AP Biology: Photosynthesis Light Reactions Practice Questions
These original AP Biology practice questions focus on how chloroplast light reactions capture light energy and make ATP and NADPH. They are study practice, not official AP exam questions.
Free original practice questions for study purposes. Open Exam Prep is an independent study resource and is not affiliated with, endorsed by, or sponsored by the makers of AP Exams.
advertisement
Answer the questions below — you get instant feedback and a full explanation for each.
1. What is the main energy-conversion outcome of the light reactions of photosynthesis?
Explanation. The light reactions use absorbed light to drive electron transfers that produce ATP and NADPH. The Calvin cycle uses those energy-rich products later to help reduce carbon dioxide; glucose is not assembled directly by the light reactions.
2. After photosystem II loses energized electrons, which substance provides replacement electrons?
Explanation. Photosystem II replaces its lost electrons by splitting water. This reaction supplies electrons, releases protons into the thylakoid lumen, and produces oxygen as a by-product; carbon dioxide enters the Calvin cycle instead.
3. The oxygen released by a photosynthesizing plant comes most directly from which process?
Explanation. Molecular oxygen is produced when water is oxidized at photosystem II. Reducing NADP+ makes NADPH, carbon fixation consumes carbon dioxide in the Calvin cycle, and ATP synthase uses an existing proton gradient.
4. Where are the photosystems and electron-transport complexes of chloroplast light reactions located?
Explanation. Photosystem II, the electron-transport chain, photosystem I, and ATP synthase are embedded in the thylakoid membrane. This membrane separates the thylakoid lumen from the stroma, allowing a proton gradient to form.
5. In noncyclic electron flow, what is the role of NADP+ near the end of the light reactions?
Explanation. NADP+ is the final electron acceptor in the usual linear light-reaction pathway. After accepting electrons and a proton, it becomes NADPH, which carries reducing power to carbon-fixation reactions.
6. How does ATP synthase in a chloroplast make ATP during the light reactions?
Explanation. Electron transport and water splitting build a higher proton concentration in the thylakoid lumen. Protons then flow down that gradient through ATP synthase toward the stroma, providing energy to phosphorylate ADP.
7. What is a likely result of cyclic electron flow involving photosystem I?
Explanation. In cyclic electron flow, electrons from photosystem I cycle back through carriers that help build the proton gradient. This can increase ATP production without making NADPH or oxygen because photosystem II water splitting is not part of the cycle.
8. Why do healthy green leaves commonly reflect more green light than red or blue light?
Explanation. Chlorophyll pigments absorb much red and blue light, which can excite electrons for photosynthesis. They absorb green wavelengths less strongly, so more green light is reflected or transmitted and reaches an observer's eyes.
9. Which change most directly helps increase the proton concentration in the thylakoid lumen during linear electron flow?
Explanation. The light-reaction electron-transport chain uses energy from electron transfers to move protons from the stroma into the thylakoid lumen. Water splitting also contributes protons there, while ATP synthase allows protons to leave the lumen.
10. If a chloroplast had no available NADP+, what effect would occur most directly in linear electron flow?
Explanation. In linear flow, NADP+ normally accepts electrons after photosystem I and is reduced to NADPH. Without NADP+, this usual terminal step is blocked, so the chain cannot continue its normal linear electron transfer pathway.
11. Which pair of light-reaction products supplies energy and reducing power to the Calvin cycle?
Explanation. ATP provides usable chemical energy and NADPH provides high-energy electrons for carbon-reduction reactions in the Calvin cycle. Oxygen and water are not the paired energy-and-reducing-power outputs used for that process.
12. A treatment prevents photosystem II from transferring energized electrons to its first electron acceptor. Which outcome is expected first?
Explanation. Blocking photosystem II stops the normal start of linear electron flow. Fewer electrons move through the chain, so less proton-gradient energy is generated and production of ATP and NADPH through the usual linear pathway falls.
📘 Want a full structured course and official-style practice tests? Browse top-rated AP Exams prep books and courses. Some links are affiliate links; we may earn a commission at no cost to you.
advertisement
FAQ
What is the difference between photosystem II and photosystem I?
In linear electron flow, photosystem II uses light energy to begin electron transfer and replaces electrons by splitting water. Photosystem I re-energizes electrons later in the chain, allowing NADP+ to be reduced to NADPH. Their names reflect discovery order, not pathway order.
Why is a proton gradient important in the light reactions?
A high proton concentration in the thylakoid lumen stores potential energy. When protons move through ATP synthase into the stroma, the enzyme uses that energy to make ATP from ADP and phosphate.
Do the light reactions make glucose?
No. The light reactions make ATP and NADPH and release oxygen from water. The Calvin cycle uses ATP and NADPH to reduce carbon dioxide into carbohydrate intermediates; several later steps are required to form glucose.