Industrial
N2 fixation
Biological
N2 fixation
Nitrification
–
NO2
Soil 'N' Pool
NH3
(Ammonification)
Denitrification
–
NO3
(Uptake)
Plant biomass
Decaying biomass
Animal biomass
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Steps of N2 Cycle :
1. N2 fixation
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2. Uptake and assimilation of nitrogen by plants
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3. Ammonification
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4. Nitrification
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5. Denitrification
Nitrogen Fixation
*
Process of conversion of atmospheric N2 into nitrogenous compounds (NH3, NO3–) is called nitrogen fixation.
*
Nitrogen fixation is of two types :
(A) Abiological / Physicochemical
(B) Biological
Abiological Nitrogen Fixation
*
Abiological Nitrogen Fixation / Physicochemical N2 fixation is further divided into two types :
(a) Atmospheric or electrical
(a)
(b) Industrial
Atmospheric or electrical : In nature, lightening and ultraviolet radiation provide enough energy to
convert nitrogen to nitrogen oxides (NO, NO2, N2O).
(i)
D
® 2NO
N2 + O2 ¾¾
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The nitrogen cycle showing relationship between the
three main nitrogen pools - atmosphere, soil and biomass
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(ii)
2NO + O2 ¾® 2NO2
(iii)
3NO2 + H2O (rain water) ¾® 2HNO3 + NO
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4NO2 + O2 +2H2O (rain water) ¾® 4HNO3
(iv)
HNO3 + NH3 ¾® NH4NO3
2HNO3 + CaO ¾® 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.
Biological Nitrogen Fixation / Diazotrophy
*
Conversion of atmospheric nitrogen ( N2) into inorganic nitrogenous compounds like - NH3 by living organisms
is called biological nitrogen fixation or Diazotrophy. Only CERTAIN PROKARYOTIC SPECIES are capable of
fixing nitrogen.
(A)
Nitrogen Fixing organisms (Diazotrophs) –
Free living or non–symbiotic –
*
Eubacteria :– Azotobacter, Beijernickia, (both aerobic) and Rhodospirillum (anaerobic).
*
Cyanobacteria (Blue green algae) – Nostoc, Anabaena. Oscillatoria, Aulosira, Cylindrospermum etc. Oscillatoria
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and Aulosira are active nitrogen fixer in paddy field and Cylindrospermum is an active nitrogen fixer in sugarcane
and maize field.
Symbiotic –
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(B)
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(ii)
2.
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Root nodules
Host
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Rhizobium &
Bradyrhizobium
Azorhizobium
Actinomycetes
(Filamentous
bacteria) –
Frankia
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(i)
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Eubacteria –
Leguminous plants
Stem nodules
Leguminous plant – Sesbania
Root nodules
Non-leguminous angiosperms
e.g. Alnus, Casuarina.
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1.
Symbiotic
structure
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Micro-organism
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Symbiotic Diazotrophs or N2 fixers
3.
Cyanobacteria –
(i)
(ii)
Anabaena azollae
A. cyacadae
–
–
Pteriophyte : Azolla (in leaves)
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 posses 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)
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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.
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Mechanism of Biological N2 fixation :
*
Nitrogenase enzyme reduces N2 by the addition of hydrogen atoms.
*
The three bonds between two nitrogen atoms N º N or dinitrogen are broken step by step & ammonia (NH3)
is formed by reduction of N º N.
*
N2 fixation has 4 requirements :
(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 bactrium while nif (nitrogenase inducing
factor) and fix present only in bactrium.
N º N + 8e– + 8H+ + 16 ATP ¾® 2NH3 + H2 + 16 ADP + 16Pi
H
N
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N
Reduction H
N
N
H
N
H
H
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Binding of +2H
substrate
n
N
H
Reduction H
+2H
H
N
H
Reduction H
N
H
H
+2H
H
H
H
N
N
N
H
H
Release
of products
H
Free nigrogenase
can bind another
molecule of N2
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Rhizobium - Legume symbiosis –
*
Rhizobium is a free-living, gram negative non sporulating, aerobic and motile rod shaped bacterium. Rhizobia are
more prominant 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 interactions between Rhizobium and roots of the host plant.
