Photosynthesis

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.

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.

30. What ecological role do photosynthetic cyanobacteria usually occupy?

[SSC MTS, 30 Oct 2024 (Shift 2)]

• (A) Primary consumers
• (B) Producers
• (C) Decomposers only
• (D) Tertiary consumers

Answer: (B) Producers

Explanation: Cyanobacteria are prokaryotes that perform oxygenic photosynthesis using chlorophyll a. They convert inorganic carbon into organic material, supporting aquatic and terrestrial food webs. Some also fix atmospheric nitrogen, but their producer status arises from making food using light.

29. Which structure within a plant leaf contains the chlorophyll used in photosynthesis?

[UPSC CDS-II, General Knowledge (1 Sep 2024)]

• (A) Nucleus
• (B) Ribosome
• (C) Chloroplast
• (D) Chromosome

Answer: (C) Chloroplast

Explanation: In higher plants, chlorophyll occurs in the thylakoid membranes of chloroplasts. These organelles house both the light reactions and the Calvin cycle. Nuclei and chromosomes store genetic information, while ribosomes synthesise proteins rather than absorb photosynthetic light.

28. Which of the following is NOT required as a reactant or essential condition for photosynthesis?

[UPSC NDA-II, General Ability (3 Sep 2023)]

• (A) Chlorophyll
• (B) Carbon dioxide
• (C) Water
• (D) Oxygen

Answer: (D) Oxygen

Explanation: Oxygen is released as a by-product when water is split during the light reactions. Photosynthesis needs water, carbon dioxide, light and the relevant photosynthetic machinery, including chlorophyll. Oxygen is used in respiration and can compete with carbon dioxide at RuBisCO.

27. After boiling a sunlit leaf in water and alcohol, what colour develops on adding iodine if starch is present?

[UPSC CDS-I, General Knowledge (16 Apr 2023)]

• (A) Colourless
• (B) Green
• (C) Blue-black
• (D) White

Answer: (C) Blue-black

Explanation: Alcohol removes chlorophyll so the iodine result can be observed clearly. Iodine forms a blue-black complex with starch accumulated in a photosynthesising leaf. Boiling in water kills the tissue and makes cell membranes more permeable; chlorophyll removal follows.

26. Which unicellular green alga has a characteristic cup-shaped chloroplast?

[SSC CHSL Tier-I, 10 Aug 2023 (Shift 4)]

• (A) Hydrodictyon
• (B) Ulva
• (C) Chlamydomonas
• (D) Oedogonium

Answer: (C) Chlamydomonas

Explanation: Chlamydomonas is a unicellular green alga with a conspicuous cup-shaped chloroplast that captures light for photosynthesis. Hydrodictyon forms colonies, Ulva is a multicellular sea lettuce, and Oedogonium grows as unbranched filaments, so they do not fit the description.

25. Which regions of visible light are most strongly absorbed by chlorophyll?

[68th BPSC Preliminary Examination 2023]

• (A) Red and blue
• (B) Green and blue
• (C) Green and red
• (D) Green and yellow

Answer: (A) Red and blue

Explanation: Chlorophyll has strong absorption bands in blue and red wavelengths but reflects or transmits much of the green region. This produces the familiar green appearance of leaves. Accessory pigments broaden overall absorption, while chlorophyll a remains the principal reaction-centre pigment.

24. Which microscopic photosynthetic organisms form the main primary-producer base of marine food webs?

[SSC MTS, 6 Sep 2023 (Shift 2)]

• (A) Phytoplankton
• (B) Zooplankton
• (C) Viruses
• (D) Most decomposer fungi

Answer: (A) Phytoplankton

Explanation: Phytoplankton include microscopic algae and cyanobacteria that fix carbon dioxide using sunlight. They produce organic matter eaten by zooplankton and other consumers and contribute substantially to global oxygen production. Zooplankton mostly graze rather than photosynthesise.

23. Which set correctly pairs cell organelles with their main functions?

[UPSC CDS, General Knowledge (3 Sep 2023)]

• (A) Mitochondria—respiration; chloroplast—photosynthesis; ribosome—protein synthesis
• (B) Mitochondria—photosynthesis; chloroplast—respiration; ribosome—protein synthesis
• (C) Mitochondria—respiration; chloroplast—protein synthesis; ribosome—photosynthesis
• (D) Mitochondria—protein synthesis; chloroplast—respiration; ribosome—starch storage

Answer: (A) Mitochondria—respiration; chloroplast—photosynthesis; ribosome—protein synthesis

Explanation: Mitochondria principally perform aerobic energy metabolism, chloroplasts perform photosynthesis, and ribosomes assemble proteins from amino acids. These functions belong to different cellular structures, although chloroplasts and mitochondria also contain their own ribosomes for synthesising some internal proteins.

