C4 (Hatch–Slack) pathway: which immediate 4-carbon product is formed when atmospheric CO2 (as bicarbonate) is first fixed by PEP carboxylase in mesophyll cells?

Introduction / Context:
The C4 pathway minimizes photorespiration by concentrating CO2 around rubisco in bundle sheath cells. The very first step—PEP carboxylase adding bicarbonate to phosphoenolpyruvate—defines the pathway and its 4-carbon intermediates.

Given Data / Assumptions:

• We consider initial fixation in mesophyll cells.
• PEP carboxylase uses HCO3−, not CO2 bound to rubisco.
• The first product is a 4-carbon acid.

Concept / Approach:
PEP carboxylase catalyzes PEP + HCO3− → oxaloacetate (OAA). OAA is then rapidly converted to malate (or aspartate) for transport to bundle sheath cells, where CO2 is released for the Calvin cycle. This spatial separation underlies the efficiency of C4 plants at high light and temperature.

Step-by-Step Solution:

Verification / Alternative check:
Standard depictions of the C4 cycle show OAA as the first 4-carbon product before malate formation.

Why Other Options Are Wrong:

• A: Malate is typically the next intermediate after OAA, not the first product of fixation.
• C/D/E: Pyruvate and α-ketoglutarate are not initial C4 products; 3-PGA is a Calvin cycle product in C3.

Common Pitfalls:
Assuming malate is the initial product; remember OAA appears first and is then reduced or transaminated.

Final Answer:
Oxaloacetate (OAA) as the first carboxylation product