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Low carb, high protein, paleo—diet trends often single out a nutrient group as the culprit of unwanted weight gain and an unhealthy lifestyle. But our body needs food for energy, and all three groups—carbohydrates, fats and proteins—have important roles to play. What is energy to the body, and how does the body turn what we’ve eaten into a form it can use?

To the body, energy is a molecule called adenosine triphosphate (ATP). Many of the body’s processes need ATP to happen. For example, for a muscle to contract, ATP needs to be on the muscle cells’ moving parts for the contraction to continue. Rigor mortis, the stiffening of muscles after death, happens because no ATP is around.

Cells have three ways to convert food into ATP: glycolysis, the Krebs cycle and oxidative phosphorylation. Each pathway processes specific nutrient groups: Glycolysis only uses carbohydrates, while the Krebs cycle and oxidative phosphorylation can use all three. In each pathway, the nutrient is broken down in a multi-step process by specialized proteins, and new ATP molecules are regenerated from used ATP along the way. Besides using different transformation steps, the reaction pathways also take place in different parts of the cells. The proteins that carry out glycolysis float throughout the cell, which means glycolysis can happen anywhere. The proteins for the Krebs cycle and oxidative phosphorylation, on the other hand, reside in cell compartments called mitochondria, so these conversions only happen there.

For most cells, glycolysis produces the least amount of ATP. Most of the ATP formation occurs through oxidative phosphorylation and the Krebs cycle. This is why the mitochondria, where these reactions occur, are commonly referred to as the powerhouses of the cell. Both oxidative phosphorylation and the Krebs cycle need oxygen to work, and the Krebs cycle releases carbon dioxide as a byproduct. As a result, most of the oxygen we breathe in goes to the mitochondria for these reactions, and most of the carbon dioxide we’re breathing out comes from them.

What’s the chemistry behind ATP’s energy? Watch this video from Khan Academy for details.

Maggie Kuo, PhD, is the former Communications and Social Media Coordinator for APS. Catch more of her writing in the Careers Section of Science Magazine.