Principle stages in the nodule formation are summarised as follows :
Stage-I : Rhizobium contact a susceptible root hair. Rhizobia multiply and colonise the surroundings of roots and
get attached to epidermal and root hair cells.
·
This attraction is chemotactic (rhicadhesin protein of bacterial cell identify host root). To attract rhizobia root of
legume releases amino acid, sugars, organic acids and flavonoids.
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Enzyme
(nitrogenase)
Product
[ammonia (NH3)]
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Substrate
[nitrogen gas (N2)]
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Stage-II : Bacteria invade the root hair (infection). Successful infection of root hair causes it to curl.
·
Curling of root hair is induced by specific complex polysaccharides found on the surface of Rhizobia, recognised
by lectins (small proteins of host plant root.)
Stage-III : An infection thread is produced carrying the bacteria into the inner cortex of the root.
Then bacteria are released from the thread into the cells. The bacteria get modified into rod shaped bacteroids
and cause inner cortical and pericycle cells to divide.
For dedifferentiation of cortical and pericycle cell PGRs (auxin and cytokinin) are required. Auxin is provided by
host root while cytokinin by bacteria.
·
Division and growth of cortical and pericycle cells lead to nodule formation.
Stage-IV : The mature nodule is formed and establishes a direct vascular connection with the host for exchange
of nutrients.
Soil particles
Hook
Root hair
Infection
thread
containing
bacteria
Bacteria
Bacteria
Mature nodule
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Inner cortex and
pericycle cells under division
:2
UPTAKE AND ASSIMILATION OF NITROGEN BY PLANTS (FATE OF AMMONIA)
Z:\NODE02\B0B0-BA\TARGET\BIO\ENG\MODULE_6\01-PLANT PHYSIOLOGY\05-MINE-E.P65
*
E
Se
ss
io
n
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 NH 4+ is used to synthesise amino acids in plants.
There are two main ways in which this can take place :
Reductive amination – NH4+ reacts with a–ketoglutaric acid to form an amino acid glutamic acid. This
process known as Reductive amination.
Glutamate
ˆˆˆˆˆˆˆˆ
†
a – Ketoglutaric acid + NH4+ + NADPH + H+ ‡ˆˆˆˆˆˆˆ
ˆ Glutamate + H2O + NADP+
dehydrogenase
*
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 + R2 – C – COO
+
NH3
Amino donor
—
O
—
R1 – C – COO + R2 – C – COO
—
O
Amino -acceptor
Glutamic acid + OAA
Glutamic acid + Pyruvic acid
Aspartic acid + a–keto glutaric acid.
Alanine + a-ketoglutaric acid
+
NH3
70
ALLEN
Pre-Medical : Biology
q
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 posses high nitrogen to carbon ratio (2N to 5C in glutamine, while
glutamic acid posses 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.
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.
21
The ammonia produced by ammonification is divided into two fractions –
Some amount volatilises
(ii)
Most of the amount is converted into nitrate in soil. (Nitrification)
0-
(i)
02
*
Oxidation of ammonia into nitrates by nitrifying bacteria (Chemoautotrophs) is called nitrification.
n
*
:2
Nitrification
ss
io
During this process energy is released with the help of which, bacteria synthesise their own food.
Nitrosomonas/Nitrococcus
2NH3 + 3O2 ¾¾¾¾¾¾¾¾¾¾¾¾
® 2NO2– + 2H2O + 2H+ + E
Ammonia
Nitrite ion
(ii)
Nitrobacter
2NO2– + O2 ¾¾¾¾¾¾
®
Se
(i)
Nitrate ion
q
Nitrate reduction :
*
The nitrate thus formed is absorbed by plants and transported to the leaves.
Nitrate reductase
Nitrite reductase
® NO–2 ¾¾¾¾¾¾¾
® NH3
NO–3 ¾¾¾¾¾¾¾
(Cytoplasm) ¾
(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.
