Making nearly half the body’s mass, muscle required a tremendous amount of energy in order for it to perform its functions. The most distinguishing functional characteristics of muscles is that they have the ability to convert ATP into mechanical energy, which they then use as fuel in doing their function.
Providing Energy for Contraction
ATP is the primary source of power of the muscles. When the muscles contract, the ATP provides them with the necessary energy to do cross bridge movement and detachment and also aids in the operation of calcium pumps. Taking this into consideration, a question arises as to why the muscles only store little amounts of ATP reserves. The reserve ATP in the muscles will only last for about 6 seconds, barely enough to get you started on what you’re doing.
Since ATP is the only source of energy the muscle uses, it must therefore be produced as fast as the rate it is being used. There are three possible ways in which ATP is regenerated and these include the interaction of creatine phosphate to ADP (the hydrolyzed form of ATP), from glycogen through glycolysis and by way of aerobic respiration.
Direct Phosphorylation of ADP
When doing vigorous activities, the muscle’s demand for ATP increases rapidly and the stored ATP is consumed in just a split second. When this happens, creatine phosphate is called into action to regenerate ATP. The creatine phosphate couples with ADP, resulting in a burst of energy transfer and forming ATP. This is the reason why the muscle only has a limited stored amount ATP because it triples the amount of stored creatine phosphate.
Anaerobic Mechanism – Glycolysis
While ATP is being produced by action of the creatine phosphate, more ATP is produced by catabolism of glucose. The glucose comes from blood or by breaking down stored glycogen in the muscles. Glycolysis is the initial phase of this breakdown and during this phase, glucose yields two pyruvic acid molecules and two ATP molecules.
Under normal conditions, the pyruvic acid is delivered to the mitochondria and interacts with oxygen, producing more ATP. However, when the vigorous activity of the muscle goes on for a long time, the muscles bulges and when this happens, the muscles compresses the blood vessels. Upon compression of the blood vessels, delivery of oxygen is impaired and the conversion of pyruvic acid to ATP is halted. Pyruvic acid is then converted to lactic acid by way of anaerobic glycolysis.
Comparing the two pathways, anaerobic pathway is able to produce only 5% ATP from glucose when compared to aerobic pathways; however, the production time in anaerobic pathway is 2 ½ times faster.
95% of the ATP being used by muscles in moderate activities and even during rest comes from aerobic respiration. This happens in the mitochondria and it requires oxygen for this to happen. As glucose is broken down, it yields carbon dioxide, water and most importantly large amounts of ATP as its final product.