Microbial Metabolism & Energy Pathways – Vocabulary Review

Fifty key vocabulary terms covering energy generation, metabolic pathways, enzymatic regulation, and photosynthesis as presented in the Chapter 6 lecture on microbial metabolism.

Metabolism

The sum total of all chemical reactions in a cell that generate energy and drive biosynthesis.

Catabolism

Metabolic processes that break down compounds, releasing energy that cells capture as ATP.

Anabolism

Biosynthetic reactions that build cell components; they consume ATP and reducing power.

Potential Energy

Stored energy, such as that in chemical bonds, a rock on a hill, or water behind a dam.

Kinetic Energy

Energy of movement; e.g., moving water or a rotating flagellum.

ATP (Adenosine Triphosphate)

The energy currency of the cell; hydrolysis of its ~P bonds powers work.

Substrate-level Phosphorylation

ATP synthesis in which a phosphate is transferred directly to ADP during an exergonic reaction.

Oxidative Phosphorylation

ATP formation driven by a proton motive force generated when electrons pass through an electron transport chain.

Photophosphorylation

ATP synthesis powered by light-generated proton motive force during photosynthesis.

Glycolysis

Central pathway that splits 1 glucose (6C) to 2 pyruvate (3C), yielding net 2 ATP and 2 NADH.

Pentose Phosphate Pathway

Glucose-oxidizing pathway that supplies NADPH and the precursor metabolites ribose-5-P and erythrose-4-P.

Transition Step

Conversion of pyruvate to acetyl-CoA with release of CO₂ and production of NADH.

Tricarboxylic Acid Cycle

Also Krebs or citric acid cycle; oxidizes acetyl-CoA to CO₂ and generates ATP, NADH, and FADH₂.

Electron Transport Chain (ETC)

Series of membrane carriers that pass electrons, eject protons, and create an electrochemical gradient.

Proton Motive Force

Electrochemical gradient of H⁺ across a membrane that drives ATP synthase, transport, and flagella.

Chemiosmotic Theory

Mitchell’s proposal that ATP synthesis is coupled to a proton gradient formed by electron transport.

Fermentation

Anaerobic metabolism that uses pyruvate (or a derivative) as an internal electron acceptor to regenerate NAD⁺.

Aerobic Respiration

Respiratory pathway that uses O₂ as the terminal electron acceptor and yields maximal ATP.

Anaerobic Respiration

Respiration using an electron transport chain but with a terminal acceptor other than O₂ (e.g., NO₃⁻).

Terminal Electron Acceptor

The molecule that ultimately receives electrons from the ETC (e.g., O₂, nitrate, sulfate).

Reducing Power

Electron carriers such as NADH, FADH₂, or NADPH that can donate electrons (reducing equivalents).

NAD⁺/NADH

Oxidized/reduced forms of nicotinamide adenine dinucleotide; NADH feeds electrons to the ETC.

FAD/FADH₂

Oxidized/reduced forms of flavin adenine dinucleotide; FADH₂ donates electrons at complex II.

NADP⁺/NADPH

Electron carrier pair used mainly in biosynthesis; NADPH supplies reducing power for anabolic reactions.

Redox Reaction

Chemical reaction involving electron transfer; one substance is oxidized while another is reduced.

Oxidation

Loss of electrons (often with loss of H); the molecule becomes more positive or less energy-rich.

Reduction

Gain of electrons (often with gain of H); the molecule becomes more negative or energy-rich.

Electron Carrier

Molecule such as NAD⁺, FAD, quinone, or cytochrome that shuttles electrons in metabolic pathways.

Precursor Metabolite

Intermediate of catabolism that serves as a carbon skeleton for biosynthesis (e.g., pyruvate, oxaloacetate).

Acetyl-CoA

Two-carbon activated acetyl group attached to coenzyme A; entry point into the TCA cycle.

Pyruvate

Three-carbon end product of glycolysis and key branching point to fermentation, respiration, or anabolism.

RuBisCO

Ribulose-1,5-bisphosphate carboxylase/oxygenase, the CO₂-fixing enzyme of the Calvin cycle.

Calvin Cycle

Light-independent pathway that fixes CO₂ into carbohydrate, consuming ATP and NADPH.

Photosystem

Pigment-protein complex that captures light energy and funnels excited electrons to a reaction center.

Cyclic Photophosphorylation

Light reaction using photosystem I alone to make ATP without producing NADPH or O₂.

Non-cyclic Photophosphorylation

Light reaction using photosystems II and I; produces ATP, NADPH, and O₂ from H₂O.

Chemolithotroph

Organism that oxidizes reduced inorganic molecules (e.g., H₂, NH₃, Fe²⁺) for energy.

Chemoorganotroph

Organism that derives energy by oxidizing organic compounds like glucose.

Autotroph

Organism that uses CO₂ as its principal carbon source for biosynthesis.

Heterotroph

Organism that obtains carbon from pre-formed organic compounds.

Allosteric Regulation

Control of enzyme activity by binding of a regulatory molecule to a site other than the active site.

Feedback Inhibition

Pathway control in which an end product acts as an allosteric inhibitor of the first enzyme.

Competitive Inhibitor

Molecule resembling a substrate that binds the active site and blocks substrate access.

Non-competitive Inhibitor

Molecule that binds an enzyme at a site other than the active site, altering its activity.

Cofactor

Non-protein helper for enzymes; may be an inorganic ion like Mg²⁺, Zn²⁺, or Cu²⁺.

Coenzyme

Organic cofactor (often vitamin-derived) that transfers atoms or electrons between reactions, e.g., NAD⁺.

Activation Energy

Energy barrier that must be lowered by an enzyme for a reaction to proceed.

Enzyme

Biological catalyst that speeds reactions by lowering activation energy without being consumed.

Substrate

Specific reactant molecule that binds to an enzyme’s active site and is converted to product.

Electron Donor

Energy source in redox reactions; a molecule with low electron affinity that gives up electrons, releasing energy.