1st PUC Biology Question Bank Chapter 12 Mineral Nutrition

Karnataka 1st PUC Biology Question Bank Chapter 11 Mineral Nutrition

1st PUC Biology Mineral Nutrition One Marks Questions and Answers

Question 1.
Name a plant which accumulates silica.
Answer:
Equisetum.

Question 2.
Mycorrhiza is a mutualistic association. How do the organisms involved in this association gain from each other?
Answer:
The fungus partner gets shelter and food from the plant, while the plant obtains minerals, and water from the fungus.

Question 3.
Name an insectivorous angiosperm.
Answer:
Nepenthes (Pitcher Plant).

Question 4.
A farmer adds azotobacter culture to soil before sowing Maize. Which mineral element is being replenished?
Answer:
Nitrogen.

Question 5.
Name the macronutrient which is a component of all organic compounds, but is not obtained from the soil.
Answer:
Carbon.

Question 6.
Name one non-symbiotic nitrogen fixing prokaryote.
Answer:
Azotobacter.

Question 7.
Rice fields produce an important green house gas. Name it.
Answer:
Methane.

1st PUC Biology Mineral Nutrition Two Marks Questions and Answers

Question 1.
‘All the elements that are present in a plant, need not be essential to its survival’. Comment.
Answer:
A number of elements are found in plants. More than 60 elements have been detected in them. However, all the elements found in plants are not essential for them. The non-essential elements enter plants because of the pathway common with some essential elements, and special conditions of the soil which make them soluble.

Question2.
Why is it that in certain plants deficiency symptoms appear first in younger parts of the plant while in others they do so in mature organs?
Answer:
Certain mineral nutrients are mobile in the plants while others are not. During deficiency, the mobile elements are mobilised from the mature leaves, and transferred to younger leaves. In such cases, the deficiency symptoms appear first in older leaves, e.g., N, P, Mg. Other nutrients are immobile inside the plants. In such cases, the deficiency symptoms appear first in young leaves, e.g., Fe, Ca, S.

Question 3.
Complete the equation for reductive amination.

(b) Photo phosphorylation:
The extra energy of ejected electrons during photoexcitation is used for phosphorylation of ‘ADP into ATP Hence it is called photophosphorylation. It is of two types namely,
(ii) Cyclic Photo phosphorylation
(a) Cyclic Photo phosphorylation :
(b) Non-cyclic Photo phosphorylation.

It is a cyclic path of electrons expelled from chlorophyll through a series of substrates that are arranged in a suitable oxidation reduction potential. The energy in the electrons is used for phosphorylation of ADP to ATP.
In PS I the absorbed photons of light excite chlorophyll-a 700 to eject energised electrons on makes it positively charged and unstable. Electrons pass through the sequence → FRS → FD → Cyt b₆ → Cytf → PC, and generate ATP at two places. Finally electron from PC returns to chl-a restoring its stability.

(b) Non-cyclic Photo phosphorylation:
– It is the non cyclic path of electrons from PS II, and PS I to release ATP and NADPH₂, with the help of protons and electrons from photolysis of water

– In PS I the absorbed photons of light excite chlorophyll-a 100 to eject energised electrons which pass through FRS, FD, and finally get locked up with NADP which becomes negatively charged, and partially reduced, while PSI becomes positively charged and unstable.

– At the, same time in PS II, the absorbed photons of light excite chlorophyll a 680 to eject energised electrons which pass through in sequence, pheophytin, PQ, cyto b₆, cytf, plastocyanin, and generate ATP. The electrons finally join P.S I to replace its lost electrons and restore its stability. Now P.S II becomes positively charged, and unstable. Meanwhile electrons of photolysed water join with PS II to replace its lost electrons, and restore its stability. Protons join with the already ionised NADP, and complete its reduction to NADPH₂

DARK REACTION / Blackmann’s Reaction / Calvin cycle / C₃ Pathway:

– It is a light independent process occurring in stroma of chloroplast. It uses the assimilatory power (ATP and NADPH₂) produced in a light reaction. It includes 4 main events namely:

(1) Carboxylative phase: (CO₂ Fixation):The five carbon ribulose mono phosphates (RUMP) present in stroma, is phosphorylated by ATP to ribulose diphosphate (RUDP). RUDP is the initial CO₂ acceptor which forms an unstable six carbon compound. It soon splits to form two molecules of a three carbon compound, namely phosphoglyceric acid (PGA), the first stable compound. At a time, six CO₂ molecules are fixed, which eventually results in the formation of 12 molecules of PGA.

