Biology · Class 11· Quiz

Photosynthesis Quiz — Test Your Knowledge

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Test yourself

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Q1. Where do the light reactions of photosynthesis occur?
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Answer: B — Thylakoid membrane
Light reactions occur on the thylakoid membranes of the chloroplast. The stroma is where the Calvin cycle (dark reactions) takes place.
Q2. The oxygen released during photosynthesis comes from:
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Answer: B — Water
Oxygen comes from the photolysis (splitting) of water during the light reactions. This was proven by Ruben and Kamen using O-18 isotope tracing.
Q3. Which enzyme fixes CO2 in the Calvin cycle?
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Answer: C — RuBisCO
RuBisCO (Ribulose bisphosphate carboxylase oxygenase) catalyzes the first step of the Calvin cycle — combining CO2 with RuBP. It is the most abundant protein on Earth.
Q4. How many CO2 molecules are needed to produce one glucose in the Calvin cycle?
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Answer: B — 6
Each turn of the Calvin cycle fixes one CO2. After 3 turns, one net G3P is produced. Two G3P molecules make one glucose, so 6 CO2 are needed per glucose.
Q5. Which photosystem acts first in the light reactions (non-cyclic)?
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Answer: B — Photosystem II
PS II acts first despite its name. The numbering reflects the order of discovery, not function. In the Z-scheme, electrons flow from PS II → ETC → PS I → NADPH.
Q6. The Calvin cycle occurs in the:
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Answer: C — Stroma
The Calvin cycle (light-independent reactions) takes place in the stroma of the chloroplast, using ATP and NADPH produced by the light reactions.
Q7. In cyclic photophosphorylation, which of the following is NOT produced?
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Answer: B — NADPH
Cyclic photophosphorylation involves only PS I. Electrons cycle back to PS I, producing only ATP. No water is split (no O2) and no NADPH is formed.
Q8. What is the first stable product of the Calvin cycle?
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Answer: C — 3-PGA (3-phosphoglycerate)
When RuBisCO fixes CO2 onto RuBP, the unstable 6C compound immediately splits into two molecules of 3-PGA (a 3-carbon compound). This is the first stable product.
Q9. Which factor does NOT directly affect the rate of photosynthesis?
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Answer: C — Oxygen concentration
Light intensity, CO2 concentration, and temperature are the three main factors affecting photosynthesis rate. While O2 can compete with CO2 at the RuBisCO active site (photorespiration), it is not considered a primary rate-limiting factor.
Q10. Chlorophyll primarily absorbs which colours of light?
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Answer: B — Red and blue
Chlorophyll absorbs red (~680nm) and blue (~450nm) wavelengths most efficiently. Green light is mostly reflected, which is why plants appear green.

Test yourself

Q1. Where do the light reactions of photosynthesis occur?

Q2. The oxygen released during photosynthesis comes from:

Q3. Which enzyme fixes CO2 in the Calvin cycle?

Q4. How many CO2 molecules are needed to produce one glucose in the Calvin cycle?

Q5. Which photosystem acts first in the light reactions (non-cyclic)?

Q6. The Calvin cycle occurs in the:

Q7. In cyclic photophosphorylation, which of the following is NOT produced?

Q8. What is the first stable product of the Calvin cycle?

Q9. Which factor does NOT directly affect the rate of photosynthesis?

Q10. Chlorophyll primarily absorbs which colours of light?

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