CBSE Class 11 Biology Mineral Nutrition Assignments

Chapter-12

MINERAL NUTRITION

POINTS TO REMEMBER

Autotroph: An organism that synthesises its required nutrients from simple and inorganic substances. Example - plants, blue green algae (cyanobacteria).

Heterotroph: An organism that cannot synthesise its own nutrients and depend on others. Example - Bacteria, protists, members of animalia.

Biological nitrogen fixation: Conversion of atmospheric nitrogen into organic compounds by living organisms.

Nitrification: Conversion of ammonia \((\text{NH}_3)\) into nitrite and then to nitrate.

Denitrification: A process of conversion of nitrate into nitrous oxide and nitrogen gas \((\text{N}_2)\).

Leg-hemoglobin: Pinkish pigment found in the root nodules of legumes. It acts as oxygen scavenger and protects the nitrogenase enzyme from oxidation.

Flux: The movement of ions is called flux. Influx is inward movement of ions into the cells and efflux is the outward movement of ions.

Necrosis: Death of tissues particularly leaf tissue due to deficiency of Ca, Mg, Cu, K.

Mineral Nutrition: Plants require mineral elements for their growth and development. The utilization of various absorbed ions by a plant for growth and development is called mineral nutrition of the plant.

Hydroponics: Soil-less culture of plants, where roots are immersed in nutrient solution (without soil) is called hydroponics. The result obtained from hydroponics may be used to determine deficiency symptoms of essential elements.

Essential Elements

Macronutrients: Macronutrients are present in plant tissues in concentrations of more than \(10 \text{ m mole Kg}^{-1}\) of dry matter.

Micro-nutrients: Micro-nutrients are needed in very low amounts : less than \(10 \text{ m mole Kg}^{-1}\) matter.

Chlorosis: Yellowing of leaves due to loss of chlorophyll.

Active Transport: Absorption occurring at the expense of metabolic energy.

Passive Transport: Absorption of minerals with concentration gradient by the process of diffusion without the expense of metabolic energy.

Role of Minerals Elements in Plants

MACRO NUTRIENTS

MICRO-NUTRIENTS

Role of microbes in nitrogen cycle:

Rhizobium, Azotobacter, Rhodospirillum: Fix atmospheric nitrogen

Nitrosomonas and / or Nitrococcus: Conversion of ammonia to nitrite

Nitrobacter: Conversion of nitrite into nitrate.

Pseudomonas and Thiobacillus: reduce nitrate into nitrogen.

Nitrogen Cycle:

Atmospheric Nitrogen \(\rightarrow\) Biological \(\text{N}_2\) fixation, Industrial \(\text{N}_2\) fixation, Electrical \(\text{N}_2\) fixation, Denitrification

Biological \(\text{N}_2\) fixation by Rhizobium \(\rightarrow\) \(\text{NH}_3\)

Ammonia \(\xrightarrow{\text{Nitrosomonas}} \text{NO}_2^-\) (Nitrite) \(\xrightarrow{\text{Nitrobacter}} \text{NO}_3^-\) (Nitrates) [SOIL 'N' Pool]

Nitrates undergo uptake into Plant Biomass.

Plant Biomass \(\rightarrow\) Animals Biomass \(\rightarrow\) Death and Decay \(\rightarrow\) Decaying Biomass \(\rightarrow\) Ammonification \(\rightarrow\) \(\text{NH}_3\).

Enzyme nitrogenase: The enzyme nitrogenase is a Mo-Fe protein and catalysis the conversion of atmospheric nitrogen to ammonia (First stable product of nitrogen fixation)

\(\text{N}_2 + 8\text{e}^- + 8\text{H}^+ + 16\text{ATP} \rightarrow 2\text{NH}_3 + \text{H}_2 + 16\text{ADP} + 16\text{Pi}\)

Steps of nodule formation:

a) Rhizobium bacteria present in soil contract a susceptible root hair.

b) Infection of the root hair cause it to cure.

c) An infection thread is produced carrying the bacteria into the cortex of the root.

d) The bacteria get modified into rod-shaped bacteriods and cause inner cortical and pericycle cells to divide.

e) Division and growth of cortical and pericycle cells lead to nodule formation.

(Refer Figure 12.4, page 203 NCERT Text Book).

Fate of Ammonia:

Ammonium ions (\(\text{NH}_4^+\)) are used to synthesize amino acids in plants. It can be taken place by two main ways:

(i) Reductive amination: ammonia reacts with \(\alpha\)-ketoglutaric acid and forms glutamic acid. This reaction is catalysed by enzyme glutamic dehyotrogenase.

(ii) Transamination: Other amino acids are formed by the transfer of amino group (\(\text{NH}_2\)) mainly from glutamic acid to the keto group of a ketoacid. The enzyme trans-aminase catalyses these reactions

QUESTIONS

Very Short Answer Questions (1 mark each)

Question 1. Name one symbiotic nitrogen-fixing bacteria.
Answer: Rhizobium.
In simple words: Rhizobium is a bacterium that lives in legume roots and fixes nitrogen.

