There is a law in physics called the Conservation of Energy which says "energy cannot be created nor destroyed." Energy simply exists and is transformed from one form to another.
The many forms of energy include mechanical, kinetic, electrical, heat, potential, etc. The type of energy found in a glucose molecule is chemical.
Glucose is the sugar byproduct of carbohydrate digestion.
Through a process called photosynthesis, plants take the solar energy from the sun and convert it into a chemical energy. It stores this energy within the chemical bonds that link the carbon (C) atom from carbon dioxide (CO2), to a water molecule (H2O), forming a carbohydrate (CH2O).
Through a process called cellular respiration, your body catalyzes glucose, splitting it into smaller and smaller molecules until the chemical bond is broken and the energy within the bond is released.
Sixty percent of the energy released through cellular respiration is converted to heat energy. The remaining 40 percent is converted into a high-energy phosphate bond called ATP (adenosine triphosphate). ATP is the form of energy that actually powers your body's cells.