Proton Gradient in Light Reactions — NEET Biology
✅ Asked in NEET 2016In a chloroplast, the highest number of protons are found in: (NEET 2016 Phase 1)
In a chloroplast, the highest number of protons are found in: (NEET 2016 Phase 1)
Answer & NCERT explanation
Correct answer: A — Lumen of thylakoids
Highest number of protons are found in thylakoid lumen. According to NCERT chemiosmotic hypothesis, protons accumulate in the lumen due to water splitting at PS II and proton pumping across thylakoid membrane. This creates the proton gradient essential for ATP synthesis. Stroma has lower proton concentration, while antenna complex and intermembrane space are not proton reservoirs.
Read more NCERT concept on the PYQ
Hence, within the chloroplast, protons in the stroma decrease in number, while in the lumen there is accumulation of protons. This creates a proton gradient across the thylakoid membrane as well as a measurable decrease in pH in the lumen.
NTA tests whether students understand how light reactions create a proton gradient across the thylakoid membrane. When photosystem II splits water, protons accumulate inside the thylakoid lumen, while protons decrease in the stroma, establishing an electrochemical gradient. The common mistake is confusing where protons accumulate—students often think it's the stroma instead of the lumen. Remember: protons move INTO the lumen (lower pH), creating the gradient that drives ATP synthase for energy production. This concept is fundamental to understanding how light energy converts to chemical energy in photosynthesis.
What does NCERT say about Hence within chloroplast protons?
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