Metabolic Processes within the Cell

What is Metabolism? §

• Metabolism refers to the sum total of all chemical reactions that occur within the cell. These reactions occur in groups of metabolic pathways that can be classified into two kinds:
  • Catabolic Pathways that are always exergonic since they involve the breaking down of complex molecules to simpler molecules.
  • Anabolic Pathways that are always endergonic since they involve building complex molecules from simpler primitives.
• Pathways can also be divided based on whether or not they require oxygen (i.e., aerobic and anaerobic reactions).
• Catabolic and anabolic pathways can be separated due to catabolic pathways being irreversible, and also because the same enzymes are involved in both pathways

What drives Metabolism? §

• The main driver for metabolism in the cell is the stripping of hydrogen molecules. More Hydrogen means more energy.
  • Usually oxidation reactions involve the release of energy
  • Reduction reactions involve the storage of energy
  • An important concept in metabolism is transfer potential wherein certain molecules with high transfer potential are better donors and those with low transfer potential are better acceptors. The transfer potential is directly proportional to the amount of free energy the molecule has and inversely proportional to the affinity for the molecule being transferred to.
• The initial stages that drive metabolic reactions in the cell are mostly the same across organisms — glycolysis or the oxidation of glucose.
  • Reactions involved in glycolysis, as well as most catabolic pathways are essentially irreversible. This allows the reaction to be driven in the forward direction.
  • This reaction is not truly spontaneous as the cell needs to invest one ATP to start the reaction. However, the end result is more ATP than what the cell began with.
  • Fermentation is a related reaction wherein the cell creates intermediates that can be activated when Oxygen is introduced again. This allows the cell to regenerate its supply of NAD.
• Energy that is produced via metabolic pathways is stored using ATP (Adenosine Triphosphate)
  • ATP synthesis entails a reduction-oxidation reaction that makes use of the cofactor NAD.
  • ATP synthesis is regulated. It is inhibited when energy is at a surplus. This is driven by the fact that the ATP / ADP supply of the cell is limited.
• NAD is a cofactor that is involved in many reduction-oxidation reactions. It is a high energy compound that drives metabolic reactions through donating high energy electrons.
  • A related compound is NADPH which can be used to measure how much energy the cell has in store. When energy is abundant, the production of NADPH is favored. This is the cofactor used by enzymes with a reductive role in anabolic pathways. This allows for the growth of the cell.
  • Another is NADP. When energy is scarce, the production of NADP is favored. This is the cofactor used by enzymes with a dehydrogenase role in catabolic pathways. This allows the cell to store the energy as ATP.

Links §

• Enzymes as Machines for Cellular Processes - describes enzymes as drivers for metabolism

• Cellular Respiration - for an example of metabolic pathways.

• Karp Ch. 3 - select sections

  • 3.10 - 3.11 - for a detailed discussion on Metabolism
  • 3.13 - for a discussion on separating catabolic and anabolic pathways