Mineral Nutrition
Welcome to HSLC Guru! This page provides complete English-medium study material for ASSEB Class 11 Biology Chapter 12 — Mineral Nutrition. You will find a clear summary, well-structured question answers (1-mark, 2-3 mark, and 5-7 mark), MCQs, fill-in-the-blanks, true/false statements, and a glossary table. The notes are designed to help students prepare confidently for the Assam State School Education Board (ASSEB) Class 11 Biology examination.
Summary
Mineral nutrition is the branch of plant physiology that studies how plants obtain, absorb, and utilise inorganic mineral elements essential for their growth, metabolism, and reproduction. Plants require a variety of mineral elements which they take up mainly from the soil through their roots, in addition to carbon, hydrogen, and oxygen which they get from air and water. Two important experimental techniques used to study the mineral requirements of plants are hydroponics and aeroponics. Hydroponics, developed by Julius von Sachs, is the technique of growing plants in a defined nutrient solution without soil, allowing scientists to identify which elements are essential by selectively omitting them. Aeroponics is a more advanced technique in which plant roots are suspended in air and sprayed periodically with a fine mist of nutrient solution.
The essential mineral elements are classified on the basis of the quantity required by the plant. Macronutrients are required in large amounts (more than 10 mmol/kg of dry matter) and include carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulphur (S). Micronutrients or trace elements are required in very small quantities (less than or equal to 10 mmol/kg of dry matter) and include iron (Fe), manganese (Mn), boron (B), zinc (Zn), copper (Cu), molybdenum (Mo), chlorine (Cl), and nickel (Ni). According to Arnon and Stout, the criteria for essentiality are: (i) the element must be absolutely necessary for normal growth and reproduction, (ii) the requirement must be specific and not replaceable by another element, and (iii) the element must be directly involved in plant metabolism.
Mineral elements perform many vital roles in plants. They serve as components of biomolecules, structural elements of cells, activators or inhibitors of enzymes, and help maintain osmotic potential. Deficiency of an essential element produces characteristic symptoms such as chlorosis (loss of chlorophyll causing yellowing of leaves), necrosis (death of tissues), stunted growth, premature fall of leaves and buds, and inhibition of cell division. Excess concentration of any mineral can also be harmful and cause toxicity, often shown as brown spots surrounded by chlorotic rings on leaves. Mineral absorption from the soil takes place in two main phases — a passive, rapid initial uptake into the apoplast (free space) and a slower, active uptake into the symplast which requires metabolic energy in the form of ATP. After absorption, mineral ions are transported to other parts of the plant through the xylem along with water.
Nitrogen is one of the most important macronutrients and is required for the synthesis of proteins, nucleic acids, vitamins, and hormones. The conversion of atmospheric nitrogen (N2) into usable forms is called nitrogen fixation, and the cycling of nitrogen through the biosphere is known as the nitrogen cycle. Biological nitrogen fixation is carried out by certain prokaryotes that possess the enzyme nitrogenase. Free-living nitrogen fixers include Azotobacter and Beijerinckia (aerobic) and Clostridium and Rhodospirillum (anaerobic). Symbiotic nitrogen fixers include Rhizobium in the root nodules of legumes, Frankia in non-legumes such as Alnus, and the cyanobacterium Anabaena in association with Azolla. The formation of root nodules in legumes involves recognition between the bacteria and the root hairs, curling of root hairs, formation of an infection thread, and finally the development of mature nodules in which nitrogenase converts N2 into ammonia (NH3), which is then assimilated into amino acids by the plant.
1-Mark Questions
Q1. What is hydroponics?
Answer: Hydroponics is the technique of growing plants in a defined nutrient solution without soil.
Q2. Who developed the technique of hydroponics?
Answer: Julius von Sachs, a German botanist, developed the technique of hydroponics in 1860.
Q3. Define essential mineral element.
Answer: An essential mineral element is one which is absolutely necessary for the normal growth and reproduction of a plant and cannot be replaced by any other element.
Q4. Name any two macronutrients required by plants.
Answer: Nitrogen (N) and phosphorus (P) are two important macronutrients required by plants.
Q5. Name any two micronutrients required by plants.
