GK: Class 12 General Q&A · Page 159
3,658 free answers on GK (General) for Class 12 students, across CBSE, ICSE and State boards.
Question & Answer
Questions
- GeneralClass 12All
How many ATP and NADPH molecules will be required to make one molecule of glucose through the Calvin pathway?
For synthesis of 1 glucose molecule (C₆H₁₂O₆): Requirement:6 turns of Calvin cycle (fixing 6 CO₂ molecules) Energy consumption per turn: 3 ATP molecules (2 for reduction, 1 for regeneration) 2 NADPH molecules (for reduction phase) Total for 1 glucose: 18 ATP molecules (6 × 3)
- GeneralClass 12All
Give comparison between the cyclic and non-cyclic photophosphorylation.
Feature Cyclic Photophosphorylation Non-cyclic Photophosphorylation Photosystems involved PS I only (P700) Both PS I and PS II (P700 + P680) El
- GeneralClass 12All
What is Blackman's law of limiting factors?
Blackman's Law of Limiting Factors (1905): Statement: "If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value. It is this factor which directly affects the process if its quantity is changed." Application to photosynthesis: Example at dawn: - Low light intensity → Light is limiting factor - Inc
- GeneralClass 12All
What are light harvesting complexes?
Light-Harvesting Complexes (LHC) are pigment-protein antenna complexes embedded in thylakoid membranes that: Structure: 200-300 chlorophyll molecules (mainly chlorophyll a and b) Carotenoid pigments (30-50 molecules) Integral membrane proteins (LHCI in PS I, LHCII in PS II) Function: Absorb phot
- GeneralClass 12All
Why do the leaves appear green in colour?
Leaves appear green because: Chlorophyll absorption spectrum: Chlorophyll a and b absorb strongly in: Blue region (430-450 nm) Red region (640-680 nm) Reflection of green light: Chlorophylls poorly absorb green wavelengths (500-600 nm), which are instead reflected and transmitted, making leaves appea
- GeneralClass 12All
What is RuBisCO? Where is it present in C3 and C4 plants? RuBisCO (Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase):
Properties: Most abundant enzyme on Earth (~50% of leaf soluble protein) Molecular weight: ~540 kDa Structure: 8 large subunits + 8 small subunits (L₈S₈) Dual activity: Carboxylase (CO₂ fixation) and oxygenase (photorespiration) Location in different plant types: P
- GeneralClass 12All
Differentiate between photorespiration and aerobic respiration.
Feature Photorespiration Aerobic Respiration Site Green photosynthetic cells only All living cells (green and non-green) Light requirement</str
- GeneralClass 12All
Complete the given below sequence of C4 pathway. CO₂ fixation → [C₄ acid] → Transport → Decarboxylation → [C₃ acid + CO₂]
Complete pathway: CO₂ fixation (mesophyll): PEP + CO₂ → Oxaloacetate (C₄ acid) Reduction (mesophyll): Oxaloacetate → Malate/Aspartate (C₄ acids) Transport (via plasmodesmata): C₄ acids move to bundle sheath cells Decarboxylation (bundle sheath): C₄ acid → Pyruvate (C₃ acid) + CO₂ Calvin cycle (bundle sheath): Released CO₂ f
- GeneralClass 12All
Describe briefly the contribution of Joseph Priestley. Joseph Priestley's experiments (1770):
Mouse + candle in sealed bell jar → both died due to "vitiated air" Mint plant + mouse in sealed bell jar → mouse survived Mint plant alone for several days → "restored" the air Conclusion: Plants release a substance (later identified as oxygen) that purifies air "fouled" by respiration and combustion. Significance: First evidence that plants and animals have complement
- GeneralClass 12All
Describe photosynthetic unit.
A photosynthetic unit (or quantasome) is the smallest functional unit capable of photosynthesis, consisting of: Components: 200-300 chlorophyll molecules (mostly chlorophyll a and b) 50-100 accessory pigment molecules (carotenoids, xanthophylls) 1 reaction center chlorophyll molecule (P700 in PS I, P680 in PS II) Pr
- GeneralClass 12All
Who gave the law of limiting factors?
