In biological systems, most metabolic pathways are interconnected. The cellular respiration processes that convert glucose to ATP—such as glycolysis, pyruvate oxidation, and the citric acid cycle—tie into those that break down other organic compounds. As a result, various foods—from apples to cheese to guacamole—end up as ATP. In addition to carbohydrates, food also contains proteins and lipids—such as cholesterol and fats. All of these organic compounds are used as energy sources to produce ATP.

Carbohydrate Digestion

The human body possesses several enzymes that break down carbohydrates into simple sugars. While glucose can enter glycolysis directly, some simple sugars, such as fructose and galactose, are first converted into sugars that are intermediates of the glycolytic pathway.

Protein Digestion

Proteins are broken down by enzymes into their constituent amino acids, which are usually recycled to create new proteins. However, if the body is starving or has a surplus of amino acids, some amino acids can lose their amino groups and subsequently enter cellular respiration. The lost amino groups are converted into ammonia and incorporated into waste products. Different amino acids enter cellular respiration at various stages, including glycolysis, pyruvate oxidation, and the citric acid cycle. Amino acids can also be produced from intermediates in cellular respiration processes.

Fat Digestion

Lipids, such as cholesterol and triglycerides, commonly known as fats, can also be produced and broken down in cellular respiration pathways. Triglycerides, for example, are composed of glycerol and three fatty acids. Phosphorylated glycerol enters glycolysis. Fatty acids enter the citric acid cycle after being converted into acetyl CoA through a series of reactions called beta-oxidation.

Biochemical energy, in the form of ATP, can be obtained from carbohydrates, proteins, or lipids.