Denitrification
*
*
Some amount of nitrate present in the soil is also reduced to nitrogen (N2) by the process of
Denitrification.
NO–3 ¾® NO–2 ¾® N2
The soil bacteria involved in this process are called denitrifying bacteria.
Ex. Pseudomonas and Thiobacillus.
Z:\NODE02\B0B0-BA\TARGET\BIO\ENG\MODULE_6\01-PLANT PHYSIOLOGY\05-MINE-E.P65
2NO3– + E
E
ALLEN
Pre-Medical : Biology
1.
The bacterium _________ belonging to group Actinomycetes, produces N2-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.
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
5.
Nitrifying bacteria :(1) oxidize ammonia to nitrates
(2) convert proteins into ammonia
21
002
:2
n
io
ss
ANSWER KEY
B EGINNE R 'S B OX-1
B EGINNE R'S BOX -2
Que.
A ns.
Que.
Ans.
(4) N2 fixation
(2) convert free nitrogen to nitrogen compounds
(4) reduce nitrates to free nitrogen.
Se
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E
71
1
3
2
3
3
3
4
2
5
1
1
1
2
3
3
4
4
2
5
1
ALLEN
Pre-Medical : Biology
Build Up Your Understanding
EXERCISE-I (Conceptual Questions)
4.
11.
work elements :-
Die back disease in citrus is due to deficiency of :(1) Mo
(2) B
(3) Cu
(4) Zn
MN0005
12.
8.
Which element is not considered as macronutrient ?
(1) Mg
(2) Ca
(3) Mn
(4) P
MN0008
Which element essential for stability of chromosome
(1) Zn
(2) Ca
(3) Mo
(4) Fe
Which element is required in comparatively less
(1) Zn
(2) N
(3) P
(4) Ca
MN0014
13.
Which mineral nutrients are called critical element
for crops ?
14.
(1) N, P, K
(2) C, H, O
(3) N, S, Mg
(4) K, Ca, Fe
MN0016
Which is most common free ion present in a cell ?
(1) P
(2) K
(3) Fe
(4) B
MN0017
15.
Which element related with Khaira disease of Paddy
& auxin synthesis ?
(1) Fe
(2) Zn
(3) B
(4) Cu
MN0020
16.
The element which can not be placed alongwith
Plants absorb nitrogen mainly in the form of :–
(1) NO2–
(2) NO3–
(3) N º N
(4) HNO2
MN0021
micronutrients :(1) Mn
(2) Mo
(3) Cu
(4) Ca
17.
MN0009
9.
(4) Zn, Mn, Cu
quantity for the growth of plant ?
n
7.
(3) Mg, Fe, Zn
MN0012
io
ss
Protoplasmic elements are :(1) C, H, O, P, N, S
(2) C, H,O, Fe, N
(3) N, S, Fe, P, K
(4) Fe, Mg, Ca, N, P
MN0007
Se
6.
(2) C, H, O
structure ?
For chlorophyll formation a plant needs :(1) Fe, Ca & light
(2) Fe, Mg & Light
(3) Ca, K & light
(4) Mn & Cu
MN0003
Brown heart rot of beets is due to deficiency of(1) B
(2) P
(3) Mg
(4) Mo
MN0004
(1) N, S, P
MN0011
:2
5.
Hydroponics is a technique in which plants are
grown in :(1) Green house
(2) Water saturated sand
(3) Balanced nutrient solution
(4) Purified distilled water
MN0002
The group of mineral nutrients known as frame
21
3.
10.
0-
2.
Which of the following group of elements is not
essential for a normal plant ?
(1) K, Ca, Mg
(2) Fe, Zn, Mn, B
(3) Pb, I, Na
(4) Mg, Fe, Mo
MN0001
02
1.
