Photosynthesis in Higher Plants (NCERT Practice Paper)

Q1. Regarding Chemiosmotic hypothesis, which are correct?
(i) ATP synthesis is linked to proton gradient.
(ii) ATP synthase is located on the stroma lamellae only.
(iii) Protons move from lumen to stroma through CF0.
Correct Answer: (a)
The chemiosmotic hypothesis states that ATP synthesis is driven by a proton gradient (i). Protons move from the lumen to the stroma through the CF₀ channel of ATP synthase (iii). ATP synthase is not restricted to stroma lamellae; it is also present in grana membranes. Hence, (ii) is false, and (i) and (iii) are correct.
Q2. Assertion: The first CO2 fixation product in C3 plants is PGA. Reason: PGA is a 3-carbon organic acid.
Correct Answer: (b)
Both statements are true, but the reason does not explain why PGA is the first product; it merely describes a property of PGA. The fact that it is 3‑carbon is a consequence of the carboxylation of RuBP, but it does not account for the sequence of reactions that leads to its formation.
Q3. Assertion: RuBisCO is the most abundant enzyme in the world. Reason: Its active site can bind to both CO2 and O2.
Correct Answer: (b)
RuBisCO is indeed the most abundant enzyme on Earth, partly because it is relatively inefficient and plants produce large amounts of it. The ability to bind both CO₂ and O₂ is a characteristic of the enzyme, but its abundance is not due to this dual affinity; it is a consequence of the need for large amounts to achieve adequate carbon fixation rates.
Q4. Assertion: In C4 plants, the bundle sheath cells have no intercellular spaces. Reason: These cells have thick walls impervious to gaseous exchange.
Correct Answer: (a)
Bundle sheath cells in C₄ plants are tightly packed without intercellular spaces, and their thick walls limit gas exchange. This structural feature is essential for maintaining a high CO₂ concentration around RuBisCO, preventing the loss of CO₂ and suppressing photorespiration. The reason correctly explains the assertion.
Q5. Which statements about C4 plants are true?
(i) They tolerate higher temperatures.
(ii) They show a response to high light intensities.
(iii) They have photorespiration.
Correct Answer: (a)
C₄ plants are adapted to high temperatures and intense light, and they can utilise high light intensities effectively. They do not have significant photorespiration because their CO₂‑concentrating mechanism suppresses the oxygenase activity of RuBisCO. Thus, (i) and (ii) are true, while (iii) is false.
Q6. Match the following for C3 and C4 plants:
Column IColumn II
A. Primary CO2 acceptor (C3)(I) PEP
B. Primary CO2 acceptor (C4)(II) RuBP
C. First stable product (C3)(III) OAA
D. First stable product (C4)(IV) PGA
Correct Answer: (a)
In C₃ plants, the primary CO₂ acceptor is RuBP (II) and the first stable product is PGA (IV). In C₄ plants, the primary acceptor is PEP (I) and the first stable product is OAA (III). These differences underlie the two distinct photosynthetic pathways.
Q7. Regarding 'Splitting of water':
(i) Occurs in PS II.
(ii) Releases O2 and protons.
(iii) Protons are released into the lumen.
Correct Answer: (a)
Water splitting is associated with PS II, releases O₂ and protons, and the protons are released into the thylakoid lumen. All three statements accurately describe the photolysis of water.
Q8. The enzyme RuBisCO has an affinity for:
Correct Answer: (c)
RuBisCO (ribulose‑1,5‑bisphosphate carboxylase/oxygenase) has a dual affinity: it can bind both CO₂ and O₂ at its active site. Depending on the relative concentrations, it acts as a carboxylase (fixing CO₂) or as an oxygenase (initiating photorespiration). This bifunctional nature makes RuBisCO a key enzyme in photosynthesis and also a source of inefficiency under certain conditions.
Q9. The CF1 part of ATP synthase:
Correct Answer: (c)
CF₁ is the hydrophilic portion of chloroplast ATP synthase that protrudes into the stroma. It contains the catalytic sites for ATP synthesis. CF₀ is the membrane‑embedded part that forms a channel for protons. CF₁ does not split water; that function belongs to the oxygen‑evolving complex of PS II. The stroma‑facing orientation allows ATP to be released directly into the stroma for use in the Calvin cycle.
Q10. Which of the following are the products of light reaction used in biosynthetic phase?
Correct Answer: (a)
The light reactions produce ATP and NADPH (reduced nicotinamide adenine dinucleotide phosphate). These two energy‑rich molecules are then used in the Calvin cycle (biosynthetic phase) to reduce CO₂ into carbohydrates. ATP provides the energy, while NADPH provides the reducing power. Water and oxygen are not products used in biosynthesis; CO₂ is the substrate, not a product.
Q11. During cyclic photophosphorylation, which of the following is produced?
Correct Answer: (a)
Cyclic photophosphorylation involves only Photosystem I and results in the synthesis of ATP without producing NADPH or oxygen. Electrons are cycled back to the reaction centre after passing through the electron transport chain, generating a proton gradient for ATP synthesis. This pathway is used when the cell needs extra ATP but not additional reducing power (NADPH) or when NADPH levels are already high.
Q12. Match the products formed in different organisms (van Niel):
Column I (Organism)Column II (Product)
A. Green plants(I) Sulphur/Sulphate
B. Purple sulphur bacteria(II) Oxygen
Correct Answer: (a)
Green plants using water as a hydrogen donor produce oxygen (II). Purple sulphur bacteria use H₂S as a donor, producing sulphur or sulphate (I). This distinction led van Niel to propose that the oxygen comes from water, not CO₂.
Q13. Identify the correct phylum/group being described:
(i) They tolerate high light intensities.
(ii) They have Kranz anatomy.
