Calvin Cycle (C3 Pathway) Practice Questions
20 free Calvin Cycle (C3 Pathway) practice questions for the NCERT Biology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which of the following is the primary CO2 acceptor molecule in the C3 pathway (Calvin Cycle)?
- A Phosphoenolpyruvate (PEP)
- B Ribulose-1,5-bisphosphate (RuBP)
- C Oxaloacetic acid (OAA)
- D 3-phosphoglyceric acid (3-PGA)
Correct answer: Ribulose-1,5-bisphosphate (RuBP)
In the C3 pathway, the primary acceptor of carbon dioxide is a 5-carbon ketose sugar called Ribulose-1,5-bisphosphate (RuBP). This reaction is catalyzed by the enzyme RuBisCO.
The first stable product of the Calvin cycle in C3 plants is a:
- A 2-carbon compound
- B 3-carbon compound
- C 4-carbon compound
- D 5-carbon compound
Correct answer: 3-carbon compound
The first stable product formed after CO2 fixation in the Calvin cycle is 3-phosphoglyceric acid (PGA), which contains three carbon atoms. This is why the cycle is referred to as the C3 pathway.
The Calvin cycle proceeds in three distinct stages. What is the correct chronological order of these stages?
- A Reduction -> Carboxylation -> Regeneration
- B Regeneration -> Reduction -> Carboxylation
- C Carboxylation -> Reduction -> Regeneration
- D Carboxylation -> Regeneration -> Reduction
Correct answer: Carboxylation -> Reduction -> Regeneration
The Calvin cycle begins with Carboxylation (fixation of CO2), followed by Reduction (formation of glucose), and ends with Regeneration of the CO2 acceptor molecule RuBP to keep the cycle going.
How many molecules of ATP and NADPH are required for every molecule of CO2 fixed during the reduction step of the Calvin cycle?
- A 2 ATP and 2 NADPH
- B 3 ATP and 2 NADPH
- C 2 ATP and 3 NADPH
- D 1 ATP and 1 NADPH
Correct answer: 2 ATP and 2 NADPH
During the reduction phase, two molecules of ATP are used for phosphorylation and two molecules of NADPH are used for reduction per CO2 molecule fixed. This results in the formation of glyceraldehyde-3-phosphate.
To produce one molecule of glucose, how many 'turns' of the Calvin cycle are required?
- A 2
- B 4
- C 6
- D 12
Correct answer: 6
Each turn of the Calvin cycle fixes one molecule of CO2. Since glucose is a 6-carbon sugar ($C_6H_{12}O_6$), six turns of the cycle are necessary to generate the net gain of one glucose molecule.
Regeneration of one molecule of RuBP requires the expenditure of how many ATP molecules?
- A 1 ATP
- B 2 ATP
- C 3 ATP
- D 0 ATP
Correct answer: 1 ATP
The regeneration step is crucial for the cycle to continue uninterrupted. It requires one molecule of ATP to phosphorylate the intermediate to reform Ribulose-1,5-bisphosphate (RuBP).
Which enzyme is the most abundant protein on Earth and catalyzes the carboxylation of RuBP?
- A PEP carboxylase
- B Hexokinase
- C RuBisCO enzyme
- D Pyruvate dehydrogenase
Correct answer: RuBisCO enzyme
Ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO) is the enzyme responsible for CO2 fixation in the Calvin cycle. It is characterized by its dual ability to bind both CO2 and O2 and is the most abundant protein in the biosphere.
What is the total energy requirement (ATP and NADPH) for the synthesis of one molecule of glucose in the C3 pathway?
- A 12 ATP and 12 NADPH
- B 18 ATP and 12 NADPH
- C 12 ATP and 18 NADPH
- D 18 ATP and 18 NADPH
Correct answer: 18 ATP and 12 NADPH
For each CO2 fixed, 3 ATP and 2 NADPH are consumed (2 ATP/2 NADPH in reduction, 1 ATP in regeneration). Multiplying this by 6 turns for one glucose molecule yields 18 ATP and 12 NADPH.
In which part of the chloroplast do the 'Dark Reactions' or biosynthetic phase occur?
- A Thylakoid lumen
- B Grana
- C Stroma
- D Inner membrane space
Correct answer: Stroma
The dark reactions involve enzymatic steps that do not directly require light but depend on the products of light reactions (ATP and NADPH). These enzymes are located in the fluid stroma of the chloroplast.