Exam Tip: Mention the genus name correctly in italics or underlined.

Question 2. Give two examples of photosynthetic micro-organisms, which also fix atmospheric nitrogen.
Answer: Anabaena and Nostoc.
In simple words: Cyanobacteria like Anabaena and Nostoc can perform both photosynthesis and nitrogen fixation.

Exam Tip: Cyanobacteria (blue-green algae) are classic examples of diazotrophic phototrophs.

Question 3. Name two organisms each which fix nitrogen asymbiotically and symbiotically.
Answer: Asymbiotically: Azotobacter, Bacillus polymyxa (or Rhodospirillum). Symbiotically: Rhizobium, Anabaena.
In simple words: Some nitrogen-fixing bacteria live freely in soil (asymbiotic), while others live inside plant roots (symbiotic).

Exam Tip: Clearly categorize free-living versus mutualistic nitrogen fixers to secure full marks.

Question 4. Which substance imparts pink colour to the root nodule of a leguminous plant and also mention its role?
Answer: Leghemoglobin. It acts as an oxygen scavenger, protecting the oxygen-sensitive enzyme nitrogenase from oxidation.
In simple words: Leghemoglobin is a pink pigment that removes excess oxygen so the nitrogen-fixing enzyme can work properly.

Exam Tip: Emphasize the term oxygen scavenger and mention nitrogenase protection.

Question 5. What is the term used for mineral deficiency symptom in plants in which leaves become yellow in different pattern?
Answer: Chlorosis.
In simple words: Chlorosis refers to the yellowing of leaves caused by the loss or failure of chlorophyll formation.

Exam Tip: Distinguish chlorosis (yellowing due to chlorophyll loss) from necrosis (tissue death).

Question 6. Define hydroponics.
Answer: The technique of growing plants in a defined nutrient solution without soil is called hydroponics.
In simple words: Hydroponics is growing plants in water containing dissolved mineral nutrients instead of soil.

Question 7. Differentiate between two types of absorption of minerals in plants from soil.
Answer: The two types of mineral absorption are active absorption and passive absorption:
1. Active absorption: It is the inward movement of ions into cells that requires an expenditure of metabolic energy (ATP) and often occurs against a concentration gradient.
2. Passive absorption: It is the absorption of minerals with the concentration gradient by the process of diffusion without the expenditure of metabolic energy.
In simple words: Passive absorption happens naturally along concentration gradients without energy, while active absorption uses cellular energy to move ions against gradients.

Exam Tip: Highlight energy requirement (ATP) as the key differentiating factor.

Question 8. Name the following:
(a) Bacteria which converts ammonia into nitrite.
(b) Bacteria which oxidises nitrite into nitrate.
Answer:
(a) Nitrifying Bacteria - Nitrosomonas (and/or Nitrococcus).
(b) Nitrifying Bacteria - Nitrobacter.
In simple words: Nitrosomonas turns ammonia to nitrite, and Nitrobacter turns nitrite to nitrate.

Exam Tip: Learn both steps of nitrification along with their specific bacterial catalysts.

Question 9. How does Leghemoglobin protect the enzyme nitrogenase?
Answer: Leghemoglobin has a very high affinity for oxygen. By acting as an oxygen scavenger, it binds free oxygen within the root nodule, keeping the microenvironment anaerobic. This is crucial because the nitrogenase enzyme is extremely sensitive to molecular oxygen and gets irreversibly inactivated in its presence.
In simple words: Nitrogenase gets damaged by oxygen. Leghemoglobin traps oxygen to keep the environment safe for nitrogen fixation.

Exam Tip: Mention that nitrogenase is oxygen-sensitive and leghemoglobin acts as an oxygen buffer/scavenger.

Short Answer Questions-I (3 marks each)

Question 10. Write the deficiency symptoms of the following three elements:
(a) Phosphorus
(b) Magnesium
(c) Potassium
Answer:
(a) Phosphorus: Poor plant growth, leaves appear dull green, delayed seed germination, appearance of purple or red spots on leaves, and premature leaf fall.
(b) Magnesium: Chlorosis between leaf veins (interveinal chlorosis) and necrotic purple-colored spots on older leaves.
(c) Potassium: Stunted growth, yellowing and scorching of leaf edges, mottled appearance of leaves, and premature death.
In simple words: Each mineral deficiency causes distinct visible symptoms like purple spots for phosphorus, interveinal chlorosis for magnesium, and leaf-edge yellowing for potassium.

Exam Tip: Memorize at least two distinct deficiency symptoms for each major macronutrient.