(iii) Examples include Maize and Sorghum.
Correct Answer: (b)
The description matches C₄ plants, which tolerate high light, exhibit Kranz anatomy, and include maize and sorghum. C₃ plants lack Kranz anatomy; bryophytes and algae do not show this anatomy either.
Q14. How many ATP molecules are required for every CO2 molecule entering the Calvin cycle?
Correct Answer: (b)
For each CO₂ molecule fixed in the Calvin cycle, 3 ATP molecules are consumed: 2 ATP are used in the reduction step (converting 1,3‑bisphosphoglycerate to G3P) and 1 ATP is used in the regeneration of RuBP. Additionally, 2 NADPH are required per CO₂. Over six turns (one glucose), the total ATP requirement is 18 ATP.
Q15. Water stress causes photosynthesis rates to decline mainly because it:
Correct Answer: (b)
Water stress triggers stomatal closure to reduce transpiration, which in turn limits the entry of CO₂ into the leaf. The reduced CO₂ availability directly slows the Calvin cycle, decreasing the rate of photosynthesis. While prolonged drought can also damage chloroplasts, the immediate effect is stomatal closure. Wilting may also occur, further reducing metabolic activity.
Q16. Which are correct for 'Chloroplast pigments'?
(i) Chl a is bright green.
(ii) Carotenoids are yellow to yellow-orange.
(iii) Pigments absorb light at specific wavelengths.
Correct Answer: (a)
Chlorophyll a appears bright green (or blue‑green in chromatograms). Carotenoids are yellow to yellow‑orange. All pigments absorb light at specific wavelengths, enabling photosynthesis. Thus, all three statements are correct.
Q17. The empirical equation for photosynthesis in oxygen-evolving organisms is:
Correct Answer: (a)
The overall empirical equation for oxygenic photosynthesis is CO₂ + H₂O → (CH₂O) + O₂. This summarizes that carbon dioxide is reduced to carbohydrate (CH₂O) while water is oxidised to molecular oxygen. It represents the net chemical change without detailing the many intermediate steps. The equation was established by the mid‑19th century, unifying the work of earlier scientists.
Q18. Match the following for CO2 concentration:
ConditionCO2 level
A. Atmospheric(I) 0.05 %
B. Fixation increases up to(II) 0.03 - 0.04 %
Correct Answer: (a)
Atmospheric CO₂ is around 0.03‑0.04% (II). In C₃ plants, photosynthesis increases with CO₂ up to about 0.05% (I), beyond which it saturates. C₄ plants saturate at lower levels (~0.036%).
Q19. Which statements regarding Photorespiration are correct?
(i) In C3 plants, O2 binds with RuBisCO.
(ii) It results in the synthesis of sugars.
(iii) ATP is utilized in this pathway.
Correct Answer: (a)
In photorespiration, RuBisCO binds O₂ (i) and the pathway consumes ATP and releases CO₂ without synthesising sugars (ii is false). It is a wasteful process that reduces photosynthetic efficiency. Thus, (i) and (iii) are correct.
Q20. Assertion: Light is rarely a limiting factor in nature. Reason: Light saturation occurs at 10% of full sunlight.
Correct Answer: (a)
Light saturation at about 10% of full sunlight means that even moderate light levels are sufficient to saturate photosynthesis; therefore, light is rarely the limiting factor under natural conditions. The reason provides a correct explanation for the assertion, as plants typically receive enough light to reach saturation.
Q21. Assertion: Splitting of water takes place on the inner side of the thylakoid membrane. Reason: Protons produced by splitting of water accumulate in the stroma.
Correct Answer: (c)
The assertion is true: water splitting occurs on the lumenal (inner) side of the thylakoid membrane. However, the reason is false because the protons produced accumulate in the lumen, not the stroma. This proton accumulation in the lumen contributes to the proton gradient used for ATP synthesis.
Q22. Match the absorption peak:
Column I (System)Column II (Peak)
A. PS I(I) P680
B. PS II(II) P700
Correct Answer: (a)
Photosystem I has a reaction centre with an absorption peak at 700 nm, hence P700 (II). Photosystem II absorbs at 680 nm, hence P680 (I). These designations reflect the wavelength of maximum absorption.
Q23. Correct statements about 'RuBisCO':
(i) Most abundant enzyme.
(ii) Acts as carboxylase in C4 plants.
(iii) Acts as oxygenase in C3 plants under low CO2.
Correct Answer: (a)
RuBisCO is the most abundant enzyme. In C₄ plants, it acts as a carboxylase in the bundle sheath cells. In C₃ plants, under low CO₂ (and high O₂), it acts as an oxygenase, initiating photorespiration. All statements are true.
Q24. The C4 acid (Malic/Aspartic) is broken down in bundle sheath cells to release:
Correct Answer: (b)
In bundle sheath cells, the C₄ acids (malate or aspartate) undergo decarboxylation, releasing CO₂ and a three‑carbon compound (pyruvate or alanine). The released CO₂ enters the Calvin cycle, while the three‑carbon molecule is transported back to the mesophyll cells to regenerate PEP. This process concentrates CO₂ around RuBisCO, suppressing photorespiration.
Q25. The first CO2 fixation product in C4 plants is:
Correct Answer: (c)
In C₄ plants, the first stable product of CO₂ fixation is oxaloacetic acid (OAA), a four‑carbon compound. This is formed by the carboxylation of PEP catalysed by PEP carboxylase in the mesophyll cells. OAA is then reduced to malate or aspartate and transported to bundle sheath cells. PGA is the first product in C₃ plants.
Q26. Select correct statements about 'limiting factors':
(i) Blackman proposed it in 1905.
(ii) Rate is limited by the factor nearest to its minimal value.
(iii) Temperature affects the light reaction more than dark reaction.
