The process by which chemical reactions occur that break down food molecules into energy to be used by organisms is called cellular respiration. This is a complex process that occurs in the cells of all living organisms. Here's a breakdown of the key steps:

1. Glycolysis: This is the first stage and occurs in the cytoplasm of the cell. It involves the breakdown of glucose (a simple sugar) into pyruvate. This process generates a small amount of ATP (adenosine triphosphate), the cell's primary energy currency, and NADH (nicotinamide adenine dinucleotide), an electron carrier.

2. The Krebs Cycle (Citric Acid Cycle): This cycle takes place in the mitochondria, the cell's powerhouse. Pyruvate is converted into acetyl-CoA, which enters the Krebs cycle. This series of reactions produces more ATP, NADH, and another electron carrier called FADH2 (flavin adenine dinucleotide).

3. Electron Transport Chain: This is the final stage of cellular respiration and occurs in the inner mitochondrial membrane. The electrons carried by NADH and FADH2 are passed along a chain of proteins, releasing energy along the way. This energy is used to pump protons across the membrane, creating a concentration gradient.

4. Oxidative Phosphorylation: The protons flow back across the membrane through an enzyme called ATP synthase, which uses the energy from this flow to synthesize ATP. This is the main way that ATP is generated in cellular respiration.

Overall, the chemical reactions of cellular respiration break down glucose and other food molecules, releasing energy that is stored in the chemical bonds of ATP. This energy is then used by the organism to power essential life processes like muscle contraction, nerve impulse transmission, and protein synthesis.

There are two main types of cellular respiration:

* Aerobic respiration: This requires oxygen as the final electron acceptor in the electron transport chain. It is much more efficient than anaerobic respiration, producing far more ATP from each glucose molecule.

* Anaerobic respiration: This does not require oxygen and uses other molecules, like sulfate or nitrate, as the final electron acceptor. This process produces much less ATP than aerobic respiration.

Understanding cellular respiration is fundamental to comprehending how living organisms obtain and utilize energy.