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About This Drill
AP Biology — Unit 3 — Photosynthesis — Drill 13 is a practice drill. It contains 5 original questions created by Brian Stewart, a Barron's test prep author with over 20 years of tutoring experience.
Practice analyzing light reactions and the Calvin cycle with this AP Biology drill. You will trace energy and electron flow through photosystems I and II, evaluate the products of each stage of photosynthesis, and analyze how environmental factors affect photosynthesis rate.
Passage
Photosynthesis converts light energy into chemical energy stored in glucose. It occurs in two main stages in the chloroplast.
Stage 1: Light Reactions (Thylakoid Membranes)
Light energy is absorbed by chlorophyll pigments organized into two photosystems. In Photosystem II (PSII), light energy excites electrons to a higher energy state, and water molecules are split (photolysis) to replace these electrons: 2H2O → 4H+ + 4e- + O2. Oxygen is released as a byproduct. The excited electrons travel through the electron transport chain, releasing energy that drives ATP synthesis via chemiosmosis (photophosphorylation). In Photosystem I (PSI), electrons are re-energized by light and ultimately reduce NADP+ to NADPH.
Products of light reactions: ATP, NADPH, O2
Stage 2: Calvin Cycle (Stroma)
ATP and NADPH from the light reactions power the fixation of CO2 into organic molecules. The enzyme RuBisCO catalyzes carbon fixation: CO2 is added to a 5-carbon molecule (RuBP) to produce two 3-carbon molecules (3-PGA). ATP and NADPH are used to reduce 3-PGA to G3P (glyceraldehyde-3-phosphate). G3P is used to regenerate RuBP and to synthesize glucose and other organic compounds.
For every 3 CO2 fixed: 9 ATP and 6 NADPH consumed; 1 net G3P produced.
Factors Affecting Photosynthesis Rate
| Factor | Effect on Rate | Limiting Condition |
|---|
| Light intensity | Increases rate up to saturation point | Low light |
| CO2 concentration | Increases rate up to saturation point | Low CO2 |
| Temperature | Increases up to optimum (~30°C), then decreases | Enzyme denaturation above optimum |
| Water availability | Stomata close under drought, limiting CO2 entry | Drought stress |
Questions in This Drill
- In the light reactions, water molecules are split during photolysis. What is the primary role of this process in photosynthesis?
- 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 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 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?
- Which of the following correctly describes the relationship between photosynthesis and cellular respiration at the level of net chemical change?