When living organisms eat, they ingest complex organic food materials. As these food materials enter the body, the process of digestion begins.
Enzymes in the body, located at different parts of the gastrointestinal tract see to the breakdown of these complex food materials ingested. The oxidation of these complex food materials lead to the release of the chemical potential energy stored in them.
A reducing agent is one which is oxidised in the reaction itself. When you take into account the oxidation numbers you will see that the Cl- ions are oxidised from an oxidation number of -1 to 0 in Cl2. Therefore Cl- ions are the reducing agent.
