Reductant

A fundamental redox reaction is "self-exchange", which involves the degenerate reaction between an oxidant and a reductant.

In industry, hydrogen is the preferred reductant, whereas in the laboratory, tin and iron are often employed.

In redox reactions one reactant, the oxidant, lowers its oxidation state and another reactant, the reductant, has its oxidation state increased.

These are linked by plastoquinone, which does require energy to reduce cytochrome f for it is a sufficient reductant.