AP Exam Prep · AP Biology

AP Biology Unit 3 Practice Test: Cellular Energetics

Unit 3 follows energy through cells: how enzymes speed up reactions, how temperature and pH change enzyme activity, and how photosynthesis and respiration capture and release energy. Many questions give you a graph or an experiment and ask you to explain the result.

Exam weighting:: 12–16%
Questions:: 12 multiple choice
Time:: about 16 minutes
Cost:: Free, no sign-up

Taken this test before? Get a brand-new set of questions for this unit.

Question 1 of 12 · 0 answered

Topic 3.1 · Concept check

How does an enzyme speed up a chemical reaction?

It lowers the activation energy needed for the reaction.It makes the reaction release more free energy.It is used up as a reactant in the reaction.It supplies the energy needed to start the reaction.

How sure are you?

Topic 3.2 · Data analysis

Activity of an enzyme from a mammal at different temperatures
Temperature (°C)	Product formed per minute (µmol)
20	12
30	20
37	31
45	18
55	2

Which explanation best accounts for the change in activity between 37 °C and 55 °C?

Molecules move more slowly, so enzyme and substrate collide less often.The substrate is used up faster at higher temperatures.The enzyme is consumed by the reaction at high temperatures.Bonds that hold the enzyme's shape are disrupted, so the active site changes and binds substrate less well.

How sure are you?

Jump to a question

You can change any answer before submitting.

What AP Bio Unit 3 covers

Unit 3, Cellular Energetics, makes up 12–16% of the multiple-choice section and usually takes ~12–14 class periods. These are the topics in the current course framework, and every question in the test above is tagged with one of them.

3.1Enzymes
3.2Environmental Impacts on Enzyme Function
3.3Cellular Energy
3.4Photosynthesis
3.5Cellular Respiration

Unit 3 key ideas

Enzymes lower activation energy

An enzyme's active site binds a specific substrate. Enzymes speed reactions without being used up and without changing the overall free-energy change.

Shape decides activity

Temperature and pH change the interactions that hold an enzyme's shape. Beyond the optimum, the enzyme denatures and activity falls. Inhibitors compete for or change the active site.

Photosynthesis stores energy

Light reactions split water, release oxygen and make ATP and NADPH. The Calvin cycle uses that ATP and NADPH to fix carbon dioxide into sugar.

Respiration releases it

Glycolysis, pyruvate oxidation and the Krebs cycle pass electrons to NADH and FADH2. The electron transport chain pumps protons, and ATP synthase uses the proton gradient to make most of the ATP.

Common Unit 3 mistakes

Thinking the oxygen released by photosynthesis comes from carbon dioxide; it comes from water.
Saying high temperature lowers activity because molecules collide less; collisions increase, but the enzyme denatures.
Forgetting that fermentation exists to regenerate NAD+, not to make much ATP.

How to study this unit

Draw where each stage of photosynthesis and respiration happens and what goes in and out of each.
For enzyme graphs, describe the trend first, then explain it using the active site.

Need notes on a topic? Make one with the AI Study Guide Maker or turn it into flashcards.

More AP Biology practice tests

Full AP Biology practice exam →60 questions from all 8 units, weighted like the real exam, with a 90-minute timer.

← Unit 2: Cells Unit 4: Cell Communication and Cell Cycle →

About these questions

StudyCoach wrote these questions for this test. Each one is mapped to a topic in the College Board's current AP Biology course framework. Every answer key was also checked by a separate AI review, and any disagreement was rechecked by hand. They have not yet been reviewed by an AP Biology teacher. If you think an answer is wrong, email support@studycoach.io.

AP® is a trademark registered by the College Board, which is not affiliated with, and does not endorse, StudyCoach.