Cellular Respiration — AP Biology Study Guide

For: AP Biology candidates sitting AP Biology.

Covers: the balanced overall reaction of aerobic cellular respiration, glycolysis, pyruvate oxidation, the citric acid cycle, oxidative phosphorylation, anaerobic fermentation, ATP yield calculations, and connections to other cellular metabolic pathways.

You should already know: Structure of the mitochondrion, ATP as the cell’s primary energy currency, enzyme function and activation energy.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Biology style for educational use. They are not reproductions of past College Board / Cambridge / IB papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official mark schemes for grading conventions.

1. What Is Cellular Respiration?

Cellular respiration is the collective term for intracellular catabolic processes that break down organic molecules (most commonly glucose) to release stored free energy, which is then used to synthesize ATP, the cell’s usable energy currency. It is an exergonic process with a negative ΔG, meaning it occurs spontaneously. For AP Biology, do not confuse this with organismal respiration (breathing); it refers exclusively to the metabolic pathway that generates ATP for cellular work.

The overall balanced reaction for aerobic cellular respiration (the most energy-efficient form) is: $C_6{H}_{12}{O}_6+6O_2\to 6C{O}_2+6H_{2}O+\text{energy (ATP + heat)}$. All steps of respiration rely on redox reactions: glucose is fully oxidized to carbon dioxide, and oxygen is reduced to water, with intermediate electron carriers NAD+ and FAD shuttling high-energy electrons between steps.

According to the AP Biology CED, Unit 3: Cellular Energetics makes up 12-16% of the total AP exam score, with cellular respiration accounting for roughly half of that unit’s content. It appears on both the multiple-choice (MCQ) and free-response (FRQ) sections: MCQs commonly test ATP yield, pathway step identification, and regulation, while FRQs often ask for structure-function connections of mitochondria, experimental analysis of respiration rates, or comparisons between aerobic and anaerobic metabolism. (248 words)

2. Stages of Aerobic Respiration and ATP Accounting

Aerobic cellular respiration (which requires oxygen as the final electron acceptor) occurs in four sequential stages in eukaryotes, each with defined inputs, outputs, and locations. All counts below are per starting glucose molecule:

1. Glycolysis: Occurs in the cytoplasm, splits one 6-carbon glucose into two 3-carbon pyruvate. Net outputs: 2 ATP (via substrate-level phosphorylation, where ATP is made directly by transferring a phosphate from an organic molecule to ADP), 2 NADH, 2 pyruvate.
2. Pyruvate oxidation: Occurs in the mitochondrial matrix. Each pyruvate is oxidized to acetyl-CoA. Net outputs: 2 acetyl-CoA, 2 CO2, 2 NADH.
3. Citric Acid (Krebs) Cycle: Occurs in the mitochondrial matrix. Each acetyl-CoA enters the cycle, and all remaining carbon is released as CO2. Net outputs: 4 CO2, 6 NADH, 2 FADH2, 2 ATP (substrate-level).
4. Oxidative Phosphorylation: Occurs on the inner mitochondrial membrane. NADH and FADH2 donate electrons to the electron transport chain (ETC), which pumps H+ into the intermembrane space to create a proton gradient. ATP synthase uses the gradient to make ATP. Modern empirical values for maximum yield are 2.5 ATP per NADH and 1.5 ATP per FADH2 (outdated 3/2 values are not used on the AP exam).

Worked Example

Calculate the maximum total ATP yield from one glucose molecule in aerobic respiration, using modern conversion factors.

1. Count total NADH: 2 from glycolysis + 2 from pyruvate oxidation + 6 from the citric acid cycle = 10 total NADH.
2. Count total FADH2: 0 from glycolysis/pyruvate oxidation + 2 from the citric acid cycle = 2 total FADH2.
3. Count substrate-level ATP: 2 from glycolysis + 2 from the citric acid cycle = 4 total substrate-level ATP.
4. Calculate ATP from oxidative phosphorylation: $(10\times 2.5)+(2\times 1.5)=25+3=28$ ATP.
5. Add substrate-level ATP for total yield: $28+4=32$ maximum ATP.

Exam tip: Always use 2.5 ATP/NADH and 1.5 ATP/FADH2 for calculations unless the question explicitly gives different conversion factors; the AP CED endorses the modern empirical values, not outdated textbook values.

3. Oxidative Phosphorylation and Chemiosmosis

Oxidative phosphorylation generates ~90% of the ATP produced in aerobic respiration, and relies on the chemiosmotic principle: energy stored in an electrochemical gradient across a membrane is used to drive cellular work. The inner mitochondrial membrane is selectively impermeable to H+ ions, so the protein complexes of the ETC can pump H+ from the matrix into the intermembrane space, creating a combined electrochemical gradient (called proton motive force) with more H+ (and a more positive charge) outside the matrix.

Electrons flow from higher to lower free energy through the ETC, with oxygen acting as the final electron acceptor due to its high electronegativity, which pulls electrons down the chain. Oxygen combines with electrons and H+ to form water, a waste product. ATP synthase is a transmembrane enzyme that acts as a channel for H+ to diffuse down their gradient back into the matrix; the flow of H+ causes conformational changes in ATP synthase that catalyze the phosphorylation of ADP to ATP. If the gradient is disrupted, no ATP can be produced via oxidative phosphorylation, even if the ETC is still functional.