The amino acid having S in its composition is(1) Methionine
(2) Cysteine
(3) Both (1) & (2)
(4) Glutamic acid
MN0010
The most abundant element present in the plants
is –
(1) Iron
(2) Carbon
(3) Nitrogen
(4) Manganese
MN0022
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72
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ALLEN
Pre-Medical : Biology
Which is essential for nitrogen metabolism ?
(1) B
(2) Mo
(3) Cu
(4) Mg
MN0023
19.
The major portion of the dry weight of plants
comprises of :(1) Nitrogen, phosphorus and potassium
(2) Calcium, magnesium and sulphur
(3) Carbon, nitrogen and hydrogen
(4) Carbon, hydrogen and oxygen
MN0024
20.
Gray spots of Oat are caused by deficiency of :
(1) Cu
(2) Zn
(3) Mn
(4) Fe
MN0025
E
io
ss
Se
Z:\NODE02\B0B0-BA\TARGET\BIO\ENG\MODULE_6\01-PLANT PHYSIOLOGY\06-MINERAL NUTRITION_EXERCISE.P65
n
:2
02
0-
21
18.
73
ANSWER KEY
EXERCISE-I (Conceptual Questions)
Que.
A ns.
Que.
A ns.
1
3
2
3
3
2
4
1
5
3
16
2
17
2
18
2
19
4
20
3
6
1
7
3
8
4
9
3
10
2
11
2
12
1
13
1
14
2
15
2
ALLEN
Pre-Medical : Biology
AIPMT/NEET
EXERCISE-II (Previous Year Questions)
AIPMT 2007
3.
About 98 percent of the mass of every living
organism is composed of just six elements including
carbon, hydrogen, nitrogen, oxygen and :
(1) Calcium and phosphorus
(2) Phosphorus and sulphur
(3) Sulphur and magnesium
(4) Magnesium and sodium
MN0028
Which one of the following elements is not an
essential micronutrient for plant growth ?
(1) Ca
(2) Mn
(3 Zn
(4) Cu
Which one of the following is not a micronutrient?
(1) Molybdenum
(2) Magnesium
(3) Zinc
(4) Boron
MN0034
10.
One of the free-living, anaerobic nitrogen - fixer is :
(1) Beijernickia
(2) Rhodospirillum
(3) Rhizobium
(4) Azotobacter
MN0035
11.
The common nitrogen - fixer in paddy fields is :
(1) Rhizobium
(2) Azospirillum
(3) Oscillatoria
(4) Frankia
MN0036
AIPMT-Mains 2010
12.
Study the cycle shown below and select the option
which gives correct words for the all the four blanks
A, B, C and D.
Atmospheric N2
io
n
4.
Which of the following is a flowering plant with
nodules containing filamentous nitrogen-fixing
microorganism ?
(1) Cicer arietinum
(2) Casuarina equisetifolia
(3) Crotalaria juncea
(4) Cycas revoluta
MN0027
9.
:2
2.
An element playing important role in nitrogen
fixation is :
(1) Molybdenum
(2) Copper
(3) Manganese
(4) Zinc
MN0033
21
MN0026
8.
0-
A plant requires magnesium for :
(1) Cell wall development
(2) Holding cells together
(3) Protein synthesis
(4) Chlorophyll synthesis
02
1.
AIPMT-Pre 2010
AIPMT 2008
5.
6.
7.
Se
ss
MN0029
Biological
N2 fixation
Nitrogen flxation in root nodules of Alnus is brought
about by :(1) Frankia
(2) Azorhizobium
(3) Bradyrhizobium
(4) Clostridium
MN0030
AIPMT 2009
Manganese is required in :(1) Plant cell wall formation
(2) Photolysis of water during photosynthesis
(3) Chlorophyll synthesis
(4) Nucleic acid synthesis
MN0031
Which of the following is a symbiotic nitrogen fixer?
(1) Azotobacter
(2) Frankia
(3) Azolla
(4) Glomus
MN0032
Industrial
N2 fixation
NH3
A
Electrical
N2 fixation
–
NO2
Soil 'N' Pool
B
NO3
–
(Uptake)
C
Decaying biomass
D
A
B
C
D
(1)
Denitrification
(2)
Nitrification
Ammonification
Plants
Animals
Denitrification
Animals
(3)
Plants
Denitrification
Nitrification
Plants
Animals
(4)
Nitrification
Ammonification
Animals
Plants
MN0037
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74
E
ALLEN
13.
