Quiz Me!

Introduction to Metabolism

  Lecture Index
  Course Resources page
Last revised: Tuesday, February 15, 2000
Ch. 8 in Prescott et al, Microbiology, 4th Ed.
Note: These notes are provided as a guide to topics the instructor hopes to cover during lecture. Actual coverage will always differ somewhat from what is printed here. These notes are not a substitute for the actual lecture!
Copyright 2000. Thomas M. Terry

Energy and Metabolism

Energy = capacity to do work


Overview of Catabolism

Thermodynamics = laws governing energy transfer

Free energy: G, Go', relationship to equilibrium


Q. Do reactions with positive G ever occur?
A. No. But many reactions with positive values of Go' occur in cells.

Q. How is that possible?
A. Remember that G is the sum of two terms (see above): the Go', which can be positive, and the RT lnK term. If the second term is sufficiently negative, it can overcome a strongly postive Go' term.

Q. Wait a minute. Isn't the Go' itself related to the equilibrium constant?
A. Yes, it's related to the equilibrium constant under standard conditions, which include everything being at 1 molar concentrations and pH 7. But no cell ever comes close to 1 molar concentrations. In fact, cells often keep the concentrations of many reactants at very small levels; as soon as a compound is produced in one reaction, it is immediately used up in another reaction. Most cell chemicals occur in mMolar amounts or less.

A consequence of this is that the RT lnK term of the G reaction (above) can be modified quickly in the cell. If the cell needs to make this a very large negative number, the concentration of product needs to be kept very small relative to the concentration of reactant. Since the log of 1 is zero, the log of numbers smaller than 1 is a negative number, which makes the RT lnK term negative. The smaller the ratio of [C] [D] / [A] [B], the more negative that number becomes.

Q. What's the bottom line here?
A. If reactions have negative G values, they're very useful to a cell -- they can release energy, they can happen spontaneously. All the cell needs to do is make an enzyme to speed up such reactions. On the other hand, if needed reactions have positive G values, they're not going to happen. In order to make such reactions occur, the cell has to change the G to a negative value, either by coupling such reactions with exergonic reactions (we'll see examples of that later), or manipulating concentrations of reactants and products to force the reaction to have a negative value. That works surprisingly well for intermediary metabolites (chemicals used only to get from one molecule to another), but not for desired end products, which by definition must accumulate in large quantity.

Biological Oxidations

Redox Carriers

Use of Redox Tower to measure Go', predict reactions

Use of ATP to store Energy

Activation Energy and the role of enzymes


Mechanisms of energy release: overview

  1. Fermentation -- oxidation of an organic compound in the absence of external electron acceptor (no oxygen required). Uses SLP (substrate-level phosphorylation)
  2. Respiration -- oxidation of an organic compound where oxygen is the final electron acceptor. Uses ETS (electron transport system) as well as SLP
  3. Anaerobic respiration (unique to bacteria) -- oxidation of organic compounds where an external substrate other than oxygen serves as final electron acceptor. Exs: nitrate, sulfate, carbon dioxide

Take a Self-Quiz on this material
Return to Lecture Index
Return to MCB 229 Course Resources page