CBSE Class 11 Biology VBQs Mineral Nutrition

Ques. Match the following concerning essential elements and their functions in plants.
  Column-I                    Column-II
(A) Iron                 (i) Photolysis of water
(B) Zinc                (ii) Pollen germination
(C) Boron             (iii) Required for chlorophyll biosynthesis
(D) Manganese   (iv) IAA biosynthesis
Select the correct option.
(A) (B) (C) (D)
(a) (ii) (i) (iv) (iii)
(b) (iv) (iii) (ii) (i)
(c) (iii) (iv) (ii) (i)
(d) (iv) (i) (ii) (iii) 

Answer: C

Ques. Which of the following elements is responsible for maintaining turgor in cells?
(a) Magnesium
(b) Sodium
(c) Potassium
(d) Calcium 

Answer: C

Ques. In which of the following forms is iron absorbed by plants?
(a) Ferric
(b) Ferrous
(c) Free element
(d) Both ferric and ferrous 

Answer: A

Ques. Which is essential for the growth of root tip?
(a) Zn
(b) Fe
(c) Ca
(d) Mn 

Answer: C

Ques. The oxygen evolved during photosynthesis, comes from water molecules. Which one of the following pairs of elements is involved in this reaction?
(a) Magnesium and Molybdenum
(b) Magnesium and Chlorine
(c) Manganese and Chlorine
(d) Manganese and Potassium 

Answer: C

Ques. Minerals known to be required in large amounts for plant growth include
(a) potassium, phosphorus, selenium, boron
(b) magnesium, sulphur, iron, zinc
(c) phosphorus, potassium, sulphur, calcium
(d) calcium, magnesium, manganese, copper.

Answer: C

Ques. Deficiency symptoms of nitrogen and potassium are visible first in
(a) senescent leaves
(b) young leaves
(c) roots
(d) buds. 

Answer: A

Ques. A few normal seedlings of tomato were kept in a dark room. After a few days they were found to have become white-coloured like albinos. Which of the following terms will you use to describe them?
(a) Mutated
(b) Embolised
(c) Etiolated
(d) Defoliated 

Answer: C

Ques. Which of the following elements is a constituent of biotin?
(a) Magnesium
(b) Calcium
(c) Phosphorus
(d) Sulphur 

Answer: D

Ques. Best defined function of manganese in green plants is
(a) photolysis of water
(b) Calvin cycle
(c) nitrogen fixation
(d) water absorption. 

Answer: A

Ques. Which one of the following elements in plants is not remobilised?
(a) Phosphorus
(b) Calcium
(c) Potassium
(d) Sulphur 

Answer: B

Ques. Which one of the following is not an essential mineral element for plants while the remaining three are?
(a) Iron
(b) Manganese
(c) Cadmium
(d) Phosphorus

Answer: C

Ques. An element playing important role in nitrogen fixation is
(a) molybdenum
(b) copper
(c) manganese
(d) zinc. 

Answer: A

Ques. Manganese is required in
(a) plant cell wall formation
(b) photolysis of water during photosynthesis
(c) chlorophyll synthesis
(d) nucleic acid synthesis. 

Answer: B

Ques. Which one of the following elements is not an essential micronutrient for plant growth?
(a) Zn
(b) Cu
(c) Ca
(d) Mn 

Answer: C

Ques. A plant requires magnesium for
(a) protein synthesis
(b) chlorophyll synthesis
(c) cell wall development
(d) holding cells together.

Answer: B

Question. Although carnivorous plants contain chlorophyll than why do they eat insects?.
Answer: Carnivorous plants contain chlorophyll and carry out photosynthesis but they eat insects to full fill the need of nitrogen.

Question. A farmer grows some leguminous crop after the main crop, even though he is not interested to get the seeds of legume. Then why is he doing so?
Answer: He is doing so because the roots of legumes contain the bacteria rhizobium which will fix the nitrogen from the atmosphere and improve the fertility of soil.