F.F. Blackman proposed the Law of Limiting Factors in 1905. The law states: "When a physiological process depends on multiple factors, its rate is limited by the factor present in minimal or near-minimal quantity, and increasing only that factor will enhance the process rate." Application to photosynthesis: At low light, increasing light intensity boosts photosy
- GeneralClass 12All
Give the chemical equation of photosynthesis.
Overall photosynthesis equation: 6CO₂ + 12H₂O --[Light/Chlorophyll]--> C₆H₁₂O₆ + 6O₂ + 6H₂O Key points: Requires 6 molecules of CO₂ and 12 molecules of water Produces 1 glucose molecule, 6 O₂ molecules, and 6 H₂O molecules The oxygen released comes from water photolysis, not CO₂ (proven by isotope studies)
- GeneralClass 12All
What is the first stable product of C4 cycle?
The first stable product of the C4 pathway is oxaloacetic acid (OAA), a 4-carbon dicarboxylic acid. It forms when PEP carboxylase (PEPcase) catalyzes CO₂ fixation to phosphoenolpyruvate (PEP) in mesophyll cells.
- GeneralClass 12All
Name two photosynthetic pigments belonging to carotenoids.
The two major carotenoid pigments are: Carotene (β-carotene) – orange pigment, C₄₀H₅₆, precursor of vitamin A Xanthophyll (lutein, zeaxanthin) – yellow pigments containing oxygen Functions: Absorb blue-green light (450-550 nm) Protect chlorophyll from photo-oxidative damage Transfer energy to chlorophyll a
- GeneralClass 12All
Name the technique used to separate leaf pigments.
Paper chromatography (or thin-layer chromatography) separates photosynthetic pigments based on their differential solubility in organic solvents and adsorption rates on paper/silica gel. Typical Rf values (in petroleum ether:acetone): Carotene: 0.95 (highest, most non-polar) Chlorophyll a: 0.65 Chlorophyll b: 0.45 Xanthophylls: 0.35 (lowest, most polar)</li
- GeneralClass 12All
What is photorespiration?
Photorespiration is the light-dependent uptake of O₂ and release of CO₂ that occurs when RuBisCO's oxygenase activity dominates over carboxylase activity. This wasteful process: Occurs primarily in C3 plants Results in 25% loss of fixed carbon Involves three organelles: chloroplasts, peroxisomes, and mitochondria Does not produce ATP or NADPH Increases wi
- GeneralClass 12All
Expand RuBisCO.
RuBisCO stands for Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase. This bifunctional enzyme is the most abundant protein on Earth and catalyzes: Carboxylation: CO₂ fixation to RuBP (photosynthesis) Oxygenation: O₂ binding to RuBP (photorespiration)
- GeneralClass 12All
Define photophosphorylation.
Photophosphorylation is the light-dependent synthesis of ATP from ADP and inorganic phosphate (Pi) in the chloroplast thylakoid membranes. This process occurs via chemiosmosis, where light energy creates a proton gradient that drives ATP synthase enzyme activity. Two types exist: Non-cyclic photophosphorylation (produces ATP + NADPH + O₂) Cyclic photop
- GeneralClass 12All
What are pigments and which is the chief pigment associated with photosynthesis?
Pigments are light-absorbing molecules that capture specific wavelengths of electromagnetic radiation. The chief pigment in photosynthesis is chlorophyll a, which absorbs maximally at 430 nm (blue) and 662 nm (red) wavelengths and appears green because it reflects green light (500-600 nm). Other photosynthetic pigments include: Chlorophyll b (accessory pigment, yellow-g
- GeneralClass 12All
Who provided evidence for production of glucose when plants grow?
Julius Von Sachs (1862) demonstrated that glucose production occurs during plant growth through his famous half-leaf starch experiment. He showed that chlorophyll-containing regions of leaves produce starch (a glucose polymer) only in the presence of light and carbon dioxide.