1. What Is Cellular Respiration?★★☆☆☆⏱ 3 min

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 spontaneous process, and should not be confused with organismal breathing (organismal respiration).

The overall balanced reaction for aerobic cellular respiration is:

All steps rely on redox reactions: glucose is fully oxidized to carbon dioxide, oxygen is reduced to water, and intermediate electron carriers $NA{D}^{+}$ and FAD shuttle high-energy electrons between steps. This topic makes up ~6-8% of the total AP Biology exam score, appearing on both multiple-choice and free-response sections.

2. Stages of Aerobic Respiration and ATP Accounting★★★☆☆⏱ 4 min

In eukaryotes, aerobic respiration occurs in four sequential stages, with the following inputs and outputs per starting glucose molecule:

1. Glycolysis (cytoplasm): Splits glucose into two pyruvate. Net outputs: 2 ATP (substrate-level phosphorylation), 2 NADH, 2 pyruvate.

2. Pyruvate Oxidation (mitochondrial matrix): Each pyruvate is oxidized to acetyl-CoA. Net outputs: 2 acetyl-CoA, 2 $C{O}_2$, 2 NADH.

3. Citric Acid (Krebs) Cycle (mitochondrial matrix): Each acetyl-CoA enters the cycle, all remaining carbon is released as $C{O}_2$. Net outputs: 4 $C{O}_2$, 6 NADH, 2 $FAD{H}_2$, 2 ATP (substrate-level).

4. Oxidative Phosphorylation (inner mitochondrial membrane): Uses electrons from NADH and $FAD{H}_2$ to generate ATP via chemiosmosis. AP endorses modern yields of 2.5 ATP per NADH and 1.5 ATP per $FAD{H}_2$ (outdated 3/2 values are not used).

3. Oxidative Phosphorylation and Chemiosmosis★★★☆☆⏱ 3 min

Oxidative phosphorylation generates ~90% of ATP in aerobic respiration, relying on the chemiosmotic principle: energy stored in an electrochemical proton gradient across a membrane drives cellular work. The inner mitochondrial membrane is impermeable to $H^{+}$ ions, so ETC protein complexes pump $H^{+}$ from the matrix into the intermembrane space, creating proton motive force (an electrochemical gradient).

Oxygen acts as the final electron acceptor due to its high electronegativity, pulling electrons down the ETC and combining with electrons and $H^{+}$ to form water. ATP synthase is a transmembrane enzyme that allows $H^{+}$ to diffuse back into the matrix; the flow causes conformational changes that catalyze phosphorylation of ADP to ATP.

4. Anaerobic Pathways: Fermentation★★★☆☆⏱ 3 min

When oxygen is unavailable, the ETC backs up because there is no final electron acceptor. Glycolysis requires $NA{D}^{+}$ as an input, which is converted to NADH during glycolysis; if NADH cannot be oxidized back to $NA{D}^{+}$, glycolysis stops. Fermentation solves this by oxidizing NADH back to $NA{D}^{+}$ to keep glycolysis running. Critically, fermentation produces no ATP beyond the 2 net ATP already generated by glycolysis.

• Lactic acid fermentation: Pyruvate is reduced by NADH to lactate, regenerating $NA{D}^{+}$. Occurs in human muscle during strenuous exercise and lactic acid bacteria. No $C{O}_2$ is produced.

• Alcohol fermentation: Pyruvate is decarboxylated to release $C{O}_2$, forming acetaldehyde, which is reduced by NADH to ethanol, regenerating $NA{D}^{+}$. Occurs in yeast, used in brewing and bread making.

Fermentation is distinct from anaerobic respiration: anaerobic respiration still uses an ETC with an alternative final electron acceptor (e.g. nitrate), while fermentation does not use an ETC at all.

5. AP-Style Concept Check★★★★☆⏱ 1 min