Answer: Iron (Fe) and zinc (Zn) are two essential micronutrients required by plants.
Q6. What is chlorosis?
Answer: Chlorosis is the loss of chlorophyll resulting in yellowing of leaves due to deficiency of certain mineral elements such as N, K, Mg, S, Fe, Mn, Zn, and Mo.
Q7. What is necrosis?
Answer: Necrosis is the death of plant tissues, particularly leaf tissues, caused by deficiency of elements such as Ca, Mg, Cu, and K.
Q8. Name the bacterium that forms root nodules in legumes.
Answer: Rhizobium is the bacterium that forms root nodules in leguminous plants and fixes atmospheric nitrogen.
Q9. Name the enzyme responsible for biological nitrogen fixation.
Answer: The enzyme nitrogenase is responsible for biological nitrogen fixation.
Q10. Give one example of a free-living nitrogen-fixing bacterium.
Answer: Azotobacter is an example of a free-living aerobic nitrogen-fixing bacterium.
2-3 Marks Questions
Q1. Differentiate between macronutrients and micronutrients with examples.
Answer: Macronutrients are essential mineral elements required by plants in large amounts, more than 10 mmol/kg of dry matter. Examples include carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulphur. Micronutrients (or trace elements) are required in very small quantities, less than or equal to 10 mmol/kg of dry matter. Examples include iron, manganese, boron, zinc, copper, molybdenum, chlorine, and nickel. Both groups are equally essential for normal plant growth, but they differ only in the quantity required.
Q2. What are the criteria for essentiality of a mineral element as proposed by Arnon and Stout?
Answer: Arnon and Stout (1939) proposed three criteria for an element to be considered essential: (i) the element must be absolutely necessary for the normal growth and reproduction of the plant; in its absence the plant cannot complete its life cycle, (ii) the requirement of the element must be specific, that is, it cannot be replaced by another element, and (iii) the element must be directly involved in the metabolism of the plant, for example as a constituent of an enzyme or biomolecule.
Q3. Distinguish between hydroponics and aeroponics.
Answer: Hydroponics is the technique of growing plants in a soil-less nutrient solution where the roots remain submerged in water containing all essential mineral nutrients. Aeroponics, on the other hand, is a method in which the roots of the plant are suspended in air inside an enclosed chamber and are periodically sprayed with a fine mist of nutrient solution. Both methods are used to study the mineral requirements of plants and for commercial cultivation of vegetables and ornamental plants without soil.
Q4. What are the common deficiency symptoms shown by plants?
Answer: The common deficiency symptoms shown by plants include: (i) chlorosis — loss of chlorophyll leading to yellowing of leaves, (ii) necrosis — death of tissues, especially leaf tissues, (iii) stunted growth — inhibition of cell division and elongation, (iv) premature fall of leaves and buds, (v) inhibition of flowering, and (vi) anthocyanin formation in some cases. The pattern of these symptoms (whether seen on younger or older leaves) helps in identifying which element is deficient.
Q5. Briefly describe the role of nitrogen in plants.
Answer: Nitrogen is one of the most important macronutrients required by plants in the largest quantity. It is absorbed mainly in the form of nitrate (NO3−) and ammonium (NH4+) ions. Nitrogen is an essential constituent of proteins, nucleic acids (DNA and RNA), vitamins, hormones, alkaloids, and chlorophyll. It is required for normal growth, leaf expansion, and grain formation. Deficiency of nitrogen causes chlorosis, stunted growth, and reduced flowering and fruiting.
Q6. What is biological nitrogen fixation? Name two symbiotic nitrogen-fixing organisms.
Answer: Biological nitrogen fixation is the conversion of atmospheric molecular nitrogen (N2) into ammonia (NH3) by certain prokaryotes through the action of the enzyme nitrogenase. This is a vital biological process that makes nitrogen available to plants in a usable form. Two examples of symbiotic nitrogen-fixing organisms are Rhizobium, which forms root nodules in leguminous plants such as pea and soybean, and Frankia, which forms nodules in non-leguminous plants such as Alnus and Casuarina.
5-7 Marks Questions
Q1. Describe the methods used to study the mineral requirements of plants.