(2) Reduction phase: The 12 PGA are phosphorylated by 12 ATP of assimilatory power to form 12 molecules of DPGA (Diphosphoglyceric acid). They soon loose 12 phosphates to an inorganic source and become reduced by 12 NADPH₂ of assimilatory power to the form 12 molecules of PGAL (Phosphoglyceraldehyde). PGAL can isomerise into DHAP (Dilhydroxy acetone phosphate) which is a reversible reaction.

(3) Regeneration Phase: Of the 12 Molecules of PGAL, 10 molecules undergo a series of complex reactions to regenerate 6 RUMP.

(4) Glucose Formation: The remaining two molecules (PGAL and DHAP) get condensed by – aldolase to form fructose 1-6 diphosphate. It is later dephosphorylated twice, and isomerised to form glucose.

C₂ Pathway Or Hatch Slack Pathway:

Pathway of CO₂ fixation in which the first stable compound formed is a 4-carbon compound oxalo acetic acid in the presence of PEP carboxylase is called C₄ pathway.
It was first observed by Hatch, and Slack.
e.g: Monocots like maize, sugarcane, ragi and dicots like euphorbia.

Mechanism:

In mesophyll cells, CO₂is fixed by phosphoenol pyruvate (PEP) to form the first stable carbon compound called oxalo acetic acid, a four carbon sugar in the presence of PEP carboxylase.
Oxalo acetic acid converts into malic acid.
Malic acid is transferred to bundle sheath cells, where it is converted into pyruvic acid and CO₂
The released CO₂is used in the calvin cycle and gets reduced to carbohydrate by the rubisco.
Pyruvic acid is transported back to mesophyll cells.

Photo Respiration:
It is found that the rate of respiration is 3 to 5 times more in green photosynthetic plant cells in the presence of light. This phenomenon is called as photo respiration. It occurs only in C₃ plants. In C₃ plants, RUBP carboxylase is the main enzyme for glucose formation, but at high temperature, and high oxygen concentration functions as oxygenase and catalyses the oxidation of RUBP into one molecule of 3C compound phosphoglyceric acid, and one molecule of 2C compound, phosphoglycolic acid.

Phosphoglycollic acid is quickly converted into glycolic acid, and transported to peroxisomes. In peroxisome, glycollic acid is oxidised to glyoxylic acid, and is converted into amino acid, ‘glycine.

Glycine enters the mitochondria, and two glycine molecples give rise to one molecule of serine and one molecule of CO₂ serine is picked up by peroxisomes, and is converted to glyceric acid by series of reactions.
Answer:

Question 5.
How is sulphur important for plants? Name the amino acids in which it is present
Answer:
Sulphur is a component of two amino acids, vitamins, thiamine, biotin, Coenzyme A, lipoic acid, glutathione, and ferredoxin (FeS). Sulphur containing Amino Acids, are Cysteine and Methionine.

Question 6.
Marne the most crucial enzyme found in root nodules for N₂ fixation? Does it require a special pink coloured pigment for its functioning? Elaborate.
Answer:
Crucial Enzyme. Nitrogenase.

1st PUC Biology Mineral Nutrition Three Marks Questions and Answers

Question 1.
How are the minerals absorbed by the plants?
Answer:
The process of absorption occurs in two distinct phases.

In the first phase, there is a rapid uptake of irons into the ‘free space’ or outer space of cell called the apoplast. It is a passive process and takes place through non-channels.
In the second phase, the ions are taken slowly into the inner space – the symp last of the cells. This is an active process and requires the expenditure of metabolic energy.

