Essential Biochemical Pathways

Essential biochemical pathways are crucial for understanding how living organisms convert nutrients into energy. These processes, like glycolysis and the citric acid cycle, highlight the intricate connections between chemistry and biology, forming the foundation for metabolic functions in all life forms.

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).
   • Involves ten enzymatic steps, with key regulatory enzymes including hexokinase and phosphofructokinase.

2. Citric Acid Cycle (Krebs Cycle)

   • Takes place in the mitochondria and processes acetyl-CoA derived from carbohydrates, fats, and proteins.
   • Produces 3 NADH, 1 FADH2, and 1 GTP (or ATP) per cycle, contributing to the electron transport chain.
   • Regulated by the availability of substrates and feedback inhibition from products like ATP and NADH.

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.
   • The flow of protons back into the mitochondrial matrix drives ATP synthesis via ATP synthase.
   • Oxygen serves as the final electron acceptor, forming water and preventing the backup of the chain.

4. Gluconeogenesis

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

5. 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, while ribose-5-phosphate is vital for nucleotide synthesis.
   • Divided into an oxidative phase (producing NADPH) and a non-oxidative phase (interconverting sugars).

6. Fatty Acid Synthesis

   • Occurs in the cytoplasm and involves the conversion of acetyl-CoA into fatty acids, primarily palmitate.
   • Requires NADPH, which is generated from the pentose phosphate pathway.
   • Key enzyme is fatty acid synthase, which catalyzes the elongation of the fatty acid chain.

7. Beta-Oxidation of Fatty Acids

   • The process of breaking down fatty acids into acetyl-CoA units in the mitochondria.
   • Each cycle of beta-oxidation produces 1 FADH2, 1 NADH, and 1 acetyl-CoA, which enters the citric acid cycle.
   • Regulated by the availability of fatty acids and the energy needs of the cell.

8. Urea Cycle

   • A series of biochemical reactions in the liver that converts ammonia, a toxic byproduct of amino acid metabolism, into urea for excretion.
   • Involves five key enzymes, including carbamoyl phosphate synthetase and arginase.
   • Essential for nitrogen balance and detoxification in the body.

9. Amino Acid Metabolism

   • Involves the synthesis and degradation of amino acids, which are the building blocks of proteins.
   • Transamination and deamination are key processes that allow for the interconversion of amino acids and the production of energy.
   • Essential for the synthesis of neurotransmitters, hormones, and other nitrogen-containing compounds.

10. Photosynthesis (Light and Dark Reactions)

    • Light reactions occur in the thylakoid membranes, converting solar energy into chemical energy (ATP and NADPH).
    • Dark reactions (Calvin cycle) take place in the stroma, using ATP and NADPH to fix carbon dioxide into glucose.
    • Chlorophyll absorbs light energy, driving the electron transport chain and ultimately producing oxygen as a byproduct.