Question 11. Describe the following three deficiency symptoms and co-relate them with concerned mineral deficiency:
(a) Chlorosis
(b) Necrosis
(c) Stunted plant growth
Answer:
(a) Chlorosis: The loss of chlorophyll leading to yellowing of leaves. It is caused by the deficiency of elements such as N, K, Mg, S, Fe, Mn, Zn, and Mo.
(b) Necrosis: The death of plant tissues, particularly leaf tissue, characterized by brown or dead spots. It is caused by the deficiency of Ca, Mg, Cu, and K.
(c) Stunted plant growth: Reduced overall vegetative growth and biomass accumulation, caused by the deficiency of elements like N, P, K, Ca, etc., which are vital for cell division, protein synthesis, and metabolic activities.
In simple words: Chlorosis is leaf yellowing, necrosis is tissue death, and stunted growth is slowed plant development due to lack of key nutrients.

Question 12. Explain the steps in biological nitrogen fixation in brief.
Answer: Biological nitrogen fixation involves the enzymatic reduction of atmospheric nitrogen gas into ammonia by prokaryotes possessing the nitrogenase enzyme complex. The main steps are:
1. The nitrogenase enzyme binds to atmospheric nitrogen (\(\text{N}_2\)).
2. Electrons (derived from reduced ferredoxin) and protons are transferred sequentially to the nitrogen molecule.
3. ATP hydrolysis provides the necessary activation energy (16 ATP per mole of \(\text{N}_2\) fixed).
4. Nitrogen is successively reduced through diimide and hydrazine intermediates until two molecules of ammonia (\(\text{NH}_3\)) are produced.
In simple words: Atmospheric nitrogen is reduced into ammonia using electrons, protons, and a large amount of ATP catalyzed by the nitrogenase enzyme.

Exam Tip: State the overall chemical equation and mention the ATP requirement (16 ATP).

Question 13. Describe the two main processes of synthesis of amino acids from Ammonium ion (\(\text{NH}_4^+\)) in plants.
Answer: Ammonia produced by nitrogen fixation is toxic to plants and is rapidly converted into amino acids through two primary pathways:
1. Reductive amination: Ammonia reacts with \(\alpha\)-ketoglutaric acid in the presence of the enzyme glutamate dehydrogenase to form glutamic acid.
\(\alpha\text{-ketoglutaric acid} + \text{NH}_4^+ + \text{NADPH} \xrightarrow{\text{glutamate dehydrogenase}} \text{Glutamate} + \text{H}_2\text{O} + \text{NADP}^+\)
2. Transamination: It involves the transfer of an amino group (\(-\text{NH}_2\)) from a donor amino acid (typically glutamic acid) to the keto group of a keto acid, catalyzed by transaminase enzymes, thereby synthesizing various other amino acids.
In simple words: Plants convert toxic ammonia into amino acids either by direct reductive amination with keto acids or by transferring amino groups via transamination.

Question 14. Describe all the steps of nitrogen cycle in nature.
Answer: The nitrogen cycle is a biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmospheric, terrestrial, and marine ecosystems. The key steps include:
1. Nitrogen Fixation: Conversion of inert atmospheric nitrogen (\(\text{N}_2\)) into usable nitrogen compounds (\(\text{NH}_3\) or nitrates) via biological fixation (by Rhizobium, Azotobacter, etc.), industrial fixation, or atmospheric electrical discharge.
2. Ammonification: The decomposition of organic nitrogenous matter from dead plants and animals and animal excretions into ammonia by ammonifying bacteria and fungi.
3. Nitrification: The biological oxidation of ammonia to nitrites by Nitrosomonas bacteria, followed by the oxidation of nitrites into nitrates by Nitrobacter bacteria.
4. Assimilation: Plants absorb nitrates and ammonium ions from the soil through their roots and incorporate them into plant proteins and nucleic acids.
5. Denitrification: The reduction of soil nitrates back into gaseous nitrogen (\(\text{N}_2\)) by denitrifying bacteria such as Pseudomonas and Thiobacillus, returning nitrogen to the atmosphere.
In simple words: The nitrogen cycle moves nitrogen from the air into soil and plants through fixation, converts waste through ammonification and nitrification, and returns nitrogen to the atmosphere via denitrification.

Exam Tip: Outline all five stages clearly with relevant bacterial examples for each conversion.

Question 15. Describe with diagrams how root nodules are formed in leguminous plants.
Answer: Root nodule formation in legumes is a complex, multi-step symbiotic interaction between host legume roots and soil bacteria (Rhizobium). The sequence of steps includes:
1. Rhizobia multiply and colonize the surroundings of roots and get attached to epidermal and root hair cells.
2. Root hairs curl and hook around the bacteria due to chemical signals secreted by both partners.
3. An infection thread is formed inward through the root hair, carrying the bacteria into the root cortex.
4. The bacteria are released into cortical cells, where they differentiate into specialized, nitrogen-fixing cells called bacteroids.
5. Simultaneously, cell division in the inner cortex and pericycle is induced, leading to the formation of the visible root nodule.
In simple words: Rhizobium bacteria attach to root hairs, cause them to curl, enter through an infection thread, and stimulate root cells to divide and form a nitrogen-fixing nodule.

Exam Tip: List the chronological steps from root hair curling to bacteroid differentiation and cortical cell division.