Photosynthesis in Higher Plants — NEET Botany MCQs with answers

Photosynthesis in Higher Plants is a Class 11 Botany chapter in the NEET (UG) syllabus. NEET720 has 986 reviewed practice questions on it, each with a quick answer and a step-by-step explanation. The 8 questions below are free and fixed, so you can bookmark this page; the full chapter, plus mistake tracking and spaced revision, is in the app.

Topics covered

Early Experiments and Pigments · Light Reactions · Calvin Cycle (C3 Pathway) · C4 Pathway and Photorespiration · Factors Affecting Photosynthesis · C3, C4 and CAM pathways · C3 Pathway (Calvin Cycle) · Quantum yield · CO2 compensation point · Measurement of photosynthesis · ATP synthase · Chemiosmosis · Early experiments · C3 vs C4 · Leaf structure · Overall process · Red drop and Emerson effect · Photosynthetic Pigments · Photorespiration · Site of Photosynthesis · Historical Experiments · C4 vs C3 Comparison · Discovery Experiments · C3 vs C4 anatomy · Photosynthetic Efficiency · Blackman's law of limiting factors · C3 vs C4 comparison · Chloroplast pigments · Photosynthesis · Photolysis of water · Photophosphorylation · C4 anatomy · Photosynthetic units · Chemiosmotic hypothesis · Emerson effect · CAM vs C4 comparison · Blackman and Matthaei experiment · Site of light and dark reactions · Calvin cycle phases · Number of turns of Calvin cycle

8 free Photosynthesis in Higher Plants practice questions with answers

Choose an answer in your head before opening it. Each explanation says why the correct option is right and, where relevant, why the tempting wrong option is wrong.

1. Question 1 · easy · Calvin Cycle (C3 Pathway)

   The Calvin cycle (C3 pathway) of photosynthesis occurs in which part of the chloroplast?

   • D.Stroma
   • A.Thylakoid lumen
   • B.Outer chloroplast membrane
   • C.Grana stack
   Show answer and explanation

   Answer: D. Stroma

   The Calvin cycle, also called the biosynthetic or dark phase, takes place in the stroma, the fluid matrix of the chloroplast surrounding the thylakoids.

   Light reactions occur on/in the thylakoid membrane and lumen, producing ATP and NADPH. These reduced coenzymes diffuse into the stroma, where the enzymes of the Calvin cycle (including RuBisCO) are dissolved, and CO2 fixation, reduction, and RuBP regeneration all occur there. Grana are stacks of thylakoids, not the cycle's location.

   Common mistake: Mixing up thylakoid-based light reactions with stroma-based dark reactions.

   Key point: Light reactions: thylakoid membrane. Calvin cycle: stroma.

2. Question 2 · easy · Calvin Cycle (C3 Pathway)

   The Calvin cycle was elucidated using which alga and which technique?

   • A.Chlorella; radioactive carbon (14CO2) tracing
   • B.Chlamydomonas; X-ray crystallography
   • C.Spirogyra; fluorescence microscopy
   • D.Ulothrix; mass spectrometry
   Show answer and explanation

   Answer: A. Chlorella; radioactive carbon (14CO2) tracing

   Melvin Calvin used the unicellular green alga Chlorella and radioactively labelled CO2 (14CO2) to trace the path of carbon through the dark reactions, work for which he received the Nobel Prize.

   By exposing Chlorella suspensions to 14CO2 for very short times and quickly killing the cells, Calvin and coworkers identified 3-phosphoglyceric acid (3-PGA) as the first stable product, establishing the C3 pathway. Chlamydomonas is famous for genetics/flagellar studies, Spirogyra for Engelmann's experiment, and Ulothrix is unrelated to this discovery.

   Common mistake: Confusing Chlorella (Calvin cycle) with Spirogyra (Engelmann's action spectrum).

   Key point: Calvin cycle = Melvin Calvin + Chlorella + radioactive 14CO2.

3. Question 3 · easy · Calvin Cycle (C3 Pathway)

   The pathway is named the "C3 pathway" because:

   • C.The first stable product of CO2 fixation, 3-phosphoglyceric acid, is a 3-carbon compound
   • D.The CO2 acceptor RuBP contains exactly 3 carbon atoms
   • A.Three molecules of ATP are used to fix one CO2
   • B.The cycle was the third pathway of photosynthesis to be discovered
   Show answer and explanation

   Answer: C. The first stable product of CO2 fixation, 3-phosphoglyceric acid, is a 3-carbon compound

   The pathway is called C3 because the first stable, detectable product of carbon fixation is 3-phosphoglyceric acid (3-PGA), a three-carbon organic acid.

