Proton Gradient Formation in Photosystems — NEET Biology
📚 Practice ConceptAs electrons move through the photosystems, protons are transported across the membrane. This happens because the primary acceptor of electron which is located towards the outer side of the membrane transfers its electron not to an electron carrier but to an H carrier. Hence, this molecule removes a proton from the stroma while transporting an electron. When this molecule passes on its electron to the electron carrier on the inner side of the membrane, the proton is released into the inner side or the lumen side of the membrane.
Which of the above statements regarding the light reaction of photosynthesis are NOT correct? S1: The primary acceptor of electrons in Photosystem II is located towards the inner side of the thylakoid membrane. S2: Both Photosystem I and Photosystem II are functionally involved in non-cyclic photophosphorylation. S3: Cyclic photophosphorylation results in the synthesis of both ATP and NADPH + H+. S4: The splitting of water (photolysis) occurs on the outer side of the thylakoid membrane, releasing protons into the chloroplast stroma. S5: The synthesis of ATP via chemiosmosis is powered by the breakdown of the proton gradient across the thylakoid membrane.
View solution & NCERT explanation
Correct answer: A — S1, S3 and S4 are NOT correct
The question assesses understanding of the detailed mechanisms and locations within the light reaction. S1: NOT correct. The primary acceptor of electrons in PS II is located towards the outer side of the membrane. This acceptor then transfers its electron to an H carrier, which removes a proton from the stroma. (NCERT Page 141, point (b)). S2: Correct. Non-cyclic photophosphorylation, also known as the Z-scheme, involves both Photosystem II (PS II) and Photosystem I (PS I) working in series. (NCERT Page 140, Para 2). S3: NOT correct. Cyclic photophosphorylation involves only PS I and results exclusively in the synthesis of ATP. It does not produce NADPH + H+. (NCERT Page 140, Para 3). S4: NOT correct. The splitting of the water molecule takes place on the inner side of the thylakoid membrane, and the protons produced accumulate within the lumen of the thylakoids, not in the stroma. (NCERT Page 140, point (a)). S5: Correct. The chemiosmotic hypothesis explains that ATP synthesis is driven by the breakdown of the proton gradient across the thylakoid membrane, facilitated by ATP synthase. (NCERT Page 141, Para 2). Therefore, statements S1, S3, and S4 are NOT correct. The tested concept is the precise details of electron transport, photophosphorylation, and chemiosmosis within the light reaction.
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