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AP Biology: Cellular Energetics, Photosynthesis and Respiration — quiz

15 multiple-choice questions covering Enzyme Structure, Activation Energy, ATP, Factors Affecting Enzyme Activity, Light Reactions. One answer each; you get a score at the end and the explanation for every question. No account needed.

Updated 2026-11-04. Keys 1–4 pick an answer, Enter moves on. Redo it any time, in order or shuffled.

The questions

Answers and explanations are shown after you finish.

1. Enzyme Structure

   A student notices that after a reaction finishes, the enzyme is recovered completely unchanged, yet the reaction occurred much faster than it would have without the enzyme. What best explains this observation?

   • The enzyme was consumed and then regenerated during the reaction
   • The enzyme shifted the reaction's equilibrium constant
   • The enzyme lowered the activation energy needed to reach the transition state, without being used up
   • The enzyme increased the free energy of the products
2. Enzyme Structure

   Which scenario best illustrates the induced-fit model of enzyme-substrate binding rather than lock-and-key?

   • The enzyme has no specific active site and binds substrates randomly
   • The active site is rigid and only one exact substrate shape can ever fit
   • The active site slightly changes shape as it closes around the substrate for a tighter fit
   • The substrate changes shape permanently to match the enzyme
3. Enzyme Structure

   An enzyme requires a bound Mg2+ ion to function but does not need any additional organic molecule. This Mg2+ ion is best classified as a:

   • Allosteric inhibitor
   • Substrate
   • Coenzyme
   • Cofactor
4. Enzyme Structure

   An enzyme exposed briefly to extreme heat stops working permanently, even after it is cooled back to normal temperature. This result is best explained by:

   • Saturation of the active site
   • Competitive inhibition
   • Feedback inhibition
   • Denaturation of the enzyme's shape
5. Activation Energy

   On an energy diagram comparing a catalyzed and an uncatalyzed version of the same reaction, what is different between the two curves?

   • The catalyzed reaction has a higher transition-state peak but reaches products faster
   • The catalyzed reaction has a lower transition-state peak, but reactants and products start and end at the same energy levels
   • The catalyzed reaction starts at a lower energy level than the uncatalyzed one
   • The catalyzed reaction changes the overall free energy change (ΔG) of the reaction
6. ATP

   ATP hydrolysis is exergonic and releases free energy. In cells, this reaction is most often coupled to which type of reaction to drive it forward?

   • An endergonic reaction that requires an input of energy
   • A reaction that does not involve phosphate transfer
   • A reaction with no net energy change
   • Another exergonic reaction
7. Factors Affecting Enzyme Activity

   An enzyme's reaction rate increases as temperature rises from 10°C to 37°C, but then drops sharply as temperature continues to climb to 60°C. What explains the drop above the optimum?

   • Molecular collisions become too infrequent at high temperature
   • The enzyme is denaturing, losing the shape of its active site
   • The reaction has reached chemical equilibrium
   • Substrate molecules are being used up faster than they can be replaced
8. Factors Affecting Enzyme Activity

   Trypsin normally functions optimally around pH 8 in the small intestine. If trypsin were instead placed into the stomach's highly acidic environment (~pH 2), what would most likely happen to its activity?

   • Activity would drop sharply because the pH is far from trypsin's optimum, disrupting its shape
   • Activity would increase because all enzymes work best at low pH
   • Activity would increase until the substrate is used up, then plateau
   • Activity would stay the same since pH does not affect enzyme structure
9. Factors Affecting Enzyme Activity

   On a graph of reaction rate vs. substrate concentration for an enzyme-catalyzed reaction, the rate levels off into a plateau at high substrate concentrations. What does this plateau represent?

   • The substrate has been completely destroyed
   • The enzyme has denatured due to excess substrate
   • Feedback inhibition has shut down the enzyme
   • All available active sites are saturated with substrate, reaching Vmax
10. Factors Affecting Enzyme Activity

    When excess substrate is added to an enzyme reaction that contains an inhibitor, the reaction rate climbs back up to the same Vmax seen without the inhibitor. This pattern indicates the inhibitor is most likely:

    • A competitive inhibitor
    • A coenzyme
    • A feedback activator
    • A noncompetitive (allosteric) inhibitor
11. Factors Affecting Enzyme Activity

    In a metabolic pathway, the final product binds to an enzyme near the start of the pathway at a site separate from the active site, shutting the enzyme off once enough product has accumulated. This mechanism is called:

    • Feedback (allosteric) inhibition
    • Denaturation
    • Competitive inhibition
    • Induced fit
12. ATP

    Why does breaking the terminal phosphate bond in ATP (forming ADP + Pi) release energy?

    • Because the products are more stable than ATP due to reduced electrostatic repulsion between phosphate groups
    • Because new chemical bonds are being formed, which always releases energy
    • Because ADP contains more potential energy than ATP
    • Because the reaction absorbs energy from the surrounding water
13. Light Reactions

    If researchers isolate just the thylakoid membranes from a chloroplast and supply them with light, water, ADP, Pi, and NADP+, which process could still occur in this isolated membrane system?

    • Glycolysis
    • The Krebs cycle
    • The light reactions
    • The Calvin cycle
14. Light Reactions

    A toxin completely blocks the splitting of water (photolysis) at Photosystem II. What would be the most direct downstream consequence?

    • NADPH production at Photosystem I would increase
    • ATP synthase would pump more H+ into the thylakoid lumen
    • No oxygen would be released, and there would be no replacement electrons to keep the electron transport chain running
    • The Calvin cycle would immediately speed up
15. Fermentation

    A muscle cell runs out of oxygen during intense exercise and switches from aerobic respiration to lactic acid fermentation. What happens to the net ATP yield per glucose, and why?

    • ATP yield stays about the same because the Krebs cycle can still run without oxygen
    • ATP production stops completely because glycolysis also requires oxygen
    • ATP yield drops drastically to only the net 2 ATP from glycolysis, since oxidative phosphorylation cannot occur without oxygen
    • ATP yield increases because fermentation generates extra ATP beyond glycolysis