Skip to content

Quiz: Photosynthesis

Test your understanding of the light-dependent reactions, the Calvin cycle, and C3/C4/CAM photosynthetic strategies with these review questions.

1. Which pigment directly participates in the photochemical charge separation at the reaction center of a photosystem?

  1. Carotenoids
  2. Chlorophyll a
  3. Chlorophyll b
  4. Xanthophylls
Show Answer

The correct answer is B. A pair of chlorophyll a molecules sits at the reaction center of each photosystem and undergoes the actual photochemical event: an excited electron is ejected and passed to a primary electron acceptor. Chlorophyll b, carotenes, and xanthophylls are accessory pigments in the antenna complex that absorb additional wavelengths and funnel energy to chlorophyll a, but do not themselves eject electrons.

Concept Tested: Chlorophyll and photosynthetic pigments

2. Which molecule is split during the light-dependent reactions to replace electrons lost from photosystem II, releasing oxygen as a byproduct?

  1. Carbon dioxide
  2. NADPH
  3. ATP
  4. Water
Show Answer

The correct answer is D. The oxygen-evolving complex of photosystem II splits water molecules (photolysis) to replace electrons ejected from the P680 reaction center, releasing protons, electrons, and O2 as byproducts. This reaction is the source of essentially all atmospheric oxygen — the carbon and oxygen in CO2 instead end up in glucose and water, not in the O2 released by photosynthesis.

Concept Tested: Photolysis of water

3. Why does the Calvin cycle require ATP and NADPH generated by the light-dependent reactions?

  1. ATP and NADPH provide the energy and reducing power needed to convert fixed carbon (3-PGA) into G3P
  2. ATP and NADPH are needed to split water and release oxygen in the stroma
  3. ATP and NADPH regenerate chlorophyll molecules after each turn of the cycle
  4. ATP and NADPH are consumed only during photorespiration, not during normal carbon fixation
Show Answer

The correct answer is A. After RuBisCO fixes CO2 onto RuBP to form 3-PGA, the cycle uses ATP to phosphorylate 3-PGA and NADPH to reduce it, converting the 3-carbon intermediate into G3P. This reduction step, along with the ATP used to regenerate RuBP, is why the Calvin cycle depends entirely on the energy and reducing power supplied by the light reactions.

Concept Tested: Calvin cycle energy requirements

4. Why is RuBisCO considered an inefficient enzyme despite being the most abundant protein on Earth?

  1. It requires an unusually high concentration of NADPH to function
  2. It can only function in the presence of chlorophyll b
  3. It catalyzes only about 3 reactions per second and can bind O2 instead of CO2, leading to wasteful photorespiration
  4. It is only active during the night in all photosynthetic organisms
Show Answer

The correct answer is C. RuBisCO is slow, fixing only about 3 CO2 molecules per second compared to roughly 1000 reactions per second for a typical enzyme, and it lacks perfect specificity — it can also bind O2, initiating the wasteful photorespiration pathway that releases CO2 and consumes ATP and NADPH rather than producing sugar.

Concept Tested: RuBisCO and photorespiration

5. How do C4 plants avoid the photorespiration that limits C3 plants under hot, dry conditions?

  1. By eliminating RuBisCO entirely and using only PEP carboxylase throughout the leaf
  2. By using PEP carboxylase in mesophyll cells to fix CO2 into a 4-carbon compound, which is transported to bundle sheath cells where CO2 is released at high concentration around RuBisCO
  3. By opening stomata only at night to fix CO2 into malate for storage
  4. By increasing the rate of photorespiration to recycle wasted carbon
Show Answer

The correct answer is B. C4 plants use PEP carboxylase, an enzyme with high affinity for CO2 and no oxygenase activity, to fix carbon in mesophyll cells into a 4-carbon compound that is shuttled to bundle sheath cells. There it is decarboxylated, releasing a high local concentration of CO2 directly around RuBisCO, which suppresses RuBisCO's competing oxygenase activity and minimizes photorespiration.

Concept Tested: C4 photosynthesis

6. A researcher grows a plant using water labeled with the heavy isotope ¹⁸O and normal (¹⁶O) CO2. In which molecule would the labeled oxygen most likely appear first?

