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catalysis requires a protonated group, consistent with a general-acid contribution. Loss of its proton as pH rises would remove carbonyl activation or the proton needed to finish the conjugate reduction. A glutamate residue with a shifted active-site $p{K}_a$ is a plausible candidate, but a pH cutoff alone neither identifies the residue nor measures a precise $p{K}_a$. Irreversible unfolding or changes in substrate binding must also be excluded by reversible pH jumps, protein-integrity checks and initial-rate measurements.

For the mechanistic notation below, the reduced cofactor is written NADPH, the established cofactor of human steroid 5α-reductase. The original PDF explicitly labels it NADH; its abbreviated N-substituent does not display the phosphate distinction. NADH and NADPH have the same reactive reduced nicotinamide ring, so the local hydride-transfer arrows are unchanged, although their biochemical specificity is different.^[ref]

The historical name fermentation covered several transformations of organic matter: alcoholic, lactic, butyric and acetous changes, as well as some processes now classified separately. In modern biochemistry, fermentation more narrowly means energy-yielding metabolism that regenerates oxidized cofactors without requiring an external respiratory electron acceptor. The older use of the word should not be forced into that modern definition.

In alcoholic fermentation, suitable yeast converts a fermentable sugar into ethanol and carbon dioxide. For glucose, the principal net balance isA vessel with an outlet for the evolved gas and a separate receiver illustrates the change: test the carbon dioxide with limewater and identify the volatile alcohol in a separated distillate. Gas evolution alone does not prove that the liquid contains ethanol. Actual yeast cultures also make biomass and minor products; the chemical equation describes the principal conversion. In modern terms, glycolysis supplies ATP, and the reduction leading to ethanol regenerates NAD+ from NADH.

In lactic acid fermentation, an ideal homolactic balance isThe souring of suitable carbohydrate-containing materials, including milk, supplied a familiar historical example. Hydrolysis of lactose supplies monosaccharides, while acid production lowers the pH and can precipitate casein. Casein coagulation is a consequence of the changed conditions, not evidence that the protein has itself been converted into lactic acid. Some organisms instead make additional products, so not every lactic fermentation obeys only this homolactic balance.

In butyric acid fermentation, carbohydrates or lactate can give butyric acid, carbon dioxide and molecular hydrogen. An idealized balance from lactic acid isIt contrasts with the simple homolactic balance because both gaseous products and a different organic acid appear. Mixed cultures and alternative pathways can alter the product distribution; odour is neither a sufficiently specific nor a necessary laboratory test.

Acetous fermentation denotes the conversion of ethanol into acetic acid, as in vinegar production:It depends on oxygen and is now understood as an aerobic microbial oxidation, rather than a fermentation in the narrow biochemical sense. This explains why access to air matters differently here than in the main alcohol-producing stage.

Other older extensions of the word include saccharification or diastatic changes, such as the hydrolysis of starch into smaller sugars by malt enzymes, and sugar inversion, in which sucrose takes up water to form glucose and fructose:The two hexose products have the same elemental formula but different structures. Such hydrolyses prepare substrates for later metabolism; they do not by themselves constitute the alcohol-producing conversion. Viscous fermentation or mucilaginous fermentation described transformations yielding gum-like material and sometimes mannitol; it is an operational historical category without one universal balanced chemical equation. Putrefactive fermentation referred to the decomposition of nitrogen-containing organic matter, including proteolysis and subsequent reactions producing Ammonia, amines and sometimes hydrogen sulfide. It likewise has no single formula covering every mixed substrate and product.

The unifying historical interest is that small amounts of a “ferment” bring about major changes in a large amount of organic substrate. Modern explanation distinguishes organisms, enzymes and metabolic pathways. The later demonstration of fermentation in cell-free yeast juice showed that alcoholic conversion need not require intact living yeast cells in the reacting liquid; that result belongs to 1897, not to knowledge to demand of an 1866 candidate.^[ref]

Cells store usable free energy in chemical disequilibria: reduced carbon compounds, redox carriers such as NADH, ATP relative to ADP and inorganic phosphate, ion concentration gradients and membrane voltage. These reservoirs are maintained away from equilibrium; the useful quantity is the chemical or electrochemical potential difference, not simply a bond's isolated energy. Some ATP-consuming reactions can be driven because a coupled overall reaction decreases Gibbs free energy, even when the uncoupled synthesis would not.