Question 2.
What are the conditions necessary for fixation of atmospheric nitrogen by Rhizobium. What is their role in Nz-fixation?
Answer:
Biological Nitrogen Fixation :

Though nitrogen is abundant in the atmosphere, only a few living organisms especially certain prokaryotes are capable of using it.
The process by which living organisms convert the free atmospheric nitrogen into ammonia, is called biological nitrogen fixation.
The nitrogen fixing micro-organisms may be free living or symbionts.

Free living nitrogen fixers include:
Azotobacter, Rhodospirillum, Clostridium (anaerobes), and Cyanobacteria like Nostoc, and Anabaena.

Symbiotic nitrogen fixers include:

Rhizobium in the root nodules of legumes, Frankia in non-legumes like Alnus, and Casurina, cyanobacteria like Nostoc, and Anabaena (cycas roots, Anthoceros, Azolla etc.)
Symbiotic nitrogen fixation takes place in the root nodules of legumes.
During this process, a nitrogen molecule is reduced by the addition of hydrogen atoms into two molecules of ammonia catalysed by the enzyme nitrogenase.

Biological nitrogen fixation requires the following three biochemical components.
(i) A reducing agent, to transfer the hydrogen atoms to dinitrogen.
(ii) ATP, to provide energy.
(iii) The enzyme system – nitrogenase, a Mo-Fe protein and leghaemoglobin.

• The leghaemoglobin is a pink colour pigment similar to haemoglobin of vertebrates, and
functions as an oxygen scavenger, and protects the nitrogenase from oxygen.
N₂+ 8e^– + 8H⁺ + 16ATP 2NH₃ + 16 ADP + 16 Pi + H₂

1st PUC Biology Question Bank Chapter 12 Mineral Nutrition.2

1st PUC Biology Mineral Nutrition Five Marks Questions and Answers

Question 1.
Explain with examples: macronutrients, micronutrients, beneficial nutrients, toxic elements and essential elements.
Answer:
Macronutrients : They are essential elements which occur in concentration of 1-10 mg/g of dry matter, e.g., C, H, O, N, P, K.

Micronutrients : They are essential elements which occur in plants in a concentration of 0.1 mg or below per gm of dry matter, e,g„ Mn, Zn, Cu, Mo, Bo.

Beneficial Nutrients : They are non-essential elements which have some specific role in some plants, e.g., Na⁺ in C₄ plants, Si in grasses.

Toxic Elements : They are non-essential elements which if absorbed reduce plant growth, and yield. Many elements become toxic only after their concentration reaches a particular level, e g., Lead, Aluminium, Mn.
Essential Elements : They are those elements which are involved in some vital structural, and functional role in plants so that their deficiency will cause disorders while their absence results in non-completion of life cycle, e.g., C, H, O, N, P K Ca, Mg, S, etc.

Question 2.
Name at least five different deficiency symptoms in plants. Describe them, and correlate them with the concerned mineral deficiency.
Answer:
Deficiency Symptoms:

An element is said to be deficient, if it is present below its critical concentration. Since each element has one or more specific structural or functional role in plants, in the absence of any particular element, the plants show certain morphological/ observable changes. They are indicative deficiency of a certain element, and are called as deficiency symptoms.

The common deficiency symptoms include :
(i) Chiorosis: It is the loss of chlorophyll leading to yellowing of leaves. Deficiency of elements nitrogen, potassium, magnesium, sulphur, iron, manganese, zinc, and molybdenum cause chlorosis.

(ii) Necrosis : It refers to the death of tissues (especially of leaves) e.g., Deficiency of elements like calcium, magnesium, copper, potassium.

(iii) Inhibition of Cell. Division : It is caused by deficiency of potassium, nitrogen, and molybdenum.

(iv) Delay in flowering: Deficiency of elements like nitrogen, sulphur, and molybdenum causes delay in flowering.

The deficiency symptoms tend to appear first in the young tissues in the case of elements that are relatively immobile, and are not transported out of the mature organs. e.g., Sulphur, and calcium, which form structural components.

The deficiency symptoms tend to appear first in older tissues, if the elements are actively mobile, they are withdrawn from the senescing organs, and transported to young tissues. e.g., Nitrogen, potassium and magnesium.