Correct Answer: (a)
Blackman proposed the Law of Limiting Factors in 1905 (i), and the rate is limited by the factor closest to its minimum (ii). Temperature primarily affects the dark (enzymatic) reactions, not the light reactions (iii is false). Therefore, (i) and (ii) are correct.
Q27. Match the scientist in Column I with their contribution in Column II:
Column IColumn II
A. Joseph Priestley(I) Action spectrum of photosynthesis
B. Jan Ingenhousz(II) Discovered oxygen
C. Julius von Sachs(III) Role of sunlight in purifying air
D. T.W Engelmann(IV) Glucose stored as starch
Correct Answer: (a)
Joseph Priestley discovered oxygen (II). Jan Ingenhousz demonstrated that sunlight is essential for plants to purify air (III). Julius von Sachs provided evidence that glucose is stored as starch (IV). T.W. Engelmann used a prism and bacteria to determine the action spectrum of photosynthesis (I).
Q28. Which scientist showed that only the green part of the plants could release oxygen?
Correct Answer: (a)
Jan Ingenhousz demonstrated that oxygen production in plants occurs only in the green parts and is dependent on sunlight. He used aquatic plants and observed that bubbles of gas (oxygen) appeared only around the green tissues when exposed to light. This was a crucial step in linking the green pigment (chlorophyll) to the photosynthetic process. His work refined Priestley's observations by identifying the role of light and green tissue.
Q29. The first step of the Calvin cycle is:
Correct Answer: (c)
The first step of the Calvin cycle is carboxylation, in which CO₂ is fixed by attaching to RuBP, producing an unstable six‑carbon intermediate that immediately splits into two molecules of 3‑PGA. Reduction and regeneration are subsequent steps. Photophosphorylation is part of the light reactions, not the Calvin cycle.
Q30. Light harvesting complexes (LHC) are also known as:
Correct Answer: (b)
Light‑harvesting complexes are often referred to as antennae because they function like an antenna, capturing light energy and transferring it to the reaction centre. They consist of hundreds of pigment molecules (chlorophylls and carotenoids) arranged to maximise light absorption. The energy is then funnelled to the reaction centre where photochemical reactions begin. This arrangement increases the efficiency of light capture.
Q31. Which enzyme is absent in the mesophyll cells of C4 plants?
Correct Answer: (b)
Mesophyll cells of C₄ plants lack RuBisCO; instead, they contain PEP carboxylase for the initial fixation of CO₂. RuBisCO is present only in the bundle sheath cells where the Calvin cycle operates. This spatial separation prevents RuBisCO from acting as an oxygenase, thereby minimising photorespiration. ATPase and carbonic anhydrase are present in both cell types.
Q32. Match the reaction with its site in the chloroplast:
Column IColumn II
A. Light reaction(I) Stroma
B. Dark reaction(II) Membranous system (Grana)
C. Splitting of water(III) Inner side of thylakoid membrane
D. ATP synthesis(IV) Across thylakoid membrane
Correct Answer: (a)
Light reactions occur in the grana (II). Dark reactions (Calvin cycle) take place in the stroma (I). Water splitting occurs on the inner (lumenal) side of the thylakoid membrane (III). ATP synthesis is driven by the proton gradient across the thylakoid membrane (IV).
Q33. Assertion: Water stress reduces photosynthesis. Reason: It causes wilting of leaves and reduces metabolic activity.
Correct Answer: (a)
Water stress leads to stomatal closure, wilting, and reduced metabolic activity, all of which decrease the rate of photosynthesis. The reason correctly explains the assertion, as the physiological effects of water deficit directly impair the photosynthetic machinery.
Q34. In C4 plants, the Calvin cycle takes place in:
Correct Answer: (b)
In C₄ plants, the Calvin cycle (or C₃ pathway) occurs exclusively in the bundle sheath cells. The mesophyll cells are responsible for the initial fixation of CO₂ into four‑carbon acids, which are then transported to the bundle sheath where they are decarboxylated, releasing CO₂ for the Calvin cycle. This spatial separation concentrates CO₂ around RuBisCO, reducing photorespiration.
Q35. The first stable product of CO2 fixation in C3 plants is:
Correct Answer: (b)
In C₃ plants, the first stable product of CO₂ fixation is 3‑phosphoglyceric acid (PGA), a three‑carbon compound. CO₂ is added to ribulose‑1,5‑bisphosphate (RuBP) by RuBisCO, and the resulting unstable six‑carbon intermediate immediately splits into two molecules of PGA. Oxaloacetic acid is the first stable product in C₄ plants, and glucose is a final product of the cycle.
Q36. The enzyme responsible for ATP synthesis in the thylakoid is:
Correct Answer: (c)
ATP synthase (also called CF₀–CF₁ in chloroplasts) is the enzyme that catalyses the formation of ATP from ADP and inorganic phosphate. It uses the energy of the proton gradient across the thylakoid membrane to drive phosphorylation. RuBisCO is involved in CO₂ fixation, NADP reductase reduces NADP⁺, and PEP carboxylase is found in C₄ plants. ATP synthase is embedded in the thylakoid membrane with its CF₁ portion protruding into the stroma.
Q37. T.W Engelmann used which organism to detect the sites of O2 evolution?
Correct Answer: (b)
Engelmann used aerobic bacteria in his experiment to detect oxygen evolution. He illuminated a filament of green alga with a spectrum of light, and the aerobic bacteria clustered around the regions where oxygen was being produced (blue and red light). These bacteria require oxygen for respiration, so their accumulation indicated the sites of active photosynthesis. This approach also helped him determine the action spectrum of photosynthesis.
