AP Biology Unit 3: Cellular Energetics practice questions

Question 1

A cell needs to break down hydrogen peroxide fast — but the reaction barely happens on its own. What does the enzyme catalase actually change about the reaction?

Accepted Answer

Catalase lowers the reaction's activation energy without being consumed. That sharply increases the rate.

Answer

Catalase adds energy to the reaction so it can proceed

Answer

Catalase changes the products formed so the reaction releases more energy

Answer

Catalase raises the temperature of the cell to speed the reaction

Question 2

A researcher synthesizes a molecule with the same charge distribution as a real substrate but a slightly different 3D shape. It never forms a stable enzyme-substrate complex. What principle explains this failure?

Both the shape and charge of the substrate must fit the enzyme's active site. Only then can a stable complex form.

Accepted Answer

Both the shape and charge of the substrate must fit the enzyme's active site. Only then can a stable complex form.

Answer

The molecule failed because it lacked the correct molecular weight to enter the active site

Answer

The enzyme could not lower activation energy without a cofactor present

Answer

The reaction failed because the substrate concentration was too low

Question 3

Two reactions: one catalyzed by an enzyme, one uncatalyzed. Same reactants, same products, same conditions. What is the ONE thing the enzyme changes — and the one thing it definitely does NOT change?

Accepted Answer

The enzyme lowers activation energy and speeds the rate, but not the free-energy change. Reactants and products keep the same energy gap.

Answer

The enzyme changes both activation energy and the free energy released by the reaction

Answer

The enzyme changes the identity of the products formed in the reaction

Answer

The enzyme increases the free energy of the products to make the reaction more favorable

Question 4

A digestive enzyme works perfectly at pH 2 in the stomach. It travels to the small intestine where pH is 8. Enzyme activity drops to nearly zero. What structurally happened to the enzyme?

Accepted Answer

The pH shift broke the hydrogen bonds holding the enzyme's shape. The active site warped so the substrate no longer fits.

Answer

The substrate was destroyed by the alkaline environment before it could bind

Answer

The enzyme ran out of cofactors needed to function at higher pH

Answer

The enzyme was consumed during the reaction in the stomach and no longer existed

Question 5

A protein is denatured by a mild drop in pH. When researchers restore the original pH, the protein refolds and regains full catalytic activity. What does this tell you about that denaturation event?

Accepted Answer

The denaturation was reversible — the protein wasn't permanently destroyed. Restoring the pH let it refold into its functional shape.

Answer

The protein was never truly denatured — it only slowed down temporarily

Answer

A new enzyme was synthesized to replace the denatured one

Answer

The cofactors reattached and rebuilt the active site independently of refolding

Question 6

Increasing temperature from 20°C to 35°C speeds up an enzyme-catalyzed reaction. Increasing it further to 60°C causes the rate to crash. Trace the two different mechanisms behind each change.

Accepted Answer

More collisions raise the rate to the optimum; past it, heat denatures the enzyme. Hydrogen bonds break and the active site loses shape, so the rate crashes.

Answer

Higher temperature always increases reaction rate because enzymes are more energized

Answer

The rate crashed at 60°C because the substrate was destroyed by heat, not the enzyme

Answer

The enzyme ran out of substrate at higher temperatures, causing the rate to drop

Question 7

Drug A blocks enzyme activity by binding directly to the active site. Drug B binds elsewhere on the enzyme and reduces activity. Same outcome, totally different mechanism — what distinguishes them?

Accepted Answer

Drug A is a competitive inhibitor (binds active site, competes with substrate). Drug B is a noncompetitive inhibitor (binds allosteric site, changes enzyme shape or activity).

Answer

Drug A is noncompetitive because it reduces activity without destroying the enzyme

Answer

Both drugs are competitive inhibitors since they both reduce enzyme activity

Answer

Drug B must be a cofactor because it binds outside the active site

Question 8

A metabolic reaction has been running for an hour and product is accumulating. You notice the reaction is slowing down even though temperature and pH are constant. What is the most likely explanation?

Accepted Answer

Rising product and falling substrate slow the enzymatic reaction. The enzyme has less substrate to act on and more product crowding it.

Answer

The enzyme was consumed during the reaction and no longer available

Answer

The temperature optimum shifted as products built up

Answer

The enzyme denatured spontaneously after extended use

AP Biology — Unit 3: Cellular Energetics

What AP Biology Unit 3 covers

AP Biology Unit 3 practice questions

Key terms in Unit 3

Drill all of Unit 3 with spaced repetition