Krebs cycle starts with the formation of six carbon compound by a reaction between

Explanation

The atoms of the elements, substitute one or more hydrogen atoms in the hydrocarbon chain and thereby, the tetravalency of carbon is satisfied. The atom of the element which is substitute for hydrogen is referred as hetero atom. e.g. `CH_3OH`. In this example, one hydrogen atom in methane `CH_4` is replaced by-OH group. OH is a functional group. So, OH is a heteroatom.