   RuBP, the CO2 acceptor, is a 5-carbon sugar bisphosphate, not 3-carbon. When CO2 (1 carbon) combines with RuBP (5 carbon) via RuBisCO, a transient 6-carbon compound forms and immediately splits into two molecules of 3-PGA (3 carbon each). Because this first stable product has 3 carbons, the pathway and the plants using it predominantly are termed C3. The ATP count (3 per CO2 fixed) is coincidentally also 3 but is not the basis of the name.

   Common mistake: Assuming the acceptor molecule RuBP is 3-carbon or that ATP stoichiometry explains the name.

   Key point: C3 naming comes from the 3-carbon first stable product (3-PGA), not from RuBP or ATP numbers.

4. Question 4 · easy · Calvin Cycle (C3 Pathway)

   Which statement about the occurrence of the Calvin cycle is correct?

   • D.It operates in every photosynthetic plant, including C4 plants, in addition to the C4 pathway
   • A.It occurs only in C3 plants; C4 plants entirely replace it with the Hatch-Slack pathway
   • B.It occurs only in CAM plants during the night
   • C.It occurs only in algae, not in higher plants
   Show answer and explanation

   Answer: D. It operates in every photosynthetic plant, including C4 plants, in addition to the C4 pathway

   The Calvin cycle is the universal biosynthetic pathway of photosynthesis, occurring in ALL photosynthetic organisms (C3, C4, and CAM plants); the C4 and CAM pathways are add-on CO2-concentrating mechanisms that feed CO2 into this same cycle.

   C3 plants fix CO2 directly via the Calvin cycle in mesophyll chloroplasts. C4 plants first fix CO2 via PEP carboxylase in mesophyll cells (forming oxaloacetate/malate), then release CO2 in bundle sheath cells where it enters the Calvin cycle exactly as in C3 plants. CAM plants fix CO2 at night and release it during the day into the same cycle. Thus the Calvin cycle itself is never absent; only the CO2-delivery mechanism differs.

   Common mistake: Believing C4 plants "skip" the Calvin cycle entirely.

   Key point: Every photosynthetic plant runs the Calvin cycle; C4/CAM are pre-concentration add-ons, not replacements.

5. Question 5 · easy · Calvin Cycle (C3 Pathway)

   Statement I: The Calvin cycle is called the "dark reaction" because it never requires light at any step. Statement II: The Calvin cycle depends on ATP and NADPH generated by the light reactions. In light of the above statements, choose the correct answer:

   • A.Both Statement I and Statement II are true
   • B.Statement I is true but Statement II is false
   • C.Statement I is false but Statement II is true
   • D.Both Statement I and Statement II are false
   Show answer and explanation

   Answer: C. Statement I is false but Statement II is true

   "Dark reaction" is a historical name meaning the steps do not directly require light photons, but the cycle is indirectly light-dependent because it consumes ATP and NADPH supplied by the light reactions.

   Statement I is false: although none of the Calvin cycle's enzymatic steps directly absorb light, several stromal enzymes (e.g., RuBisCO activity, some Calvin cycle enzymes) are light-activated, and more importantly the cycle cannot proceed for long in true darkness because its ATP and NADPH supply is cut off. Statement II is true: reduction of 3-PGA to G3P requires ATP and NADPH generated during the light reactions in the thylakoid membrane. Hence the cycle is only 'dark' in the sense of not directly using light energy itself.

   Common mistake: Believing the Calvin cycle can run indefinitely in complete darkness.

   Key point: Dark reaction = no direct light absorption, but indirectly dependent on light via ATP/NADPH.

