Principles Module 22

Principles Module 23


  • Describe what a metabolic pathway is, provide examples
  • Discuss the concepts of energy and thermodynamics as they relate to biochemical reactions

Key points

  • Cells transform energy and matter in a series of enzyme-mediated reactions that make up a metabolic pathway
    • a pathway is when the product of one reaction acts as the substrate for a subsequent reaction
    • reactions that break down molecules are catabolic
    • reactions that synthesize molecules are called anabolic
  • chemical energy is a form of potential energy represented by the bonds between atoms
  • laws of thermodynamics govern the process of energy transformation in a cell
    • energy is neither created nor destroyed
    • a reaction only proceeds spontaneously if it results in increased entropy
  • in biology we talk about energy in terms of the change in net free energy, ΔG
    • reactions that require net input of energy have positive ΔG, endergonic
    • reactions that release potential energy have negative ΔG, exergonic
    • endergonic and exergonic reactions are coupled

Questions for Practice

  • Define the phrase metabolic pathway.
  • What is the difference between endergonic and exergonic reactions?
  • What is ΔG, and what is the significance of its sign (positive vs. negative)?
  • What is energy? Provide examples of the cellular use of energy.