AP Bio · Unit 3 Progress Check

AP Biology Unit 3 Progress Check Cellular Energetics walkthrough.

AP Biology Unit 3 is cellular energetics, and the Unit 3 Progress Check MCQ tests whether you can track energy and electrons through photosynthesis and respiration rather than just name the stages. This walkthrough covers what the questions ask, the traps, and how to reason through them. Explanations only — no AP Classroom answer keys.

Updated August 2026Written by Mahmudul HasanFree · No signup

What Unit 3 covers

Unit 3 is Cellular Energetics. Cellular energetics is roughly 12–16% of the AP Biology exam and one of the most conceptually demanding units.

Enzymes

Active site specificity, and how temperature, pH, and inhibitors change reaction rate.

Photosynthesis

Light-dependent reactions in the thylakoid, and the Calvin cycle in the stroma.

Cellular respiration

Glycolysis, pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation.

Electron transport and chemiosmosis

How electron carriers build a proton gradient that drives ATP synthase.

Anaerobic pathways

Fermentation, and why it regenerates NAD⁺.

Energy flow

Coupled reactions, ATP as the currency, and feedback regulation.

The Progress Check MCQ: what each question type tests

These are the question patterns that recur on this Progress Check, and what each one is really asking.

Locate the stage

Where does each step occur? Cytosol, mitochondrial matrix, inner membrane, thylakoid, or stroma.

Enzyme graphs

Rate versus temperature or pH. Beyond the optimum the enzyme denatures and the curve falls sharply.

Inhibitor identification

Competitive inhibition is overcome by more substrate; non-competitive is not.

Track the electrons

Follow NADH and FADH₂ to the electron transport chain, and NADPH into the Calvin cycle.

Predict the disruption

“If ATP synthase is blocked, what happens?” — trace the gradient, not just the product.

The Progress Check FRQ

Unit 3 free response often presents experimental data on enzyme activity or photosynthetic rate and asks you to interpret and predict. Points come from identifying the variable, explaining the mechanism at the molecular level, and justifying a prediction with a causal chain rather than a restatement of the trend.

Where students lose the most points

Mixing up NADH and NADPH

NADH belongs to respiration; NADPH is produced in the light reactions and spent in the Calvin cycle.

Confusing where stages occur

Glycolysis is in the cytosol, not the mitochondrion. This is a standard trap.

Saying the Calvin cycle is the “dark reaction”

It does not require light directly but depends on ATP and NADPH from the light reactions, so it stops in darkness.

Treating denaturation as reversible

Past the optimum, changing shape destroys function — the curve does not simply mirror.

Describing a trend instead of a mechanism

The free response asks why, not what.

How to work through this unit

For any disruption question, follow the electrons and the proton gradient. If a step upstream stops, everything downstream of it stops too — that chain is the answer.

Once you have finished the Progress Check, put your raw score into our AP Biology Calculator to see roughly where that pace puts you on the 1–5 scale, and use the AP Biology Review for the full exam format and study plan.

Frequently asked questions

Quick answers — written by humans, not a chatbot.

What does the AP Biology Unit 3 Progress Check cover?

Cellular energetics: enzyme structure and regulation, photosynthesis, cellular respiration, electron transport and chemiosmosis, anaerobic pathways, and energy flow.

What is the difference between NADH and NADPH?

NADH is an electron carrier in cellular respiration, while NADPH is produced by the light-dependent reactions and used to build sugar in the Calvin cycle.

Where does glycolysis take place?

In the cytosol, not inside the mitochondrion. Only pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation occur in the mitochondrion.

Do you post AP Biology Unit 3 answers?

No. We publish walkthroughs of the reasoning and common traps rather than AP Classroom answer keys.