Q38. The major limiting factor for photosynthesis in nature is:
Correct Answer: (c)
Carbon dioxide is the major limiting factor for photosynthesis in natural environments because its atmospheric concentration is very low (about 0.03‑0.04%). Even small increases in CO₂ can significantly boost photosynthesis, especially in C₃ plants. While light and water can also be limiting, CO₂ is often the primary constraint under normal conditions.
Q39. The pigment chlorophyll a is called the chief pigment because:
Correct Answer: (b)
Chlorophyll a is considered the chief pigment because its absorption spectrum most closely matches the action spectrum of photosynthesis, with peaks in the blue and red regions. It is the primary electron donor in both photosystems, directly involved in the photochemical reactions. Other pigments are accessory and transfer energy to chlorophyll a. It is not the only pigment, nor does it absorb all wavelengths.
Q40. The primary CO2 acceptor in C4 plants is:
Correct Answer: (c)
In C₄ plants, the primary CO₂ acceptor is phosphoenol pyruvate (PEP), a three‑carbon compound. PEP carboxylase catalyses the addition of CO₂ to PEP, forming oxaloacetic acid (OAA), a four‑carbon compound. RuBP is the acceptor in C₃ plants, while PGA and malic acid are intermediates, not primary acceptors.
Q41. Light saturation for most plants occurs at ____ of full sunlight.
Correct Answer: (b)
Most plants reach light saturation at about 10% of full sunlight. Beyond this intensity, further increases in light do not significantly increase the rate of photosynthesis, and may even cause damage. This is because the enzymes and electron carriers become saturated, and the plant cannot utilise the extra light energy efficiently. Shade‑adapted plants may saturate at even lower intensities.
Q42. Which of the following is the basis of all life on earth?
Correct Answer: (a)
Photosynthesis is the primary mechanism by which solar energy is captured and converted into chemical energy stored in organic molecules. This energy drives nearly all ecosystems, directly or indirectly supporting all living organisms. Without the light‑driven conversion, life as we know it would not exist. Oxygen release is a vital by‑product, but the energy conversion itself is the foundational process.
Q43. Which type of plants lack photorespiration?
Correct Answer: (b)
C₄ plants do not show photorespiration because their CO₂‑concentrating mechanism ensures that RuBisCO is constantly supplied with high levels of CO₂ in the bundle sheath cells, suppressing its oxygenase activity. While photorespiration can still occur at a very low level, it is negligible compared to C₃ plants. C₃ plants are prone to photorespiration, especially under hot, dry conditions.
Q44. Within the chloroplast, where do the 'dark reactions' (carbon reactions) take place?
Correct Answer: (c)
The dark reactions, also called the Calvin cycle, occur in the stroma of the chloroplast. The stroma is the fluid‑filled matrix surrounding the thylakoid membranes, containing the enzymes necessary for CO₂ fixation and sugar synthesis. Although these reactions do not directly require light, they depend on the ATP and NADPH produced in the light reactions. The grana and stroma lamellae are parts of the thylakoid membrane system where light reactions take place.
Q45. During the biosynthetic phase, ATP and NADPH are used to convert:
Correct Answer: (b)
The ATP and NADPH generated in the light reactions are utilised in the biosynthetic (dark) phase to reduce CO₂ into carbohydrates (sugars). Specifically, NADPH provides reducing power for the reduction of 1,3‑bisphosphoglycerate to G3P, while ATP provides energy for various steps, including the regeneration of RuBP. This is the main purpose of the Calvin cycle.
Q46. In Photosystem I (PS I), the reaction centre chlorophyll a has an absorption peak at:
Correct Answer: (b)
Photosystem I has a reaction centre chlorophyll a that absorbs light most effectively at 700 nm, hence it is called P700. This longer wavelength corresponds to a lower energy level compared to PS II. The absorption peak at 700 nm is characteristic of the special pair of chlorophyll a molecules in PS I. This wavelength is in the far‑red region of the spectrum.
Q47. In C3 plants, photorespiration occurs because RuBisCO binds with:
Correct Answer: (c)
Photorespiration is initiated when RuBisCO binds O₂ instead of CO₂ at its active site. This oxygenase activity leads to the formation of phosphoglycolate, which is then processed through a series of reactions that release CO₂ and consume ATP. The process competes with photosynthesis and reduces efficiency. In C₄ plants, a CO₂‑concentrating mechanism minimises this oxygenase activity.
Q48. The CO2 concentration in the atmosphere is between:
Correct Answer: (b)
The atmospheric concentration of CO₂ is approximately 0.03‑0.04% (about 300‑400 ppm). This low level makes CO₂ a major limiting factor for photosynthesis in many plants. The concentration has varied over geological time, but current levels are around 0.04% (400 ppm) due to anthropogenic emissions.
Q49. Photorespiration is a 'wasteful' process because:
Correct Answer: (b)
Photorespiration is considered wasteful because it does not produce any ATP or NADPH and does not result in the net fixation of carbon into sugars. Instead, it oxidises RuBP and releases CO₂, effectively undoing some of the photosynthetic work. The process also consumes ATP and reduces the efficiency of photosynthesis, especially in C₃ plants under high oxygen conditions.
Q50. Match the metabolic pathway with its environment:
Column IColumn II
A. C3 plants(I) Dry tropical regions
B. C4 plants(II) Temperate regions
Correct Answer: (a)
C₃ plants are typically found in temperate regions (II), while C₄ plants are adapted to dry tropical regions (I). The C₄ pathway is more efficient under high temperature and low CO₂ conditions, common in tropical climates.
Q51. Which pigment appears as 'bright or blue-green' in a chromatogram?
Correct Answer: (a)
In paper chromatography, chlorophyll a typically appears as a bright blue‑green band because it is the most abundant and has a distinctive absorption spectrum. Its colour is due to its strong absorption in the blue and red regions, reflecting green light. Chlorophyll b appears yellow‑green, xanthophylls are yellow, and carotenoids are yellow‑orange. The distinct colour helps in identifying the pigments during separation.