During the Calvin cycle, the reduction of 3-phosphoglyceric acid (PGA) leads to the formation of:
- A Dihydroxyacetone phosphate
- B Triose phosphate
- C Fructose-1,6-bisphosphate
- D Oxaloacetate
Correct answer: Triose phosphate
The reduction step involves the use of ATP and NADPH to convert 3-PGA into triose phosphates (G3P). These triose phosphates are the precursors for the synthesis of sucrose and starch.
Which of the following statements about the Calvin cycle is FALSE?
- A It occurs in all photosynthetic plants
- B It requires ATP and NADPH from light reactions
- C It is also called the reductive pentose phosphate pathway
- D It occurs exclusively in the dark, independent of light
Correct answer: It occurs exclusively in the dark, independent of light
The term 'dark reaction' is a misnomer; while it does not directly use light, it is most active during the day because it relies on the immediate supply of ATP and NADPH from the light reactions.
The carboxylation reaction of the Calvin cycle results in two molecules of:
- A RuBP
- B PGA
- C Glucose
- D PEP
Correct answer: PGA
When RuBP (5C) reacts with CO2 (1C), it forms a highly unstable 6-carbon intermediate that immediately splits into two molecules of 3-phosphoglyceric acid (3-PGA).
In the C3 pathway, if the concentration of CO2 becomes very low and O2 is high, RuBisCO starts acting as an:
- A Isomerase
- B Oxygenase
- C Dehydrogenase
- D Hydrolase
Correct answer: Oxygenase
RuBisCO has an affinity for both CO2 and O2. When O2 levels are high and CO2 levels are low, it functions as an oxygenase, leading to the process of photorespiration, which is wasteful for the plant.
Which of the following is utilized during the regeneration phase of the Calvin cycle?
- A NADPH
- B ATP
- C CO2
- D Oxygen
Correct answer: ATP
The regeneration of the CO2 acceptor RuBP is essential for the cycle to continue. This specific step requires the input of one ATP molecule per RuBP regenerated.
Melvin Calvin used which radioactive isotope to trace the path of carbon in algal photosynthesis?
- A 15N
- B 32P
- C 14C
- D 18O
Correct answer: 14C
Melvin Calvin and his coworkers used the radioactive isotope 14C in algal photosynthesis studies. This allowed them to identify the first carbon fixation product and map the entire pathway.
For the net export of one molecule of triose phosphate (G3P) from the Calvin cycle, how many turns of the cycle are needed?
- A 1
- B 3
- C 6
- D 9
Correct answer: 3
To export one 3-carbon triose phosphate without depleting the cycle's intermediates, 3 turns are required (fixing 3 CO2). This produces 6 molecules of G3P, 5 of which are used to regenerate RuBP, leaving 1 for export.
What is the primary reason the 'Reduction' phase is called so?
- A It reduces the amount of RuBP available in the stroma of chloroplasts.
- B It reduces the carbon chain from five carbons down to three carbons.
- C It involves the addition of electrons from NADPH to carbon compounds.
- D It reduces the internal temperature of the chloroplast during daytime.
Correct answer: It involves the addition of electrons from NADPH to carbon compounds.
The phase is named 'Reduction' because the 3-phosphoglyceric acid is reduced to triose phosphate using the reducing power of NADPH and the energy from ATP.
Which of these is NOT a direct product of the Calvin cycle?
- A Triose phosphate (G3P)
- B ADP molecule
- C NADP+ coenzyme
- D Oxygen gas
Correct answer: Oxygen gas
Oxygen is a byproduct of the light-dependent reactions (photolysis of water) in the thylakoid. The Calvin cycle produces triose phosphate, and regenerates ADP and NADP+ to be reused by light reactions.
If a C3 plant is provided with CO2 containing 14C, which molecule would show radioactivity first?
- A Glucose
- B 3-phosphoglycerate
- C Ribulose bisphosphate
- D Phosphoenolpyruvate
Correct answer: 3-phosphoglycerate
As 3-phosphoglycerate (3-PGA) is the very first stable product formed after CO2 fixation in C3 plants, it would be the first molecule to incorporate the radioactive 14C.
In the C3 pathway, the conversion of 3-phosphoglyceric acid to 1,3-bisphosphoglyceric acid involves:
- A Decarboxylation of 3-PGA
- B Phosphorylation using ATP
- C Reduction using NADPH
- D Oxidation of 3-PGA
Correct answer: Phosphorylation using ATP
Before reduction, each molecule of 3-PGA is phosphorylated by ATP to form 1,3-bisphosphoglyceric acid. This prepares the molecule for the subsequent reduction by NADPH.