1. IN 1860, JULIUS VON SACHS (GERMAN BOTANIST) → Demonstrated, for the first time, that plants could be grown to maturity in a defined nutrient solution in complete absence of soil.

2. THIS TECHNIQUE OF GROWING PLANTS IN A NUTRIENT SOLUTION IS KNOWN AS HYDROPONICS.

3. Through hydroponics technique, impact of a specific minerals can be studied by adding/substituting or removing that mineral.

4. By this method, essential elements were identified and their deficiency symptoms discovered.

5. Hydroponics technique has been successfully used in commercial production of vegetables such as tomato, seedless cucumber and lettuce.

ESSENTIAL MINERAL ELEMENTS

6. Plants obtain their inorganic nutrients from air, water and soil. Plants absorb a wide variety of mineral elements.

7. Not all the mineral elements that they absorb are required by plants.

8. Out of the more than 105 elements discovered so far, less than 21 are essential and beneficial for normal plant growth and development.

9. CRITERIA FOR ESSENTIALITY:
o 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.
o The requirement of the element must be specific and not replaceable by another element means no other element can fulfil the requirement of that mineral.
o The element must be directly involved in the metabolism of the plant.

10. VERY FEW MINERALS FULFIL THE CRITERIA OF ESSENTIALITY. These minerals again divided in two groups according to their quantitative requirement.
o (I) MACRONUTRIENTS, AND
o (II) MICRONUTRIENTS

11. MACRONUTRIENTS
o Macronutrients are generally present in plant tissues in large amounts (in excess of 10 mmole Kg –1 of dry matter).
o THE MACRONUTRIENTS include
o Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorous, Sulphur, Potassium, Calcium And Magnesium.
o Of these, carbon, hydrogen and oxygen are mainly obtained from CO₂ and H₂O, while the others are absorbed from the soil as mineral nutrition.

12. NITROGEN:
o Nitrogen is required by plants in the greatest amount.
o It is absorbed mainly as NO₃^– (some are also taken up as NO₂^– or NH₄⁺)
o Nitrogen is required by all parts of a plant, particularly the meristematic tissues and the metabolically active cells.
o It is one of the major constituents of proteins, nucleic acids, vitamins and hormones.

13. PHOSPHORUS
o It is absorbed by the plants from soil in the form of phosphate ions.
o Phosphorus is a constituent of cell membranes, certain proteins, all nucleic acids and nucleotides, and is required for all phosphorylation reactions.

14. POTASSIUM:
o It is absorbed as potassium ion (K+).
o It is required in more abundant quantities in the meristematic tissues, buds, leaves and root tips.
o 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.

15. CALCIUM:
o Plant absorbs calcium from the soil in the form of calcium ions (Ca₂⁺).
o Calcium is required by MERISTEMATIC AND DIFFERENTIATING TISSUES.
o During cell division it is used in the synthesis of cell wall, particularly as calcium pectate in the middle lamella.
o It is also needed during the formation of mitotic spindle.
o It accumulates in older leaves.
o It is involved in the normal functioning of the cell membranes.

16. MAGNESIUM:
o It is absorbed by plants in the form of divalent Mg2+.
o It activates the enzymes of respiration, photosynthesis
o Involved in the synthesis of DNA and RNA.
o Magnesium is a constituent of the ring structure of chlorophyll
o helps to maintain the ribosome structure.

17. SULPHUR:
o Plants obtain sulphur in the form of sulphate .
o Sulphur is present in two amino acids –
• cysteine and
• methionine
o Main constituent of several coenzymes, vitamins (thiamine, biotin, Coenzyme
A) and ferredoxin.

MICRONUTRIENTS

18. MICRONUTRIENTS OR TRACE ELEMENTS, are needed in very small amounts (less than 10 mmole Kg ^–1 of dry matter).

19. These include Iron, Manganese, Copper, Molybdenum, Zinc, Boron, Chlorine and Nickel.

20. IRON:
o Plants obtain iron in the form of ferric ions (Fe³⁺)
o It is required in larger amounts in comparison to other micronutrients.
o It is an important constituent of proteins involved in the transfer of electrons like ferredoxin and cytochromes.
o It is reversibly oxidised from Fe²⁺ to Fe³+ during electron transfer.
o It activates catalase enzyme and is essential for the formation of chlorophyll.

