We've all experienced the feeling of having "no energy." Sometimes it's because we haven't had enough sleep. More often, though, it's because we haven't eaten recently enough. Our bodies need food to survive. Food provides nutrients -- vitamins, minerals, and certain amino acids, to name a few -- that our bodies can't produce on their own. Even more important, food provides our bodies with energy -- the fuel our cells must have in order to continue functioning.

But our cells don't just burn tiny bits of food. Hamburgers and pizza are of no direct use to our cells. Food must first be digested, a process that breaks down food both physically, into smaller pieces, and chemically, into substances that are more easily absorbed by the blood. This process takes place in the stomach and the intestines. From there, food molecules, including carbohydrates, fats, and proteins, move into the bloodstream and are carried to the liver.

The liver breaks down food molecules even further. In many cases it converts carbohydrates, fats, and, if necessary, proteins into a simple sugar called glucose. Glucose is the molecule that every cell in your body must have in order to function. It is the raw material that drives a multistep process called cellular respiration. This chemical conversion, which is made up of the Krebs cycle and the electron transport chain, transforms glucose and oxygen into high-energy molecules of a substance called ATP. This important chemical fuels virtually every chemical reaction that takes place inside cells.

More than five dozen steps are required to convert glucose into ATP. Thankfully, this is not a one-to-one conversion. In fact, one molecule of glucose yields up to 38 molecules of ATP. Each of these molecules can be used, or "spent," easily and efficiently inside the cells to drive even the smallest chemical reactions. It is this efficiency, the ability to parcel out units of energy as needed, that drives the cellular respiration process. Just as it would be silly to leave a $500 bill for a tip because you haven't got anything smaller, it would be wasteful for a cell to use a whole glucose molecule to drive a small chemical reaction. Digestion and cellular respiration are the processes cells use to "get change" for their "big currency" food molecules.