Question 3.
What are the steps involved in the formation of a root nodule?
Answer:

When the root hair of a legummous plant comes in contact with Rhizobium, it becomes curled or deformed due to the chemicals secreted by the bacterium.
The rhizobia enter these deformed root hair and proliferate within the root hair.

1st PUC Biology Question Bank Chapter 12 Mineral Nutrition.3

Root Nodule Formation in a leguminous plant.

The pIant responds by forming an infection thread, that grows inwards to deliver the bacteria to the tissues.
Its believed that the cytokinin produced by the bacteria, and the auxin produced by the plant cells stimulate cell division, and enlargement to form a nodule.
The nodule establishes contact with the vascular tissues of the host for absorption of nutrients.
The formation of root nodules, and nitrogen fixation occur under the control of nod genes of legumes, and the nod, nif, and fix genes of bacteria.

1st PUC Biology Mineral Nutrition Text Book Questions and Answers

1. Methods’to Study The Mineral Requirements Of Plants.

Julius Van Sachs (I860) demonstrated that plants could be grown to maturity in a defined nutrient solution and in the complete absence of soil. Hydroponics has the following uses :

The essential elements have been identified, and their deficiency symptoms discovered.
It has been employed for the commercial production of crops like tomato, seedless cucumber, and lettuce.

To achieve optimum growth of plants,

The solution must be adequately aerated and
The concentration of mineral nutrients must be maintained constant.

2. Criteria Of Essentiality Of An Element.

The element must be absolutely necessary for supporting normal growth and reproduction, and in the absence of the element, the plants do not complete their life cycle.
The requirement of the element must be specific and not replaceable by another element, i.e. 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.

3. Essential Elements.
Only a few elements have been found to be absolutely essential for plant growth and metabolism.
(A) They are classified into two broad categories:
(i) Macronutrients and
(ii) Micronutrients

(i) Macronutrients : Macronutrients are those elements, which are generally present in large amounts in the plant tissues, i.e., in excess of 10 m. mole kg^-1 of dry matter, e.g., Carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, potassium, and magnesium.

(ii) Micronutrients: Micronutrients or trace elements are those elements, which are needed in minute quantities, i.e., 0.1 mg per gram of dry matter. i.e., Boron, copper, iron, manganese, molybdenum, zinc, nickel and chlorine. Some plants also require traces of cobalt, selenium, and silicon.

(B) Essential elements can also be classified into four categories, based on their diverse functions, as given below:

As components of bimolecular, and hence structural elements of cells e.g., carbon, hydrogen, oxygen, and nitrogen.
As components of energy related chemical compounds. e.g., Phosphorus in ATP, Magnesium in chlorophyll.
As activator or inhibitor of enzymes. e.g., Mg⁺⁺ is an activator of ribulose bisphosphate carboxylase, and phosphoenolpyruvate carboxylase. Zn⁺⁺ is an activator of alcohol dehydrogenase, Mo activates nitrogenase.
Elements that maintain the osmotic potential of a cell. e.g., Potassium is involved in the opening, and closing of stomata. Chlorine maintains the anion-cation balance, and the osmotic potential of the cell.

4. Role Of Macroelements And Their Deficiency Symptoms In Plants:

(A) Nitrogen :
It is absorbed as NO₃⁺, NO₂⁺ and NH₄⁺ ions.

Functions:
It is a major constituent of amino acids, proteins, nucleic acids,vitamins, etc.

Deficiency symptoms:

Chlorosis, observed first in older leaves.
Stunted growth in plants.
Purple colouration due to anthocyanin development in the shoot axis surface.
Dormancy of lateral buds.
Delayed flowering.
Wrinkling of cereal grains.

(B) Sulphur :
It is obtained as sulphates (50/- ions).

Functions:
(i) It is the constituent of amino acids, methionine and cystine.
(ii) It also forms part- of ferredoxin, and vitamins like thiamine, biotin, and coenzyme-A. Deficiency

symptoms :
(i) Chlorosis of younger leaves.
(ii) Stunted growth of plants.
(iii) Accumulation of anthocyanin (purple colouration).

