C3 and C4 Photosynthetic Pathways — NEET Biology
✅ Asked in NEET 2025NEET 2025: Which compound is the first stable product of C4 photosynthesis?
NEET 2025: Which compound is the first stable product of C4 photosynthesis?
The first stable product of CO₂ fixation in sorghum is: (NEET 2021)
Answer & NCERT explanation
Correct answer: A — Oxaloacetic acid
In C₄ photosynthesis, the primary CO₂ acceptor PEP combines with CO₂ via PEP carboxylase to form oxaloacetic acid, which is the first stable 4-carbon product. This distinguishes C₄ plants from C₃ plants where 3-phosphoglyceric acid is the first stable product.
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Scientists also tried to know whether all plants have PGA as the first product of CO₂ fixation, or whether any other product was formed in other plants. Experiments conducted over a wide range of plants led to the discovery of another group of plants, where the first stable product of CO₂ fixation was again an organic acid, but one which had 4 carbon atoms in it. This acid was identified to be oxaloacetic acid or OAA. Since then CO₂ assimilation during photosynthesis was said to be of two main types: those plants in which the first product of CO₂ fixation is a C₃ acid (PGA), i.e., the C₃ pathway, and those in which the first product was a C₄ acid (OAA), i.e., the C₄ pathway. These two groups of plants showed other associated characteristics that we will discuss later.
NTA tests whether students know that C3 plants (like rice, wheat) fix CO₂ into PGA (3-carbon compound first), while C4 plants (like maize, sugarcane) fix CO₂ into OAA (4-carbon compound first). The biggest trap: students confuse which acid comes first in each pathway or mix up the carbon numbers. Remember: C3 pathway → PGA first, C4 pathway → OAA first. This distinction determines how efficiently plants use CO₂ under different temperature and light conditions—crucial for understanding tropical vs. temperate plant adaptation.
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