Metabolism Notes

Metabolism

  • Metabolism is derived from the Greek word meaning "change."
  • It involves chemical changes that convert "raw materials"/nutrients into energy and complex cellular products.

Metabolic Diversity: Classifications of Organisms by Energy and Carbon Source

  • Organisms are classified based on their energy and carbon sources.
  • Chemotrophs: Obtain energy from chemical compounds.
    • Chemoautotrophs: Use chemical energy and inorganic compounds as a carbon source (e.g., hydrogen-, sulfur-, iron-, nitrogen-, and carbon monoxide-oxidizing bacteria).
    • Chemoheterotrophs: Use chemical energy and organic compounds as a carbon source (e.g., all animals, most fungi, protozoa, and bacteria).
  • Phototrophs: Obtain energy from light.
    • Photoautotrophs: Use light energy and inorganic compounds as a carbon source (e.g., all plants, algae, cyanobacteria, and green and purple sulfur bacteria).
    • Photoheterotrophs: Use light energy and organic compounds as a carbon source (e.g., green and purple non-sulfur bacteria, heliobacteria).

Anabolism and Catabolism

  • Metabolism comprises two main processes: catabolism and anabolism.
  • Catabolism: An energy-generating process involving the breakdown/degradation of complex molecules. It is an oxidative process.
  • Anabolism: An energy-requiring process that synthesizes complex molecules. It is a reductive process.
  • The energy produced by catabolism is utilized for anabolism.

Molecules Linking Anabolism and Catabolism

  • Adenosine Triphosphate (ATP)
    • In phototrophs, light energy is converted to ATP.
    • In chemotrophs, chemical energy is converted to ATP.
    • ATP stores energy in its energy-rich phosphoric anhydride bonds.
    • ATP serves as the "energy currency of the cell."
  • NADP+/NADPH
    • Substrates of catabolism are in a reduced state.
    • Catabolism is oxidative, releasing reducing equivalents.
    • NADP+/NADPH serves as a source of reducing equivalents in the cell.
  • NAD+/NADH
    • Catabolism is oxidative, and the released reducing equivalents are transferred to NAD+ to form NADH.
    • NADH is used in mitochondria (oxidative phosphorylation) to produce ATP.

Regulation of Metabolism

  • Metabolism is highly regulated for efficiency.
  • Regulation ensures that molecules are degraded to meet the cell's energy and reducing power needs.
  • This regulation is achieved through the regulation of enzymes, employing three main mechanisms:
    • Allosteric regulation
    • Covalent modification
    • Regulation of expression (synthesis/degradation), i.e., controlling the amount of enzyme in the cell.

Compartmentalization of Metabolism

  • Prokaryotes (no organelles)
    • Some processes are localized to the plasma membrane (e.g., oxidative phosphorylation).
    • Multi-enzyme complexes facilitate the transfer of intermediates between enzymes, limiting diffusion.
  • Eukaryotes
    • Some processes are localized to organelles (e.g., the TCA cycle and oxidative phosphorylation in mitochondria).
    • Active transport, if required, can serve as a regulatory mechanism.