21. MANGANESE:
o It is absorbed in the form of manganous ions (Mn²⁺).
o It activates many enzymes involved in photosynthesis, respiration and nitrogen metabolism.
o It is required in the splitting of water to liberate oxygen during photosynthesis.

22. ZINC:
o Plants obtain zinc as Zn2+ ions.
o Activates carboxylases enzyme.
o Needed in the synthesis of auxin.

23. COPPER:
o It is absorbed as cupric ions (Cu₂⁺).
o It is associated with certain enzymes involved in redox reactions and is reversibly oxidised from Cu⁺ to Cu₂⁺.

24. BORON:
o It is absorbed as BO₃³⁻ or B₄O₇^{2− .}
o Boron is required for uptake and utilisation of Ca2+ as well as membrane functioning, pollen germination, cell elongation, cell differentiation and carbohydrate translocation.

25. MOLYBDENUM:
o Plants obtain it in the form of molybdate ions.
o It is a component of nitrogenase and nitrate reductase both of which participate in nitrogen metabolism.

26. CHLORINE:
• It is absorbed in the form of chloride anion (Cl^–).
• Na+ and K+ & chlorine help 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.

27. There are some beneficial elements such as sodium, silicon, cobalt and selenium.
They are required by higher plants.

28. DEFICIENCY SYMPTOMS OF ESSENTIAL ELEMENTS:

✓ critical concentration: The concentration of the essential element below which plant growth is retarded.

✓ The element is said to be deficient when present below the critical concentration.

✓ The morphological changes are indicative of certain element deficiencies and are called deficiency symptoms.

✓ The deficiency symptoms disappear when the deficient mineral nutrient is provided to the plant.

✓ The deficiency symptoms appear first in the older tissues because of remobilization of element to younger parts from older part.

✓ For example, the deficiency symptoms of nitrogen, potassium and magnesium 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.

✓ When the elements are relatively immobile and part of structural component of cell then they are not transported out of the mature organs, in this condition the deficiency symptoms tend to appear first in the young tissues for example, element 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 as farmers can not delay the supply of structural element in case of deficiency of these elements in soil otherwise the growth will be affected.

✓ Chlorosis is the loss of chlorophyll leading to yellowing in leaves and it is caused by the deficiency of elements N, K, Mg, S, Fe, Mn, Zn and Mo.
✓ 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 flowering if there concentration in plants is low.

TOXICITY OF MICRONUTRIENTS 
✓ Any mineral ion concentration in tissues that reduces the dry weight of tissues by about 10 per cent is considered toxic.
✓ Toxicity concentration varies for different micronutrients.
✓ Many a times excess of an element may inhibit the uptake of another element.
✓ For example, the prominent symptom of manganese toxicity is the appearance of brown spots surrounded by chlorotic veins.
✓ Manganese competes with iron and magnesium for uptake
✓ Manganese also competes with magnesium for binding with enzymes.
✓ Manganese also inhibit calcium translocation in shoot apex.
✓ Therefore, excess of manganese may induce deficiencies symptoms of iron, magnesium and calcium(even they are present in soil in sufficient amount).
✓ Thus, what appears as symptoms of manganese toxicity may actually be the deficiency symptoms of iron, magnesium and calcium.
On the basis of their diverse functions essential elements can also be grouped into four broad categories. These categories are:

✓ STRUCTURAL ELEMENTS OF CELLS:
o Essential elements as components of biomolecules and hence (e.g., carbon, hydrogen, oxygen and nitrogen).

✓ COMPONENTS OF ENERGY-RELATED CHEMICAL COMPOUNDS:
o e.g., magnesium in chlorophyll and phosphorous in ATP.