Oxidation of nutrients transfers reducing equivalents to electron carriers and then to an electron-transport chain. Coupled proton pumping converts part of the redox free-energy drop into a protonmotive force. ATP synthase can convert that force into ATP chemical potential, and ATP hydrolysis drives biosynthesis, transport and mechanical motion. Substrate-level phosphorylation is another ATP source; photosynthetic cells can initially supply the disequilibrium with absorbed light. These are flows with unavoidable dissipation and heat release, not independent energy-creation processes. A reservoir is depleted unless metabolism replenishes it.

Glycolysis converts glucose into two pyruvate molecules, with a net two ATP and reduced NADH; it does not directly consume molecular oxygen. Pyruvate processing and the citric acid cycle, also called the TCA cycle, release carbon dioxide and supply reduced carriers. Thus releasing carbon dioxide and consuming molecular oxygen occur at different stages.^[ref]

An electron transport chain reoxidizes carriers such as NADH. Coupling some of its redox steps to proton transport establishes an electrochemical gradient; ATP synthase can use that gradient to form ATP from adenosine diphosphate and inorganic phosphate. This is oxidative phosphorylation. A separate route, substrate-level phosphorylation, transfers a phosphate group from a reacting substrate to adenosine diphosphate. In typical eukaryotic biological cells, the respiratory chain occupies the inner membrane of a mitochondrion.^[ref]

The useful achievement is controlled transfer: fuel oxidation can replenish an ATP supply that supports active transport and synthesis. ATP is a reusable chemical carrier, not newly created energy; ATP hydrolysis and the coupled cellular process have their own net free energy balance. Some fuel energy is dissipated as heat.^[ref]

There is no universal yield of $36$ or $38$ ATP per glucose. Actual ATP recovery depends on carrier transfer routes, coupling and diversion of intermediates to other metabolism. In particular, an oxygen uptake rate cannot be converted into an ATP production rate by assuming one fixed multiplier.^[ref]

In 1997, Hiroyuki Noji and colleagues attached a fluorescent actin filament to the central $\gamma$ shaft of isolated $F_1$. With ATP supplied, the marker made sustained rotations. This exposed rotation directly, but used ATP hydrolysis: there was no membrane $F_0$ motor producing ATP from a proton-motive force in this assay.^[ref]

A subsequent primary experiment made the optical readout much faster. Purified Escherichia coli $F_1$ was immobilized on glass in a $30\mu \mathrm{m}$-deep flow cell, with a $60\mathrm{nm}$ gold bead attached to $\gamma$. With $2\mathrm{mM}$ ATP and magnesium supplied, laser illumination and dark-field microscopy recorded bead positions at $4000$ or $8000$ frames per second. The position trajectory resolved turns and intervening dwells. This apparatus follows a shaft-attached marker, rather than imaging the shaft's chemical atoms.^[ref]

Controls addressed perturbation and chemical activity: comparison of $40$–$200\mathrm{nm}$ probes found similar rates for the two smallest sizes, supporting low drag; a separate coupled-enzyme assay followed ATP hydrolysis through NADH absorbance. But the rotational analysis excluded pauses longer than $0.1\mathrm{s}$, so it characterizes continuously active intervals, not total output including long inhibited periods. It neither counts transported protons nor measures simultaneous single-molecule ATP synthesis.^[ref]

The experimental distinction is essential. Sustained directional marker motion tests a rotary mechanism. Demonstrating proton-driven synthesis additionally requires intact $F_0{F}_1$ in a membrane with a controlled proton-motive force and a chemical ATP-production readout; dependence on that driving force and inhibition or respiratory uncoupling controls then test the proposed energy coupling. A rotation movie alone cannot establish the complete material and free energy ledger.