Your body uses sugar, known as glucose, as the raw material for a form of energy called adenosine triphosphate, or ATP. You get this sugar from your diet -- primarily from carbohydrate sources such as fruits, vegetables and whole grains. When you don't get enough carbohydrates from your diet, your body also breaks down fats to make energy. The process of making energy is called cellular respiration. In humans, the process of cellular respiration is also called the Krebs cycle.
Glucose Absorption
Your body absorbs glucose into your bloodstream through your mouth, stomach and intestines. Once the glucose enters your blood stream your pancreas releases insulin to trigger the cells to absorb the glucose. Inside the cells are structures called mitochondria, also called the powerhouse of the cell. These structures are responsible for converting glucose to ATP.
Fat Absorption
Your body absorbs fats into your bloodstream through your intestines. Once the fat, in the form of fatty acids, enters your bloodstream your body uses it for several functions, such as making hormones, nerve conduction and making cell walls. Whatever your body does not use is stored in your fat cells. Excess glucose is also converted and stored as fat. If necessary, your body will also use dietary fat and break down body fat to convert to ATP.
Cellular Respiration: Sugar
Sugar is broken down in the cytoplasm, or fluid matrix, of the cell by a process called glycolysis. The end result is a substance called pyruvate, another carbon-based molecule. Each six-carbon chain of sugar converts to two molecules of pyruvate. The pyruvate enters the nucleus of the cell and is transported to the mitochondria. Inside the mitochondria, the pyruvate loses one carbon dioxide to become acetyl CoA. The mitochondria converts the acetyl CoA to adenosine diphosphate, ADP, and then to ATP. The mitochondria makes two or three ATP for every molecule of pyruvate.
Cellular Respiration: Fat
Your body cannot use raw fat for energy production; it has to transform the fat into substances that can be converted into energy. When your body taps into its fat stores for energy, it transports the fat to your liver where enzymes break it down into glycerol and fatty acids. Glycerol is a type of sugar alcohol that the body uses in the Krebs cycle in place of sugar. The body converts the fatty acids into acetyl CoA, which enters another part of the Krebs cycle to become ATP.
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