✓ ESSENTIAL ELEMENTS THAT ACTIVATE OR INHIBIT ENZYMES:
o for example, Mg2+ is an activator for both ribulose bisphosphate carboxylase oxygenase and phosphoenol pyruvate carboxylase, both of which are critical enzymes in photosynthetic carbon fixation.
o Zn₂⁺ is an activator of alcohol dehydrogenase.
o Mo of nitrogenase during nitrogen metabolism.

✓ ESSENTIAL ELEMENTS WHICH ALTER THE OSMOTIC POTENTIAL OF A CELL:
o Sodium and chloride ions and potassium.
o Potassium plays an important role in the opening and closing of stomata.

MECHANISM OF ABSORPTION OF ELEMENTS
✓ The process of absorption → two main phases.
✓ In the first phase → an initial rapid uptake of ions into the ‘free space’ or ‘outer space’ of cells – the APOPLAST, is passive.
o The passive movement of ions into the apoplast usually occurs through ion-channels, the trans-membrane proteins that function as selective pores.
✓ In the second phase of uptake → the ions are taken in slowly into the inner space which is called THE SYMPLAST of the cells.
o The entry or exit of ions to and from the symplast requires the expenditure of metabolic energy, which is an ACTIVE PROCESS.
o The movement of ions is usually called flux; the inward movement into the cells is influx and the outward movement, efflux.

NITROGEN METABOLISM
✓ Plants compete with microbes for the limited nitrogen that is available in soil.
✓ Nitrogen is a limiting nutrient for both natural and agricultural eco-systems.
✓ Abundant nitrogen (79%) is present in atmosphere but plant absorb it in nitrate form but some are also taken up it as NO₂^– or NH₄⁺.
✓ In nature, lightning and ultraviolet radiation provide enough energy to convert nitrogen to nitrogen oxides (NO, NO₂, N₂O).
✓ Industrial combustions, forest fires, automobile exhausts and powergenerating stations are also sources of atmospheric nitrogen oxides.
✓ The process of conversion of nitrogen (N₂) to ammonia is termed as nitrogen fixation.
✓ This conversion of ammonia is done by biological fixation and industrial N₂ fixation.
✓ Decomposition of organic nitrogen of dead plants and animals into ammonia is called AMMONIFICATION.
✓ Some of this ammonia volatilises and re-enters the atmosphere but most of it is converted into nitrate by soil bacteria.
✓ Conversion of ammonia into nitrate is called NITRIFICATION.
✓ Nitrification is completed in two steps.
o First, ammonia is oxidized into nitrites by NITROSOMONAS and/or

NITROCOCCUS bacteria.
o Second, nitrite is oxidized into nitrate by NITROBACTAR bacteria.
✓ THESE NITRIFYING BACTERIA ARE CHEMOAUTOTROPHS.
✓ Then nitrate is absorbed by plants and is transported to the leaves.
✓ In leaves, it is reduced to form ammonia that finally forms the amine group of amino acids.

✓ DENITRIFICATION:
o Nitrate present in the soil is also reduced to nitrogen by the process of denitrification.
o Denitrification is carried by bacteria Pseudomonas and Thiobacillus.

✓ BIOLOGICAL NITROGEN FIXATION:
o REDUCTION OF NITROGEN TO AMMONIA BY LIVING ORGANISMS IS CALLED BIOLOGICAL NITROGEN FIXATION.
o The enzyme, nitrogenase which is capable of nitrogen reduction is present exclusively in prokaryotes.
o Biological fixation is property of prokaryotic. Such microbes are called N₂^- fixers.
o The nitrogen-fixing microbes could be free-living or symbiotic.
o Free-living nitrogen-fixing aerobic microbes are Azotobacter and Beijernickia.
o Rhodospirillum is anaerobic and Bacillus free-living.
o In addition, a number of cyanobacteria such as Anabaena and Nostoc are also free-living nitrogen-fixers.