22. Which statement correctly describes photosynthesis?

[UPSC CDS-II, General Knowledge (4 Sep 2022)]

• (A) Light energy becomes kinetic energy that oxidises carbon dioxide
• (B) Light energy directly reduces water to carbon dioxide
• (C) Chlorophyll absorbs chemical energy to oxidise carbon dioxide
• (D) Chlorophyll captures light; water is split and carbon dioxide is reduced

Answer: (D) Chlorophyll captures light; water is split and carbon dioxide is reduced

Explanation: Photosynthetic pigments capture light and drive electron transfer. Water loses electrons during photolysis, whereas carbon dioxide gains reducing equivalents in the Calvin cycle to form carbohydrate. Thus water is oxidised and carbon dioxide is reduced; the other choices reverse these events.

21. Which natural compound enables green plants to absorb the light needed for photosynthesis?

[SSC MTS, 6 Jul 2022 (Shift 3)]

• (A) Chlorophyll
• (B) Glucose
• (C) Curcumin
• (D) Lectin

Answer: (A) Chlorophyll

Explanation: Chlorophyll is the key light-absorbing pigment in plants. Its porphyrin-like head contains magnesium and is embedded with associated proteins in thylakoid membranes. Glucose is a photosynthetic product, while curcumin and lectins do not form the normal plant photosynthetic antenna.

20. Approximately what percentage of sunlight falling on leaves is captured as food energy by green plants in a terrestrial ecosystem, according to the standard school-level estimate?

[SSC CHSL 2022 cycle, held 17 Mar 2023 (Shift 4)]

• (A) 20%
• (B) 10%
• (C) 1%
• (D) 5%

Answer: (C) 1%

Explanation: The conventional ecology textbook approximation is that green plants convert about one percent of incident sunlight into stored food energy at ecosystem scale. Actual conversion efficiency varies greatly with vegetation, climate and measurement method; this is an approximate examination figure, not a universal constant.

19. Which gas is required as a raw material for photosynthesis?

[RRB NTPC CBT-I, 8 Jan 2021 (Shift 2)]

• (A) Carbon dioxide
• (B) Oxygen
• (C) Hydrogen gas
• (D) Carbon monoxide

Answer: (A) Carbon dioxide

Explanation: Plants take carbon dioxide through stomata and incorporate its carbon into sugars via the Calvin cycle. Water is the source of electrons in oxygenic photosynthesis. Oxygen is released during water splitting, while carbon monoxide and hydrogen gas are not ordinary raw materials.

18. Which organelle uses light energy to support photosynthesis?

[RRB NTPC CBT-I, 7 Apr 2021 (Shift 1)]

• (A) Golgi apparatus
• (B) Mitochondrion
• (C) Ribosome
• (D) Chloroplast

Answer: (D) Chloroplast

Explanation: Chloroplasts contain thylakoids with photosystems that convert light energy into ATP and NADPH. The stroma then uses those molecules to support carbon fixation. Mitochondria mainly mediate aerobic respiration, ribosomes make proteins, and the Golgi apparatus modifies and sorts cellular products.

17. How many ATP molecules does the Calvin cycle require for the net synthesis of one glucose from six CO₂ molecules?

[CGPSC (Chhattisgarh PCS), Prelims 2020]

• (A) 16
• (B) 18
• (C) 20
• (D) 24

Answer: (B) 18

Explanation: For six carbon dioxide molecules, the Calvin cycle requires 18 ATP and 12 NADPH to produce two net molecules of glyceraldehyde-3-phosphate, sufficient to make one glucose. This ATP expenditure covers both reduction reactions and regeneration of RuBP.

16. Which element is NOT a constituent of the chlorophyll molecule?

[66th BPSC Preliminary Re-examination 2020]

• (A) Calcium
• (B) Carbon
• (C) Magnesium
• (D) Hydrogen

Answer: (A) Calcium

Explanation: Chlorophyll contains carbon, hydrogen, oxygen, nitrogen and a central magnesium ion. Calcium is important to plant physiology but is not part of the basic chlorophyll molecular structure. Magnesium deficiency commonly causes leaf yellowing, called chlorosis, because chlorophyll production is impaired.

