50. Which process is involved in the light-independent reactions of photosynthesis?
[UPSC CDS-II, General Knowledge (13 Sep 2026)]
- (A) Calvin cycle
- (B) Hill reaction
- (C) Excitation of Photosystem II
- (D) Absorption of light by pigments
Answer: (A) Calvin cycle
Explanation: The Calvin cycle uses ATP and NADPH generated during the light reactions to fix carbon dioxide into organic molecules. It operates in the chloroplast stroma. Hill reaction, photosystem excitation and light absorption belong to the light-dependent stage.
49. Which of the following is NOT a function of carotenoid pigments?
[UPSC CDS-I, General Knowledge (12 Apr 2026)]
- (A) Producing yellow-orange leaf colours
- (B) Protecting chlorophyll against photo-oxidation
- (C) Helping protect human eyes from light damage
- (D) Acting as the reaction centre of a photosystem
Answer: (D) Acting as the reaction centre of a photosystem
Explanation: Carotenoids are accessory pigments that absorb additional wavelengths and protect photosynthetic systems from excessive light. The reaction centres P680 and P700 instead contain specialised chlorophyll a molecules. Carotenoids also contribute yellow-orange colours and certain eye-protective pigments.
48. Splitting of water during photosynthesis ultimately produces which pair of substances?
[RRB Group D, 3 Aug 2026 (2025 recruitment cycle)]
- (A) Oxygen and carbon
- (B) Nitrogen and oxygen
- (C) Hydrogen and oxygen
- (D) Nitrogen and carbon
Answer: (C) Hydrogen and oxygen
Explanation: Photolysis of water at Photosystem II yields oxygen, electrons and hydrogen ions (protons). Thus the conventional answer is hydrogen and oxygen, although free hydrogen gas is not produced. Electrons replace those lost by chlorophyll, while protons contribute to ATP formation.
47. What is the primary role of stomata during photosynthesis?
[RRB Group D, 11 Aug 2026 (2025 recruitment cycle)]
- (A) Maintaining a constant leaf temperature
- (B) Preventing carbon dioxide entry
- (C) Allowing gaseous exchange
- (D) Absorbing oxygen as the only input
Answer: (C) Allowing gaseous exchange
Explanation: Stomata are small epidermal pores controlled by guard cells. They admit carbon dioxide required for carbon fixation and permit oxygen and water vapour to diffuse out. Water stress can close stomata, decreasing carbon dioxide supply and hence photosynthetic rate.
46. Why does a desert plant that takes in CO₂ at night lose less water?
[RRB Group D, 25 Aug 2026 (2025 recruitment cycle)]
- (A) It manufactures all carbohydrates only at night
- (B) Its stomata open during cooler night-time hours
- (C) It no longer needs chlorophyll
- (D) It never carries out photosynthesis during the day
Answer: (B) Its stomata open during cooler night-time hours
Explanation: Many succulents use crassulacean acid metabolism (CAM). Their stomata open at night when evaporative demand is lower, and absorbed carbon dioxide is stored as organic acids. During daylight, released carbon dioxide enters the Calvin cycle while stomata remain mostly closed.
45. Which pigment is chiefly responsible for absorbing light for photosynthesis?
[RRB Group D, 21 Aug 2026 (2025 recruitment cycle)]
- (A) Anthocyanin
- (B) Carotene alone
- (C) Keratin
- (D) Chlorophyll
Answer: (D) Chlorophyll
Explanation: Chlorophyll pigments in thylakoid membranes capture visible light, especially in blue and red regions. Chlorophyll a forms each reaction centre, whereas chlorophyll b and carotenoids broaden the usable spectrum. Keratin is an animal protein, not a plant photosynthetic pigment.
44. In which form are excess carbohydrates produced by photosynthesis generally stored in plants?
[RRB Group D, 10 Aug 2026 (2025 recruitment cycle)]
- (A) Cellulose
- (B) Glycogen
- (C) Protein
- (D) Starch
Answer: (D) Starch
Explanation: Plants convert surplus photosynthetic sugar into starch, an insoluble glucose polymer that can be stored without greatly increasing osmotic pressure. Cellulose serves mainly as a structural cell-wall component; glycogen is characteristic of animals and fungi, not the principal plant reserve.
43. What is another name for the light-harvesting complex in photosynthesis?
[SSC CGL, 25 Sep 2025 (Shift 2)]
- (A) Antenna complex
- (B) ATP synthase complex
- (C) Ribosome complex
- (D) Spliceosome complex
Answer: (A) Antenna complex
Explanation: An antenna complex contains chlorophylls and accessory pigments bound to proteins. Together they absorb photons across several wavelengths and transfer excitation energy to a reaction-centre chlorophyll a. ATP synthase, ribosomes and spliceosomes perform entirely different cellular functions.
42. Which process is usually negligible in C₄ plants because of their carbon-concentrating mechanism?
[RRB NTPC Undergraduate CBT-I, 22 Aug 2025 (Shift 2)]
- (A) Transpiration
- (B) Photorespiration
- (C) Photosynthesis
- (D) Cellular respiration
Answer: (B) Photorespiration
Explanation: C₄ plants concentrate carbon dioxide around RuBisCO in bundle-sheath cells, strongly suppressing its oxygenase activity. Consequently photorespiration is negligible, especially under favourable warm conditions. Transpiration, photosynthesis and respiration still occur; the mechanism does not eliminate them.
41. What role does RuBisCO play in photosynthesis?
[BPSC Assistant Section Officer, Prelims 2025]
- (A) Catalysing carbon dioxide fixation
- (B) Opening and closing stomata
- (C) Splitting water directly to liberate oxygen
- (D) Transferring electrons to make ATP
Answer: (A) Catalysing carbon dioxide fixation
Explanation: RuBisCO catalyses the addition of carbon dioxide to ribulose-1,5-bisphosphate in the Calvin cycle, yielding a short-lived six-carbon intermediate that divides into two 3-phosphoglycerate molecules. The enzyme can also bind oxygen, leading to photorespiration in C₃ plants.