Q52. Match the photophosphorylation type:
Column IColumn II
A. Non-cyclic(I) PS I only
B. Cyclic(II) PS I and PS II
Correct Answer: (a)
Non‑cyclic photophosphorylation involves both PS II and PS I (II) and produces ATP and NADPH. Cyclic photophosphorylation uses only PS I (I) and produces only ATP. The pathways differ in their electron flow and products.
Q53. Greenhouse crops like tomatoes are grown in CO2 enriched atmospheres to:
Correct Answer: (b)
Elevating CO₂ levels in greenhouses enhances photosynthesis, particularly in C₃ plants, because CO₂ is often a limiting factor. This leads to increased growth and higher crop yields. The enhanced CO₂ also reduces photorespiration, improving the efficiency of carbon fixation. Other factors like water loss and pest control are not direct benefits of CO₂ enrichment.
Q54. Match the following for Calvin cycle turns:
ProductTurns required
A. 1 Glucose(I) 6
B. 1 CO2 fixed(II) 1
Correct Answer: (a)
One glucose molecule (six carbons) requires six turns of the Calvin cycle (I). Each turn fixes one CO₂ (II). This stoichiometry is fundamental to understanding the efficiency of the cycle.
Q55. Julius von Sachs provided evidence in 1854 that glucose is usually stored as:
Correct Answer: (c)
Julius von Sachs showed that the glucose produced during photosynthesis is not stored as glucose itself but is polymerised into starch. He demonstrated that starch grains accumulate in chloroplasts and can be detected by iodine staining. This observation was pivotal because it linked the immediate product of photosynthesis (glucose) to a visible storage form. Starch serves as a reserve carbohydrate in many plants, especially in leaves and storage organs.
Q56. Regarding the Calvin Cycle, identify the correct statements:
(i) RuBP is a 5-carbon ketose sugar.
(ii) Six molecules of CO2 are required for one glucose.
(iii) Reduction requires 18 ATP for one glucose.
Correct Answer: (a)
Statements (i) and (ii) are correct. RuBP is a 5‑carbon ketose sugar, and six turns of the cycle (fixing six CO₂) are needed for one glucose. Statement (iii) is false because the reduction step per glucose uses 12 ATP (2 ATP per CO₂ × 6), not 18. The total ATP for the whole cycle is 18 (including regeneration).
Q57. In C4 plants, the large cells around the vascular bundles are called:
Correct Answer: (b)
In C₄ plants, the vascular bundles are surrounded by a layer of large, specialised cells called bundle sheath cells. These cells have thick walls, lack intercellular spaces, and contain many chloroplasts where the Calvin cycle takes place. The mesophyll cells are smaller and arranged around the bundle sheath; they perform the initial CO₂ fixation via PEP carboxylase. Kranz anatomy refers to this wreath‑like arrangement.
Q58. Where is the proton accumulation (gradient) created during ATP synthesis in chloroplasts?
Correct Answer: (c)
During photosynthesis, protons (H⁺) accumulate inside the thylakoid lumen as a result of water splitting and the pumping of protons across the thylakoid membrane by the electron transport chain. This creates a proton gradient across the membrane, with a higher concentration in the lumen. The gradient drives ATP synthesis as protons flow back to the stroma through ATP synthase. The intermembrane space is not involved in this process.
Q59. In the 'Half-leaf' experiment using KOH, the leaf portion inside the tube tested negative for starch because:
Correct Answer: (b)
In the half‑leaf experiment, potassium hydroxide (KOH) soaked cotton is placed inside the tube to absorb carbon dioxide from the enclosed air. This depletion of CO2 prevents photosynthesis in that part of the leaf, so no starch is synthesised. The starch test with iodine remains negative, proving that CO2 is essential for the dark reactions. The leaf portion outside the tube, exposed to atmospheric CO2, tests positive for starch.
Q60. Green plants are called autotrophs because they:
Correct Answer: (b)
Autotrophs are organisms that produce their own organic nutrients from inorganic substances. Green plants perform photosynthesis, using light energy to synthesise carbohydrates from carbon dioxide and water. This self‑sustaining ability is the hallmark of autotrophy, not simply consuming minerals or releasing gases.
Q61. The primary acceptor of CO2 in C3 plants is a:
Correct Answer: (c)
The primary CO₂ acceptor in C₃ plants is ribulose‑1,5‑bisphosphate (RuBP), which is a five‑carbon ketose sugar (a ketopentose). It combines with CO₂ to form an unstable six‑carbon intermediate that immediately splits into two molecules of 3‑phosphoglycerate. The 5‑carbon nature of RuBP distinguishes it from the 3‑carbon (PEP) or 4‑carbon (OAA) acceptors in other pathways.
Q62. Which scientist used radioactive isotopes to prove that oxygen comes from water?
Correct Answer: (c)
Cornelius van Niel first inferred that oxygen comes from water based on his studies of sulphur bacteria, but the definitive proof came later using radioactive isotopes of oxygen (¹⁸O). The experiments with ¹⁸O‑labelled water confirmed that the evolved oxygen originates from water, not from CO₂. Although the exact scientists (Ruben and Kamen) are not listed, the option referring to van Niel's work and subsequent isotope confirmation is the closest match.
Q63. Identify the correct statements for 'Z-scheme':
(i) Movement of electrons is downhill on redox potential scale.
(ii) PS I electrons are excited by 700 nm wavelength.
(iii) Electrons are used up as they pass through ETS.
