Gluconeogenesis: Making Glucose When You’re Fasting – MCAT Essentials
What happens when you run out of glucose but still need energy? Your body makes its own through a process called gluconeogenesis. It’s not just a reversed glycolysis—there are unique enzymes, key checkpoints, and regulatory controls the MCAT wants you to understand. Let’s break it down clearly.
🧬 Gluconeogenesis: Where and When?
Occurs in: Liver (mostly), kidneys (minor role)
Activated during: Fasting, starvation, intense exercise
Hormonal trigger: ↑ Glucagon, ↓ Insulin
Purpose: Maintain blood glucose for brain/RBCs
🧠 MCAT Tip: Brain and red blood cells rely on glucose. That’s why this pathway matters!
🔄 Glycolysis vs Gluconeogenesis
| Feature | Glycolysis | Gluconeogenesis |
|---|---|---|
| Overall direction | Glucose → Pyruvate | Pyruvate → Glucose |
| Energy | Produces ATP | Consumes ATP, GTP, NADH |
| Key tissue | All cells | Liver, kidney |
| Hormonal control | Insulin activates | Glucagon activates |
🔬 Key Enzymes to Know
| Step Blocked in Glycolysis | Enzyme in Gluconeogenesis | Notes |
|---|---|---|
| Pyruvate → PEP | Pyruvate carboxylase → PEPCK | Requires ATP and GTP |
| Fructose-1,6-bisP → F6P | Fructose-1,6-bisphosphatase | Major regulation point |
| G6P → Glucose | Glucose-6-phosphatase | Final step, only in liver/kidney |
🧠 These 3 bypass enzymes are MCAT favorites.
🧪 MCAT Passage Application
✅ Example Question:
“A defect in glucose-6-phosphatase leads to hypoglycemia...”
→ Gluconeogenesis is blocked → glucose can't exit liver
✅ Experimental Setup:
“Cells treated with glucagon increase PEPCK activity...”
→ Indicates active gluconeogenesis in response to fasting signal
🎯 Final MCAT Tips
Don’t memorize every step—focus on unique enzymes and regulation
Recognize how it’s different from glycolysis (ATP use, tissue, direction)
Know when and why the body uses this process (e.g., fasting, hormone cues)
Watch for hormonal context: glucagon = think gluconeogenesis
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Frequently Asked Questions (FAQs)
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