AP Biology cellular respiration: Glycolysis, Krebs, ETC and ATP yield

Cellular respiration is Unit 3 territory in AP (Advanced Placement) Biology, which means it sits inside the largest content unit (Cellular Energetics, 12-16% of the exam). It also generates some of the highest-frequency FRQ (free-response question) topics, because the pathway has clear reactants, products, locations and yields that graders can score cleanly against a rubric.

This guide covers all four stages, glycolysis, pyruvate oxidation, the Krebs cycle and the electron transport chain, plus ATP yield, aerobic vs anaerobic pathways, and the FRQ angles most likely to show up. If you want structured content to match, Cognito's AP Biology notes walk through each stage in short lessons.

The overall reaction for aerobic cellular respiration:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (ATP + heat)

One glucose molecule is fully oxidized to carbon dioxide and water, releasing energy that gets captured in ATP (adenosine triphosphate). The exam expects you to know this equation cold, not just to write it down, but to trace where each carbon and each oxygen atom ends up across the four stages.

The process is a redox reaction: Glucose is oxidized (loses electrons), oxygen is reduced (gains electrons and becomes water). The electron carriers NAD+ and FAD shuttle electrons from glucose through the intermediate stages to the electron transport chain, where oxygen finally accepts them.

Glycolysis happens in the cytosol and doesn't need oxygen, so it runs in both aerobic and anaerobic conditions. It splits one 6-carbon glucose molecule into two 3-carbon pyruvate molecules.

The process has two phases. The investment phase uses 2 ATP to phosphorylate glucose and rearrange it into fructose-1,6-bisphosphate, which is then split into two 3-carbon molecules. The payoff phase oxidizes those 3-carbon molecules to pyruvate, producing 4 ATP (through substrate-level phosphorylation) and 2 NADH along the way.

Net yield per glucose: 2 ATP, 2 NADH, 2 pyruvate.

Substrate-level phosphorylation is worth calling out, that's ATP made directly by transferring a phosphate from an intermediate onto ADP. Most of your ATP comes from oxidative phosphorylation later, but glycolysis and Krebs both include substrate-level ATP that graders often ask you to distinguish.

Between glycolysis and Krebs, pyruvate has to be prepped. Each 3-carbon pyruvate enters the mitochondrial matrix (through the outer and inner membranes) and gets oxidized to a 2-carbon acetyl group, which is attached to coenzyme A to form acetyl-CoA.

The removed carbon leaves as CO2, that's the first place carbon dioxide is released in respiration. NAD+ is reduced to NADH in the process. Because glycolysis produced 2 pyruvate, this stage runs twice per glucose, producing 2 acetyl-CoA, 2 CO2 and 2 NADH.

Pyruvate oxidation is sometimes lumped in with the Krebs cycle in older textbooks. The College Board treats it as its own stage on the CED, so know it separately.

The Krebs cycle (also called the citric acid cycle or TCA cycle) happens in the mitochondrial matrix. Each acetyl-CoA (2-carbon) combines with oxaloacetate (4-carbon) to form citrate (6-carbon), which is then oxidized through a series of steps back to oxaloacetate. Along the way, 2 CO2 molecules are released, 3 NADH and 1 FADH2 are produced, and 1 ATP is made through substrate-level phosphorylation.

Because each glucose produces 2 acetyl-CoA, the cycle runs twice per glucose. Total yield from Krebs per glucose: 4 CO2, 6 NADH, 2 FADH2, 2 ATP.

At this point, all six carbons from the original glucose have been released as CO2 (2 in pyruvate oxidation, 4 in Krebs). The energy is now stored in NADH and FADH2, which carry electrons to the last stage.

The electron transport chain (ETC) is a series of protein complexes embedded in the inner mitochondrial membrane. NADH and FADH2 drop off their electrons at the start of the chain, and those electrons are passed from one complex to the next, releasing energy at each step.

