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AP Biology

AP Bio Unit 3 MCQ Set A Concepts

Shared by Ashton · Nov 12, 2026 · 0 plays

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18 questions across 5 sections. After every answer you’ll see why each choice is right or wrong, not just the one you picked.

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Sections (none selected means all)

Sample questions

Open a question to see the answer choices and the correct answer.

Which statement best describes how an enzyme increases the rate of a reaction?
  1. A. It lowers the activation energy of the reaction ✓
  2. B. It makes the products lower in free energy than before
  3. C. It shifts the reaction's equilibrium toward the products
  4. D. It supplies the extra energy the reactants need to react

Correct answer: A. Enzymes are catalysts that lower Eₐ (3.1.A.1); more molecules can react per second.

An enzyme hydrolyzes maltose but not sucrose, even though both are disaccharides. What is the best explanation?
  1. A. Sucrose's activation energy is too high for any enzyme to lower
  2. B. Sucrose is too large to enter the cell where the enzyme is
  3. C. The enzyme is used up after it hydrolyzes the maltose
  4. D. Only maltose has the shape and charge that fit the active site ✓

Correct answer: D. The substrate's shape and charge must be compatible with the active site (3.1.A.2).

The rate of a human enzyme's reaction increases as temperature rises from 10 °C to 30 °C. Which explanation is best?
  1. A. The active site partly unfolds, making it easier to enter
  2. B. Faster molecules collide with active sites more often ✓
  3. C. Hydrogen bonds in the enzyme break and expose the active site
  4. D. The reaction's ΔG becomes more negative as temperature rises

Correct answer: B. Below the optimum, more kinetic energy means more enzyme–substrate collisions (3.2.B.2).

Isoleucine is the final product of a five-step pathway. It binds the first enzyme of the pathway at a site other than the active site and slows the enzyme. This is an example of
  1. A. denaturation of the first enzyme caused by product buildup
  2. B. positive feedback that increases isoleucine production
  3. C. competitive inhibition, because isoleucine resembles the substrate
  4. D. allosteric inhibition by the pathway's end product (feedback) ✓

Correct answer: D. It binds an allosteric site, so it's noncompetitive (3.2.B.3), and it's the end product, so it's negative feedback.

A purified enzyme is unfolded with a chemical denaturant. When the chemical is removed, the enzyme refolds and regains most of its activity. This result best supports which claim?
  1. A. The chemical acted as a competitive inhibitor of the enzyme
  2. B. Some denaturation is reversible ✓
  3. C. Enzymes are denatured only by heat, never by chemicals
  4. D. Chemical denaturants break the enzyme's peptide bonds

Correct answer: B. 3.2.A.2. The primary structure holds the information for the active fold.