15. Which gas is released as a by-product of oxygenic photosynthesis?

[65th BPSC Preliminary Examination 2019]

• (A) Carbon dioxide
• (B) Oxygen
• (C) Nitrogen
• (D) Sulphur dioxide

Answer: (B) Oxygen

Explanation: Oxygen is formed when water molecules are oxidised by the oxygen-evolving complex attached to Photosystem II. Carbon dioxide is used to build carbohydrates. This photosynthetic oxygen release underpins much of atmospheric oxygen available for aerobic organisms.

14. In which cell organelle is solar energy converted into ATP during photosynthesis?

[65th BPSC Preliminary Examination 2019]

• (A) Mitochondrion
• (B) Chloroplast
• (C) Ribosome
• (D) Peroxisome

Answer: (B) Chloroplast

Explanation: Photophosphorylation takes place across thylakoid membranes of chloroplasts. Light-powered electron transport establishes a proton gradient that drives ATP synthase, producing ATP. The mitochondrion also makes ATP, but principally by oxidative phosphorylation rather than by using captured sunlight.

13. Succulent plants that fix CO₂ at night as malic acid and use it during daytime follow which pathway?

[RPSC Biology Paper-II 2018]

• (A) Cyclic plants
• (B) CAM pathway
• (C) Pitcher-plant pathway
• (D) Photorespiratory pathway

Answer: (B) CAM pathway

Explanation: Crassulacean acid metabolism allows CAM plants to capture carbon dioxide at night and store it temporarily as organic acids such as malate. Releasing carbon dioxide during the day permits carbon fixation while stomata stay relatively closed, conserving water in arid habitats.

12. Which enzymes catalyse primary carbon dioxide fixation in C₃ and C₄ plants, respectively?

[RPSC Biology Paper-II 2018]

• (A) RuBisCO in C₃; PEP carboxylase in C₄
• (B) PEP carboxylase in C₃; RuBisCO in C₄
• (C) RuBisCO exclusively in both initial fixation steps
• (D) PEP carboxylase exclusively in both initial fixation steps

Answer: (A) RuBisCO in C₃; PEP carboxylase in C₄

Explanation: C₃ plants initially fix carbon dioxide by RuBisCO in the Calvin cycle. C₄ plants first fix bicarbonate by PEP carboxylase in mesophyll cells, producing oxaloacetate; RuBisCO subsequently operates in bundle-sheath cells. Therefore both enzymes occur in C₄ metabolism, but their roles differ.

11. During photosynthesis, sunlight energy is transformed chiefly into which form?

[MPPSC (Madhya Pradesh PCS), Prelims 2018]

• (A) Chemical energy
• (B) Mechanical energy
• (C) Sound energy
• (D) Gravitational energy

Answer: (A) Chemical energy

Explanation: Photosynthesis converts absorbed light energy into chemical energy, temporarily held in ATP and NADPH and ultimately stored in organic compounds. This energy supplies plant metabolism and food chains. Light is not primarily converted to sound or mechanical energy during this process.

10. Which element forms the central metal atom in chlorophyll?

[WBCS Mains, Science and Technology 2018]

• (A) Zinc
• (B) Iron
• (C) Magnesium
• (D) Cobalt

Answer: (C) Magnesium

Explanation: The porphyrin-like ring of chlorophyll coordinates a magnesium ion at its centre. This pigment is essential for photon absorption in photosynthesis. Iron and other micronutrients are needed for various plant functions, but magnesium is the characteristic metal of chlorophyll.

9. In photosynthesis, solar energy is ultimately converted into which form of energy?

[CGPSC (Chhattisgarh PCS), Prelims 2017]

• (A) Heat energy only
• (B) Kinetic energy
• (C) Potential energy stored in chemicals
• (D) Mechanical energy

Answer: (C) Potential energy stored in chemicals

Explanation: The correct idea is chemical potential energy: plants use absorbed sunlight to build energy-rich molecules from carbon dioxide and water. That energy remains stored in molecular bonds and can later be used through respiration. Conversion solely to heat or motion is not photosynthesis.

8. Consider two statements about cactus:
1. Leaves are modified into spines.
2. The green stem carries out photosynthesis.

Which is correct?

