Where does the energy released in cellular respiration originally come from?

Answer 1

The energy that is released during cellular respiration comes from chemical bonds. When these bonds are broken, free energy is released.

Much of this energy is lost as heat, but some is trapped in new bonds, especially in the molecule ATP. Each time a large packet of energy is released during respiration, the cell synthesizes a molecule of ATP.

The cell needs a high-energy chemical compound to start with, just as a car needs a high-energy fuel in the tank. One such compound is glucose, which has a significant amount of energy in its bonds. That energy got there when a plant trapped some light energy and used it to synthesize sugar from simpler compounds.

Other high-energy compounds, such as glycogen and fats (lipids) can be used for cellular respiration. They are converted to glucose (in the case of glycogen and some other polysaccharides) or to some other compound in the respiration pathway.

Answer 2

When the covalent bonds break (from the glucose splitting), electrons are released.