Question 1

The primary CO₂ acceptor in the Calvin (C3) cycle is:

Accepted Answer

RuBP (ribulose-1,5-bisphosphate). RuBP (5-C) accepts CO₂; the first stable product is 3-PGA (a 3-carbon acid) — hence "C3".

Question 2

The first stable product of carbon fixation in the C3 pathway is:

Accepted Answer

3-phosphoglyceric acid (3-PGA). CO₂ + RuBP → 2 molecules of 3-PGA (3-carbon), catalysed by RuBisCO.

Question 3

The enzyme catalysing CO₂ fixation in the Calvin cycle is:

Accepted Answer

RuBisCO (ribulose bisphosphate carboxylase-oxygenase). RuBisCO is the most abundant enzyme; it carboxylates RuBP but can also oxygenate it (photorespiration).

Question 4

To fix ONE molecule of CO₂ in the Calvin cycle, the cell uses:

Accepted Answer

3 ATP and 2 NADPH. Per CO₂: 3 ATP + 2 NADPH; six turns (6 CO₂) make one glucose using 18 ATP and 12 NADPH.

Question 5

The three phases of the Calvin cycle are, in order:

Accepted Answer

Carboxylation → reduction → regeneration (of RuBP). Carboxylation (CO₂ + RuBP), reduction (3-PGA → G3P using ATP/NADPH), and regeneration of RuBP.

Question 6

To synthesise one glucose molecule, the Calvin cycle must turn ______ times, fixing ______ CO₂:

Accepted Answer

6; 6. Six turns fix six CO₂ molecules, consuming 18 ATP and 12 NADPH to produce one hexose.

Question 7

The Calvin cycle is described as having three phases. Their correct order beginning from the entry of CO₂ is:

Accepted Answer

carboxylation → reduction → regeneration. CO₂ is first fixed onto RuBP (carboxylation), the fixed carbon is then reduced using ATP and NADPH (reduction), and finally RuBP is regenerated to keep the cycle running.

Question 8

To regenerate RuBP and keep the Calvin cycle turning, ATP is consumed during the regeneration phase in addition to the reduction phase. Overall, to fix 6 CO₂ and make one glucose, the cycle uses:

Accepted Answer

18 ATP and 12 NADPH. Six turns of the Calvin cycle fix six CO₂ into one glucose using 18 ATP (12 in reduction + 6 in regeneration) and 12 NADPH.

Question 9

The observation that in Calvin's ¹⁴CO₂ experiment the first radioactive compound detected was a 3-carbon acid led to the pathway being called the C3 cycle. That first product is:

Accepted Answer

3-phosphoglyceric acid (3-PGA). Fixation of CO₂ onto the 5-carbon RuBP yields two molecules of the 3-carbon 3-PGA, the first stable product; OAA (4-C) is the first product of the C4 pathway instead.

Question 10

Assertion (A): RuBisCO fixes CO₂ onto a 5-carbon acceptor yet the first detectable product is a 3-carbon compound. Reason (R): The 6-carbon intermediate formed is unstable and immediately splits into two 3-carbon molecules.

Accepted Answer

Both A and R are true and R is the correct explanation of A. CO₂ adds to 5-C RuBP to give a transient 6-C compound that is unstable and at once breaks into two 3-C 3-PGA molecules — so R correctly explains A.

Question 11

In the reduction phase of the Calvin cycle, the conversion of 3-PGA to glyceraldehyde-3-phosphate (a triose sugar) requires:

Accepted Answer

both ATP and NADPH. The reduction of the acid 3-PGA to the sugar G3P uses ATP for phosphorylation and NADPH as the reducing agent; both products of the light reaction are needed here.

Question 12

For each single CO₂ molecule fixed in the Calvin cycle, the number of RuBP molecules that must ultimately be regenerated to sustain the cycle is:

Accepted Answer

one. The cycle is self-perpetuating: for every CO₂ fixed onto one RuBP, one RuBP must be regenerated so fixation can continue; six turns therefore regenerate six RuBP.

Question 13

RuBisCO carries out the carboxylation step of the Calvin cycle by attaching CO₂ to:

Accepted Answer

ribulose-1,5-bisphosphate (a 5-carbon molecule). RuBisCO catalyses the addition of CO₂ to the 5-carbon RuBP; PEP is the acceptor only in the C4 pathway, and it is fixed by PEP carboxylase, not RuBisCO.

Question 14

The Calvin cycle is often called the "dark reaction," yet it does not actually occur only in darkness. The reason it can proceed is that it:

Accepted Answer

depends on the ATP and NADPH supplied by the light reaction rather than on light directly. The Calvin cycle needs no light itself but is fully dependent on the ATP and NADPH made in the light reaction, so it normally runs in the day; "dark" only means light-independent.

Question 15

The first product of CO2 fixation was identified to be _____ in the Calvin cycle.

Accepted Answer

PGA. In the Calvin (C3) cycle the first stable product of CO2 fixation is the 3-carbon compound 3-phosphoglyceric acid (PGA).

Question 16

In C4 pathway, first CO2 fixation product is

Accepted Answer

OAA. In the C4 pathway CO2 is first fixed into the 4-carbon compound oxaloacetic acid (OAA) in mesophyll cells.

Question 17

OAA and PGA stands for-

Accepted Answer

3-phosphoglyceric acid and oxaloacetic acid respectively. PGA is 3-phosphoglyceric acid and OAA is oxaloacetic acid; only the first option gives both correct chemical names.

Question 18

The products of light reaction are

Accepted Answer

ATP, NADPH, O2. The light reaction produces ATP, NADPH (assimilatory power) and releases O2 from the photolysis of water.

Question 19

O2 is-

Accepted Answer

diffused out of chloroplast. O2 evolved during the light reaction is not used in the Calvin cycle; it diffuses out of the chloroplast.

Question 20

Statement A: Biosynthetic reaction is independent of direct presence of light. Statement B: Biosynthetic process continues for some time after the light becomes unavailable and then stops.

Accepted Answer

Both A & B are correct. The biosynthetic (dark) phase does not directly need light and can continue briefly on stored ATP/NADPH before stopping, so both statements are correct.

C3 Pathway (Calvin Cycle) — NEET Biology MCQs with Solutions

Questions with solutions