In order for muscles to contract, ATP must be available in the muscle fiber. ATP is available from the following sources:
- Within the muscle fiber. ATP available within the muscle fiber can maintain muscle contraction for several seconds.
- Creatine phosphate. Creatine phosphate, a high‐energy molecule stored in muscle cells, transfers its high‐energy phosphate group to ADP to form ATP. The creatine phosphate in muscle cells is able to generate enough ATP to maintain muscle contraction for about 15 seconds.
- Glucose stored within the cell. Glucose within the cell is stored in the carbohydrate glycogen. Through the metabolic process of glycogenolysis, glycogen is broken down to release glucose. ATP is then generated from glucose by cellular respiration.
- Glucose and fatty acids obtained from the bloodstream. When energy requirements are high, glucose from glycogen stored in the liver and fatty acids from fat stored in adipose cells and the liver are released into the bloodstream. Glucose and fatty acids are then absorbed from the bloodstream by muscle cells. ATP is then generated from these energy‐rich molecules by cellular respiration.
Cellular respiration is the process by which ATP is obtained from energy‐rich molecules. Several major metabolic pathways are involved, some of which require the presence of oxygen. Here's a summary of the important pathways:
- In glycolysis, glucose is broken down to pyruvic acid, and two ATP molecules are generated even though oxygen is not present. The production of ATP without the use of oxygen is called anaerobic respiration, and, because no oxygen is used during the various metabolic steps of this pathway, glycolysis is called an anaerobic process.
- During anaerobic respiration, pyruvic acid is converted to lactic acid. Lactic acid (via liver enzymes) can be converted back to pyru‐vic acid and, with the presence of oxygen, pyruvic acid can enter the mitochondria.
Anaerobic respiration has advantages and disadvantages:
- Advantages: Anaerobic respiration is relatively rapid, and it does not require oxygen.
- Disadvantages: Anaerobic respiration generates only two ATPs and produces lactic acid. Most lactic acid diffuses out of the cell and into the bloodstream and is subsequently absorbed by the liver. Some of the lactic acid remains in the muscle fibers, where it contributes to muscle fatigue. During strenuous exercise, a lot of ATP needs to be produced. Since a person is exercising faster than they are bringing in oxygen, the body tries to make ATP using the anaerobic pathway. This results in the production of ATP and lots of lactic acid. After exercise, the liver and muscles need to convert the lactic acid back to pyruvic acid. In order to do that, a lot of the oxygen the body is now taking in does the conversion instead of being used elsewhere. This is known as “repaying the debt,” hence the term “oxygen debt.”
- In aerobic respiration, pyruvic acid (from glycolysis) and fatty acids (from the bloodstream) are broken down, producing H 2O and CO 2 (carbon dioxide) and regenerating the coenzymes for glycolysis. A total of 36 ATP molecules are produced (including the two from glycolysis). However, oxygen is required for this pathway.
Aerobic respiration also has advantages and disadvantages:
- Advantages: Aerobic respiration generates a large amount of ATP.
- Disadvantages: Aerobic respiration is relatively slow and requires oxygen.
When the ATP generated from creatine phosphate is depleted, the immediate requirements of contracting muscle fibers force anaerobic respiration to begin. Anaerobic respiration can supply ATP for about 30 seconds. If muscle contraction continues, aerobic respiration, the slower ATP‐producing pathway, begins and produces large amounts of ATP as long as oxygen is available. Eventually, oxygen is depleted, and aerobic respiration stops. However, ATP production by anaerobic respiration may still support some further muscle contraction. Ultimately, the accumulation of lactic acid from anaerobic respiration and the depletion of resources (ATP, oxygen, and glycogen) lead to muscle fatigue, and muscle contraction stops.