Answer: Two main techniques are used to study the mineral requirements of plants: hydroponics and aeroponics.
(i) Hydroponics: This is the technique of growing plants in a defined nutrient solution without soil. It was developed by the German botanist Julius von Sachs in 1860. In this method, plants are grown in a container filled with a nutrient solution containing all the essential mineral elements in known concentrations. The roots remain submerged in this solution, and the solution is aerated regularly to provide oxygen to the roots. By selectively omitting one element at a time from the nutrient medium, scientists can determine which element is essential and identify deficiency symptoms produced by its absence.
(ii) Aeroponics: Aeroponics is a more advanced technique in which the roots of the plant are suspended in air inside a closed chamber. The roots are sprayed periodically with a fine mist of nutrient solution. This technique gives better aeration to the roots and allows easier observation of root growth and nutrient uptake. Aeroponics is used both for research and for commercial production of high-quality vegetables.
Importance: These soil-less techniques have helped researchers identify essential elements, study deficiency symptoms, and produce crops in regions where soil is poor or unavailable, such as deserts, rooftops, and even space stations.
Q2. Explain the role of various macronutrients in plants.
Answer: Macronutrients are required by plants in large quantities and play many essential roles:
(i) Nitrogen (N): Constituent of proteins, nucleic acids, chlorophyll, vitamins, and hormones. Required for vegetative growth.
(ii) Phosphorus (P): Component of cell membranes (phospholipids), nucleic acids, ATP, and NADP. Important in energy transfer reactions.
(iii) Potassium (K): Maintains turgor pressure of cells, regulates opening and closing of stomata, activates enzymes, and helps in protein synthesis.
(iv) Calcium (Ca): Required for the formation of cell walls (calcium pectate of middle lamella) and proper functioning of cell membranes; activates certain enzymes and is needed for cell division.
(v) Magnesium (Mg): Central atom of the chlorophyll molecule, activates enzymes of respiration and photosynthesis, helps in DNA and RNA synthesis.
(vi) Sulphur (S): Component of amino acids cysteine and methionine, several coenzymes, vitamins (thiamine, biotin), and ferredoxin.
Carbon, hydrogen, and oxygen, although classified as macronutrients, are obtained from CO2 and H2O. They form the structural framework of all organic molecules in the plant.
Q3. Describe the role of micronutrients and the deficiency symptoms caused by their absence.
Answer: Micronutrients, although required in very small quantities, are essential for many physiological functions of plants:
(i) Iron (Fe): Required for the synthesis of chlorophyll and is a component of cytochromes, ferredoxin, and several enzymes. Deficiency causes chlorosis in young leaves.
(ii) Manganese (Mn): Activates many enzymes; required for splitting of water during photosynthesis. Deficiency causes chlorosis with grey spots between veins.
(iii) Boron (B): Necessary for uptake and utilisation of Ca2+, pollen germination, cell elongation, cell differentiation, and carbohydrate translocation. Deficiency causes death of growing tips, brittle leaves, and “heart rot” of beet root.
(iv) Zinc (Zn): Activates several enzymes, especially carboxylase; needed for synthesis of auxin (IAA). Deficiency causes “little leaf” disease and stunted growth.
(v) Copper (Cu): Component of certain enzymes such as plastocyanin and is needed for the overall metabolism of the plant. Deficiency causes “exanthema” or “die-back” disease of citrus.
(vi) Molybdenum (Mo): Component of nitrogenase and nitrate reductase enzymes; essential for nitrogen metabolism. Deficiency causes mottling of leaves and “whiptail” disease of cauliflower.
(vii) Chlorine (Cl): Required along with Na+ and K+ for solute concentration; essential for the splitting of water during photosynthesis (Hill reaction). Deficiency causes wilting and chlorosis.
(viii) Nickel (Ni): Component of the enzyme urease; required for nitrogen metabolism in legumes.
Both deficiency and excess (toxicity) of these micronutrients lead to abnormal growth and reduced yield. Toxicity is often shown as brown spots surrounded by chlorotic rings on leaves.
Q4. Describe the mechanism of absorption and transport of mineral elements in plants.
