Electron Transport Chain & Oxidative Phosphorylation Vocabulary

Fifty key vocabulary cards covering ETC complexes, electron carriers, proton pumping, ATP synthase mechanics, reactive oxygen species, and bioenergetic yields from the lecture.

Electron Transport Chain (ETC)

Series of inner-mitochondrial membrane complexes that pass electrons to oxygen while pumping protons.

Reduction Potential

Measure of a molecule’s tendency to gain electrons; higher value indicates stronger oxidant and lower free energy.

Oxidative Phosphorylation

Coupling of electron transport–driven proton pumping to ATP synthesis via ATP synthase.

NADH

Two-electron carrier produced in catabolic pathways; donates electrons to Complex I.

NAD⁺

Oxidized form of NADH regenerated after electron donation to the ETC.

Oxygen (final acceptor)

Molecule that receives electrons at Complex IV and is reduced to water.

Complex I (NADH:CoQ Oxidoreductase)

First ETC complex; transfers electrons from NADH to CoQ and pumps 4 protons.

FMN (Flavin Mononucleotide)

Flavin cofactor in Complex I that can accept or donate one electron at a time.

Riboflavin

Vitamin B₂ precursor of FMN and FAD.

Iron–Sulfur (Fe–S) Cluster

Inorganic prosthetic group (Fe + S) that transfers single electrons within complexes I, II, and III.

Coenzyme Q (Ubiquinone)

Mobile hydrophobic electron carrier with an isoprene tail; cycles between oxidized (Q), semiquinone (•QH), and reduced (QH₂) forms.

Proton Pump

Protein complex that moves protons against their gradient using redox energy.

Proton Gradient

Electrochemical difference in [H⁺] and charge across the inner membrane, also called proton-motive force.

Intermembrane Space

Region between inner and outer mitochondrial membranes where protons accumulate.

Matrix (Mitochondrial)

Innermost compartment; site of TCA cycle and lower [H⁺] during respiration.

Complex II (Succinate Dehydrogenase)

TCA enzyme embedded in ETC; passes electrons from FADH₂ to CoQ without pumping protons.

Succinate

TCA substrate oxidized by Complex II to fumarate, donating electrons to FAD.

Fumarate

Product of succinate oxidation in Complex II.

FAD / FADH₂

Flavin cofactor that accepts two electrons and two protons; embedded in Complex II.

Heme Group

Porphyrin ring with central Fe atom capable of redox cycling.

Heme b

Specific heme variant found in Complex II and some cytochromes.

Cytochrome

Heme-containing protein that transfers single electrons in ETC complexes III and IV.

Cytochrome c

Small soluble protein that shuttles electrons from Complex III to Complex IV; also an apoptosis signal when released to cytosol.

Complex III (CoQ:Cytochrome c Oxidoreductase)

Middle ETC complex executing the Q cycle and pumping 4 protons per QH₂ oxidized.

Q Cycle

Two-step mechanism in Complex III that splits electron pairs from QH₂, reduces two cytochrome c molecules, and translocates protons.

Complex IV (Cytochrome c Oxidase)

Final ETC complex; transfers electrons from cytochrome c to oxygen, pumps 2 protons, and forms H₂O.

Copper Centers (CuA / CuB)

Metal sites in Complex IV that participate in electron transfer to O₂.

Reactive Oxygen Species (ROS)

Partially reduced oxygen derivatives (e.g., superoxide) that can damage biomolecules or signal mitochondrial status.

Superoxide (O₂•⁻)

One-electron reduction product of oxygen; primary mitochondrial ROS.

Superoxide Dismutase (SOD)

Enzyme that converts superoxide to hydrogen peroxide and oxygen.

Catalase

Enzyme that decomposes hydrogen peroxide to water and oxygen, limiting ROS damage.

Proton-Motive Force

Sum of membrane potential and pH gradient driving ATP synthesis and transport processes.

ATP Synthase (Complex V)

Rotary enzyme that uses proton flow to regenerate ATP from ADP + Pi.

F₀ Subunit

Membrane-embedded portion of ATP synthase containing the rotating c-ring and proton half-channels.

F₁ Subunit

Catalytic head of ATP synthase composed of alternating α and β subunits with nucleotide-binding sites.

Gamma (γ) Subunit

Asymmetric shaft that rotates inside the α₃β₃ head, inducing conformational changes for ATP formation.

Alpha (α) / Beta (β) Subunits

Stator components of F₁; β carries catalytic sites while α assists in binding and structure.

Loose (L) Conformation

β-subunit state that binds ADP and Pi.

Tight (T) Conformation

β-subunit state that catalyzes formation of ATP.

Open (O) Conformation

β-subunit state that releases ATP and allows new substrates to enter.

P/O Ratio

Number of ATP molecules synthesized per oxygen atom reduced; ≈2.5 for NADH, ≈1.5 for FADH₂.

β-Oxidation

Fatty-acid degradation pathway whose electrons can reduce CoQ directly, bypassing Complex I.

Protonation/Deprotonation

Gain or loss of a proton by an amino acid side chain, central to proton pumping mechanics.

pH Gradient (~0.75 units)

Approximate difference between matrix (higher pH) and intermembrane space (lower pH) during respiration.

Isoprene Tail

Ten-unit hydrophobic chain anchoring CoQ in the lipid bilayer.

Hydrophobicity

Property enabling CoQ and other lipid-soluble carriers to diffuse within the membrane.

Cytochrome c Release

Leakage of cytochrome c into cytosol, initiating the intrinsic apoptotic pathway.

Apoptosis

Programmed cell death often triggered by mitochondrial dysfunction signals such as cytochrome c.

Protons per NADH (10 H⁺)

Total number of protons pumped by Complexes I, III, and IV for each pair of electrons from NADH.

ATP Yield per NADH (≈2.5)

Approximate number of ATP molecules synthesized when 10 protons re-enter through ATP synthase.