AP Biology · Lesson 5 of 7 · Unit 3

Cellular respiration: glycolysis, the Krebs cycle and fermentation

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Cellular respiration and fermentation are tested on the AP Biology exam through pathway models, respirometer data, and metabolic inhibitor scenarios. You are never asked to memorize intermediate names or enzymes; instead, you must track inputs, outputs, locations, and the recycling of electron carriers across both multiple-choice sets and free-response questions.

By the end you’ll be able to

Describe how glycolysis in the cytosol releases energy from glucose to form ATP, NADH and pyruvate.
Describe how pyruvate is transported into the mitochondrion and oxidized, and how the Krebs cycle in the matrix releases carbon dioxide, synthesizes ATP and reduces NAD+ to NADH and FAD to FADH2.
Explain how fermentation allows glycolysis to proceed in the absence of oxygen, producing organic molecules such as alcohol or lactic acid.

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Core ideas (continued)
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What the exam asks

Pathway tracking: Identifying where glycolysis, pyruvate oxidation, the Krebs cycle, and fermentation occur, along with their net inputs and outputs.
Disruption predictions (FRQ Q1, Q4, Q5): Predicting what happens to $\text{ATP}$ , $\text{NADH}$ , or $\text{CO}_2$ production when an enzyme is inhibited or $\text{O}_2$ is removed.
Evolutionary conservation: Explaining why glycolysis is shared across all domains of life.
Experimental analysis (MC sets & FRQ Q1/Q2/Q3): Interpreting $\text{O}_2$ consumption or $\text{CO}_2$ production rates in yeast, germinating seeds, or muscle tissue.

Core ideas

1. Glycolysis: The universal cytosolic pathway

Glycolysis is a biochemical pathway occurring in the cytosol of almost all organisms. It breaks down one molecule of glucose (a 6-carbon sugar) into two molecules of pyruvate (3 carbons each) without requiring oxygen.

Energy investment vs. payoff: $\text{ATP}$ is initially invested to phosphorylate and destabilize glucose, but more $\text{ATP}$ is synthesized later via substrate-level phosphorylation (direct enzymatic transfer of a phosphate group from a substrate to $\text{ADP}$ ).
Net outputs per glucose: $2\text{ pyruvate}$ , $2\text{ ATP}$ (net), and (formed by reducing with electrons stripped from glucose).

2. Pyruvate oxidation and the Krebs cycle

When $\text{O}_2$ is present in eukaryotes, pyruvate is actively transported from the cytosol into the mitochondrial matrix (in aerobic prokaryotes, this occurs in the cytosol).

Pyruvate oxidation (link reaction): Pyruvate is oxidized; a carbon atom is removed and released as $\text{CO}_2$ , $\text{NAD}^+$ is reduced to $\text{NADH}$ , and the remaining 2-carbon acetyl group attaches to coenzyme A to form acetyl-CoA.
The Krebs cycle (citric acid cycle): Acetyl-CoA enters the cycle in the mitochondrial matrix. Through a series of enzyme-catalyzed steps, the remaining carbons are completely oxidized and released as .