(C) Phosphorus:
It is absorbed by plants in the form of phosphate ions, either as H₂PO₄^– or HPO₄^3- Functions:
(i) It is a constituent of nucleotides, and nucleic acids.
(ii) It is present in cell membranes as phospholipids.
(iii) It is involved in phosphorylation reactions, and energy metabolism a~ A TP.

Deficiency Symptoms :
(i) Deficiency causes purple or red spots on leaves.
(ii) Leaves also become dark (dull) green.
(iii) There is a delay in seed germination.
(iv) Premature fall of leaves and flower buds also occurs.

(D) Potassium:
Potassium is taken in as potassium ions (K⁺). It is more abundant in meristematic tissues like buds, root-tips, and young leaves.

Functions:
(i) It determines the anion-cation benefice in cells.
(ii) It controls the opening and closing of stomata.
(iii) It maintains turgidity (osmotic balance) in cells.
(iv) It activates a number of enzymes.
(v) It is also involved in protein synthesis.

Deficiency symptoms:
(i) Interveinal chlorosis.
(ii) Scorched leaf tips.
(iii) Loss of apical dominance and a bushy appearance of plants.
(iv) Loss of cambial activity.
(v) Shorter intemodes .
(vi) Disintegration of plastids.
(vii) Increased rate of respiration.

(E) Magnesium:
It is absorbed as Mg⁺⁺ ions by the plants.

Functions:
(i) It is a constituent of chlorophyll (forms the central atom of the porphyrin ring).
(ii) It maintains the ribosome structure for protein synthesis.
(iii) It activates the enzymes of phosphate metabolism in respiration and photosynthesis.
(iv) It is also involved in the synthesis of DNA, RNA, and activation of enzymes.

Deficiency symptoms:
(i) Interveinal chlorosis and necrosis, first occurs in of older leaves.
(ii) Premature leaf fall.

(F) Calcium :
Calcium is obtained as calcium ions (Cat) from the soil.

Functions:
(i) It is necessary for selective permeability of cell membranes.
(ii) It occurs as calcium pectate in the middle lamella of cell wall, hence is necessary for cell enlargement.
(iii) It is used in the formation of mitotic spindles during cell division, hence is required in the meristematic, and differentiating tissues (root apex, and shoot apex).
(iv) It activates certain enzymes in the metabolism.

Deficiency symptoms :
(i) Stunted growth.
(ii) Necrosis of meristematic regions.

5. Role Of Micronutrients And Their Deficiency Symptoms:

(A) Iron: Iron is absorbed mainly in the form of ferric ions (Fe³⁺).

Functions:
(i) It is an important constituent of cytochromes and ferredoxin, involved in the electron transport.
(ii) It activates catalase.
(iii) It is involved in the synthesis of chlorophyll.

Deficiency symptoms :
(i) Chlorosis starts with the intravenous regions, and results in chlorosis of the complete leaf.

(B) Copper: It is absorbed as cupric ions (Cu⁺⁺).
Functions:
(i) It is associated with certain enzymes involved in the redox reactions.
(ii) It is essential for the overall metabolism.

Deficiency symptoms :
(i) Necrosis starts from the tip of young leaves and extends downwards.
(ii) In fruit trees, it causes die back of shoot (death of shoot meristem).
(iii) Leaves fall off, and bark becomes rough, and splits exuding gummy substances.

(C) Boron: It is absorbed as BO₃^3- or B₄O₇^2-.
Functions:
(i) It is required for carbohydrate translocation (as sugar borate).
(ii) It is necessary for uptake and utilisation of calcium ions.
(iii) It is needed for pollen germination.
(iv) It is involved in cell elongation and cell differentiation.

Deficiency symptoms:
(i) Death of root and shoot tips.
(ii) Abscission of flowers.
(iii) Loss of apical dominance.
(iv) Absence of root nodules in leguminous plants.
(v) Small sized fruits.
(vi) Stunted roots.

(D) Zinc : It is obtained by plants as Zn⁺⁺ ions.
Functions:
(i) It is necessary for the synthesis of auxins.
(ii) It activates many enzymes, especially carboxylases.

