AP Biology Unit 3 review: Cellular Energetics

• "Plants photosynthesize, animals respire." Plant cells respire all the time in their mitochondria. College Board's course description lists "only animals conduct cellular respiration" as a common misconception.
• "Only plants photosynthesize." Algae and cyanobacteria do too. Photosynthesis first evolved in prokaryotes, and the course description credits cyanobacteria with giving Earth an oxygen atmosphere.
• "Photosynthesis creates oxygen." No atoms are created. The O₂ is released when water is split in the light reactions.
• "Hotter is always faster." Rates rise with temperature only up to the enzyme's optimum. Past it, the hydrogen bonds holding the enzyme's shape break and the active site stops fitting.
• Treating all inhibitors the same. A competitive inhibitor binds the active site, so extra substrate can outcompete it. A noncompetitive inhibitor binds an allosteric site, and extra substrate doesn't help.
• Getting the gradient backward. Protons pile up inside the thylakoid and in the mitochondrial intermembrane space. ATP is made as they flow back through ATP synthase.
• "Fermentation makes ATP." Its job is to regenerate NAD⁺ so glycolysis can keep making its 2 ATP per glucose. Fermentation itself adds none.
• Rates without units. The exam can ask you to graph lab data and calculate a reaction rate. The course description tells students to show the calculation and include units in the answer.

1. Question 1 · 3.2 Environmental impacts on enzymes

   An enzyme from a human cell is tested at 25 °C, 37 °C and 60 °C. The reaction rate is highest at 37 °C and close to zero at 60 °C. Which best explains the result at 60 °C?

   • AMolecules move too slowly at 60 °C to collide
   • BThe heat changed the enzyme's amino acid sequence
   • CHeat broke the bonds holding the enzyme's shape, so the substrate no longer fits the active site
   • DThe enzyme used up its supply of activation energy
   Show the answer

   C. Above the optimum, heat disrupts the hydrogen bonds that hold the enzyme's 3D shape. The active site changes and the substrate no longer fits, so the rate falls. Heat doesn't change the amino acid sequence, only the folding. Molecules move faster when it's hotter, not slower. Enzymes lower activation energy; they don't carry a supply of it.

2. Question 2 · 3.2 Environmental impacts on enzymes

   A drug lowers an enzyme's reaction rate. When the researcher adds much more substrate, the rate climbs back close to its original maximum. What does this suggest about the drug?

   • AIt binds an allosteric site and changes the enzyme's shape
   • BIt competes with the substrate for the active site
   • CIt denatures the enzyme
   • DIt raises the activation energy of every reaction in the cell
   Show the answer

   B. A competitive inhibitor binds the active site reversibly. Flood the reaction with substrate and the substrate wins most of the binding, so the rate recovers. A noncompetitive inhibitor binds an allosteric site, and adding substrate doesn't undo it. A denatured enzyme wouldn't recover either.

3. Question 3 · 3.4 Photosynthesis · rate calculation

   A sprig of pondweed in bright light gives off oxygen, collected in a measuring tube. The tube holds 2 mL of O₂ at 10 seconds and 11 mL at 40 seconds. What is the average rate of O₂ production between 10 and 40 seconds?

   • A0.28 mL/s
   • B0.37 mL/s
   • C0.30 mL/s
   • D3.3 mL/s
   Show the answer

   C. Rate = change in volume ÷ change in time = (11 − 2) mL ÷ (40 − 10) s = 9 ÷ 30 = 0.30 mL/s. 0.28 divides the final volume by the total time, which counts gas collected before the interval. 0.37 forgets to subtract the 2 mL. 3.3 divides time by volume. Keep the units in your answer.

4. Question 4 · 3.4 Photosynthesis

   Algae are given water made with the heavy oxygen isotope ¹⁸O and ordinary CO₂. During the light reactions, where does the labeled oxygen from the water go?

   • AInto the O₂ the algae release
   • BInto the glucose the algae make
   • CInto the CO₂ the algae release
   • DNowhere, because water only supplies hydrogen
   Show the answer

   A. In the light reactions, water is split to replace the electrons photosystem II loses, and its oxygen atoms leave as O₂. That's the sense in which photosynthesis doesn't create oxygen: it releases it from water. The oxygen in the sugar comes from CO₂.

5. Question 5 · 3.5 Cellular respiration

   A chemical makes the inner mitochondrial membrane leaky to H⁺ ions. Electron transport keeps running. What happens?

   • AATP production drops, O₂ use continues and more energy is released as heat
   • BATP production rises because protons cross the membrane faster
   • CO₂ use stops because the electron transport chain shuts down
   • DGlycolysis stops because it needs the proton gradient
   Show the answer

   A. ATP synthase makes ATP as protons flow back through it. If protons leak across some other way, the gradient runs down without making ATP. Electrons still flow to oxygen, so O₂ use goes on, and the energy leaves as heat. The course description calls this decoupling oxidative phosphorylation from electron transport, and endotherms use the heat to regulate body temperature.

6. Question 6 · 3.5 Cellular respiration

   Yeast cells are moved into a sealed flask with no oxygen. Why does fermentation let them keep making ATP?

   • AFermentation makes more ATP per glucose than the Krebs cycle
   • BFermentation regenerates NAD⁺, which glycolysis needs to keep running
   • CFermentation uses ethanol as the final electron acceptor of the electron transport chain
   • DFermentation pumps protons for ATP synthase
   Show the answer

   B. Glycolysis turns NAD⁺ into NADH. Without oxygen, the electron transport chain can't take electrons from NADH, so the cell runs out of NAD⁺ and glycolysis stops. Fermentation hands those electrons to pyruvate or a molecule made from it, which frees up NAD⁺. The only ATP still comes from glycolysis: 2 per glucose.

7. Question 7 · 3.4 Photosynthesis

   A plant that has been in bright light is moved into darkness. What happens to the Calvin cycle soon after?

   • AIt speeds up, because it doesn't need light
   • BIt continues at the same rate, because it only needs CO₂
   • CIt starts releasing O₂
   • DIt slows and stops as the ATP and NADPH from the light reactions run out
   Show the answer

   D. The Calvin cycle runs on ATP and NADPH made in the light reactions. In the dark, the plant stops making them, so the cycle slows and stops. Light-independent means it doesn't use light directly, not that it can run without it. O₂ comes from splitting water in the light reactions, not from the Calvin cycle.

Learn the two pathways as a story about energy and electrons, not a list of steps. For each stage, know where it happens, what goes in, what comes out and what breaks if one input disappears. The course description says students should predict how a change in reactants affects a pathway, and that's what most questions test.

Then practice with data. Expect graphs of reaction rate against temperature, pH or substrate concentration, and lab setups that measure oxygen or carbon dioxide. Calculate rates with units every time.

OpenStax's free Biology 2e covers the unit in three sections: enzymes, cellular respiration and photosynthesis. Khan Academy's AP Biology unit on cellular energetics has videos and practice. To see ATP synthase as an actual molecular machine, RCSB's Molecule of the Month on ATP synthase is worth five minutes. The topic list here comes from College Board's AP Biology course and exam description.

Ashore's Biology course has a short lesson for every topic in this unit, each ending with a few questions on what it taught.

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