Worked Example

The chemical dinitrophenol (DNP) makes the inner mitochondrial membrane freely permeable to H+ ions. Predict how DNP affects ATP production, and explain where the energy from the ETC goes.

1. First, recall that the proton gradient across the inner mitochondrial membrane is required for chemiosmosis; the potential energy stored in the gradient is what powers ATP synthesis by ATP synthase.
2. If the membrane is leaky to H+, H+ ions diffuse back into the matrix without passing through ATP synthase, so the proton motive force is completely dissipated.
3. The ETC can still transfer electrons from NADH/FADH2 to oxygen, so the ETC continues to run, but no ATP can be synthesized from the energy released by electron transfer.
4. All the energy released by the ETC that would have been captured as ATP is instead released as heat.

Exam tip: Any question about toxins or uncouplers that affect respiration will almost always test the link between the proton gradient and ATP synthesis; remember uncouplers separate ETC activity from ATP production, so ETC runs but no ATP is made.

4. Anaerobic Pathways: Fermentation

When oxygen is not available (or when ATP demand outpaces oxygen delivery), the ETC backs up because there is no final electron acceptor to accept electrons. Glycolysis requires NAD+ as an input, and NAD+ is converted to NADH during glycolysis; if the ETC cannot oxidize NADH back to NAD+, glycolysis stops, and no more ATP is produced. Fermentation solves this problem: it is an anaerobic process that oxidizes NADH back to NAD+ to keep glycolysis running. Critically, fermentation produces no ATP beyond the 2 net ATP already generated by glycolysis.

There are two common types of fermentation: 1) Lactic acid fermentation: pyruvate is directly reduced by NADH to form lactate, regenerating NAD+. This occurs in human muscle cells during strenuous exercise and in lactic acid bacteria used to make yogurt. No CO2 is produced as a byproduct. 2) Alcohol fermentation: pyruvate is first decarboxylated to release CO2, forming acetaldehyde, which is then reduced by NADH to ethanol, regenerating NAD+. This occurs in yeast and is used for brewing and bread making. Note that fermentation is distinct from anaerobic respiration: anaerobic respiration still uses an ETC with an alternative final electron acceptor (like nitrate or sulfate), while fermentation does not use an ETC at all.

Worked Example

A yeast culture grown in a sealed container consumes 9.0 grams of glucose exclusively via alcohol fermentation. Molar mass of glucose is 180 g/mol. How many moles of CO2 are released by the culture?

1. Recall the stoichiometry of alcohol fermentation: 1 mole of glucose produces 2 moles of CO2, as each of the two pyruvate molecules releases one CO2.
2. Calculate moles of glucose consumed: $\frac{9.0\text{g}}{180\text{g/mol}}=0.05\text{mol glucose}$.
3. Multiply by the mole ratio of CO2 to glucose: $0.05\text{mol glucose}\times \frac{2{\text{mol CO}}_2}{1\text{mol glucose}}=0.10{\text{mol CO}}_2$.
4. Confirm: This matches the expected output, as no CO2 is produced in any step before fermentation in anaerobic conditions.

Exam tip: Always remember that fermentation produces no new ATP beyond the 2 net ATP from glycolysis. Many exam questions trick students into thinking fermentation generates more ATP, so memorize this key distinction from aerobic respiration.

5. Common Pitfalls (and how to avoid them)

• Wrong move: Calling fermentation "anaerobic respiration" on the AP exam. Why: Students often use the terms interchangeably, but AP Biology explicitly distinguishes them: anaerobic respiration uses an ETC with an alternative final electron acceptor, while fermentation does not use an ETC at all. Correct move: Always use "fermentation" for the NAD+-regenerating process without ETC, and "anaerobic respiration" only when referencing ETC-based respiration without oxygen.
• Wrong move: Stating that the citric acid cycle occurs on the inner mitochondrial membrane. Why: Students confuse the location of oxidative phosphorylation with the citric acid cycle, since both occur in mitochondria. Correct move: Memorize that pyruvate oxidation and the citric acid cycle occur in the mitochondrial matrix, while oxidative phosphorylation occurs on the inner mitochondrial membrane.
• Wrong move: Claiming lactic acid fermentation produces CO2 as a byproduct. Why: Students confuse lactic acid fermentation with alcohol fermentation, which does release CO2. Correct move: Remember only alcohol fermentation releases CO2; lactic acid fermentation has no CO2 byproduct.
• Wrong move: Stating that oxygen is required for glycolysis. Why: Students associate oxygen with all of cellular respiration, so they incorrectly assume all steps require it. Correct move: Remember glycolysis can run with or without oxygen; it is the first step for both aerobic respiration and fermentation.
• Wrong move: Claiming ATP synthase uses energy from electron transfer directly to make ATP. Why: Students confuse the ETC's role with ATP synthase's role in oxidative phosphorylation. Correct move: Remember the ETC creates the proton gradient, and ATP synthase uses potential energy from that gradient to make ATP.
• Wrong move: Counting 4 ATP from glycolysis as net ATP yield. Why: Students forget that glycolysis requires an initial investment of 2 ATP to split glucose. Correct move: Always use 2 net ATP from glycolysis in yield calculations, not 4 gross ATP.

