Brock Biology of Microorganisms: Microbial Metabolism

Metabolism

All biochemical reactions needed for life, encompassing both catabolism (breaking down molecules to obtain energy) and anabolism (synthesizing cellular material).

Free Energy ($\Delta G^{\circ\prime}$)

The energy available to do work during a chemical reaction, defined under standard conditions of $pH\,7$, $1\,\text{atmosphere}$, and $1\,\text{M}$ concentration for all reactants and products.

Exergonic Reaction

A chemical reaction that releases free energy, characterized by a negative change in free energy ($\Delta G^{\circ\prime} < 0$).

Endergonic Reaction

A chemical reaction that requires an input of energy, characterized by a positive change in free energy ($\Delta G^{\circ\prime} > 0$).

Reducing Power

The capacity of a cell or molecule to donate electrons during reduction-oxidation (redox) transfer reactions.

Phototroph

An organism that obtains metabolic energy directly from light rather than from chemical compounds.

Chemotroph

An organism that conserves energy from chemical reactions, utilizing either organic or inorganic electron donors.

Chemoorganotroph

An organism that obtains its metabolic energy and reducing power from the oxidation of organic chemicals (e.g., glucose, acetate).

Chemolithotroph

An organism that obtains its metabolic energy and reducing power from the oxidation of inorganic chemicals (e.g., $H_2$, $H_2S$, $Fe^{2+}$, $NH_4^+$).

Heterotroph

An organism that obtains its required cellular carbon from organic chemicals.

Autotroph

An organism (primary producer) that obtains its cellular carbon by assimilating inorganic carbon dioxide ($CO_2$).

Reduction Potential ($E_0^\prime$)

The inherent tendency of a chemical substance to donate or accept electrons, expressed in volts ($\text{V}$) compared to a standard reference.

Redox Couple

A pair consisting of the oxidized and reduced forms of a substance, conventionally represented with the oxidized form on the left and reduced form on the right (e.g., $NO_3^- / NO_2^-$).

The Redox Tower

A vertical ordering of redox couples ranging from the most negative reduction potentials at the top to the most positive at the bottom; the further electrons drop between couples, the greater the free energy released.

Nicotinamide Adenine Dinucleotide ($NAD^+ / NADH$)

A freely diffusible soluble redox coenzyme that facilitates electron transfers between different catabolic enzymes via reversible redox cycling.

Substrate-Level Phosphorylation

A mechanism of energy conservation in which a high-energy phosphate bond on an organic substrate is directly transferred to ADP to form ATP.

Oxidative Phosphorylation

A mechanism of energy conservation wherein electron transfer through a respiratory chain produces an electrochemical proton gradient across the membrane that drives ATP synthesis.

Photophosphorylation

A mechanism of energy conservation in which light-driven electron transport generates a proton motive force that drives ATP synthesis.

Activation Energy

The minimum energy barrier required to align reactive chemical bonds and initiate a chemical reaction.

Enzyme

A highly specific biological catalyst (usually a protein, occasionally RNA) that accelerates reaction rates by lowering activation energy without being consumed.

Active Site

The specific region of an enzyme that binds reactant substrates and participates directly in catalysis.

Prosthetic Group

A nonprotein, nonsubstrate molecule that is permanently and tightly (often covalently) bound to an enzyme to participate in catalytic activity (e.g., heme in cytochromes).

Coenzyme

A small, nonprotein organic molecule (typically a vitamin derivative) that binds loosely and transiently to an enzyme during catalysis.

Glycolysis (Embden–Meyerhof–Parnas Pathway)

A ubiquitous glucose catabolism pathway consisting of a preparatory stage (Stage I) and an energy-conserving redox stage (Stage II) that breaks down glucose into two molecules of pyruvate.

Citric Acid Cycle (CAC)

A central metabolic cycle that condenses acetyl-CoA with oxaloacetate