Pre-Medical : Biology
Leguminous plants are able to fix atmospheric
AIPMT-Mains 2011
nitrogen through the process of symbiotic nitrogen
fixation. Which one of the following statements is not
19.
Which one of the following is not an essential mineral
element for plants while the remaining three are ?
(1) Phosphorus
(2) Iron
(3) Manganese
(4) Cadmium
MN0044
20.
Which one of the following is essential for photolysis
of water ?
(1) Boron
(2) Manganese
(3) Zinc
(4) Copper
MN0045
correct during this process of nitrogen fixation ?
(1) Nodules act as sites for nitrogen fixation
(2) Th e en zyme nit roge nase catalyses t he
converison of atmospheric N2 to NH3
(3) Nitrogenase is insensitive to oxygen
(4) Leghaemoglobin scavenges oxygen and is
pinkish in colour
MN0038
AIPMT-Pre 2011
14.
A prokaryotic autotrophic nitrogen fixing symbiont
is found in :(1) Alnus
(2) Cycas
(3) Cicer
AIPMT-Pre 2012
21.
Best defined function of Manganese in green plants
is :(1) Nitrogen fixation
(2) Water absorption
(3) Photolysis of water
(4) Calvin cycle
MN0046
22.
Which one of the following is wrong statement ?
(1) Phosphorus is a constituent of cell membranes,
certain nucleic acids and all proteins
(2)Nit roso monas an d Nitro bact er are
chemoautotrophs
(3) Anabaena and Nostoc are capable of fixing
nitrogen in free-living state also
(4) Root nodule forming nitrogen fixers live as
aerobes under free-living conditions
MN0047
(4) Pisum
Which one of the following elements in plants is not
remobilised ?
(1) Phosphorus
(2) Calcium
MN0040
:2
An organism used as a biofertilizer for raising
soyabean crop is :(1) Azotobacter
E
(4) Nostoc
io
ss
(2) Azospirillum
(3) Rhizobium
Se
Z:\NODE02\B0B0-BA\TARGET\BIO\ENG\MODULE_6\01-PLANT PHYSIOLOGY\06-MINERAL NUTRITION_EXERCISE.P65
n
16.
0-
(4) Sulphur
02
(3) Potassium
21
MN0039
15.
AIPMT-Mains 2012
MN0041
17.
Nitrifying bacteria :(1) Oxidize ammonia to nitrates
(2) Convert free nitrogen to nitrogen compounds
23.
(3) Convert proteins into ammonia
(4) Reduce nitrates to free nitrogen
For its action, nitrogenase requires :(1) Mn2+
(2) Super oxygen redicals
(3) High input of energy
(4) Light
MN0048
MN0042
18.
The function of leghaemoglobin in the root nodules
of legumes is :(1) Inhibition of nitrogenase activity
(2) Oxygen removal
(3) Nodule differentiation
(4) Expression of nif gene
MN0043
75
NEET-UG 2013
24.
The first stable product of fixation of atmospheric
nitrogen in leguminous plants is :
(1) Glutamate
(2) NO2–
(3) Ammonia
(4) NO3–
MN0049
ALLEN
Pre-Medical : Biology
AIPMT 2014
25.
Deficiency symptoms of nitrogen and potassium are
visible first in :
(1) Senescent leaves
(2) Young leaves
(3) Roots
(4) Buds
MN0050
30.
NEET(UG) 2017
Select the mismatch :
(1) Rhodospirillum - Mycorrhiza
(2) Anabaena - Nitrogen fixer
(3) Rhizobium - Alfalfa
(4) Frankia - Alnus
MN0059
AIPMT 2015
Re-AIPMT 2015
In which of the following forms is iron absorbed by
plants ?
