Major Metabolic Pathways to Know for General Biology II

Major metabolic pathways are essential for understanding how our bodies convert food into energy. These processes, like glycolysis and the citric acid cycle, play a crucial role in nutrition, biochemistry, and cellular function, impacting overall health and energy balance.

1. Glycolysis

   • Converts glucose into pyruvate, producing a net gain of 2 ATP and 2 NADH.
   • Occurs in the cytoplasm and does not require oxygen (anaerobic process).
   • Key regulatory enzymes include hexokinase, phosphofructokinase, and pyruvate kinase.

2. Citric Acid Cycle (Krebs Cycle)

   • Takes place in the mitochondria and processes acetyl-CoA to produce NADH, FADH2, and GTP.
   • Completes the oxidation of carbohydrates, fats, and proteins.
   • Regulated by the availability of substrates and the energy needs of the cell.

3. Electron Transport Chain and Oxidative Phosphorylation

   • Located in the inner mitochondrial membrane, it uses electrons from NADH and FADH2 to create a proton gradient.
   • ATP is synthesized via ATP synthase as protons flow back into the mitochondrial matrix.
   • Oxygen serves as the final electron acceptor, forming water.

4. Gluconeogenesis

   • The synthesis of glucose from non-carbohydrate precursors, primarily in the liver.
   • Key enzymes include pyruvate carboxylase and phosphoenolpyruvate carboxykinase.
   • Important for maintaining blood glucose levels during fasting or intense exercise.

5. Glycogenesis and Glycogenolysis

   • Glycogenesis is the process of converting glucose to glycogen for storage, primarily in the liver and muscle.
   • Glycogenolysis is the breakdown of glycogen back into glucose when energy is needed.
   • Regulated by hormones such as insulin (promotes glycogenesis) and glucagon (promotes glycogenolysis).

6. Pentose Phosphate Pathway

   • A metabolic pathway parallel to glycolysis that generates NADPH and ribose-5-phosphate.
   • NADPH is crucial for biosynthetic reactions and antioxidant defense.
   • Ribose-5-phosphate is essential for nucleotide synthesis.

7. Fatty Acid Synthesis

   • Occurs in the cytoplasm and involves the conversion of acetyl-CoA into fatty acids.
   • Key enzyme is fatty acid synthase, which catalyzes the elongation of fatty acid chains.
   • Requires NADPH and is regulated by insulin and energy status.

8. Fatty Acid Oxidation (Beta-oxidation)

   • Takes place in the mitochondria, breaking down fatty acids into acetyl-CoA units.
   • Produces NADH and FADH2, which enter the electron transport chain for ATP production.
   • Regulated by the availability of fatty acids and energy needs of the cell.

9. Ketogenesis

   • The production of ketone bodies from excess acetyl-CoA, primarily in the liver.
   • Occurs during prolonged fasting or low-carbohydrate diets when glucose is scarce.
   • Ketone bodies serve as an alternative energy source for tissues, especially the brain.

10. Urea Cycle

    • A series of biochemical reactions in the liver that convert ammonia to urea for excretion.
    • Helps detoxify ammonia, a byproduct of amino acid metabolism.
    • Key enzymes include carbamoyl phosphate synthetase and arginase.

11. Amino Acid Metabolism

    • Involves the synthesis and degradation of amino acids, which are the building blocks of proteins.
    • Transamination and deamination are key processes for amino acid interconversion.
    • Amino acids can be used for energy, converted to glucose, or used in biosynthesis.

12. Cholesterol Biosynthesis

    • A multi-step process primarily occurring in the liver, converting acetyl-CoA into cholesterol.
    • Key regulatory enzyme is HMG-CoA reductase, which is targeted by statin drugs.
    • Cholesterol is essential for cell membrane structure and the synthesis of steroid hormones.

13. Purine and Pyrimidine Metabolism

    • Involves the synthesis and degradation of purines (adenine and guanine) and pyrimidines (cytosine, thymine, and uracil).
    • Nucleotides are synthesized de novo or salvaged from breakdown products.
    • Important for DNA and RNA synthesis, as well as energy metabolism (ATP).

14. Photosynthesis (Calvin Cycle and Light Reactions)

    • Light reactions capture solar energy to produce ATP and NADPH, occurring in the thylakoid membranes.
    • The Calvin Cycle uses ATP and NADPH to convert carbon dioxide into glucose in the stroma.
    • Essential for converting solar energy into chemical energy, supporting life on Earth.