Correct Answer: (a)
In the Z‑scheme, the movement of electrons from PS II to PS I through the electron transport chain is downhill in terms of redox potential (energy is released). PS I indeed has a reaction centre that absorbs at 700 nm. Electrons are not used up; they are passed to NADP⁺ to form NADPH or recycled. Thus, only (i) and (ii) are correct.
Q64. Which plants show saturation at lower CO2 levels (around 360 µlL-1)?
Correct Answer: (b)
C₄ plants show CO₂ saturation at about 360 µL L⁻¹ (or µlL⁻¹) because they have a CO₂‑concentrating mechanism that efficiently supplies RuBisCO with CO₂. In contrast, C₃ plants continue to respond to increasing CO₂ up to about 450 µL L⁻¹. This difference reflects the higher efficiency of C₄ plants in utilising low CO₂ concentrations.
Q65. A milestone contribution showing that O2 evolved comes from H2O and not CO2 was made by:
Correct Answer: (a)
Cornelius van Niel proposed that the oxygen evolved during photosynthesis comes from water, not from carbon dioxide. He based this on his studies of purple and green sulphur bacteria, which use H2S instead of H2O as a hydrogen donor and release sulphur, not oxygen. This led to the generalised equation: CO2 + 2H2O → (CH2O) + O2 + H2O. His inference was later confirmed using isotope‑labelled water (¹⁸O).
Q66. Regarding 'Photosynthetic Rate':
(i) At low light, it is linearly related to intensity.
(ii) CO2 concentration of 0.05% is ideal for C3 productivity.
(iii) Water stress directly increases rate.
Correct Answer: (a)
At low light intensity, the rate increases linearly with light (i). C₃ plants show increased photosynthesis up to about 0.05% CO₂ (ii). Water stress decreases, not increases, the rate (iii false). Hence, (i) and (ii) are correct.
Q67. In Photosystem II (PS II), the reaction centre chlorophyll a has an absorption peak at:
Correct Answer: (a)
Photosystem II has a reaction centre chlorophyll a that absorbs maximally at 680 nm, and is therefore designated P680. This shorter wavelength corresponds to a higher energy level than PS I. P680 is involved in the splitting of water and the initial electron transfer. The absorption peak at 680 nm is a key distinguishing feature of PS II.
Q68. Match the pigment with its color in chromatogram:
Column I (Pigment)Column II (Color)
A. Chlorophyll a(I) Yellow green
B. Chlorophyll b(II) Bright or blue green
C. Xanthophylls(III) Yellow to yellow-orange
D. Carotenoids(IV) Yellow
Correct Answer: (a)
Chlorophyll a appears bright or blue‑green (II). Chlorophyll b is yellow‑green (I). Xanthophylls are yellow (IV). Carotenoids range from yellow to yellow‑orange (III). These colour differences help in identifying the pigments during paper chromatography.
Q69. Kranz anatomy is a characteristic feature of:
Correct Answer: (b)
Kranz anatomy, meaning 'wreath' in German, describes the distinctive arrangement of two types of photosynthetic cells in C₄ plants: bundle sheath cells surrounded by mesophyll cells. This structure facilitates the efficient concentration of CO₂ around RuBisCO, minimising photorespiration. C₃ plants lack this specialised anatomy, and it is not found in aquatic plants or fungi.
Q70. The 3-carbon molecule formed after decarboxylation in C4 plants is transported back to:
Correct Answer: (c)
After decarboxylation of the C₄ acid in bundle sheath cells, the resulting three‑carbon molecule (pyruvate or alanine) is transported back to the mesophyll cells. In the mesophyll, it is converted back to PEP using ATP, allowing the cycle to continue. This shuttle maintains the spatial separation of the two carboxylation steps. The three‑carbon molecule is not transported to stroma or phloem in this context.
Q71. Who discovered oxygen in 1774?
Correct Answer: (b)
Joseph Priestley, an English chemist, is credited with the discovery of oxygen in 1774. He conducted experiments with a bell jar, a candle, and a mint plant, showing that plants could restore the air that supported combustion. His work laid the foundation for understanding the role of plants in purifying the atmosphere. Priestley’s discovery predates the detailed understanding of photosynthesis.
Q72. Match the steps of the Calvin Cycle with requirements per CO2:
Column I (Step)Column II (Requirement)
A. Reduction(I) 1 ATP
B. Regeneration(II) 2 ATP + 2 NADPH
Correct Answer: (a)
The reduction step (conversion of 1,3‑BPG to G3P) requires 2 ATP and 2 NADPH per CO₂. The regeneration step (conversion of G3P to RuBP) requires 1 ATP per CO₂. Thus, per CO₂ fixed, the cycle consumes 3 ATP and 2 NADPH.
Q73. The wavelength of light at which maximum absorption by chlorophyll a occurs is in:
Correct Answer: (b)
Chlorophyll a shows maximum absorption in the blue (around 430‑450 nm) and red (around 660‑680 nm) regions of the spectrum. It reflects green light, which is why plants appear green. The absorption peaks correspond to the two main energy levels of the chlorophyll molecule. These wavelengths drive the light reactions most effectively.
Q74. The pigment responsible for the 'yellow' color in leaves is:
Correct Answer: (c)
Xanthophylls are yellow pigments that appear yellow in chromatograms. They are oxygenated derivatives of carotenes and belong to the class of carotenoids. In leaves, xanthophylls are usually masked by the green chlorophylls, but they become visible in autumn when chlorophyll breaks down. Carotenoids as a group are yellow‑orange, but specifically xanthophylls are yellow.
Q75. Match the enzyme with its function:
Column IColumn II
A. RuBisCO(I) Fixes CO2 in mesophyll of C4
B. PEPcase(II) ATP synthesis
C. ATP synthase(III) Fixes CO2 in C3 pathway
D. Carbonic anhydrase(IV) Speeds up H2CO3 formation
Correct Answer: (a)
RuBisCO is the carboxylase in the C₃ pathway (III). PEP carboxylase (PEPcase) fixes CO₂ in C₄ mesophyll cells (I). ATP synthase synthesises ATP (II). Carbonic anhydrase catalyses the interconversion of CO₂ and H₂CO₃ (IV).