Deficiency symptoms:
(i) Malformed (little) leaves.
(ii) Interveinal chlorosis.
(iii) Stunted growth.

(E) Manganese : It is obtained by plants as Mn⁺⁺ ions.
Functions:
(i) It is necessary for photolysis of water in photosynthesis.
(ii) It activates many enzymes involved in photosynthesis, nitrogen metabolism, and respiration.

Deficiency symptoms:
(i) Chlorosis.
(ii) Grey spots on leaves.

(F) Molybdenum : Plants obtain molybdenum as molybdate ions (MoO₂^2-).

Functions:
(i) It is a co-factor for enzyme nitrate reductase.
(ii) It is also necessary for nitrogenase.
Deficiency symptoms:
(i) Interveinal chlorosis.
(ii) Whiplike disease in cauliflower.
(iii) Necrosis, first observed in older leaves.

(G) Chlorine : It is taken in the form of chloride ions.
Functions:
(i) Along with Na⁺ and K⁺ ions, it determines the anion-cation balance in cells, and
determines the solute concentration.
(ii) It is necessary for photolysis of water in photosynthesis.
(iii) It may be required for cell division in leaves, and roots.

Deficiency symptoms :
(i) Wilted leaves.
(ii) Stunted root growth.
(iii) Reduced fruit formation.

6. Deficiency Symptoms:

An element is said to be deficient, if it is present below its critical concentration.
Since each element has one or more specific structural or functional role in plants, in the absence of any particular element, the plants show certain morphological/ observable changes. They are indicative deficiency of a certain element, and are called as deficiency symptoms.

• The common deficiency symptoms include :
(i) Chlorosis : It is the loss of chlorophyll leading to yellowing of leaves.
Deficiency of elements nitrogen, potassium, magnesium, sulphur, iron, manganese, zinc, and molybdenum cause chlorosis.

(ii) Necrosis : It refers to the death of tissues (especially of leaves)
e.g., Deficiency of elements like calcium, magnesium, copper, potassium.

(iii) Inhibition of Cell Division : It is caused by deficiency of potassium, nitrogen, and molybdenum.
(iv) Delay in flowering: Deficiency of elements like nitrogen, sulphur, and molybdenum causes delay in flowering.

• The deficiency symptoms tend to appear first in the young tissues in the case of elements that are relatively immobile, and are not transported out of the mature organs. e.g., Sulphur, and calcium, which form structural components.

7. Toxicity Of Micronutrients:

An element or mineral ion is considered toxic, if it reduces the dry weight of plant tissues by about 10%.
The toxicity symptoms are difficult to identify.
Toxicity levels of an element varies with the plant species.
Excess of an element may inhibit the uptake of another element e.g., manganese toxicity produces brown spots surrounded by chlorotic veins.
Manganese competes with iron and magnesium for uptake and with magnesium for binding with enzymes.
Manganese also inhibits calcium translocation in shoot apex.
As a result, excess of manganese may induce deficiencies of iron, magnesium, and calcium.

8. Mechanism Of Absorption Of Elements:

The process of absorption occurs in two distinct phases.
In the first phase, there is a rapid uptake of irons into the ‘free space’ or outer space of cell called the apoplast It is a passive process and takes place through non-channels.
In the second phase, the ions are taken slowly into the inner space – the symplast of the cells. This is an active process and requires the expenditure of metabolic energy.

9. Translocation Of Solutes:

Mineral salts are transported through the xylem along with the ascending stream of water (ascent of sap), which is pulled by the transpiration pull.

10. Metabolism Of Nitrogen:
A. Nitrogen Cycle:
• Nitrogen is a limiting nutrient for both natural, and agricultural ecosystems.
• It exists as two nitrogen atoms held together by strong triple covalent bonds (N = N).

Nitrogen cycle involves the following steps :
(i) Nitrogen fixation
(ii) Ammonification
(iii) Nitrification and
(iv) Denitrification.

(i) Nitrogen Fixation: –

The process of conversion of nitrogen into ammonia and / or other nitrogen compounds, is known as nitrogen fixation.
In nature, lighting and ultraviolet radiation provide energy, to convert nitrogen into nitrogen compound like N₂O, NO and NO₂.
Atmosphere also gets some amount of nitrogen oxides from forest fires, automobile exhausts, industrial combustion, and power generating stations.