6. Question 6 · easy · Calvin Cycle (C3 Pathway)

   Arrange the following phases of the Calvin cycle in the correct order in which they occur: (i) Reduction of 3-PGA to G3P (ii) Regeneration of RuBP (iii) Carboxylation of RuBP by CO2

   • B.(iii) → (i) → (ii)
   • C.(i) → (iii) → (ii)
   • D.(ii) → (iii) → (i)
   • A.(iii) → (ii) → (i)
   Show answer and explanation

   Answer: B. (iii) → (i) → (ii)

   The Calvin cycle proceeds in three sequential phases: carboxylation (CO2 fixation onto RuBP forming 3-PGA), reduction (3-PGA to G3P using ATP and NADPH), and regeneration (most G3P is used to regenerate RuBP).

   Carboxylation must occur first because it produces the initial 3-carbon product (3-PGA) from the 5-carbon acceptor RuBP and CO2, catalysed by RuBisCO. Reduction follows, converting 3-PGA into G3P using ATP and NADPH from light reactions. Finally, regeneration uses the majority of G3P molecules (5 out of 6) along with ATP to reconstitute RuBP, allowing the cycle to continue; the remaining 1 G3P is used for glucose/starch synthesis.

   Common mistake: Reversing reduction and regeneration order.

   Key point: Order: Carboxylation → Reduction → Regeneration.

7. Question 7 · easy · Calvin Cycle (C3 Pathway)

   A student claims: "RuBisCO always acts only as a carboxylase, fixing CO2, and never behaves as an oxygenase." This claim is:

   • A.Correct, because RuBisCO is highly specific for CO2 under all conditions
   • B.Incorrect, because RuBisCO is bifunctional and can also act as an oxygenase when O2 concentration is relatively high, leading to photorespiration
   • C.Incorrect, because RuBisCO only acts as an oxygenase and never fixes CO2
   • D.Correct, because oxygenase activity is carried out by a separate enzyme, not RuBisCO
   Show answer and explanation

   Answer: B. Incorrect, because RuBisCO is bifunctional and can also act as an oxygenase when O2 concentration is relatively high, leading to photorespiration

   RuBisCO (ribulose bisphosphate carboxylase-oxygenase) is bifunctional: it fixes CO2 (carboxylase activity, favoured at high CO2/low O2) or fixes O2 onto RuBP (oxygenase activity, favoured at high O2/low CO2), the latter initiating the wasteful photorespiration pathway.

   The relative concentrations of CO2 and O2 at the enzyme's active site determine which reaction predominates. In normal atmospheric conditions carboxylation dominates, but as O2 rises relative to CO2 (e.g., on hot, dry days with closed stomata), oxygenase activity increases, consuming RuBP and O2 to eventually release CO2 without net carbon or energy gain — this is photorespiration. This dual nature is the biochemical basis for the evolutionary advantage of C4 and CAM CO2-concentrating mechanisms.

   Common mistake: Assuming RuBisCO is a pure carboxylase with no oxygenase activity.

   Key point: RuBisCO name itself signals dual carboxylase/oxygenase activity, the basis of photorespiration.

8. Question 8 · medium · Calvin Cycle (C3 Pathway)

   The primary CO2 acceptor of the Calvin cycle is:

   • C.Ribulose 1,5-bisphosphate (RuBP), a 5-carbon sugar
   • D.Phosphoenolpyruvate (PEP), a 3-carbon compound
   • A.3-phosphoglyceric acid (3-PGA), a 3-carbon acid
   • B.Glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar
   Show answer and explanation

   Answer: C. Ribulose 1,5-bisphosphate (RuBP), a 5-carbon sugar

   RuBP (ribulose 1,5-bisphosphate), a 5-carbon sugar, is the molecule to which CO2 is added by RuBisCO in the carboxylation step of the Calvin cycle.

   In C3 plants, RuBisCO catalyses the addition of CO2 to RuBP, forming an unstable 6-carbon intermediate that splits into two molecules of 3-PGA. PEP is the CO2 acceptor specific to the C4 pathway (catalysed by PEP carboxylase in mesophyll cells), not the Calvin cycle. 3-PGA and G3P are downstream products of carboxylation and reduction respectively, not acceptors.

   Common mistake: Confusing RuBP (C3 acceptor) with PEP (C4 acceptor).

   Key point: RuBP (5C) + CO2 (1C) → unstable 6C → 2 × 3-PGA (3C each), via RuBisCO.

Questions about Photosynthesis in Higher Plants for NEET

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Questions are original NEET720 compositions reviewed for correctness, syllabus fit and option quality. Counts update as the bank grows (986 active practice questions in this chapter today).