NEETBotanyPhotosynthesis in Higher Plants
Read the following statement regarding the first step of the Calvin cycle and select the option that correctly fills the blanks (X), (Y), and (Z). In the Calvin cycle, the enzyme (X) catalyzes the addition of C O₂ to a (Y)-carbon acceptor molecule, resulting in the formation of two molecules of a (Z)-carbon compound.
Options
- A(X) PEP carboxylase, (Y) 3, (Z) 4
- B(X) RuBisCO, (Y) 5, (Z) 3
- C(X) RuBisCO, (Y) 3, (Z) 5
- D(X) RuBisCO, (Y) 6, (Z) 3
Correct answer
B. (X) RuBisCO, (Y) 5, (Z) 3
Step-by-step solution
The first step of the Calvin cycle is carboxylation. During this step, the enzyme RuBisCO (Ribulose bisphosphate carboxylase-oxygenase) catalyzes the addition of CO₂ to Ribulose-1,5-bisphosphate (RuBP). RuBP is a 5-carbon (5C) acceptor molecule. This reaction results in an unstable 6-carbon intermediate that immediately splits into two molecules of 3-phosphoglyceric acid (3-PGA), which is a 3-carbon (3C) compound. Thus, (X) is RuBisCO, (Y) is 5, and (Z) is 3. Selecting PEP carboxylase, 3, and 4 incorrectly describe