The complexes use that energy to pump protons (H+) from the matrix into the intermembrane space, creating a steep electrochemical gradient. Protons then flow back into the matrix through ATP synthase, and that flow drives the enzyme to phosphorylate ADP into ATP. This coupling of proton flow to ATP synthesis is called chemiosmosis, and it's the source of most of your ATP.

At the end of the chain, oxygen accepts the electrons and combines with protons to form water. Without oxygen, the whole chain backs up, NADH and FADH2 can't be re-oxidized to NAD+ and FAD, so the Krebs cycle stops, and the cell has to fall back on anaerobic pathways.

ATP yield from oxidative phosphorylation: Roughly 26-28 ATP per glucose (about 2.5 ATP per NADH, 1.5 ATP per FADH2).

The total ATP yield per glucose in aerobic respiration is approximately 30-32 ATP. That's a range, not a fixed number, because the exact yield depends on how NADH from glycolysis gets shuttled into the mitochondrion (which varies by cell type) and on membrane leakiness.

Breakdown: 2 ATP from glycolysis (substrate-level) + 2 ATP from Krebs (substrate-level) + about 26-28 ATP from oxidative phosphorylation = roughly 30-32 ATP.

Older textbooks and older FRQ rubrics sometimes cite 36-38 ATP, using an idealized 3:1 ratio for NADH and 2:1 for FADH2. The current CED and recent rubrics have shifted to 30-32. If you're asked for a total, state 30-32 with a brief note that yield varies.

When oxygen is absent, cells can't run the ETC or Krebs cycle. They still need to regenerate NAD+ so glycolysis can keep producing that small trickle of ATP. The way they do this is fermentation.

Lactic acid fermentation (in animals and some bacteria): Pyruvate is reduced directly to lactate using NADH, regenerating NAD+. This is what happens in your muscles during hard exercise when oxygen can't be delivered fast enough. Lactate buildup contributes to muscle fatigue.

Alcoholic fermentation (in yeast and some bacteria): Pyruvate is first decarboxylated (releasing CO2) to acetaldehyde, then reduced to ethanol using NADH, regenerating NAD+.

Either way, the ATP yield stays at just 2 ATP per glucose, the same 2 from glycolysis. That's why anaerobic conditions can't sustain cells with high energy demands (like heart muscle) for long.

A few angles come up repeatedly in released AP FRQs, and they're worth rehearsing before the exam.

Inhibitor scenarios: You're told a poison blocks a specific complex in the ETC (cyanide blocks Complex IV, for instance) and asked to predict what happens to NADH concentration, ATP production, and cellular respiration overall. The reasoning: If Complex IV is blocked, electrons can't flow, the proton gradient collapses, ATP synthesis stops, NADH accumulates because it can't be re-oxidized, and the Krebs cycle grinds to a halt.

Radioactive tracer questions: You're given a glucose molecule with a labeled (radioactive) carbon and asked to predict where that atom ends up. Six carbons in glucose, six CO2 out, with 2 released in pyruvate oxidation (from the C1 position of pyruvate) and 4 in the Krebs cycle.

Comparing organisms or conditions: You're given data on ATP production or oxygen consumption for two conditions (yeast with vs without O2, muscle at rest vs during exercise) and asked to explain the differences in terms of pathway usage.

Graphing respiration rate: You're given data on CO2 production or O2 consumption over time or temperature and asked to construct a graph, describe the trend and explain it using respiration mechanisms.

Cellular respiration rewards diagram-based recall. Draw the four stages from memory with labels for location, inputs, outputs and ATP yield. Do it three times over a week without looking between attempts. By the third round the pathway should flow without hesitation.

Pair the diagram work with 3-4 released FRQs on respiration from AP Central. Score yourself against the rubric with a highlighter, highlight every rubric point you actually hit. This calibrates how specific your language needs to be. "The cell makes ATP" isn't a rubric point; "Protons flow through ATP synthase, driving phosphorylation of ADP" is.

Cognito's AP Biology course covers respiration alongside photosynthesis and enzyme kinetics in short video lessons, which is useful if you learn better by watching the pathway drawn out than by reading it. Cognito also covers other AP subjects like Chemistry and Environmental Science on the AP hub.