Organisms contain compounds such as proteins starches and fats the chemical bonds in these compounds can be a source of

These compounds (i.e. proteins, carbohydrates, and fats) are the nutrient building blocks of life. These compounds can be a source of energy in different ways.

Carbohydrates are digested to the most simple monosacharrides such as glucose, fructose, and maltose. Glucose is the main source of energy in life. Glucose can be used as energy when the body breaks it down by glycolysis, and cycles the end product (Acetyl-CoA) in the tricarboxylic acid cycle. These two processes involve the production of ATP (energy) and reducing compounds such as NAD and FAD. These reducing compounds are then used in the electron transport chain to produce more ATP. 

Fats, on the other hand, energy is derived from the process called beta-oxidation where chemical bonds of fats are oxidized to produce energy. The end product of beta oxidation is also Acetyl-CoA but more Acetyl-CoA is produced in fats than glucose. Acetyl-CoA will then enter the tricarboxylic acid cycle and produce ATP and reducing compounds in which the latter will be used in the electron transport chain. Some fats also produce ketone bodies and can also be used as energy.

Lastly, proteins are digested into their most simple form, amino acids. These amino acids, depending on what kind, is either glucogenic (can produce glucose by gluconeogenesis) or ketogenic (can produce ketone bodies). Glucogenic amino acids such as glycine, alanine, valine etc. can be used to form glucose to a process called gluconeogenesis. The glucose formed from these amino acids can now be used as energy by glycolysis, tricarboxylic acid cycle, and electron transport chain. Ketogenic amino acids (i.e. leucine, isoleucine) can produce ketone bodies through ketogenesis. 

If it doesn't have a nucleus its a prokaryote