Biology · Energy, Enzymes & Metabolism

Oxidation, reduction and electron carriers

When a cell processes glucose, not every electron is passed straight to oxygen. Carriers connect reactions. Follow what gives electrons, what receives them and what must happen before the carrier can be used again.

Read the Chemistry of Life and Cells & Membranes chapters first if atoms, molecular interactions and membrane transport are unfamiliar. The necessary ideas are also introduced here. You do not need to memorise complete biochemical pathways or calculate thermodynamic quantities to follow the first reading.

Back to the Biology map

By the end, you can

Track an electron donor, acceptor and reusable carrier without equating oxidation with oxygen use.

Two changes belong to one transfer

Oxidation is loss of electrons; reduction is gain of electrons. In a redox reaction, the donor is oxidised and the acceptor is reduced. In covalent molecules, changes in electron sharing can be described using oxidation states, rather than imagining each electron as a free particle swimming between molecules.

Oxygen is one possible acceptor, but oxidation does not require oxygen. The terms describe electron changes. A species that donates electrons is a reducing agent because it reduces another species; it is itself oxidised. Naming the agent can seem backwards until you keep the donor and recipient in the same account.

A carrier must be restored

NAD+ and NADH are oxidised and reduced forms of the same carrier. In many enzyme-catalysed reactions, NAD+ accepts a pair of electrons with a proton equivalent, forming NADH; another proton may enter the solution. A useful half-reaction account is NAD+ + 2 e− + H+ → NADH. A complete reaction also includes the molecule donating those electrons.

NADH can transfer reducing equivalents in another reaction and return to NAD+. The diagram shows the cycle without copying the carbon-containing donor into NADH. If NADH accumulates because it cannot be reoxidised, the supply of NAD+ becomes limiting for reactions that require it. Carrier recycling connects pathway steps.

Use a carrier, then restore its oxidised form

The half-reaction is electron bookkeeping. Actual transfers occur through specific enzyme-catalysed reactions, not free electrons circulating in a cell.

Read the diagram in words

A donor is oxidised while NAD+ accepts reducing equivalents to become NADH. At a second enzyme-catalysed step, NADH donates reducing equivalents to an acceptor and returns to NAD+. Solid arrows track the carrier cycle; dashed arrows show the transfer of reducing equivalents. The donor's carbon skeleton is not transferred into NADH.

Donor: Oxidised at the first step
NAD+: Oxidised carrier
NADH: Reduced carrier
Acceptor: Reduced at the second step

Use a carrier, then restore its oxidised form
Description	Process
Donor → NAD+	Donates reducing equivalents
NAD+ → NADH	Reduction
NADH → Acceptor	Donates reducing equivalents
NADH → NAD+	Oxidation restores the carrier

Carrier half-reaction: NAD+ + 2 e− + H+ → NADH
First reaction
Second reaction

Electron flow and energy transfer are related accounts

NADPH is a related carrier used widely in biosynthesis and photosynthesis; the additional phosphate helps enzymes distinguish the carrier pools. FAD is another carrier with a reduced form called FADH2. These names are not interchangeable labels for one substance. Their actual roles depend on enzymes, compartments and pathways.

Whether a transfer is favourable depends on both redox partners and conditions. 'An electron carries energy' is incomplete unless we specify the states and the transfer it participates in. In aerobic respiration, a sequence of transfers towards oxygen can support proton pumping and ATP synthesis. The electrons do not turn into ATP atoms: energy transfer and matter transfer remain distinguishable.

Worked example

Read a donor–carrier pair

Imagine an enzyme oxidising a carbon-containing substrate while reducing NAD+ to NADH. The substrate loses reducing equivalents; the carrier receives them. At a later step, NADH gives reducing equivalents to a suitable acceptor and is oxidised back to NAD+.

The carrier has linked two transformations without being permanently consumed. If the later step stops, replenishing substrate alone does not solve the bottleneck: the first enzyme still needs an oxidised carrier. This is a qualitative pathway argument, not a prediction of a particular rate or an identified experimental outcome.

Check your understanding

A molecule gives electrons to NAD+. Which partner is oxidised and which is reduced?
Show reasoning
The donor is oxidised; NAD+ is reduced to NADH. Both descriptions refer to the same electron-transfer event.
Must a reaction use oxygen to count as oxidation?
Show reasoning
No. Oxidation concerns electron loss or increased oxidation state. Other acceptors can participate.
Why can a shortage of NAD+ slow a pathway even when there is plenty of glucose?
Show reasoning
Some reaction steps require NAD+ as an electron acceptor. A supply of substrate cannot substitute for the needed carrier; NADH must be reoxidised or NAD+ otherwise replenished.

What to carry forward

Follow donor, acceptor and carrier regeneration together; oxidation and reduction are paired descriptions.

References

Resources supporting this lesson’s explanations and examples.