  1. The O2 released during the light-dependent reactions
  2. The glucose produced by the Calvin cycle
  3. The CO2 fixed by RuBisCO
  4. The ATP synthesized by ATP synthase
Show Answer

The correct answer is A. Classic isotope-labeling experiments using ¹⁸O-labeled water demonstrated that the oxygen gas released during photosynthesis originates from the photolysis of water in photosystem II, not from CO2. Since the water was labeled, its oxygen atoms would appear in the O2 byproduct released during the light-dependent reactions rather than in the glucose or CO2 pathway.

Concept Tested: Photolysis of water (isotope tracing)

7. On a hot, dry day, a C3 plant partially closes its stomata to conserve water. What effect does this have on internal CO2 and O2 concentrations, and on photorespiration?

  1. The CO2:O2 ratio inside the leaf increases, decreasing photorespiration
  2. Stomatal closure has no effect on internal gas concentrations
  3. The plant switches immediately to C4 metabolism
  4. The CO2:O2 ratio inside the leaf decreases, increasing the rate of photorespiration
Show Answer

The correct answer is D. When stomata close to reduce water loss, CO2 entry from the atmosphere slows while O2 produced by ongoing light reactions accumulates in the leaf, lowering the internal CO2:O2 ratio. This favors RuBisCO's oxygenase activity over its carboxylase activity, increasing photorespiration and reducing the plant's net carbon gain — a key trade-off C3 plants face in hot, arid conditions.

Concept Tested: Environmental effects on photorespiration

8. Producing one molecule of glucose requires six turns of the Calvin cycle. How much total ATP and NADPH does this consume?

  1. 3 ATP and 2 NADPH
  2. 6 ATP and 6 NADPH
  3. 18 ATP and 12 NADPH
  4. 36 ATP and 24 NADPH
Show Answer

The correct answer is C. Each single turn of the Calvin cycle fixes one CO2 and consumes 3 ATP and 2 NADPH. Because six turns are required to accumulate the six carbons needed for one glucose molecule, the total cost is 6 × 3 = 18 ATP and 6 × 2 = 12 NADPH, all of which must be supplied by the light-dependent reactions.

Concept Tested: Calvin cycle stoichiometry

9. How do the mechanisms C4 and CAM plants use to concentrate CO2 around RuBisCO fundamentally differ?

  1. C4 plants use a spatial separation between mesophyll and bundle sheath cells, while CAM plants use a temporal separation between nighttime CO2 fixation and daytime Calvin cycle activity
  2. Both C4 and CAM plants use the identical spatial separation between two cell types
  3. CAM plants use PEP carboxylase only in bundle sheath cells, exactly as C4 plants do
  4. C4 and CAM plants both eliminate RuBisCO from their carbon fixation pathway entirely
Show Answer

The correct answer is A. C4 plants achieve CO2 concentration through spatial separation, fixing carbon in mesophyll cells and running the Calvin cycle in bundle sheath cells. CAM plants instead separate the two processes in time, fixing CO2 into malate at night when stomata are open and running the Calvin cycle during the day using the stored malate, keeping stomata closed to conserve water.

Concept Tested: C4 vs. CAM carbon-concentrating mechanisms

10. A researcher compares a desert cactus (CAM) and a corn plant (C4) grown under identical hot, sunny, water-limited conditions. Both minimize photorespiration, yet the corn plant achieves higher total daily carbon fixation. Why?

  1. The cactus fixes more carbon overall because CAM plants are more water-efficient
  2. Both plants fix identical amounts of carbon because photorespiration is eliminated in both
  3. The corn plant loses more water than the cactus, reducing its carbon fixation
  4. The corn plant keeps its stomata open throughout the day, allowing continuous CO2 fixation, while the cactus fixes CO2 only at night, resulting in a lower overall daily carbon fixation rate despite superior water conservation
Show Answer

The correct answer is D. C4 plants like corn keep their stomata open during daylight, allowing continuous CO2 uptake and carbon fixation whenever light is available, at the cost of greater water loss. CAM plants restrict CO2 uptake to nighttime hours to minimize water loss, which conserves water very effectively but limits the total amount of CO2 that can be fixed in a day, yielding lower overall productivity than C4 plants.

Concept Tested: Trade-offs between C4 and CAM strategies