(1) Ferric
(2) Ferrous
(3) Free element
(4) Both ferric and ferrous
MN0060
32.
Which of the following elements is responsible for
maintaining turgor in cells ?
(1) Magnesium
(2) Sodium
(3) Potassium
(4) Calcium
MN0061
During biological nitrogen fixation, inactivation of
nitrogenase by oxygen poisoning is prevented by:
(1) Cytochrome
(2) Leghaemoglobin
(3) Xanthophyll
(4) Carotene
MN0052
The oxygen evolved during photosynthesis comes
from water molecules. Which one of the following
pairs of elements is involved in this reaction?
(1) Magnesium and Chlorine
(2) Manganese and Chlorine
(3) Manganese and Potassium
(4) Magnesium and Molybdenum
MN0053
33.
NEET(UG) 2019
Thiobacillus is a group of bacteria helpful in carrying
out :
io
n
:2
28.
31.
21
27.
NEET(UG) 2018
0-
Minerals known to be required in large amounts for
plant growth include :(1) calcium, magnesium, manganese, copper
(2) potassium, phosphorus, selenium, boron
(3) magnesium, sulphur, iron, zinc
(4) phosphorus, potassium, sulphur, calcium
MN0051
02
26.
(1) Nitrogen fixation
(2) Chemoautotrophic fixation
ss
(3) Nitrification
Se
(4) Denitrification
MN0122
NEET(UG) 2019 (Odisha)
NEET-II 2016
29.
Which is essential for the growth of root tip ?
(1) Ca
(2) Mn
(3) Zn
(4) Fe
34.
MN0054
Which of the following bacteria reduce nitrate in soil
into nitrogen ?
(1) Nitrobacter
(2) Nitrococcus
(3) Thiobacillus
(4) Nitrosomonas
MN0123
ANSWER KEY
EXERCISE-II (Previous Year Questions)
Que.
Ans.
Que.
Ans.
Que.
Ans.
1
4
2
2
3
1
4
1
5
1
6
2
7
2
8
1
9
2
10
2
11
3
12
1
13
3
14
2
15
2
16
3
17
1
18
2
19
4
20
2
21
3
22
1
23
3
24
3
25
1
26
4
27
2
28
2
29
1
30
1
31
1
32
3
33
4
34
3
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76
E
ALLEN
Pre-Medical : Biology
Master Your Understanding
EXERCISE-III (Analytical Questions)
EXERCISE-III(A) (NCERT BASED QUESTIONS)
1.
Which element is an activator of enzyme used in
nitrogen fixation ?
(1) Mg
(2) Mo
(3) Zn
(4) Cu
MN0068
2.
Microbe producing nitrogen fixing nodule on the
roots of non-leguminous plant (eg. Alnus) is –
(1) Rhizobium
(3) Rhodospirillum
(2) Azotobacter
(4) Frankia
8.
Which of the following is not an anaerobic N2 fixing
organism ?
(1) Azotobacter
(2) Clostridium
(3) Rhodospirillium
(4) Rhodopseudomonas
MN0078
9.
Passive absorption of ions usually occurs through:(1) Pumps
(2) Phospholipids
(3) Ion channels
(4) All the above
MN0079
MN0070
In the following which is not a function of calcium?
(1) Synthesis of cell wall
(2) Formation of mitotic spindle
(3) Formation of chlorophyll
(4) Normal functioning of cell membrane
MN0071
10.
Which of following mineral element is needed during
the formation of mitotic spindle, accumulates in
older leaves & is involved in normal functioning
of cell membrane ?
(1) Fe
(2) S
(3) Ca
(4) P
MN0080
4.
Symptoms of manganese toxicity may actually be
the deficiency symptoms of :(1) Iron
(2) Magnesium
(3) Calcium
(4) All the above
MN0072
11.
Which mineral element is needed for synthesis of
Auxin?
(1) B
(2) Zn
(3) Cu
(4) Cl
MN0081
5.
Which of the following is/are free living in soil, but
as symbionts, can fix atmospheric nitrogen ?