SYMBIOTIC BIOLOGICAL NITROGEN FIXATION RHIZOBIUM AND LEGUMES SYMBIOTIC RELATION:
✓ Species of rod-shaped Rhizobium has such relationship with the roots of several legumes such as alfalfa, sweet clover, sweet pea, lentils, garden pea, broad bean, clover beans, etc.
✓ These nodules are small outgrowths on the roots.
✓ Rhizobium and Frankia are free-living in soil, but as symbionts, can fix atmospheric nitrogen.
✓ FRANKIA PRODUCES ROOT NODULE IN NON-LEGUMINOUS PLANT.
✓ Nodule Formation: stages in the nodule formation:
o Rhizobia multiply and colonise the surroundings of roots and get attached to epidermal and root hair cells.
o The root-hairs curl and the bacteria invade the root-hair.
o An infection thread is produced carrying the bacteria into the cortex of the root, where they initiate the nodule formation in the cortex of the root.
o Then the bacteria are released from the thread into the cells which
leads to the differentiation of specialised nitrogen fixing cells.
o The nodule establishes a direct vascular connection with the host for
exchange of nutrients.
o The nodule contains enzyme nitrogenase and leghaemoglobin.

o THE ENZYME NITROGENASE IS A MO-FE PROTEIN AND CATALYSES THE CONVERSION OF ATMOSPHERIC NITROGEN TO AMMONIA (THE FIRST STABLE PRODUCT OF NITROGEN FIXATION).
o The enzyme nitrogenase is highly sensitive to the molecular oxygen and it requires anaerobic conditions.
o To protect these enzymes, the nodule contains an oxygen scavenger called leg-haemoglobin.
o Rhizobium and Frankia microbes live as aerobes under free-living conditions but during nitrogen-fixing events (in symbiotic association), they become anaerobic to protect the nitrogenase enzyme.
o The energy require in nitrogen fixation by symbiotic bacteria is fulfilled by host plant.

o 8 ATP is required to produce one ammonia molecule.
o At physiological pH, the ammonia is protonated to form NH₄⁺ (ammonium) ion.
o Most of the plants can assimilate nitrate as well as ammonium ions.
o Ammonium ions are toxic to plants and cannot accumulate in plant so the NH₄⁺ is used to synthesise amino acids in plants.

✓ THERE ARE TWO MAIN WAYS IN WHICH THIS CAN TAKE PLACE.
o REDUCTIVE AMINATION: In these processes, ammonia reacts with α- ketoglutaric acid and forms glutamic acid and the enzyme for this
reaction is GLUTAMATE DEHYDROGENASE.

o TRANSAMINATION: It involves the transfer of amino group from one amino acid to the keto group of a keto acid. Glutamic acid is the main amino acid from which the transfer of NH2, the amino group takes place and other amino acids are formed through transamination. The enzyme transaminase catalyses all such reactions.
o The two most important amides – asparagine and glutamine – found
in plants are a structural part of proteins.
o They are formed from two amino acids, namely aspartic acid and glutamic acid, respectively, by addition of another amino group to each.
o in this reaction, the hydroxyl part of the acid is replaced by another NH₂ radicle.
o Amides contain more nitrogen than the amino acids, they are transported to other parts of the plant via xylem vessels.
o THE NODULES OF SOME PLANTS LIKE SOYBEAN EXPORT THE FIXED NITROGEN AS UREIDES. THESE COMPOUNDS ALSO HAVE A PARTICULARLY HIGH NITROGEN TO CARBON RATIO

Mineral Nutrition Notes

Mineral Nutrition

Mineral nutrition in plants

Every living organisms need carbohydrates, proteins, fats, water, and minerals to live. Similarly, plants need nutrients for growth and development.

Methods to study the mineral requirement of plants

• Hydroponics is a technique in which plants are grown in nutrient solution instead of soil. This technique is used at the time of commercial production of vegetables. It is also used to study the mineral deficiency diseases in plants.
• Aeroponics is the technique in which nutrients are sprayed suspended in the air.

Essential mineral elements

Different plants have different mineral requirements. There is criterion for the essentiality of an element. It includes-

• The element must be necessary for normal growth and reproduction. In the absence of that element the plants will not be able to complete its life cycle.
• The requirement of the element must be specific, and it should not be replaceable by any another element.
• The element must be directly involved in the metabolism of the plant.

