40. What is the point at which the rate of photosynthesis equals the rate of respiration?
[BPSC Assistant Section Officer, Prelims 2025]
- (A) Respiratory quotient
- (B) Equivalence point
- (C) Photosynthetic quotient
- (D) Compensation point
Answer: (D) Compensation point
Explanation: At the compensation point, carbon dioxide absorbed by photosynthesis balances that released by respiration, so net carbon dioxide exchange is zero. Below the light-compensation point respiration exceeds photosynthesis; above it, photosynthetic carbon gain generally becomes positive.
39. Which plant tissue primarily carries out photosynthesis?
[RRB Group D, 27 Nov 2025 (Shift 1)]
- (A) Sclerenchyma
- (B) Chlorenchyma
- (C) Phloem
- (D) Xylem
Answer: (B) Chlorenchyma
Explanation: Chlorenchyma is parenchyma containing chloroplasts and is abundant in leaf mesophyll and green stems. Its chlorophyll enables light capture and carbon fixation. Xylem transports water, phloem distributes sugars, and sclerenchyma mainly provides mechanical strength.
38. Which statement about plants is INCORRECT?
[SSC CGL, 26 Sep 2025 (Shift 1)]
- (A) Roots absorb water from soil
- (B) Phloem is specialised mainly for water transport
- (C) Chlorophyll in leaves absorbs light
- (D) Stomata permit gaseous exchange
Answer: (B) Phloem is specialised mainly for water transport
Explanation: Xylem primarily transports water and dissolved mineral salts from roots toward shoots. Phloem translocates organic food, especially sucrose, from sources to sinks. The remaining statements correctly describe root absorption, light capture by chlorophyll and gas exchange through stomata.
37. What is the principal function of subsidiary cells associated with stomata?
[RRB NTPC Graduate CBT-I, 13 Jun 2025 (Shift 1)]
- (A) Absorbing sunlight in place of chloroplasts
- (B) Supporting guard cells during stomatal operation
- (C) Secreting digestive enzymes
- (D) Transporting sugars through phloem
Answer: (B) Supporting guard cells during stomatal operation
Explanation: Subsidiary cells lie adjacent to guard cells in many leaves. They provide mechanical and physiological support, helping guard cells regulate stomatal aperture. This indirectly controls the carbon dioxide entry essential for photosynthesis and the water-vapour loss linked with transpiration.
36. How can a red or purple, non-green-looking leaf manufacture food?
[WBCS Preliminary Examination 2024]
- (A) Its coloured pigments directly replace photosynthesis
- (B) Chlorophyll is present but masked by other pigments
- (C) It absorbs glucose from nearby leaves through air
- (D) It cannot photosynthesise at all
Answer: (B) Chlorophyll is present but masked by other pigments
Explanation: Many coloured leaves retain green chlorophyll in chloroplasts, although red anthocyanins or other pigments conceal its appearance. Chlorophyll continues to absorb light, enabling photosynthesis. A visible green colour is therefore not required for a leaf to be photosynthetically active.
35. Which enzyme is widely regarded as the most abundant enzyme in the living world?
[WBCS Preliminary Examination 2024]
- (A) Lipase
- (B) DNase
- (C) Zymase
- (D) RuBisCO
Answer: (D) RuBisCO
Explanation: Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is extremely abundant because photosynthetic organisms require large amounts to fix atmospheric carbon dioxide. Its relatively modest catalytic rate contributes to this abundance. RuBisCO operates in the Calvin cycle and also catalyses oxygenation.
34. What is the site of the carbon-fixation or dark reactions inside a chloroplast?
[70th BPSC Preliminary Examination 2024]
- (A) Granum
- (B) Grana membrane
- (C) Stroma
- (D) Outer chloroplast membrane
Answer: (C) Stroma
Explanation: The Calvin cycle occurs in the stroma, the fluid matrix surrounding the thylakoids. Enzymes there consume ATP and NADPH made by light reactions in thylakoid membranes. The term dark reaction does not imply the cycle normally occurs only in darkness.
33. What is the primary function of chlorophyll in photosynthesis?
[70th BPSC Preliminary Examination 2024]
- (A) Directly reducing carbon dioxide without energy
- (B) Absorbing light energy
- (C) Having no role in photosynthesis
- (D) Acting as a storage carbohydrate
Answer: (B) Absorbing light energy
Explanation: Chlorophyll absorbs photons and excites electrons, starting the conversion of light energy into chemical energy. The resulting electron transport enables ATP and NADPH formation. Carbon dioxide reduction occurs later through enzymes of the Calvin cycle, using these energy-rich compounds.
32. Which statement about the light-dependent reactions of photosynthesis is NOT correct?
[SSC Stenographer, 10 Dec 2024 (Shift 2)]
- (A) They support ATP and NADPH synthesis
- (B) Chloroplast pigments absorb sunlight
- (C) They split water and release oxygen
- (D) They directly use light energy to synthesise glucose
Answer: (D) They directly use light energy to synthesise glucose
Explanation: Light-dependent reactions take place in thylakoid membranes, where water splitting, electron transport and photophosphorylation produce oxygen, ATP and NADPH. Sugar synthesis follows through carbon fixation in the stroma; glucose is not directly produced by the light reactions themselves.
31. In which leaf tissue does photosynthesis mainly occur?
[SSC Junior Engineer, 6 Jun 2024 (Shift 1)]
- (A) Phloem
- (B) Mesophyll
- (C) Xylem
- (D) Epidermis
Answer: (B) Mesophyll
Explanation: Palisade and spongy mesophyll cells contain numerous chloroplasts and carry out most leaf photosynthesis. Palisade cells are efficient at intercepting light, whereas air spaces in spongy mesophyll facilitate carbon dioxide diffusion. Epidermis usually has fewer chloroplasts, excluding guard cells.
