Question

how is respiration a combustion?

Answer 1

Combustion is the process of spontaneous oxidation (burning) of a substance in presence of oxygen to release energy.

Respiration is also a similar process, where glucose reacts with oxygen (from inhalation) and gets converted to CO₂ and water

with release of energy.

However it should be noted that combustion is a single step process whereas respiration is multi step process.