Summary: Energy and Metabolism

Learning Outcomes

  • Identify different types of metabolic pathways
  • Distinguish between an open and a closed system
  • State the first law of thermodynamics
  • State the second law of thermodynamics
  • Explain the difference between kinetic and potential energy
  • Describe endergonic and exergonic reactions
  • Discuss how enzymes function as molecular catalysts

Cells perform the functions of life through various chemical reactions. A cell’s metabolism refers to the combination of chemical reactions that take place within it. Catabolic reactions break down complex chemicals into simpler ones and are associated with energy release. Anabolic processes build complex molecules out of simpler ones and require energy.

In studying energy, the term system refers to the matter and environment involved in energy transfers. Entropy is a measure of the disorder of a system. The physical laws that describe the transfer of energy are the laws of thermodynamics. The first law states that the total amount of energy in the universe is constant. The second law of thermodynamics states that every energy transfer involves some loss of energy in an unusable form, such as heat energy. Energy comes in different forms: kinetic, potential, and free. The change in free energy of a reaction can be negative (releases energy, exergonic) or positive (consumes energy, endergonic). All reactions require an initial input of energy to proceed, called the activation energy.

Enzymes are chemical catalysts that speed up chemical reactions by lowering their activation energy. Enzymes have an active site with a unique chemical environment that fits particular chemical reactants for that enzyme, called substrates. Enzymes and substrates are thought to bind according to an induced-fit model. Enzyme action is regulated to conserve resources and respond optimally to the environment.

Practice Questions

  1. Look at each of the processes shown in Figure 1, and decide if it is endergonic or exergonic.
    Four photos, from left to right, show a compost pile, a baby chick emerging from a fertilized egg, a teabag’s dark-colored contents diffusing into a clear mug of water, and a ball rolling downhill.

    Figure 1. Shown are some examples of endergonic processes (ones that require energy) and exergonic processes (ones that release energy). (credit a: modification of work by Natalie Maynor; credit b: modification of work by USDA; credit c: modification of work by Cory Zanker; credit d: modification of work by Harry Malsch)

  2. Does physical exercise to increase muscle mass involve anabolic and/or catabolic processes? Give evidence for your answer.
  3. Explain in your own terms the difference between a spontaneous reaction and one that occurs instantaneously, and what causes this difference.
  4. With regard to enzymes, why are vitamins and minerals necessary for good health? Give examples.