An electron-transport chain (ETC) is an ordered set of membrane-associated redox carriers that pass electrons through a sequence of favorable oxidation-reduction reactions.

Instead of transferring reducing equivalents from one donor directly to a final acceptor in a single step, an ETC divides the overall redox process into several steps:

electron donor → carrier 1 → carrier 2 → carrier 3 → terminal acceptor

Each carrier is reduced when it accepts electrons and oxidized when it passes them onward.

Redox free energy can be captured as a proton gradient

If the overall electron transfer is thermodynamically favorable, part of that free-energy decrease can be coupled to moving protons across a membrane in the unfavorable direction.

Conceptually,

$$\text{favorable electron transfer}+\text{uphill H}^+\text{ transport}\rightarrow\text{coupled favorable process}.$$

The membrane then stores part of the original redox free energy as a proton-motive force.

Not every electron-transfer component must itself pump protons. Some carriers mainly transfer electrons, while particular protein complexes couple electron transfer to proton translocation.

Mobile and fixed carriers connect the chain

An ETC can contain both protein-bound cofactors and mobile carriers that move between redox complexes within or along the membrane. What makes them a chain is not physical contact in one rigid complex but repeated cycles of

carrier accepts electrons
        ↓
carrier changes position or redox state
        ↓
carrier donates electrons

that create net electron flow from donor toward acceptor.

Electron flow and proton flow are different processes

Electrons are transferred through redox reactions within the chain. Protons are moved across the membrane by coupled conformational or chemical changes in selected complexes.

It is therefore misleading to imagine that protons simply ride along with electrons through the same path.

The terminal acceptor determines the redox endpoint

An ETC requires a final species that can accept the electrons. Different biological systems use different terminal electron acceptors.

In aerobic respiration, molecular oxygen is the terminal acceptor and is reduced to water. Other respiratory systems can use different acceptors. Light-energy-capturing systems also use electron-transport chains, but their electron flow is initiated and driven differently.

The reusable architecture is

redox donor
   ↓ electrons
ordered electron carriers
   ↓ coupled proton pumping
proton-motive force
   ↓
terminal electron acceptor receives electrons

An electron-transport chain therefore converts redox free energy into a membrane electrochemical gradient rather than making ATP directly.