When you eat food such as a hamburger, your digestive system sets out to reduce the solid form of the food to a liquid state called chyme. It does this by tearing the hamburger into smaller pieces, mixing it with saliva, stomach acid, and digestive enzymes in the stomach. The chyme is introduced into the small intestine where the glucose (carbohydrate sugar) is absorbed into the bloodstream, where it is delivered to every cell in your body. The glucose is used by the cells in a process called cellular respiration, which combines the glucose with oxygen, illustrated in the following equation:
glucose + oxygen ---> carbon dioxide + water + energy
Breaking apart the large glucose molecule gives the cell the energy it needs, plus gives off carbon dioxide and water vapor, which are exhaled as waste products.
Your carbon atom is probably part of a carbohydrate, so it will begin to be digested in the mouth. Salivary amylase, an enzyme secreted by the salivary glands, will start to break down starchy carbohydrates into simpler sugars while you chew. These simpler sugars are further reduced to glucose in the small intestine. The glucose molecule containing your carbon atom (along with five other carbons) will be passed through the wall of the small intestine into the blood stream. Here it becomes part of the blood glucose.
A molecule of blood glucose can have one of two fates: It can be stored in an adipose cell in the form of fat and saved for use later, or it can be taken up by a working cell. If the glucose your carbon is part of is absorbed from the bloodstream by a working cell, it will be broken down into two three-carbon molecules called pyruvate, by a process called glycolysis.
The pyruvate will then enter a mitochondrion, and will be passed through the Kreb cycle. The three carbons will be seaprated, and the energy released by the separation will be used to power the cell. Each carbon will be attached to an oxygen molecule, creating a molecule of carbon dioxide.
The carbon dioxide is passed back out to the cell, back to the bloodstream, and from the bloodstream to the air sacs inside the lungs, from which it is exhaled.