AP Biology: Photosynthesis Calvin Cycle Practice Questions
These original AP Biology practice questions focus on how the Calvin cycle in the chloroplast stroma uses ATP and NADPH to turn carbon dioxide into carbohydrate intermediates. 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.
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1. Where in a chloroplast does the Calvin cycle take place?
Explanation. The Calvin cycle enzymes are located in the chloroplast stroma, the fluid surrounding the thylakoids. ATP and NADPH made by light reactions on the thylakoid membrane are used in the stroma to support carbon fixation and reduction.
2. At the start of the Calvin cycle, rubisco catalyzes a reaction between carbon dioxide and which molecule?
Explanation. Rubisco catalyzes carbon fixation by adding carbon dioxide to RuBP, a five-carbon sugar with phosphate groups. The unstable six-carbon product rapidly splits into two three-carbon molecules, beginning the pathway toward carbohydrate production.
3. What is the immediate fate of the unstable six-carbon compound formed when carbon dioxide combines with RuBP?
Explanation. The six-carbon intermediate is unstable and promptly splits into two molecules of 3-phosphoglycerate, each with three carbons. This is why fixation of one carbon dioxide ultimately supplies material to the reduction phase rather than making a six-carbon sugar at once.
4. Which statement best describes the role of NADPH in the Calvin cycle?
Explanation. NADPH carries high-energy electrons from the light reactions. During the reduction phase, those electrons help convert phosphorylated three-carbon intermediates into G3P; carbon dioxide, rather than NADPH, is the source of the newly fixed carbon.
5. Why is ATP required during the Calvin cycle?
Explanation. ATP transfers usable chemical energy to Calvin-cycle intermediates during the reduction phase and during regeneration of RuBP. It does not provide carbon, act as an enzyme, or absorb light; those functions belong to carbon dioxide, proteins, and pigments respectively.
6. For every three molecules of carbon dioxide fixed by the Calvin cycle, what is the net carbohydrate product that can leave the cycle?
Explanation. Fixing three carbon dioxide molecules adds three carbons to the cycle. After accounting for the molecules needed to regenerate RuBP, one net three-carbon G3P molecule can leave and later contribute to synthesis of glucose or other carbohydrates.
7. If a plant's light reactions stop producing ATP while the Calvin-cycle enzymes remain intact, which Calvin-cycle process is most directly limited?
Explanation. RuBP regeneration requires ATP, so an ATP shortage eventually prevents the cycle from restoring the carbon-dioxide acceptor needed for continued carbon fixation. Water splitting is a light-reaction process, and gas movement through stomata is not the ATP-requiring Calvin-cycle step asked about here.
8. A researcher supplies chloroplasts with carbon dioxide but removes NADPH. Which change is expected in the Calvin cycle?
Explanation. NADPH donates electrons needed in the reduction phase that produces G3P from phosphorylated three-carbon intermediates. Without it, carbon may initially be fixed, but the pathway cannot efficiently reduce those intermediates or sustain the normal cycle.
9. Which material is the direct source of the carbon atoms found in G3P produced by the Calvin cycle?
Explanation. Carbon dioxide enters the Calvin cycle through rubisco-mediated carbon fixation, so it supplies the carbon atoms later present in G3P and larger carbohydrates. Water supplies electrons to light reactions, while NADPH supplies reducing power rather than carbon atoms.
10. What is the central purpose of the RuBP regeneration phase?
Explanation. Most of the cycle's three-carbon intermediates are rearranged, using ATP, to regenerate RuBP. Restoring this five-carbon acceptor lets rubisco keep fixing incoming carbon dioxide; without regeneration, the cycle would stop after existing RuBP was used.
11. A leaf receives bright light but is placed in air with almost no carbon dioxide. Which Calvin-cycle outcome is most likely?
Explanation. Carbon dioxide is the substrate rubisco fixes, so little available carbon dioxide directly reduces net carbon fixation. Bright light can still support ATP and NADPH production for a time, but it cannot replace the carbon input required to make carbohydrate intermediates.
12. Why does a plant generally need to fix six carbon dioxide molecules to provide enough net G3P for one glucose molecule?
Explanation. Three fixed carbon dioxide molecules yield one net G3P with three carbons after RuBP regeneration. A six-carbon glucose can be assembled from two such G3P molecules, so six carbon dioxide molecules must be fixed to provide the two net three-carbon outputs.
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FAQ
Does the Calvin cycle require light directly?
The Calvin cycle does not capture photons directly, but it depends on ATP and NADPH made by the light reactions. In most plants, when light reactions slow, the supply of those molecules drops and carbon fixation cannot continue normally.
What does rubisco do in the Calvin cycle?
Rubisco catalyzes carbon fixation: it attaches carbon dioxide to RuBP. The resulting unstable six-carbon compound splits into two three-carbon molecules that enter the reduction phase.
Is G3P the same thing as glucose?
No. G3P is a three-carbon sugar phosphate produced as a net output of the Calvin cycle. Two net G3P molecules can later be combined and processed to form one six-carbon glucose molecule.