# DBT JRF 2020 — Question 130

Exam: DBT JRF 2020 · Section: B · Topic: Plant Biology · Difficulty: Easy

## Question

The first stable product of C3 cycle is

## Options

- **A.** ribulose bisphosphate
- **B.** phospho enol phosphate
- **C.** dihydroxy acetone phosphate
- **D.** 3-phosphoglycerate

## Answer

Option D

## Explanation

The Calvin cycle (light-independent reactions, dark reactions) of C3 photosynthesis: Step 1 (Carbon fixation): $\text{CO}_2 + \text{RuBP}$ (ribulose-1,5-bisphosphate, C5) $\xrightarrow{\text{RuBisCO}}$ [unstable 6-carbon intermediate, C6] $\rightarrow$ 2 molecules of 3-phosphoglycerate (3-PGA, C3). 3-PGA is the FIRST STABLE product because the C6 intermediate is immediately hydrolysed. Subsequent steps: 3-PGA $\xrightarrow{\text{ATP, NADPH}}$ glyceraldehyde-3-phosphate (G3P) $\xrightarrow{\text{ATP}}$ RuBP regenerated. For every 3 CO $_2$ fixed: 6 G3P produced, 5 used to regenerate 3 RuBP, 1 G3P net output for sugar synthesis. RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) is the most abundant protein on Earth.

Source: https://mypyqbuddy.com/questions/the-first-stable-product-of-c3-cycle-is-6a05a8a02adafec47029329a