Answer: Absorption of mineral elements by plants takes place mainly through the young, growing region of the roots called the root hair zone. The process occurs in two phases:
(i) Passive absorption: This is the first, rapid phase in which mineral ions move from the soil solution into the apoplast (intercellular spaces and cell walls of the root) along the concentration gradient. It does not require metabolic energy and occurs by diffusion or mass flow with water.
(ii) Active absorption: This is the slower, second phase in which mineral ions are taken into the symplast (cytoplasm of cells connected through plasmodesmata). This movement is against the concentration gradient and requires metabolic energy in the form of ATP. Specific transport proteins (carrier proteins or pumps) embedded in the plasma membrane facilitate this transport.
Transport of minerals: Once minerals enter the symplast of root cells, they pass through the cortex, endodermis (regulated by Casparian strips), pericycle, and finally enter the xylem vessels. From the xylem of the root, mineral ions are transported upward to the stem and leaves through the transpiration pull along with water. After reaching the leaves and other organs, minerals are unloaded from the xylem and used in metabolism. Some minerals are mobile (e.g., N, P, K) and can be re-translocated through the phloem from older leaves to younger growing parts, while others such as calcium are relatively immobile.
Q5. Describe the nitrogen cycle and explain the process of biological nitrogen fixation including nodule formation in legumes.
Answer: The nitrogen cycle is the cyclic flow of nitrogen between the atmosphere, soil, and living organisms. The atmosphere contains about 78% nitrogen as N2, but plants cannot use it directly. The cycle includes the following steps:
(i) Nitrogen fixation: Conversion of N2 into ammonia (NH3). This may be physical (lightning), industrial (Haber’s process), or biological (by microbes).
(ii) Ammonification: Decomposition of dead organic matter and release of NH3 by saprophytic bacteria and fungi.
(iii) Nitrification: Conversion of NH3 into nitrites (NO2−) by Nitrosomonas and then into nitrates (NO3−) by Nitrobacter. Plants absorb nitrogen mainly as NO3−.
(iv) Denitrification: Conversion of nitrates back into N2 by denitrifying bacteria such as Pseudomonas and Thiobacillus, returning nitrogen to the atmosphere.
Biological nitrogen fixation is carried out by certain prokaryotes that possess the enzyme nitrogenase. They may be free-living (e.g., Azotobacter, Beijerinckia — aerobic; Clostridium, Rhodospirillum — anaerobic) or symbiotic (e.g., Rhizobium in legumes, Frankia in non-legumes such as Alnus, Anabaena with Azolla).
Nodule formation in legumes: The development of root nodules in leguminous plants such as pea, gram, and soybean involves the following steps:
(a) Rhizobium bacteria multiply in the soil around the roots and come into contact with root hairs.
(b) The root hairs curl, and bacteria invade them through an “infection thread”.
(c) The infection thread carries bacteria into the cortex of the root, where the cells divide and form a nodule.
(d) Inside the nodule, bacteria become non-motile, enlarged forms called bacteroids.
(e) The mature nodule contains the pink pigment leghaemoglobin, which protects nitrogenase from oxygen by maintaining a low O2 concentration.
(f) Nitrogenase converts atmospheric N2 to NH3, which is then assimilated by the plant into amino acids, while the bacteria receive carbohydrates and shelter from the host plant.
Multiple Choice Questions (MCQs)
Q1. The technique of growing plants in a nutrient solution without soil is called:
(a) Aeroponics (b) Hydroponics (c) Geoponics (d) Biotechnology
Answer: (b) Hydroponics
Q2. Which of the following is a micronutrient?
(a) Nitrogen (b) Calcium (c) Boron (d) Sulphur
Answer: (c) Boron
Q3. The central atom of the chlorophyll molecule is:
(a) Iron (b) Magnesium (c) Calcium (d) Manganese
Answer: (b) Magnesium
Q4. Yellowing of leaves due to deficiency of mineral elements is called:
(a) Necrosis (b) Chlorosis (c) Etiolation (d) Wilting
Answer: (b) Chlorosis
Q5. Which enzyme is responsible for biological nitrogen fixation?
