Hello Bharat,

Cellular respiration is a process by which cells of plants and animals breakdown sugars and turn them into energy. Cellular respiration makes use of NAD (Nicotinamide adenine dinucleotide) coenzymes which are present in the cells in its oxidized form NAD+. NAD+ acts as an electron carrier that accepts the electrons from other molecules and is converted into a reduced form NADH. Therefore it also acts as a reducing agent.

Hope this is clear to you!