thick filament consisting of 300 separate but attached myosin molecules. Interestingly, rigor mortis after death is the result of “running out of ATP molecules,” which are necessary to have actin and myosin separate from each other. This leads to permanent contraction of the muscles.
MUSCLE METABOLISM ATP is necessary for muscle contraction and must be available in large numbers for the muscle to function. ATP is necessary for both the actin-myosin contraction and for the active transport of calcium across the sarcoplasmic reticulum. There isn’t much ATP stored in muscle cells—only enough for about one second of a muscle contraction. For this reason, there are multiple mitochondria inside the muscle cell—each of which is a powerhouse for making new ATP molecules. These are essentially three mechanisms for ATP synthesis in the muscle cell. These are creatine phosphate metabolism, anaerobic metabolism (called glycolysis), and aerobic respiration (in the mitochondria). Creatine phosphate is a molecule that stores energy inside its phosphate bonds. When the muscle is resting, ATP transfers its energy to creatine in order to make creatine phosphate and ADP. This is the immediate muscle reserve for ATP. It’s the front line for making ATP molecules when the muscle contracts. The creatine phosphate transfers its stored energy back to ADP to make creatine and ATP through the action of the enzyme called creatine kinase. This gives only about 15 seconds of muscle energy. When ATP is depleted, the muscles will undergo glycolysis, which is an anerobic process, meaning it is independent of oxygen. It is not as quick as creatine phosphate metabolism and involves glucose metabolizing into two pyruvate molecules, leading to the production of two ATP molecules. When oxygen levels are low, the pyruvate becomes lactic acid as an end-product. If oxygen is available, the pyruvate will go on to the mitochondria to make many more molecules of ATP. Glycolysis can be sustained for one minute so it is useful in facilitating short bursts of energy output. About 95 percent of ATP for the exercising muscle is provided through the activity of aerobic respiration in mitochondria. This is 11