The mineral elements needed by the plant is divided into- macronutrients and micronutrients

Macronutrients

These elements are needed by the plants in large quantities. It includes carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium and magnesium. Carbon, hydrogen and oxygen are obtained mainly from \( \text{CO}_2 \) and \( \text{H}_2\text{O} \). Others are absorbed from the soil as mineral nutrition.

Micronutrients

These are the nutrients that are needed by the plants in small quantity. It includes iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.

Essential elements are grouped in different categories based on their diverse functions such as-

• Essential elements such as carbon, hydrogen, oxygen and nitrogen serves as components of various biomolecules such as amino acids, lipids and hence structural elements of cells
• Essential elements are major components of chemical compounds related to energy such as magnesium ion in chlorophyll and phosphorous in ATP.
• Essential elements also influence enzyme action by acting as activator and inhibitor of certain enzymes. For example, \( \text{Mg}^{2+} \) activates both ribulose bisphosphate carboxylase oxygenase and phosphoenol pyruvate carboxylase. Both of these are essential in carbon fixation. \( \text{Zn}^{2+} \) activates alcohol dehydrogenase and \( \text{Mo} \) is essential for nitrogenase activity during nitrogen metabolism.
• Some essential elements control osmolarity of a cell. Potassium ions control the opening and closing of stomata.

Role of macronutrients and micronutrients

The role of different macro and micronutrients are given below-

Nitrogen

• Essential element required by the plants in large quantities.
• It is absorbed by the plants in the form of nitrate ions (\( \text{NO}_3^- \)) and some plants also absorb in the form of nitrite ions (\( \text{NO}_2^- \)) or ammonium ions (\( \text{NH}_4^+ \))
• Needed by the plants for actively growing tissues such as meristematic tissues.
• It is a necessary component of vitamins.
• Directly involved in photosynthesis.

Phosphorous

• It is absorbed by the plants in the form of phosphate ions (\( \text{PO}_4^{3-} \))
• It is major component of cell membranes, proteins, nucleic acids and nucleotides.
• Promotes root formation and growth.
• Required for seed formation.
• Involved in energy storage and transfer.

Potassium

• It is absorbed by the plants in the form of potassium ions.
• It is abundant in actively growing tissues such as meristematic tissues, buds, leaves and root tips.
• It is required for maintaining osmotic potential in a cell which is responsible for opening and closing of the stomata.
• Increases the rate of photosynthesis
• Essential for protein synthesis

Calcium

• It is absorbed by the plants in the form of calcium ions (\( \text{Ca}^{2+} \))
• Required for continuous cell division and growth.
• Involved in nitrogen metabolism.
• It is present in the middle lamella in the form of calcium pectate.
• It is also required for the formation of mitotic spindle
• It is also required for the activation of certain enzymes.

Magnesium

• It is absorbed by the plants in the form of magnesium ions (\( \text{Mg}^{2+} \))
• It is required for the activation of the enzymes involved in respiration, photosynthesis.
• It is involved in the synthesis of RNA and DNA.
• Key component of chlorophyll.
• It also maintains ribosome structure.

Sulphur

• Absorbed in the form of sulphate ions (\( \text{SO}_4^{2-} \))
• Integral component of amino acids such as methionine.
• Component of several coenzymes vitamins such as thiamine, biotin, and ferrodoxin.
• Helps in seed production
• Required for chlorophyll formation

Iron

• Plants absorb iron mainly in the form of ferric ions (\( \text{Fe}^{3+} \))
• Important for cell division and growth.
• Participates in electron transfer during various metabolic reactions.
• It is important for the activation of an enzyme catalase that is required for chlorophyll formation.
• Serves as oxygen carrier.

Manganese

• It is absorbed in the form of manganese ions (\( \text{Mn}^{2+} \)).
• It is involved in photolysis of water during non-cyclic photophosphorylation.
• One of the components of nitrogenase enzyme required for nitrogen metabolism.

