# DBT JRF 2020 — Question 89

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

## Question

The specific energy source for the reaction $ADP + phosphate \rightarrow ATP$ by the enzyme ATP synthetase (CF1 Coupling Factor) in thylakoid membranes is

## Options

- **A.** oxidation of NADPH
- **B.** oxidation of water
- **C.** higher concentration of $H^+$ inside versus outside the thylakoid membranes
- **D.** movement of electrons between photosystem II and photosystem I

## Answer

Option C

## Explanation

During light reactions (photophosphorylation) in plant thylakoids: The electron transport chain (Photosystem II → plastoquinone → Cyt $b_6f$ → plastocyanin → Photosystem I → ferredoxin → NADP $^+$ reductase) pumps $\text{H}^+$ into the thylakoid LUMEN, creating a proton-motive force (pmf): $\Delta\mu_{\text{H}^+} = \Delta\psi + (-2.3RT/F)\Delta\text{pH}$ . The lumen pH drops to ~5 while stroma remains ~8 — a $\Delta$ pH of ~3. H $^+$ flows from lumen (high concentration) back to stroma (low concentration) through $\text{CF}_0$ - $\text{CF}_1$ ATP synthase (coupling factor 1), driving the rotary F-type ATP synthase mechanism: $\text{ADP} + \text{P}_i \rightarrow \text{ATP}$ . This is photophosphorylation. Oxidation of water releases $\text{O}_2$ and $\text{H}^+$ but is a consequence, not the direct energy source for ATP synthase.

Source: https://mypyqbuddy.com/questions/the-specific-energy-source-for-the-reaction-adp-6a05a8a02adafec470293271