Metabolic Pathways Overview for the MCAT

Introduction

• Focus on eukaryotic metabolism
• 13 major metabolic pathways crucial for MCAT
• Overview of pathway locations, precursors, and products

Key Metabolic Pathways

1. Glycolysis

   • Location: Cytoplasm
   • Precursors: Glucose, NAD+, 2 ATP
   • Products: 2 Pyruvate, 4 ATP (Net 2 ATP), NADH
   • Function: Breaking down glucose for energy

2. Pyruvate Dehydrogenase Complex (PDH)

   • Location: Mitochondria
   • Precursor: Pyruvate
   • Products: Acetyl-CoA, NADH, CO2
   • Function: Links glycolysis to the citric acid cycle

3. Citric Acid Cycle (Krebs Cycle)

   • Location: Mitochondria
   • Precursor: Acetyl-CoA, NAD+, FAD+
   • Products: NADH, FADH2, CO2
   • Function: Energy production

4. Electron Transport Chain (ETC)

   • Location: Inner mitochondrial membrane
   • Inputs: NADH, FADH2, Oxygen
   • Products: Water, ATP
   • Function: Major site of ATP production

5. Lactic Acid Fermentation

   • Condition: Low oxygen
   • Function: Regenerates NAD+ so glycolysis can continue
   • Product: Lactate

6. Gluconeogenesis

   • Location: Liver
   • Precursor: Pyruvate
   • Function: Production of glucose from non-carbohydrate sources

7. Glycogenesis

   • Location: Liver
   • Function: Storage of glucose as glycogen

8. Glycogenolysis

   • Location: Liver
   • Function: Breakdown of glycogen to glucose

9. Pentose Phosphate Pathway

   • Diversion from glycolysis
   • Products: Nucleotides, NADPH
   • Function: Produces ribose-5-phosphate for nucleotide synthesis and NADPH

10. Beta-Oxidation

    • Precursor: Fatty acids
    • Location: Mitochondria
    • Products: Acetyl-CoA, NADH, FADH2
    • Function: Fatty acid breakdown for energy

11. Fatty Acid Synthesis

    • Location: Cytoplasm
    • Precursor: Acetyl-CoA (shuttled via citrate shuttle)
    • Products: Fatty acids, utilizes NADPH
    • Function: Storage of excess energy

12. Ketogenesis

    • Location: Mitochondria
    • Function: Formation of ketone bodies as alternative energy source for the brain

13. Ketolysis

    • Function: Breakdown of ketone bodies to supply energy, especially to the brain

Additional Points

• Regulation: Study hormonal and homeostatic regulations
• Intermediates and Enzymes: Memorize key intermediates and enzymes for each pathway
• Practical Advice: Take a practice exam to identify strengths and weaknesses

Call to Action

• Resources: Mention of a free course to learn effective practice exam techniques
• Encourage continuous learning and practice