Question

Read the given statements and select the correct option.
Statement 1: Carboxylation is the most crucial step of the Calvin cycle where CO$_2$ is utilized for the carboxylation of $RuBP$.
Statement 2: Carboxylation is catalyzed by the enzyme $RuBisCO$ which results in the formation of two molecules of $3PGA$.
A. Both statement 1 and 2 are correct
B. Statement 1 is correct but statement 2 is incorrect
C. Statement 1 is incorrect but statement 2 is correct
D. Both statement 1 and 2 are incorrect

Answer 1

Melvin Calvin uses photosynthesis experiments with radioactive carbon 14 isotope in algal. This led to the discovery that a three-carbon organic acid was the first CO$_2$ fixation substance. He also helped to define the entire biosynthetic pathway, which is why it is called the Calvin Cycle. The first stable substance is known as the C$_3$ pathway, 3-phosphoglyceric acid ($3PGA$).

Complete answer: The Calvin cycle has 3 important steps:
i) Carboxylation (of $RuBP$)
ii) Reduction
iii) Regeneration (of $RuBP$)
It is the fixation into a stable organic intermediate of CO$_2$.
CO$_2$ is used for the carboxylation of RuBP in this process. The enzyme Rubisco catalyzes this reaction. Reduction:- These reactions lead to glucose formation. The steps include the use of two molecules of ATP for phosphorylation and two molecules of NADPH for reduction per fixed CO$_2$ molecule.
Regeneration: - For phosphorylation to form RuBP, this step requires one ATP. Six turns of the cycle are necessary to make one molecule of glucose. To build a glucose molecule, ATP & NADPH molecules are used.
The fixation of CO$_2$ into a stable organic intermediate is carboxylation. It is the most important step in the Calvin cycle, where CO$_2$ is formed. The use of $ATP$ and $NADPH$ produced by the light reaction is used for the carboxylation of RuBP. This reaction is catalyzed by the $RuBisCO$ enzyme, which results in two molecules of 3-phosphoglyceric acid or $3PGA$, the first stable photosynthesis substance, being created. One can now make out that both the given statements are appropriate.
Hence, the correct option is A, i.e., Both statement 1 and 2 are correct.

Note: A five-carbon ketose sugar-Ribulose bisphosphate ($RuBP$) is the primary acceptor molecule throughout the C$_3$ cycle. RuBisCO (Ribulose Bisphosphate Carboxylase Oxygenase) is the enzyme for CO$_2$ fixation. It is the earth's most abundant enzyme. It is distinguished by the fact that its active site, hence the name Rubisco, can bind to both CO$_2$ and O$_2$.