Metabolic Pathways for the MCAT

Overview

• Focus on eukaryotic cell metabolism
• Identification of pathway locations, precursors, and products
• Discussion on why and when each pathway is used

Metabolic Pathways

1. Glycolysis

• Location: Cytoplasm
• Precursors: Glucose, NAD+, 2 ATP
• Products: 2 Pyruvate, NADH, 4 ATP (Net 2 ATP)

2. Pyruvate Dehydrogenase Complex (PDH)

• Conditions: High oxygen, low ATP
• Location: Mitochondria
• Products: Acetyl-CoA, NADH, CO2

3. Citric Acid Cycle (Krebs Cycle)

• Location: Mitochondria
• Precursors: Acetyl-CoA, NAD+, FAD+
• Products: NADH, FADH2, CO2
• Notes: Cycle process

4. Electron Transport Chain (ETC)

• Location: Inner Mitochondrial Membrane
• Precursors: NADH, Oxygen
• Products: Water, ATP
• Notes: Complex II is Succinate Dehydrogenase from Citric Acid Cycle

5. Lactic Acid Fermentation

• Conditions: Low oxygen, low ATP
• Location: Cytoplasm
• Precursors: Pyruvate, NADH
• Products: Lactate, NAD+
• Purpose: Regenerate NAD+ for glycolysis

6. Gluconeogenesis

• Conditions: High energy, low glucose
• Location: Liver cells
• Precursors: Pyruvate
• Products: Glucose

7. Glycogenesis

• Conditions: High energy, high glucose
• Location: Liver cells
• Product: Glycogen (storage form of glucose)

8. Glycogenolysis

• Conditions: Low glucose
• Location: Liver cells
• Product: Free glucose

9. Pentose Phosphate Pathway

• Conditions: High energy
• Location: Cytoplasm
• Precursors: Glycolytic intermediates
• Products: Nucleotides, NADPH

10. Beta-Oxidation (Fatty Acid Breakdown)

• Conditions: Low glucose, low ATP
• Location: Partially mitochondria
• Precursors: Fatty acids, Oxygen
• Product: Acetyl-CoA

11. Fatty Acid Synthesis

• Conditions: High energy, high glucose
• Location: Cytoplasm (requires citrate shuttle to move Acetyl-CoA)
• Products: Fatty acids, NADPH from pentose phosphate pathway

12. Ketogenesis

• Conditions: Low glucose
• Location: Mitochondria
• Precursors: Acetyl-CoA, Citric acid cycle intermediates
• Product: Ketone bodies
• Purpose: Energy source for the brain when glucose is low

13. Ketolysis

• Conditions: Need for energy in the brain
• Location: Mitochondria
• Precursors: Ketone bodies
• Products: Citric acid cycle intermediates

Additional Notes

• Understanding these pathways includes knowing their regulatory mechanisms (hormonal and homeostatic regulation).
• Importance of memorizing key intermediates and enzymes for each pathway before the MCAT.

Final Steps

• Take and review a practice exam to identify strengths and weaknesses.