Q76. Which of the following is correct?
(i) PS I absorption peak is 700 nm.
(ii) Non-cyclic flow produces ATP and NADPH.
(iii) Photosynthesis occurs only in green leaves.
Correct Answer: (a)
PS I has an absorption peak at 700 nm (i). Non‑cyclic photophosphorylation produces both ATP and NADPH (ii). Photosynthesis is not restricted to green leaves; it also occurs in other green parts like stems (iii is false). Therefore, (i) and (ii) are correct.
Q77. The splitting of water is associated with:
Correct Answer: (b)
Water splitting (photolysis) is catalysed by the oxygen‑evolving complex associated with Photosystem II. This occurs on the inner (lumenal) side of the thylakoid membrane, releasing protons, electrons, and oxygen. The electrons from water are used to replace the electrons lost by P680 upon excitation. PS I does not directly split water; it accepts electrons from the electron transport chain.
Q78. Match the following for C4 leaf anatomy:
Column IColumn II
A. Bundle sheath cells(I) Large number of chloroplasts
B. Mesophyll cells(II) PEP carboxylase present
C. Kranz(III) Wreath
Correct Answer: (a)
Bundle sheath cells have many chloroplasts (I). Mesophyll cells contain PEP carboxylase (II). Kranz anatomy refers to the wreath‑like arrangement of cells (III). This organisation is characteristic of C₄ plants.
Q79. Assertion: Cyclic photophosphorylation results only in ATP synthesis. Reason: Stroma lamellae lack PS II and NADP reductase.
Correct Answer: (a)
Cyclic photophosphorylation involves only PS I and produces only ATP. The absence of PS II and NADP reductase in stroma lamellae means that electrons cannot be transferred to NADP⁺, so no NADPH is formed. Instead, electrons return to PS I, generating a proton gradient for ATP synthesis. Thus, the reason correctly explains the assertion.
Q80. Match the factor with its effect:
Column IColumn II
A. High light intensity(I) Chlorophyll breakdown
B. Water stress(II) Stomatal closure
C. Low temperature(III) Inactive enzymes
Correct Answer: (a)
Excess light can lead to chlorophyll breakdown (I). Water stress causes stomata to close (II). Low temperatures reduce enzyme activity (III). Each factor affects photosynthesis in a distinct way, as per the Law of Limiting Factors.
Q81. Which is the most abundant plant pigment in the world?
Correct Answer: (c)
Chlorophyll a is the most abundant photosynthetic pigment on Earth. It is the primary pigment involved in converting light energy into chemical energy, present in all oxygen‑evolving photosynthetic organisms. Its abundance reflects its central role in the light reactions, acting as the reaction‑centre pigment. Other pigments like chlorophyll b and carotenoids serve as accessory pigments, but chlorophyll a is the chief pigment.
Q82. Identify the correct features of 'Hatch and Slack Pathway':
(i) Primary fixation occurs in mesophyll.
(ii) OAA is converted to malic acid.
(iii) RuBisCO is absent in bundle sheath.
Correct Answer: (a)
In the Hatch–Slack (C₄) pathway, primary CO₂ fixation occurs in mesophyll cells (i), and OAA is reduced to malate or aspartate (ii). RuBisCO is present in bundle sheath cells, not absent (iii is false). Hence, only (i) and (ii) are correct.
Q83. Total ATP required to make one molecule of glucose in C3 plants is:
Correct Answer: (b)
To synthesise one molecule of glucose (six carbons), the Calvin cycle must fix six CO₂ molecules, requiring 3 ATP per CO₂, giving 18 ATP in total. This includes 12 ATP for the reduction steps and 6 ATP for regeneration of RuBP. The ATP is supplied by photophosphorylation during the light reactions.
Q84. The primary CO2 acceptor in the Calvin cycle is:
Correct Answer: (c)
The primary CO₂ acceptor in the Calvin cycle is ribulose‑1,5‑bisphosphate (RuBP), a five‑carbon ketose sugar. RuBP combines with CO₂ in a reaction catalysed by RuBisCO to form an unstable six‑carbon intermediate that immediately breaks down into two molecules of 3‑PGA. PEP is the primary acceptor in C₄ plants, and OAA is the first stable product in C₄.
Q85. Assertion: C4 plants are more productive than C3 plants. Reason: C4 plants lack the process of photorespiration.
Correct Answer: (a)
C₄ plants are generally more productive because their CO₂‑concentrating mechanism suppresses photorespiration, allowing RuBisCO to act mainly as a carboxylase. This results in higher photosynthetic efficiency, especially under high temperature and light conditions. The absence of photorespiration is the key reason for their higher productivity.
Q86. What happens when light intensity is increased beyond the point of saturation?
Correct Answer: (b)
When light intensity exceeds the saturation point, the rate of photosynthesis may decline due to photoinhibition, which involves the breakdown of chlorophyll and damage to the photosynthetic apparatus. The excess light energy cannot be safely dissipated and can cause oxidative damage. Stomata do not open wider, and CO₂ fixation does not stop immediately, but the overall rate decreases.
Q87. Stroma lamellae lack which of the following?
Correct Answer: (c)
Stroma lamellae (the unstacked thylakoid membranes) lack Photosystem II and the NADP reductase enzyme. They contain Photosystem I and ATP synthase. The absence of PS II means that non‑cyclic electron transport cannot occur there, but cyclic electron flow can take place. Pigments are still present because PS I is embedded in these membranes.
