Q.50. In a typical green plant, the first stable product of Calvin cycle is
(A) Oxaloacetic acid
(B) Succinic acid
(C) Maleic acid
(D) 3-Phosphoglyceric acid

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Correct Answer Revealed

In a typical green plant, the first stable product of the Calvin cycle is (D) 3-Phosphoglyceric acid (3-PGA). This molecule marks the initial stable compound formed during the carbon fixation phase of photosynthesis, making it a key topic for biology exams like NEET or undergraduate courses in molecular biology and biochemistry.

The Calvin cycle, also known as the light-independent reactions, occurs in the stroma of chloroplasts. It fixes atmospheric CO₂ into organic sugars using ATP and NADPH from the light reactions. Understanding this helps grasp how plants convert sunlight into energy-storing carbohydrates.

Why 3-Phosphoglyceric Acid is the First Stable Product

The Calvin cycle starts with carbon fixation, where the enzyme RuBisCO attaches CO₂ to ribulose-1,5-bisphosphate (RuBP), a 5-carbon molecule. This unstable 6-carbon intermediate quickly splits into two molecules of 3-phosphoglyceric acid (3-PGA), a 3-carbon compound.

• 3-PGA is stable and the first fixed carbon product.

• It then gets phosphorylated to 1,3-bisphosphoglycerate using ATP, reduced to glyceraldehyde-3-phosphate (G3P) with NADPH, and some G3P regenerates RuBP while others form glucose.

This process ensures efficient CO₂ assimilation. For visual learners, imagine RuBP as a “catcher” grabbing CO₂, releasing 3-PGA as the stable “catch.”

Explanation of All Options

Let’s break down why the other choices are incorrect, helping you eliminate distractors in multiple-choice questions.

• (A) Oxaloacetic acid: This 4-carbon compound is the first stable product in C4 plants (e.g., maize) during the Hatch-Slack pathway, not the Calvin cycle in typical C3 green plants like wheat or rice. C4 plants use PEP carboxylase to form it before shuttling to bundle sheath cells for Calvin cycle entry.

• (B) Succinic acid: A 4-carbon intermediate in the Krebs cycle (TCA cycle) of cellular respiration, involved in breaking down glucose for energy. It plays no role in photosynthesis or the Calvin cycle.

• (C) Maleic acid: This is a dicarboxylic acid used in organic synthesis or as a food additive, but it’s not a natural product of photosynthesis or the Calvin cycle. It’s unrelated to plant carbon fixation.

Only 3-PGA fits the Calvin cycle in standard green plants.

Calvin Cycle Phases: Quick Recap

To solidify your knowledge:

1. Carbon Fixation: RuBP + CO₂ → unstable intermediate → 2 × 3-PGA.

2. Reduction: 3-PGA → G3P (using ATP and NADPH).

3. Regeneration: G3P → RuBP (using ATP).

For exam prep, memorize: First stable = 3-PGA in C3 plants.

This distinction is crucial for questions comparing C3 and C4 pathways in genetics and plant physiology.