(a) Nitrate reductase (b) Nitrogenase (c) Urease (d) Carbonic anhydrase
Answer: (b) Nitrogenase
Q6. Root nodules of legumes contain a pink pigment called:
(a) Haemoglobin (b) Leghaemoglobin (c) Phycocyanin (d) Anthocyanin
Answer: (b) Leghaemoglobin
Q7. Which of the following is a free-living aerobic nitrogen-fixing bacterium?
(a) Rhizobium (b) Clostridium (c) Azotobacter (d) Frankia
Answer: (c) Azotobacter
Q8. The criteria of essentiality of mineral elements were proposed by:
(a) Sachs (b) Knop (c) Arnon and Stout (d) Liebig
Answer: (c) Arnon and Stout
Q9. Whiptail disease of cauliflower is caused by deficiency of:
(a) Boron (b) Molybdenum (c) Zinc (d) Copper
Answer: (b) Molybdenum
Q10. Which of the following symbiotic associations fixes atmospheric nitrogen?
(a) Mycorrhiza (b) Lichen (c) Rhizobium and legume (d) Algae and fungi
Answer: (c) Rhizobium and legume
Fill in the Blanks
Q1. The technique of growing plants with their roots suspended in air and sprayed with nutrient solution is called __________.
Answer: aeroponics
Q2. The pink pigment present in root nodules of legumes is __________.
Answer: leghaemoglobin
Q3. The central metallic atom of the chlorophyll molecule is __________.
Answer: magnesium
Q4. The bacterium that forms a symbiotic association with non-leguminous plants such as Alnus is __________.
Answer: Frankia
Q5. The death of plant tissues due to mineral deficiency is called __________.
Answer: necrosis
True or False
Q1. Hydroponics is the technique of growing plants in nutrient solution without soil.
Answer: True
Q2. Boron is a macronutrient required by plants in large amounts.
Answer: False (Boron is a micronutrient.)
Q3. Nitrogenase is the enzyme that catalyses the conversion of atmospheric nitrogen into ammonia.
Answer: True
Q4. Anabaena forms a symbiotic association with the water fern Azolla.
Answer: True
Q5. Calcium is a highly mobile element in plants and is easily re-translocated to younger leaves.
Answer: False (Calcium is relatively immobile in plants.)
Glossary
| Term | Meaning |
|---|---|
| Hydroponics | Technique of growing plants in nutrient solution without soil. |
| Aeroponics | Technique of growing plants with roots suspended in air and sprayed with nutrient mist. |
| Macronutrient | Mineral element required by plants in large quantities (more than 10 mmol/kg dry matter). |
| Micronutrient | Mineral element required by plants in trace amounts (less than or equal to 10 mmol/kg dry matter). |
| Essential element | Mineral element that is indispensable for plant growth and reproduction. |
| Chlorosis | Yellowing of leaves due to loss of chlorophyll from mineral deficiency. |
| Necrosis | Death of plant tissues caused by deficiency of certain mineral elements. |
| Toxicity | Harmful effects on plants caused by excess concentration of mineral elements. |
| Apoplast | Continuous system of cell walls and intercellular spaces through which water and ions move freely. |
| Symplast | Continuous system of cytoplasm of cells connected by plasmodesmata. |
| Nitrogen fixation | Conversion of atmospheric N2 into ammonia. |
| Nitrogenase | Enzyme that catalyses biological nitrogen fixation. |
| Rhizobium | Symbiotic nitrogen-fixing bacterium found in root nodules of legumes. |
| Frankia | Actinomycete that forms nitrogen-fixing nodules in non-leguminous plants such as Alnus. |
| Azotobacter | Free-living aerobic nitrogen-fixing bacterium. |
| Anabaena | Cyanobacterium that fixes nitrogen, often in symbiosis with Azolla. |
| Leghaemoglobin | Pink, oxygen-scavenging pigment in legume root nodules that protects nitrogenase. |
| Bacteroid | Modified, non-motile form of Rhizobium inside legume root nodules. |
| Nitrification | Conversion of ammonia to nitrites and then to nitrates by soil bacteria. |
| Denitrification | Conversion of nitrates back into atmospheric N2 by denitrifying bacteria. |