ATP, which stands for adenosine triphosphate, is the sole source of energy for all human metabolism, yet very little of this fuel is actually stored in the body. Instead, the body has three different systems of ATP production: ATP-PC, anaerobic glycolysis, and aerobic phosphorylation.
Each system uses different starting fuels, each provides ATP at different rates, and each has its own downside (like fatigue). These differences mean that each method of energy production is best suited for particular kinds of activities.
The following table outlines the key characteristics of the body’s different ATP-producing methods.
| ATP-PC | Anaerobic Glycolysis | Aerobic (Oxidative) Phosphorylation | |
|---|---|---|---|
| Description | Provides ATP at a very fast rate. Your body holds limited stores of ATP-PC. | Provides ATP fast, but not as fast as ATP-PC. | Provides ATP at a slower rate than the other systems, but is great for endurance activities. |
| Starting Fuel | Phosphocreatine (PC) stored in the sarcomere. PC combines creatine and phosphate by using high-energy bonds. | Glucose stored in the muscle and liver in a concentrated form called glycogen. Glucose can be taken from muscle glycogen or transported from the blood via the liver. | Fats, carbohydrates, and proteins. |
| How Energy Is Produced | The chemical bonds that hold creatine and phosphate together are broken, a process that releases energy that can remake new ATP. | Enzymes in the cells convert glucose into lactic acid, producing ATP. Although ATP is needed to get glucose into the cell, you ultimately produce double the amount of ATP. | Fats and carbohydrates are delivered to the mitochondria and broken down to yield ATP. The waste product of a hydrogen ion (H+) is bonded to oxygen to form water. The other waste product is carbon dioxide (CO2), which can be breathed off. |
| Amount of Energy Produced | Enough for about 10 seconds of very high-intensity exercise. Total amount depends on stores of PC and enzymes to convert it to ATP. | Enough to power heavy exercise for extended periods (2 minutes or more). The amount depends on the availability of glucose and enzymes needed for energy production, and the levels of lactic acid. | The amount depends on enzymes, the availability of oxygen to the mitochondria, and the availability of carbohydrates and fats. With training, high levels of intensity for very long periods of time are possible (running a marathon at a 5 min/mile pace, for example). |
| Used Most for Activities Like | 100-meter sprint, short sprint, high jump, swinging a bat. | Intense activities lasting under 3 minutes, or during short bouts of heavy work. | Long-duration, low-to-moderate–intensity activities, like walking, jogging running, hiking, and swimming. |
| Cost or Tradeoff | When you run out of PC, you slow down or weaken. | Lactic acid builds up and causes the muscles to fatigue; it also shuts down glycolysis. | Work intensity is lower; running pace can’t be as fast as a sprint. Altitude or another condition that limits available oxygen (mountain climbing above 5,000 feet, for example) reduces performance. |
| How Training Maximizes these Fuel Sources | Increases stores and enzymes to make ATP faster. | Increases stores of glycogen and enzymes to make ATP faster and to better neutralize lactic acid. | Increases size and number of mitochondria and the number of enzymes to make ATP. |