Living organisms are composed of killions of organic compounds each having a unique structure what element

Carbon

Explanation:

Carbon has the ability to form a wide range of compounds. It has four valence electrons and thus can form single, double and triple bonds. It also has a tendency to bond with itself, forming long chains or cyclic structures. These bonding capabilities allow for many different combinations, resulting in the possibility of multiple unique compounds.

For example, a 4 carbon compound with hydrogens attached peripherally can still have 3 alternatives; it can be an alkane, an alkene or an alkyne, due to the fact that carbon can form different types of bonds.

the way that the mass of carbon can be conserved through cellular respiration is through a process/law called the law of conservation of mass which even tho it undergoes a process, mass is not be destroyed or changed

oxygen +1 most likely