AP Biology: Unit 3, Photosynthesis (Drill 13)

Question 1. In the light reactions, water molecules are split during photolysis. What is the primary role of this process in photosynthesis?

• A) To provide CO2 molecules that will be fixed by RuBisCO in the Calvin cycle.
• B) To replenish electrons lost from Photosystem II when chlorophyll absorbs light energy. ✓
• C) To produce NADPH directly from the split water molecules in the thylakoid membrane within the process described.
• D) To generate ATP through substrate-level phosphorylation in the thylakoid membrane.

Explanation: Correct: (B) When chlorophyll in Photosystem II absorbs light energy, electrons are excited to a higher energy state and leave the reaction center to enter the electron transport chain. These electrons must be replaced to keep PSII functioning. The splitting of water (photolysis: 2H2O → 4H+ + 4e- + O2) provides these replacement electrons. Oxygen is released as a byproduct. Water is not the source of CO2 (A). NADPH is produced at PSI, not from water splitting (C). ATP in the light reactions is produced by chemiosmosis, not substrate-level phosphorylation (D).

Question 2. A researcher isolates chloroplasts and provides them with 14C-labeled CO2. After a short incubation in the light, radioactive 14C is detected in G3P molecules. Which conclusion is most directly supported by this finding?

• A) CO2 is fixed into organic carbon in the Calvin cycle, with RuBisCO catalyzing the incorporation of CO2 into RuBP to produce 3-carbon intermediates that are converted to G3P. ✓
• B) CO2 is incorporated into organic molecules during the light reactions by Photosystem I, which fixes carbon as it transfers electrons.
• C) CO2 is split by photolysis in the thylakoid membrane, and the carbon atoms are transferred to NADPH.
• D) CO2 is directly converted to G3P by ATP synthase using the proton gradient across the thylakoid membrane.

Explanation: Correct: (A) The radioactive 14C in CO2 appears in G3P, which is the product of the Calvin cycle. RuBisCO catalyzes carbon fixation in the stroma by adding CO2 to the 5-carbon RuBP, producing two 3-carbon molecules of 3-PGA. Using ATP and NADPH, 3-PGA is reduced to G3P. The 14C label traces this path from CO2 to organic carbon in G3P. The light reactions do not fix CO2 (B). Photolysis splits water, not CO2 (C). ATP synthase synthesizes ATP but does not fix carbon (D).

Question 3. A plant is placed in a sealed chamber under bright light. CO2 concentration in the chamber then drops to near zero. Which of the following best predicts what will happen to photosynthesis and why?

• A) Light reactions will stop first because low CO2 causes the proton gradient across the thylakoid to collapse.
• B) Photosynthesis rate will increase because the absence of CO2 allows more light energy to reach the photosystems.
• C) The Calvin cycle will be unable to proceed because RuBisCO requires CO2 as a substrate; ATP and NADPH from the light reactions will accumulate and overall photosynthesis rate will decline. ✓
• D) The plant will immediately switch to carrying out only cellular respiration, which will restore CO2 to the chamber and allow the cycle to continue.

Explanation: Correct: (C) CO2 is the substrate for carbon fixation by RuBisCO. When CO2 is depleted, RuBisCO cannot catalyze the fixation step and the Calvin cycle stalls. As a consequence, ADP and NADP+ are not regenerated from the Calvin cycle, causing ATP and NADPH to accumulate and the light reactions to slow. Overall photosynthesis rate declines dramatically when CO2 is limiting. The light reactions themselves do not directly require CO2, so the proton gradient does not collapse immediately (A). Plants continue both photosynthesis and respiration simultaneously and cannot simply "switch" to respiration alone (D).

Question 4. A graph of photosynthesis rate vs. light intensity rises steeply at low intensities, then levels off at a plateau at high intensities. Which of the following best explains why the rate plateaus?

• A) At high light intensity, the chlorophyll molecules become permanently bleached and stop absorbing photons, which is what causes the rate to plateau.
• B) At high light intensity, the rate decreases because excess photons generate reactive oxygen species that destroy Photosystem II.
• C) At high light intensity, water molecules are split faster than the plant can absorb them through roots, causing stomata to close.
• D) At high light intensity, the light reactions produce ATP and NADPH faster than the Calvin cycle can use them, so the rate becomes limited by the enzymatic capacity of the Calvin cycle rather than by light availability. ✓

Explanation: Correct: (D) At low light intensities, the rate is limited by light. As intensity increases, more ATP and NADPH are produced and rate rises. However, at high intensities the light reactions produce ATP and NADPH more rapidly than the Calvin cycle enzymes (particularly RuBisCO) can process them. The rate becomes limited by the enzymatic capacity of the Calvin cycle rather than by light. Adding more light beyond this saturation point produces no additional increase in rate. Photobleaching (A) is not the standard AP Biology explanation for saturation; photoinhibition (B) describes a damage response, not the plateau itself.

Question 5. Which of the following correctly describes the relationship between photosynthesis and cellular respiration at the level of net chemical change?

• A) Both processes produce ATP and consume glucose, making them functionally identical reactions that simply occur in different organelles of the cell.
• B) Photosynthesis consumes CO2 and H2O and produces glucose and O2; cellular respiration consumes glucose and O2 and produces CO2 and H2O, making the two processes essentially reverse reactions at the level of net chemical change. ✓
• C) Photosynthesis produces CO2 and O2 that are consumed by cellular respiration, while cellular respiration produces glucose that is consumed by photosynthesis.
• D) Photosynthesis and cellular respiration both occur in chloroplasts: photosynthesis during the day and respiration at night.

Explanation: Correct: (B) Photosynthesis overall: 6CO2 + 6H2O + light → C6H12O6 + 6O2. Cellular respiration overall: C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP. These are chemical reverses of each other at the level of net reactants and products, though the actual biochemical pathways and organelles involved differ greatly. Cellular respiration does not occur in chloroplasts (D); it occurs primarily in the mitochondria. Photosynthesis consumes CO2 rather than producing it (C).