Q88. Assertion: Dark reactions are not light-dependent. Reason: They occur only in darkness.
Correct Answer: (c)
Both the assertion and the reason are false. Dark reactions are not directly light‑driven, but they depend on the products (ATP and NADPH) of the light reactions, so they are light‑dependent in an indirect sense. Moreover, they do not occur only in darkness; they can occur in the light as long as ATP and NADPH are available.
Q89. To make one molecule of glucose, how many turns of the Calvin cycle are required?
Correct Answer: (c)
One turn of the Calvin cycle fixes one molecule of CO₂ (into two molecules of PGA). To produce one glucose molecule (six carbons), six turns are required because each turn provides one carbon atom. The six turns also generate the necessary intermediates for regeneration of RuBP. Therefore, the cycle must run six times to net synthesise one hexose sugar.
Q90. The 'Law of Limiting Factors' was proposed by:
Correct Answer: (a)
The Law of Limiting Factors was proposed by F.F. Blackman in 1905. It states that when a process is conditioned by several factors, its rate is limited by the factor that is closest to its minimum value. Blackman applied this to photosynthesis, showing that at any given time, one factor (light, CO₂, or temperature) can be the limiting one. This concept is fundamental to understanding how environmental factors influence photosynthetic rate.
Q91. Photosynthesis is a _____ process.
Correct Answer: (c)
Photosynthesis is described as a physico‑chemical process because it involves both physical (light absorption, electron transfer) and chemical (enzyme‑catalysed reactions) components. The conversion of light energy into chemical energy is a physical step, while the fixation of CO₂ into carbohydrates is a chemical (enzymatic) process. It is not purely physical, purely chemical, or only biological, as it integrates all these aspects.
Q92. Who worked out the complete biosynthetic pathway of photosynthesis using radioactive 14C?
Correct Answer: (a)
Melvin Calvin, along with his colleagues, used the radioactive isotope ¹⁴C to trace the pathway of carbon fixation in photosynthesis. By exposing algae to ¹⁴CO₂ and analysing the labelled intermediates at various time intervals, he elucidated the Calvin cycle. This work earned him the Nobel Prize in Chemistry in 1961. The Hatch–Slack pathway describes C₄ photosynthesis, not the main carbon fixation pathway.
Q93. The movement of electrons in the Z-scheme from PS II to the acceptor is:
Correct Answer: (a)
In the Z‑scheme, after PS II is excited by light, electrons are transferred to a primary acceptor (pheophytin) and then move down an electron transport chain to plastoquinone and cytochrome b6f. This transfer from the excited state to the acceptor is energetically downhill because the electrons lose energy as they move to carriers with lower redox potentials. The overall path from water to NADP⁺ is uphill, but the step from PS II to its acceptor is downhill.
Q94. Match the part of ATP synthase:
Column IColumn II
A. CF0(I) Facing stroma
B. CF1(II) Embedded in membrane
Correct Answer: (a)
CF₀ is the hydrophobic portion embedded in the thylakoid membrane (II), forming a proton channel. CF₁ protrudes into the stroma (I) and contains the catalytic sites for ATP synthesis. This structure is analogous to the F₀F₁ ATP synthase in mitochondria.
Q95. Which plant shows a higher temperature optimum?
Correct Answer: (c)
C₄ plants are adapted to high‑temperature environments such as dry tropical regions, and they have a higher temperature optimum for photosynthesis compared to C₃ plants. This is partly because their CO₂‑concentrating mechanism reduces photorespiration, which becomes more severe at high temperatures. C₃ plants generally perform better in cooler, temperate climates.
Q96. Which of the following statements about light reactions are correct?
(i) They occur in the membranous system of chloroplast.
(ii) They include water splitting and O2 release.
(iii) They result in the formation of ATP and NADPH.
Correct Answer: (a)
All three statements are correct. Light reactions occur in the thylakoid membranes (grana and stroma lamellae). They involve photolysis of water, releasing O₂, and produce ATP and NADPH as energy‑carrying molecules. These products are then used in the biosynthetic phase.
Q97. Assertion: Photosynthesis is the primary source of all food on earth. Reason: It releases oxygen into the atmosphere.
Correct Answer: (b)
Both statements are true, but the reason given (oxygen release) is not the explanation for why photosynthesis is the primary source of food. The correct explanation is that photosynthesis converts inorganic carbon into organic compounds that form the base of food chains. Oxygen release is a separate, though important, by‑product.
Q98. Correct statements about 'Action Spectrum':
(i) It shows the rate of photosynthesis at different wavelengths.
(ii) Maximum rate is in blue and red light.
(iii) It has no relation to the absorption spectrum.
Correct Answer: (a)
The action spectrum plots the rate of photosynthesis against light wavelength (i). The rate is highest in blue and red regions (ii), matching the absorption peaks of chlorophyll a. It is closely related to the absorption spectrum (iii is false), especially that of chlorophyll a.
Q99. Cyclic photophosphorylation occurs when only light of wavelengths beyond _____ is available.
Correct Answer: (b)
Cyclic photophosphorylation can occur when only far‑red light of wavelengths beyond 680 nm is available, because such light can excite Photosystem I (P700) but not Photosystem II (P680). Under these conditions, PS II cannot function, and electrons cycle only through PS I to generate ATP. This is a way to produce ATP when NADPH is not needed.
Q100. Accessory pigments like chlorophyll b and carotenoids help by:
Correct Answer: (c)
Accessory pigments, including chlorophyll b and carotenoids, expand the range of light wavelengths that can drive photosynthesis and also protect chlorophyll a from photo‑oxidation. They absorb light energy and transfer it to chlorophyll a, but they also dissipate excess excitation energy as heat, preventing damage. They do not split water, fix CO₂, or form ATP directly.