Zinc

• It is absorbed in the form of zinc ions (\( \text{Zn}^{2+} \)).
• Required for chlorophyll formation.
• Involved in activation of various enzymes such as carboxylases.
• Needed for carbohydrate formation.

Copper

• It is absorbed by the plants in the form of cupric ions (\( \text{Cu}^{2+} \)).
• It is involved in redox reactions during metabolic reactions.
• Works during photosynthesis and reproductive stages of the plants.
• Involved in flavor of fruits and vegetables.
• Increases the sugar content of the plants.

Boron

• It is absorbed by the plants in the form of \( \text{BO}_3^{3-} \) or \( \text{B}_4\text{O}_7^{2-} \).
• Involved in the uptake of calcium by the plants.
• Functions in pollen tube formation.
• Carbohydrate translocation in plants.
• Involved in cell differentiation.

Molybdenum

• Plants obtain it in the form of molybdate ions (\( \text{MoO}_2^{2-} \)).
• Major component of nitrogenase enzyme. It forms a catalytic site of the enzyme along with iron.
• It is an important component of nitrate reductase. Nitrate reductase is an enzyme involved in nitrogen assimilation.
• Takes part in nodule formation

Chlorine

• Absorbed as chloride ions (\( \text{Cl}^{1-} \)).
• Maintains the osmotic potential of the cell.
• It is involved in photolysis of water for evolution of oxygen

Deficiency Symptoms of Essential Elements

• Nitrogen is one of the important essential elements. Deficiency of nitrogen leads to yellowing of leaves.
• Phosphorus is another element of which deficiency will lead to burnt leaf tips and yellowing of the tips.
• Potassium deficiency causes interveinal chlorosis and older leaves to wilt.
• Calcium deficiency leads to blossom end rot.
• Magnesium causes older leaves to become yellow, that is, chlorosis of leaf due to degradation of chlorophyll.
• Sulphur deficiency leads to yellowing of younger leaves compared to older leaves.
• Iron deficiency causes chlorosis of young leaves. Dieback disease is the characteristic disease of deficiency of iron in plants.
• Manganese causes yellowing in between the veins of the leaves and reduces plant parts with dead spots.
• Zinc causes rosette formation, yellowing between the veins as well as stunted growth of the plant.
• Copper is another mineral element, deficiency of which causes weakening of cell wall, dieback of stems and twigs.
• Boron affects the reproductive as well as vegetative growth of the plants and, death of the meristem.
• Molybdenum causes stunted growth. Leaves appear pale and there is necrosis in the tissues.
• Chlorine causes wilting, chlorosis, and highly branched root system.

Metabolism of nitrogen

Nitrogen exists as two nitrogen atoms joined by a triple covalent bond (\( \text{N} \equiv \text{N} \)). The process of conversion of nitrogen into ammonia is known as nitrogen fixation.

There are two methods for the conversion of nitrogen into ammonia. They are as follows-

• Physical nitrogen fixation occurs at the time of lightning. \( \text{N}_2 \) and \( \text{O}_2 \) present in the atmosphere react in the presence of lightning to form nitric oxide (\( \text{NO} \)). \( \text{NO} \) then further gets oxidized to form nitrogen peroxide (\( \text{NO}_2 \)).

Formation of ammonia by the decomposition of dead plants and animals is known as ammonification.

The process of conversion of ammonia in nitrite and then into nitrate is known as nitrification. The formation of nitrite occurs in the presence of Nitrosomonas bacteria whereas nitrate formation occurs in the presence of Nitrobacter.