[UPSC NDA-II, General Ability 2016]

• (A) Statement 1 only
• (B) Statement 2 only
• (C) Both statements 1 and 2
• (D) Neither statement

Answer: (C) Both statements 1 and 2

Explanation: In cacti, the leaves are usually transformed into spines, reducing exposed surface and deterring herbivores. Green succulent stems contain chloroplasts and undertake photosynthesis. Their CAM adaptation further helps conserve water by moving much carbon dioxide uptake to nighttime.

7. Which gas is released by green plants during photosynthesis?

[UPSC NDA-II, General Ability 2015]

• (A) Carbon dioxide
• (B) Oxygen
• (C) Hydrogen
• (D) Methane

Answer: (B) Oxygen

Explanation: Photosynthetic water splitting releases molecular oxygen as a by-product. Green plants use light energy to fix carbon dioxide into carbohydrates. The oxygen evolved comes from water rather than directly from carbon dioxide, a fact established by isotopic tracer experiments.

6. Which energy transformation correctly describes photosynthesis?

[UPSC Civil Services Preliminary Examination 2014]

• (A) Potential energy becomes free energy and is released
• (B) Free energy is converted into stored potential energy
• (C) Food is oxidised to release carbon dioxide and water
• (D) Oxygen is consumed as the major raw material

Answer: (B) Free energy is converted into stored potential energy

Explanation: Photosynthesis captures incoming solar energy and stores some of it as chemical potential energy in synthesised organic compounds. Food oxidation with oxygen describes aerobic respiration, not photosynthesis. Energy storage in carbohydrates supports the transfer of energy through ecosystems.

5. From which substance does the oxygen released in photosynthesis originate?

[UP Review Officer/Assistant Review Officer, Mains 2014]

• (A) Water
• (B) Carbon dioxide
• (C) Chlorophyll
• (D) 3-phosphoglycerate

Answer: (A) Water

Explanation: Photosystem II extracts electrons from water through the oxygen-evolving complex, releasing protons and molecular oxygen. Carbon dioxide supplies carbon for sugar production, not the evolved oxygen. Oxygen-isotope labelling experiments confirmed that photosynthetic oxygen originates from water.

4. Some photosynthetic organisms can use an invisible, infrared region of light. These include certain:

[UPPSC Combined State/Upper Subordinate Services, Prelims 2013]

• (A) Trees
• (B) Algae only
• (C) Bacteria
• (D) Fungi

Answer: (C) Bacteria

Explanation: Certain anoxygenic photosynthetic bacteria possess bacteriochlorophyll pigments capable of absorbing longer wavelengths, including near-infrared light. Higher plants predominantly use visible wavelengths through chlorophylls a and b. Fungi lack the photosynthetic machinery needed to capture light for food production.

3. Which mineral element is present at the centre of the chlorophyll molecule?

[UPPSC Combined State/Upper Subordinate Services, Prelims 2012]

• (A) Iron
• (B) Copper
• (C) Magnesium
• (D) Manganese

Answer: (C) Magnesium

Explanation: A magnesium ion sits in the central ring structure of chlorophyll, the light-absorbing pigment of green plants. Magnesium availability is important for chlorophyll synthesis and normal leaf colour. Iron, copper and manganese take part in other plant processes but are not the central metal.

2. Which mechanism helps remove part of the excess atmospheric carbon dioxide through the oceans?

[UPSC Civil Services Preliminary Examination 2011]

• (A) Escape of carbon dioxide into the upper stratosphere
• (B) Photosynthesis by oceanic phytoplankton
• (C) Permanent trapping of most carbon dioxide in polar ice
• (D) Conversion of carbon dioxide directly into nitrogen gas

Answer: (B) Photosynthesis by oceanic phytoplankton

Explanation: Marine phytoplankton absorb dissolved carbon dioxide during photosynthesis and convert it into organic carbon. Some carbon enters deep water through the biological pump. This contributes to carbon sequestration; the other proposed mechanisms do not explain substantial oceanic photosynthetic carbon uptake.

1. Which gas is absorbed by green plants during photosynthesis?

[Uttarakhand PCS Preliminary Examination 2002]

• (A) Ammonia
• (B) Carbon dioxide
• (C) Chlorine
• (D) Sulphur dioxide

Answer: (B) Carbon dioxide

Explanation: Carbon dioxide enters leaves, usually through stomata, and is fixed into carbohydrates in the Calvin cycle. Water supplies electrons, and light supplies energy for ATP and NADPH synthesis. Oxygen is typically released as a by-product of photosynthetic water splitting.