Question Breakdown

The query tests knowledge of pulse-labeling experiments in the Calvin cycle (C3 pathway). Melvin Calvin’s team used short exposures (5-30 seconds) of ¹⁴CO₂ on algae like Chlorella to identify the earliest labeled compound after CO₂ fixation. The correct answer is (C) 3-phosphoglycerate, as it forms directly from RuBP carboxylation.

Option Analysis

(A) Glyceraldehyde

This is glyceraldehyde-3-phosphate (G3P), a later triose phosphate formed after 3-PGA reduction by ATP and NADPH. Labeling appears after seconds to minutes, not first.

(B) 1,3-bisphosphoglycerate

An unstable intermediate post-phosphorylation of 3-PGA; not detected early in labeling due to rapid conversion.

(C) 3-phosphoglycerate

Correct. RuBisCO fixes CO₂ to RuBP, forming an unstable 6C intermediate that splits into two 3-PGA molecules, labeled at C1 within seconds.

(D) Ribulose 1,5-bisphosphate

The CO₂ acceptor; unlabeled initially as ¹⁴C incorporates into 3-PGA first. Labeling occurs later during regeneration.

Calvin Cycle Basics

CO₂ fixation begins with RuBP accepting ¹⁴CO₂, yielding two 3-PGA molecules labeled at the carboxyl carbon. In algae suspensions illuminated briefly, 3-PGA shows radioactivity first (5-30s), before G3P or RuBP.

Pulse Labeling Evidence

Calvin’s “lollipop” setup exposed Chlorella/Scenedesmus to ¹⁴CO₂ for seconds: only 3-PGA labeled initially, proving its role as the first stable product. Longer pulses label downstream intermediates like 1,3-BPG and G3P.

Exam Relevance

For CSIR NET Life Sciences, recognize 3-PGA as the answer in such MCQs; RuBP is acceptor, not first labeled. Key for molecular biology, genetics sections.