Let's trace how your body processes a piece of bread. Step 1: You eat the bread, which contains starch. Step 2: Your digestive system breaks starch down into small sugar molecules called Glucose. Step 3: Glucose travels in your blood to your muscle cells. Step 4: Inside the cell, in a part called the mitochondria, Catabolism happens. Step 5: The cell breaks down Glucose using oxygen. Step 6: This process creates ATP (energy) and releases Carbon Dioxide as waste. Step 7: You breathe out that waste, and your muscles use the ATP to contract so you can walk.