General Q&A for Class 12 · Page 239
5,215 free answers in General for Class 12 students, across CBSE, ICSE and State boards.
Question & Answer
Questions
- GeneralClass 12All
Name the enzyme capable of reducing nitrogen into ammonia. Which cells divide to form root nodules?
Enzyme:Nitrogenase—a Mo-Fe protein complex that catalyzes the reduction of N₂ to NH₃ under anaerobic conditions. Cell Division for Nodule Formation: Cortical cells Pericycle cells Division and growth of these root cells, stimulated by bacterial Nod factors, lead to the formation of root nodules in leguminous plants.
- GeneralClass 12All
Describe the steps involved in nitrification with suitable reactions and names of microbes.
Nitrification is the oxidation of ammonium nitrogen to nitrate nitrogen through a two-step microbial process: Step 1: Ammonia → Nitrite Catalyzed by Nitrosomonas and/or Nitrococcus: 2NH₃ + 3O₂ → 2NO₂⁻ + 2H⁺ + 2H₂O + Energy Step 2: Nitrite → Nitrate Catalyzed by Nitrobacter and/or
- GeneralClass 12All
Give examples of the following:
Category Examples (i) Free-living aerobic N₂-fixing bacteria Azotobacter, Beijerinckia (ii) Free-living anaerobic N₂-fixing bacteria Bacillus<
- GeneralClass 12All
(i) What do you mean by nitrogen fixation? (ii) How does N₂-fixation occur in the atmosphere?
(i) Nitrogen Fixation:Nitrogen fixation is the process of converting chemically inert atmospheric nitrogen (N₂) into biologically usable forms, primarily ammonia (NH₃). This process is catalyzed by the enzyme nitrogenase in biological systems. (ii) Atmospheric N₂-Fixation: In the atmosphere, nitrogen fixation occurs through non-biological processes: <st
- GeneralClass 12All
Briefly describe the phases involved in the mechanism of mineral absorption by plants.
Mineral absorption occurs in two distinct phases: Initial Phase (Passive Phase): Rapid uptake of ions into the outer free space (apoplast)—comprising cell walls and intercellular spaces Absorption is passive, requiring no metabolic energy Ions move through ion channels down concentration gradients <l
- GeneralClass 12All
State three effects of manganese if absorbed in excess by plants.
Manganese toxicity occurs when Mn is absorbed in excessive amounts, causing: Reduction in uptake of Mg and Fe: Competitive inhibition at uptake sites. Prevention of Mg binding with enzymes: Interferes with enzyme activation processes. Inhibition of Ca translocation to shoot apex: Disrupts calcium movement, affecting meristematic growth.
- GeneralClass 12All
List four common deficiency symptoms observed in plants.
Chlorosis: Loss of chlorophyll leading to yellowing of leaves, typically due to deficiency of N, K, Mg, S, or Fe. Necrosis: Death of plant tissue, especially leaf tissue, caused by deficiency of Ca, Mg, Cu, or K. Delay in flowering: Prolonged vegetative phase and delayed reproductive development, associated with deficiency of N, S, or Mo. Inhibitio
- GeneralClass 12All
. Identify the mineral element that is a constituent of methionine and cysteine amino acids. State one other physiological role of the same.
Sulphur (S) Additional Physiological Role: Sulphur is essential for the synthesis of: Vitamins (e.g., thiamine, biotin) Coenzyme A (involved in cellular respiration) Ferredoxin (electron carrier in photosynthesis) These compounds participate in critical metabolic processes including respiration, photosynthesis, and nitrogen metabolism.
- GeneralClass 12All
Name the ionic form in which the following mineral elements are absorbed by plant roots.
Element Ionic Form Absorbed (i) Phosphorus H₂PO₄⁻ or HPO₄²⁻ (ii) Sulphur SO₄²⁻ (sulfate ion) <td w
- GeneralClass 12All
Differentiate between macronutrients and micronutrients (three points).
Macronutrients Micronutrients Present in plants in easily detectable quantities Present in very small amounts or traces Concentration above 10 mmol/kg of dry matter Co
- GeneralClass 12All
List any three criteria for determining the essentiality of any element.
Criteria of Essentiality (Arnon and Stout Criteria): Absolute Necessity: The element must be absolutely necessary for supporting normal growth and reproduction. Plants cannot complete their life cycle without it. Direct Metabolic Role: The element must be directly involved in plant metabolism—either as a constituent of essential molecules or as an enzyme activator. <st
- GeneralClass 12All
What is hydroponics? Give one application of this technique.
Hydroponics is the technique of growing plants from seeds to maturity in a soil-less environment using a defined nutrient solution containing all essential mineral elements in proper concentrations. Application: Hydroponics is commercially employed for producing: Tomatoes Seedless cucumbers Lettuce This method allows
- GeneralClass 12All
Name the root nodule pigment involved in symbiotic nitrogen fixation.
Leghaemoglobin (Leghemoglobin) This pink or red pigment in root nodules acts as an oxygen scavenger, maintaining the anaerobic conditions essential for nitrogenase enzyme activity during symbiotic nitrogen fixation in legumes.
- GeneralClass 12All
What is the difference between influx and efflux?
Influx Efflux Inward movement of ions into cells Outward movement of ions from cells Involves active or passive uptake mechanisms Involves active or passive release mechanisms </t
- GeneralClass 12All
Why do the deficiency symptoms of nitrogen appear first in the senescent leaves?
Nitrogen is a mobile element in plants, meaning it can be remobilized from older tissues to younger, actively growing regions. During nitrogen deficiency, nitrogenous compounds in senescent (older) leaves are degraded, and nitrogen is exported to young developing tissues. Therefore, deficiency symptoms like chlorosis (yellowing) appear first in older leaves while younger leaves remain relatively normal until
- GeneralClass 12All
Write the overall reaction involved in nitrogen fixation.
N₂ + 8e⁻ + 8H⁺ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pᵢ This reaction demonstrates that nitrogen fixation is highly energy-intensive, requiring 16 ATP molecules to produce just two molecules of ammonia. The enzyme nitrogenase catalyzes this reduction under strictly anaerobic conditions.
- GeneralClass 12All
Why are nitrifying bacteria called chemoautotrophs?
Nitrifying bacteria are called chemoautotrophs because they obtain energy through chemosynthesis—oxidizing inorganic nitrogen compounds (ammonia to nitrite, nitrite to nitrate) rather than using solar energy. This liberated chemical energy is used to synthesize organic compounds from CO₂ and hydrogen donors, making them independent of photosynthesis. Examples: Nitrosomonas and <st
- GeneralClass 12All
Give two examples of free-living N₂-fixing cyanobacteria.
Nostoc Anabaena Both are free-living cyanobacteria capable of biological nitrogen fixation, converting atmospheric nitrogen (N₂) into ammonia (NH₃) using the enzyme nitrogenase.
- GeneralClass 12All
What is the average concentration of trace elements required by plants?
Trace elements (also called micronutrients) are required at concentrations of less than 10 mmol/kg of dry matter. Despite their low concentrations, these elements are essential for enzyme activation and various metabolic functions.
- GeneralClass 12All
Complete the following: Mn²⁺ is involved in _________ of water.
Manganese ions (Mn²⁺) play a crucial role in the photolysis of water during the light-dependent reactions of photosynthesis, specifically in the oxygen-evolving complex of Photosystem II, where water molecules are split to release oxygen.