Cellular Metabolism Pathways to Know for Cell Biology

Cellular metabolism pathways are vital for energy production and biosynthesis in living organisms. These processes, including glycolysis, the citric acid cycle, and oxidative phosphorylation, work together to convert nutrients into usable energy, supporting cellular functions and overall life.

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 like hexokinase and phosphofructokinase.

2. Citric Acid Cycle (Krebs Cycle)

   • Takes place in the mitochondrial matrix 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. Oxidative Phosphorylation

   • Occurs in the inner mitochondrial membrane, utilizing NADH and FADH2 to generate ATP through the electron transport chain.
   • Involves the transfer of electrons and the pumping of protons to create a proton gradient, driving ATP synthesis via ATP synthase.
   • Oxygen serves as the final electron acceptor, forming water as a byproduct.

4. Pentose Phosphate Pathway

   • Functions in the cytoplasm, generating NADPH for biosynthetic reactions and ribose-5-phosphate for nucleotide synthesis.
   • Divided into an oxidative phase (producing NADPH) and a non-oxidative phase (interconverting sugars).
   • Plays a crucial role in cellular redox balance and the synthesis of fatty acids and nucleotides.

5. Gluconeogenesis

   • The synthesis of glucose from non-carbohydrate precursors, primarily occurring in the liver and kidneys.
   • Utilizes key enzymes like pyruvate carboxylase and phosphoenolpyruvate carboxykinase to bypass irreversible steps of glycolysis.
   • Essential for maintaining blood glucose levels during fasting or intense exercise.

6. Fatty Acid Oxidation (Beta-oxidation)

   • Occurs in the mitochondrial matrix, breaking down fatty acids into acetyl-CoA units.
   • Each cycle of beta-oxidation produces NADH and FADH2, which enter the electron transport chain for ATP production.
   • Regulated by the availability of fatty acids and the energy needs of the cell.

7. Fatty Acid Synthesis

   • Takes place in the cytoplasm, converting acetyl-CoA into fatty acids through a series of enzymatic reactions.
   • Involves the enzyme fatty acid synthase and requires NADPH, primarily generated from the pentose phosphate pathway.
   • Regulated by hormonal signals (insulin promotes synthesis) and the availability of substrates.

8. Amino Acid Metabolism

   • Involves the synthesis and degradation of amino acids, which are the building blocks of proteins.
   • Transamination and deamination reactions are key processes, allowing for the interconversion of amino acids and the production of ammonia.
   • Essential for nitrogen balance and the synthesis of neurotransmitters and other nitrogen-containing compounds.

9. Urea Cycle

   • Occurs in the liver, converting toxic ammonia into urea for excretion in urine.
   • Involves a series of enzymatic reactions that utilize ornithine, carbamoyl phosphate, and aspartate.
   • Plays a critical role in nitrogen metabolism and detoxification of ammonia produced from amino acid catabolism.

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.
    • Essential for converting light energy into chemical energy, supporting life on Earth by providing oxygen and organic compounds.