The term "redox" is a combination of the words reduction and oxidation. It refers to all reactions that involve the transport of electrons from an electron donor to an electron acceptor.

A molecule loses electrons when it is oxidized, and gains electrons when it is reduced. It is easy to remember the processes with the two words OIL RIG: Oxidation Is Loss and Reduction Is Gain.

Each electron transfer in the electron transport chain is a redox reaction. The electron donor (i.e. Plastoquinone) gets oxidized and the electron acceptor (i.e. Cytochrome b6f) is reduced. In the case of the hill reaction, plastoquinone reduces DCPIP!

Each redox reaction of the electron transport chain is exothermic. This means the potential energy of the electrons decreases. The energy released through these reactions is converted into a different form of energy, a proton gradient (or, proton motive force).

In photosynthesis, in photosystem II for example, the plastoquinone is reduced with two high-energy electrons, which leads to the uptake of two protons from the stroma. In close proximity to the cytochrome complex, the plastoquinone is oxidized because the electrons are able to jump to a lower energy state. Imagine this like rolling balls on stairs. The plastoquinone is the top stair and cytochrome is one stair further down. If the balls have the chance to drop down to the lower stair, they won’t come back up anymore. The reaction site of the cytochrome is towards the thylakoid side of the membrane. That’s why the protons will be released into the thylakoid lumen; hence, the energy is converted into a proton gradient.