Steps of physical nitrogen fixation:

• The micro-organisms do not take place
• Found in rainy season during lightning

1. \( \text{N}_2 + \text{O}_2 \xrightarrow{\text{lightning}} 2\text{NO} \) (Nitric oxide)
2. \( 2\text{NO} + \text{O}_2 \xrightarrow{\text{oxidation}} 2\text{NO}_2 \) (Nitrogen peroxide)
3. \( 2\text{NO}_2 + \text{H}_2\text{O} \rightarrow \text{HNO}_2 + \text{HNO}_3 \)
4. \( 4\text{NO}_2 + 2\text{H}_2\text{O} + \text{O}_2 \rightarrow 4\text{HNO}_3 \) (Nitric acid)
5. \( \text{CaO} + 2\text{HNO}_3 \rightarrow \text{Ca(NO}_3\text{)}_2 + \text{H}_2\text{O} \) (Calcium nitrate)
6. \( \text{HNO}_3 + \text{NH}_3 \rightarrow \text{NH}_4\text{NO}_3 \) (Ammonium nitrate)
7. \( \text{HNO}_2 + \text{NH}_3 \rightarrow \text{NH}_4\text{NO}_2 \) (Ammonium nitrite)

Fig.14. Steps of physical nitrogen fixation

• Biological nitrogen fixation occurs in the presence of nitrogen fixing bacteria such as Rhizobium. The prokaryotes that fix nitrogen biologically are known as diazotrophs. Rhizobium lives in symbiotic association with the roots of the leguminous plants. The enzyme that fixes nitrogen biologically is known as nitrogenase. Nitrogen fixing bacteria can be free living as well as symbiotic. Examples of free-living nitrogen-fixing aerobic microbes are Azotobacter, Rhodospirillum, Anabaena and Nostoc etc.

Rhizobium lives in symbiotic association with the leguminous plants such as pea, beans, clover, alfalfa, etc. Nitrogen fixation involves characteristic nodule formation. Nodule formation begins with the interaction of Rhizobium with the roots of the plants. These plants release certain chemicals that attract the bacteria towards the roots of the plants. The Rhizobium secretes root hair curling factor that helps the bacteria to further invade the roots. Nitrogenase enzyme that catalyzes nitrogen fixation is sensitive to oxygen. Oxygen is therefore scavenged by the enzyme leghemoglobin (belonging to hemoglobin family) during nitrogen fixation.

Steps of nodule formation during biological nitrogen fixation:

1. Recognition and attachment: (rhicadhesin-mediated) - Rhizobial cell
2. Excretion of nod factors: by bacterium causing root hair curling
3. Invasion: Rhizobia penetrate root hair and multiply within an "infection thread"

Fig.14. Steps of nodule formation during biological nitrogen fixation

The reaction catalyzed by the nitrogenase is as follows-

Nitrogenase reaction during biological nitrogen fixation:

\( \text{N}_2 + 8e^- + 8\text{H}^+ + 16\text{ATP} \xrightarrow{\text{nitrogenase}} 2\text{NH}_3 + \text{H}_2 + 16\text{ADP} + 16\text{P}_i \)

It takes 12 ATPs to provide sufficient energy to break the strong triple bond betwen the two nitrogen atoms of \( \text{N}_2 \) gas: \( \text{N} \equiv \text{N} \)

Fig.15. Nitrogenase reaction during biological nitrogen fixation

Nitrogen fixation is an energy intensive process. 16 ATP molecules are required to convert one molecule of nitrogen into two molecules of ammonia.

The ammonia formed as a result of nitrogen fixation is toxic to the plants. Thus, this ammonia undergoes further reactions-

• Reductive amination: The process in which ammonia reacts with alpha-ketoglutaric acid and forms glutamic acid or glutamate as shown below-

1. Reductive Amination:

\( \text{Alpha keto glutarate} + \text{Ammonia} + \text{NAD(P)H} \xrightarrow{\text{Glutamate dehydrogenase}} \text{Glutamate} + \text{NAD(P)} \)

2. Transamination reaction:

\( \text{Amino acid-1} + \text{Alpha keto acid-2} \xrightarrow[\text{PLP}]{\text{Transaminase}} \text{Alpha keto acid-1} + \text{Amino acid-2} \)

Fig.16. Reductive amination and Transamination

• Transamination: The transfer of amino group from one amino acid to keto acid. This reaction is catalyzed by enzymes known as transaminases. This forms asparagine amino acids.

Thus, toxic ammonia is converted into amino acids that are essential for the plant growth and development.