Question

First product of C$_4$ cycle

• A. OAA
• B. PGA
• C. PEP
• D. RuBP

Hint:

In C$_4$ plants, PEP is the first carbon dioxide acceptor and OAA is the first stable product formed after fixation.

Answer 1

The Correct Option is A

Concept: The C$_4$ pathway is also called the Hatch-Slack pathway. It is characterized by the formation of a four-carbon compound immediately after carbon dioxide fixation. The initial fixation of carbon dioxide occurs in mesophyll cells and results in the formation of oxaloacetic acid (OAA), which is the first stable product of the pathway.

Step 1: Study the sequence of reactions. The first reaction in C$_4$ photosynthesis is \[ PEP + CO_2 \xrightarrow{\text{PEP Carboxylase}} OAA \] where OAA stands for oxaloacetic acid.

Step 2: Identify the first stable product. Since OAA is produced immediately after carbon dioxide fixation and contains four carbon atoms, it is considered the first stable product of the C$_4$ pathway.

Step 3: Examine other options. PGA is the first stable product of the Calvin cycle in C$_3$ plants. PEP is the carbon dioxide acceptor and not the first product. RuBP participates in the Calvin cycle and is not the first product of the C$_4$ pathway.

Step 4: Conclude. The first stable product of the C$_4$ cycle is oxaloacetic acid. \[ \boxed{\text{OAA}} \] Hence, the correct answer is \[ \boxed{(A)\ \text{OAA}} \]