(ii) Ammonification:

1st PUC Biology Question Bank Chapter 12 Mineral Nutrition.1

The process by which organic nitrogenous compounds arc decomposed to produce ammonia, is known as ammonification.
Some ammonia volatilizes, and re-enters the atmosphere.
Most of the ammonia is converted into nitrites and nitrates.

(iii) Nitrification:
-Nitrification is the process of converting ammonia first into nitrite, and then into nitrate.
2NH₃ + 3O₂ → 2NO₂ + 2H⁺ + 2H₂O
2NO₂ + O₂ →2NO₃

These processes are carried out by soil bacteria that are chemoautotrophs.
Ammonia is oxidised into nitrite by Nitrosomonas and Nitrococcus.
Nitrite is oxidised to nitrate by Nitrobacter,
The nitrates are absorbed by the plants, and reduced to nitrites.
The nitrites are transported to the leaves, and reduced to ammonia, which forms the amino group of amino acids.

(iv) Denitrification :

It is the process of conversion or reduction of the nitrates into free nitrogen.
It is carried out by bacteria like Pseudomonas and Thiobacillus.

B. Biological Nitrogen Fixation :

Though nitrogen is abundant in the atmosphere, only a few living organisms especially certain prokaryotes are capable of using it.
The process by which living organisms convert the free atmospheric nitrogen into ammonia, is called biological nitrogen fixation.
The nitrogen fixing micro-organisms may be free living or symbionts.

Free living nitrogen fixers include:
Azotobacter, Rhodospirillum, Clostridium (anaerobes), and Cyanobacteria like Nostoc, and Anabaena.

Symbiotic nitrogen fixers include:

Rhizobium in the root nodules of legumes, Frankia in non-legumes like Alnus, and Casurina, cyanobacteria like Nostoc, and Anabaena (cycas roots, Anthoceros, Azolla etc.)
Symbiotic nitrogen fixation takes place in the root nodules of legumes.
During this process, a nitrogen molecule is reduced by the addition of hydrogen atoms into two molecules of ammonia catalysed by the enzyme nitrogenase.

11. Formation Of Root Nodules In Legumes:

When the root hair of a leguminous plant comes in contact with Rhizobium, it becomes curled or deformed due to the chemicals secreted by the bacterium.
The rhizobia enter these deformed root hair and proliferate within the root hair.

1st PUC Biology Question Bank Chapter 12 Mineral Nutrition.3

Root Nodule Formation in a leguminous plant.

The plant responds by forming an infection thread, that grows inwards to deliver the bacteria to the tissues.
It is believed that the cytokinin produced by the bacteria, and the auxin produced by the plant. Cells stimulate cell division, and enlargement to form a nodule.
The nodule establishes contact with the vascular tissues of the host for absorption of nutrients.
The formation of root nodules, and nitrogen fixation occur under the control of nod genes of legumes, and the nod, nif, and fix genes of bacteria.

12. Synthesis Of Amino Acids

• The ammonia formed by nitrogen fixation is used for the synthesis of amino acids
• There are two processes by which amino acids are synthesised.

(i) Reductive Amination :
In this process ammonia reacts with alpha ketoglutaric acid and forms glutamic acids, a-ketoglutaric acid + NH³⁺ + NAD(P)H → Glutamic acid + H₂O + NAD (P).
The reaction is catalysed by glutamate dehydrogenase.

(ii) Transamination:
In this process, the amino group is transferred from one amino acid to the keto group of a keto-acid.
Glutamic acid is the main amino acid which transfers its NH₂ group to form seventeen other amino acids.

1st PUC Biology Question Bank Chapter 12 Mineral Nutrition.4

13. Amides:

Amides are formed by the replacement of hydroxyl ions of the amino acid by NH^–₂radical.
Asparagine and glutamine are the two major amides found in plants. They are formed aspartic acid and glutamic acid, in the presence of enzymes asparagine synthetase, and glutamine synthetase respectively.
Amides have more nitrogen than amino acids.