Photosynthesis Photosynthesis Electron Transport Chain 2 — Questions and Answers
Question 1: What drives the synthesis of ATP by ATP synthase (CF0-CF1) in chloroplasts during the light reactions?
- A proton (H+) electrochemical gradient across the thylakoid membrane (Correct answer)
- Direct transfer of high-energy electrons from PSII
- The redox reaction between NADPH and O2
- Substrate-level phosphorylation using ADP
Correct answer: A proton (H+) electrochemical gradient across the thylakoid membrane
Protons accumulate in the thylakoid lumen through water splitting and plastoquinone activity, creating a gradient that drives protons through ATP synthase to phosphorylate ADP to ATP.
Question 2: In cyclic photophosphorylation, electrons cycle around which photosystem?
- Photosystem I (PSI) (Correct answer)
- Photosystem II (PSII)
- Both PSI and PSII equally
- Neither — electrons are transferred to NADP+
Correct answer: Photosystem I (PSI)
In cyclic photophosphorylation, electrons from PSI are transferred to ferredoxin, then to the cytochrome b6f complex, and back to PSI via plastocyanin, generating ATP without producing NADPH or O2.
Question 3: What is the primary difference between cyclic and noncyclic photophosphorylation?
- Cyclic produces only ATP, while noncyclic produces both ATP and NADPH (and releases O2) (Correct answer)
- Cyclic produces NADPH only, while noncyclic produces ATP only
- Cyclic requires both PSI and PSII, while noncyclic uses only PSI
- Cyclic occurs in the stroma, while noncyclic occurs in the thylakoid lumen
Correct answer: Cyclic produces only ATP, while noncyclic produces both ATP and NADPH (and releases O2)
Cyclic photophosphorylation uses only PSI to generate extra ATP without producing NADPH or evolving oxygen, while noncyclic flow through both photosystems produces ATP, NADPH, and O2.
Question 4: The cytochrome b6f complex transfers electrons between which two mobile carriers in the electron transport chain?
- Plastoquinone (PQ) and plastocyanin (PC) (Correct answer)
- Ferredoxin and NADP+
- Water and plastoquinone
- Plastocyanin and ferredoxin
Correct answer: Plastoquinone (PQ) and plastocyanin (PC)
The cytochrome b6f complex accepts electrons from reduced plastoquinone (PQH2) and passes them to the copper-containing protein plastocyanin, while pumping protons into the thylakoid lumen.
Question 5: The P700 designation for the Photosystem I reaction center indicates what?
- The special pair chlorophyll absorbs maximally at 700 nm (far-red light) (Correct answer)
- There are 700 chlorophyll molecules in PSI
- The reaction center has a molecular weight of 700 kDa
- PSI generates 700 mV of electrochemical potential
Correct answer: The special pair chlorophyll absorbs maximally at 700 nm (far-red light)
P700 refers to the absorption maximum of the special pair of chlorophyll a molecules at the PSI reaction center, which is 700 nm in the far-red region.
Question 6: What byproduct is released by the oxygen-evolving complex when it splits two molecules of water?
- Molecular oxygen (O2) (Correct answer)
- Carbon dioxide (CO2)
- Glucose
- ATP
Correct answer: Molecular oxygen (O2)
The oxidation of two water molecules by the OEC yields four electrons, four protons (for the gradient), and one molecule of O2, which is released as a byproduct.
Photosynthesis Photosynthesis Electron Transport Chain 2
What drives the synthesis of ATP by ATP synthase (CF0-CF1) in chloroplasts during the light reactions?