6. Practice Questions (AP Biology Style)

Question 1 (Multiple Choice)

Researchers measure ATP production in isolated mitochondria supplied with excess ADP, Pi, and oxygen. FADH2 is added as the only electron donor. What is the expected maximum ATP yield per 2 molecules of FADH2, using modern conversion factors? A) 1.5 ATP B) 3 ATP C) 4 ATP D) 5 ATP

Worked Solution: Each FADH2 molecule donates one pair of electrons to the ETC, which generates a proton gradient that yields 1.5 ATP per FADH2. Two molecules of FADH2 would therefore produce $2\times 1.5=3$ ATP total. There is no substrate-level ATP production in oxidative phosphorylation, so no additional ATP is added. The correct answer is B.

Question 2 (Free Response)

A student uses a respirometer to measure respiration rate in germinating pea seeds. CO2 released by the seeds is absorbed by solid potassium hydroxide in the sealed respirometer. (a) Explain why the volume of gas in the respirometer decreases as respiration proceeds. (b) Predict how the rate of volume change will differ between seeds incubated at 10°C vs 30°C, and justify your prediction. (c) Predict what would happen to the volume change if all oxygen is removed from the container, and justify your prediction.

Worked Solution: (a) Aerobic respiration consumes 6 moles of O2 and produces 6 moles of CO2 per mole of glucose, so the moles of gas consumed equal the moles produced before CO2 absorption. The potassium hydroxide absorbs all CO2 produced, removing it from the gas phase. As O2 is consumed, the total moles of gas in the sealed container decreases, causing the measured volume to decrease. (b) The rate of volume decrease (which corresponds to respiration rate) will be faster at 30°C than at 10°C. All steps of respiration are catalyzed by enzymes, and reaction rate increases with temperature between 0°C and the optimal temperature for plant enzymes (~30-35°C). Higher temperature increases kinetic energy, leading to more frequent successful collisions between enzymes and substrates, increasing reaction rate. (c) There will be almost no change in gas volume after oxygen is removed. Pea seeds are obligate aerobes that cannot carry out alcohol fermentation, so respiration stops when oxygen is gone. No O2 is consumed, so no change in gas volume occurs. If the peas could carry out fermentation, there would still be no volume change because fermentation produces no net change in moles of gas (1 glucose → 2 ethanol + 2 CO2, so 2 moles of CO2 produced, no O2 consumed, but CO2 is absorbed by potassium hydroxide, so volume would decrease slowly; but pea seeds do not ferment, so no change).

Question 3 (Application / Real-World Style)

During intense exercise, a runner’s leg muscles can only get enough oxygen to aerobically oxidize 20% of the glucose they use; the remaining 80% is processed via lactic acid fermentation. The runner’s muscles need 186 mmol of ATP per minute to power contraction. How much more glucose is used per minute during intense exercise compared to rest, when 100% of glucose is aerobically oxidized (assume 30 ATP per glucose aerobically, 2 ATP per glucose fermented)?

Worked Solution:

1. At rest, all ATP comes from aerobic oxidation: Glucose needed at rest = $\frac{186\text{mmol ATP}}{30\text{ATP per mmol glucose}}=6.2\text{mmol glucose per minute}$.
2. During exercise, 20% of ATP (37.2 mmol) is aerobic, 80% (148.8 mmol) is fermented: Glucose for aerobic portion = $\frac{37.2}{30}=1.24\text{mmol}$, Glucose for fermentation portion = $\frac{148.8}{2}=74.4\text{mmol}$.
3. Total glucose during exercise = $1.24+74.4=75.64\text{mmol per minute}$.
4. Additional glucose used = $75.64-6.2=69.44\text{mmol glucose per minute}$. In context, this means muscles use ~12 times more glucose during intense anaerobic exercise to meet the same ATP demand, which explains why glycogen stores are depleted quickly during prolonged intense activity.

7. Quick Reference Cheatsheet

8. What's Next

Cellular respiration is the foundation for understanding how cells generate energy required for all other cellular processes. Immediately after mastering cellular respiration in AP Biology Unit 3, you will study photosynthesis, the anabolic counterpart to respiration that captures sunlight energy to build glucose, which is then broken down by respiration to release usable energy for the cell. Many core mechanisms (chemiosmosis, redox reactions, ATP synthesis) are shared between the two processes, so without mastering the concepts here, photosynthesis will be much harder to learn. This topic also feeds into broader concepts across the course: metabolic regulation, cell bioenergetics, energy flow in ecosystems, and evolutionary adaptations to low-oxygen environments. Follow-up topics to study next: Photosynthesis, Metabolic Pathway Regulation, Mitochondrial Structure and Function, Energy Flow in Ecosystems