Unit 3 Cellular Energetics: How Cells Harvest Energy and Why It Matters for Evolutionary Success

Cellular respiration

A set of metabolic pathways that convert chemical energy in food (especially glucose) into ATP, releasing some energy as heat; often aerobic in eukaryotes.

ATP (adenosine triphosphate)

The cell’s main usable energy currency that directly powers cellular work (e.g., active transport, movement, biosynthesis).

Aerobic respiration

Cellular respiration that uses oxygen as the final electron acceptor in the electron transport chain.

Electron carriers

Molecules (mainly NADH and FADH2) that store and transport high-energy electrons captured from food molecules to the ETC.

NADH

The reduced form of NAD+; an electron carrier that donates high-energy electrons to the electron transport chain.

FADH2

The reduced form of FAD; an electron carrier that donates electrons to the electron transport chain (typically at a different entry point than NADH).

Redox (reduction-oxidation)

Electron-transfer reactions central to respiration; oxidation is loss of electrons and reduction is gain of electrons.

Oxidation

Loss of electrons (e.g., glucose is oxidized as it is broken down to CO2).

Reduction

Gain of electrons (e.g., oxygen is reduced as it gains electrons and forms water).

Final electron acceptor

The molecule that accepts electrons at the end of an electron transport chain; in aerobic respiration, oxygen fills this role, allowing electron flow to continue.

Glycolysis

An anaerobic pathway in the cytosol that splits one glucose (6C) into two pyruvate (3C) while producing ATP and NADH.

Substrate-level phosphorylation

ATP production by direct transfer of a phosphate group to ADP (occurs in glycolysis and the citric acid cycle).

Pyruvate oxidation (link reaction)

Conversion of pyruvate to acetyl-CoA in the mitochondrial matrix, releasing CO2 and producing NADH.

Acetyl-CoA

A 2-carbon molecule formed from pyruvate that enters the citric acid cycle; the “entry ticket” to the cycle.

Citric acid cycle (Krebs cycle)

A cyclic pathway in the mitochondrial matrix that completes oxidation of the acetyl group to CO2 and produces lots of NADH and FADH2 (plus a small amount of ATP/GTP).

Oxidative phosphorylation

ATP production powered by the ETC and chemiosmosis; the major source of ATP in aerobic respiration.

Electron transport chain (ETC)

A series of inner-membrane protein complexes that transfer electrons from NADH/FADH2 and use released energy to pump protons across the membrane.

Chemiosmosis

Movement of protons down their electrochemical gradient through ATP synthase, driving phosphorylation of ADP to ATP.

Proton gradient (electrochemical gradient)

Stored potential energy created when the ETC pumps H+ from the matrix to the intermembrane space; this gradient drives ATP synthase.

ATP synthase

Membrane enzyme that makes ATP by using energy from protons flowing down their gradient (acts like a turbine).

Uncoupler

A factor that makes the membrane “leaky” to protons, dissipating the proton gradient so energy is released as heat rather than used to make ATP.

Fermentation

Anaerobic pathways that regenerate NAD+ from NADH by transferring electrons to an organic molecule, allowing glycolysis to continue when oxygen is absent.

Lactic acid fermentation

Fermentation pathway (common in animal muscles under low oxygen and some bacteria) where pyruvate is reduced to lactate to regenerate NAD+.

Alcohol fermentation

Fermentation pathway (common in yeast) where pyruvate is converted to ethanol and CO2 while regenerating NAD+.

Fitness (evolutionary biology)

An organism’s relative reproductive success—how effectively it passes genes to the next generation compared with others in its population, in a specific environment.