Glycolysis on the MCAT: Enzymes, ATP Yield, and Regulation
Glycolysis is the first step in cellular respiration, and the MCAT loves to test it in both standalone questions and experimental passages. From enzymes and regulation to energy yield and intermediates, this process is foundational. Let’s break it down clearly—with King of the Curve visuals, MCAT tips, and fast-recall tables.
🔬 What Is Glycolysis?
Glycolysis is the anaerobic breakdown of glucose (a 6-carbon sugar) into two molecules of pyruvate, yielding energy in the form of ATP and NADH.
🧠 MCAT Tip: Glycolysis occurs in the cytoplasm and does not require oxygen—a favorite trick question!
🧪 Net Yield of Glycolysis
| Molecule | Per Glucose Molecule |
|---|---|
| ATP (gross) | 4 |
| ATP (net) | 2 (after 2 used) |
| NADH | 2 |
| Pyruvate | 2 |
✅ Each NADH can produce ~2.5 ATP in oxidative phosphorylation (MCAT assumption)
🔑 Key Enzymes to Know
| Enzyme | Function | MCAT Note |
|---|---|---|
| Hexokinase | Traps glucose by phosphorylating it | First ATP used |
| Phosphofructokinase-1 (PFK-1) | Rate-limiting step | Activated by AMP, inhibited by ATP/citrate |
| Glyceraldehyde-3-P dehydrogenase | Produces NADH | Requires NAD⁺ |
| Pyruvate kinase | Forms pyruvate and ATP | Second ATP-producing step |
🔁 Regulation of Glycolysis
PFK-1 is the key regulatory enzyme
Activated by: AMP, fructose-2,6-bisphosphate
Inhibited by: ATP, citrate
Hexokinase and pyruvate kinase are also regulated to prevent wasteful glucose usage.
🧠 High-yield test clue: Passage shows "increased AMP levels" → This activates PFK-1 → Glycolysis speeds up
🧬 Anaerobic vs Aerobic Fate of Pyruvate
| Condition | Pyruvate Converted To | Why It Matters on MCAT |
|---|---|---|
| Aerobic | Acetyl-CoA | Enters TCA cycle |
| Anaerobic | Lactate (in humans) | Regenerates NAD⁺ for glycolysis to continue |
MCAT passages often test hypoxia, muscle activity, or mitochondrial defects—all of which shift pyruvate fate.
🧠 Final MCAT Tips
Know where glycolysis happens: cytoplasm
Remember net 2 ATP, 2 NADH, and 2 pyruvate
Expect PFK-1 regulation to be embedded in experimental setups
Practice interpreting ATP/NADH graphs, especially under drug treatments
✅ Call-to-Action (CTA)
Don’t just memorize glycolysis—visualize it, quiz it, and connect it to experimental logic. Use King of the Curve’s glycolysis flashcards, pathway visuals, and timed quiz sets to solidify this core process for the MCAT.
Frequently Asked Questions (FAQs)
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