(1) Rhizobium
(2) Frankia
(3) Both (1) and (2)
(4) Azotobacter
MN0073
02
0-
21
3.
Which mineral element is required for uptake &
utilisation of Ca+2, membrane functioning & pollen
germination ?
(1) B
(2) Zn
(3) Cu
(4) Fe
MN0082
13.
Which process results in decrease of nitrogen content
in soil ?
io
n
:2
12.
ss
Se
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E
77
6.
7.
(1) Ammonification
Which element delay flowering if its concentration
in plants is low ?
(1) Nitrogen
(2) Sulphur
(3) Molybdenum
(4) All the above
MN0074
Which of the following group is associated with
beneficial elements ?
(1) Na, K, Cl, Ca
(2) Na, Se, Si Ca
(3) Na, Se, Si, Co
(4) N, Se, Si, Co
MN0075
(2) Biological nitrogen fixation
(3) Denitrification
(4) Nitrification
MN0083
14.
Any mineral ion concentration in tissues that reduces
the dry weight of tissues by about 10 percent is
considered as :(1) Critical concentration
(2) Toxic concentration
(3) Beneficial concentration
(4) Transition concentration
MN0084
ALLEN
Pre-Medical : Biology
19.
Molybdenum is essential :(1) For RuBisCO of cyanobacteria
(2) For nitrogenase enzyme
(3) For RuBisCO of eukaryotic organism
(4) For transaminase activity
MN0089
Which of the following criteria not exhibits
(i)
Manganese
(a)
Little leaf disease
(ii)
Zinc
(b)
Stout axis
(iii)
Boron
(c)
Splitting of water
(iv)
Molybdenum
(d)
Nitrogen metabolism
(1)
(2)
(3)
(4)
MN0093
21.
Toxicity of (A) affects the activity and uptake of
(B) .
(1) A = Mg, B = Fe, Ca and Zn
(2) A = Mn, B = Mg, Ca and Zn
(3) A = Mn, B = Mg and Fe
(4) A = Mg, B = Mn, Fe and Ca
MN0094
22.
n
essentiality of any mineral to plants ?
i-c, ii-b, iii-a, iv-d
i-c, ii-a, iii-b, iv-d
i-b, ii-a, iii-d, iv-c
i-c, ii-b, iii-d, iv-a
Glutamate dehydrogenase is an important enzyme
involved in :(1) Krebs cycle
(2) amino acid biosynthesis
(3) nitrogen fixation
(4) nitrate reduction
MN0095
Se
ss
io
(1) Never substituted by any other mineral
(2) Must be directly involved in metabolism of the
plant
(3) Plants do not complete their life cycle properly
in the absence of this mineral
(4) Mineral must be present for activity of all
enzymes
MN0091
EXERCISE-III (Analytical Questions)
Que.
Ans.
Que.
Ans.
ANSWER KEY
1
2
2
4
3
3
4
4
5
3
6
4
7
3
16
4
17
3
18
2
19
4
20
2
21
3
22
2
8
1
9
3
10
3
11
2
12
1
13
3
14
2
15
2
Z:\NODE02\B0B0-BA\TARGET\BIO\ENG\MODULE_6\01-PLANT PHYSIOLOGY\06-MINERAL NUTRITION_EXERCISE.P65
18.
Mineral absorption is :
(1) Mostly passive with water absorption
(2) Mostly passive without water absorption
(3) Mostly active
(4) Always active
MN0088
Identify the correct match :
:2
17.
20.
21
The enzyme, nitrogenase which is capable of
nitrogen reduction, is present :(1) only in some eukaryotes
(2) exclusively in prokaryotes
(3) exclusively in eukaryotes
(4) in all the living organisms
MN0085
16. Rhizobium has symbiotic relationship with the roots
of :(1) Garden pea
(2) Alfalfa
(3) Sweet clover
(4) All of the above
MN0086
EXERCISE-III(B) (ANALYTICAL QUESTIONS)
0-
15.
02
78
E