9.Pathways That Help Harvest Chemical Energy

9.1

• How do cells obtain energy from glucose?

  • Cells obtain *energy from glucose by the chemical process of oxidation, which is carried out through a series of metabolic pathways.
  • five principles that govern metabolic pathways:
    • A complex chemical transformation occurs in a series of separate reactions that form a metabolic pathway.
    • Each reaction is catalyzed by a specific enzyme.
    • Many metabolic pathways are similar in all organisms, from bacteria to humans.
    • In eukaryotes, many metabolic pathways are compartmentalized, with certain reactions occurring inside specific organelles, or even specific regions of an organelle.
    • Some key enzymes in each metabolic pathway can be inhibited or activated to alter the rate of the pathway.
  • If glucose is burned in a flame or is in a typical cell, it reacts with oxygen gas (O₂), forming carbon dioxide and water and releasing energy in the form of heat.

  •  The balanced equation for the complete reaction is

    C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + free energy (ΔG = –686 kcal/mol)

  • This is an oxidation–reduction reaction (more on this shortly), in which glucose loses electrons (becomes oxidized) and oxygen gains them (becomes reduced).

  • The large standard free energy change (ΔG) is negative, indicating that the overall conversion is highly exergonic and can drive the endergonic formation of a lot of ATP from ADP and phosphate:

    ADP + P_i + free energy → ATP

  • glucose catabolism happens in many steps in a pathway. 
  • Each step is catalyzed by an enzyme, and the process is compartmentalized.
  • Three catabolic processes harvest the energy in the chemical bonds of glucose: glycolysis, cellular respiration, and fermentation
  •  All three processes involve pathways made up of many distinct chemical reactions.
    • Glycolysis begins glucose catabolism.
      • Through a series of chemical rearrangements, glucose is converted to two molecules of the three-carbon product pyruvate, and a small amount of energy is captured in usable forms. Glycolysis is an anaerobic process because it does not require O₂.
    • Cellular respiration uses O₂ from the environment and thus is aerobic.
      • Each pyruvate molecule is completely converted into three molecules of CO₂ through a set of catabolic pathways including pyruvate oxidation, the citric acid cycle, and an electron transport system (the respiratory chain).
      • In the process, a great deal of the energy…