Together creatine phosphate and ATP supply enough energy to sustain muscle contraction for approximately 15 seconds (100m race).
Anaerobic (without O2) Cellular Respiration
When muscle activity continues and the supply of creatine phosphate within muscle fibre is depleted, ATP is generated through a produce called anaerobic cellular respiration. This is the breakdown of glucose to produce ATP in the absence of oxygen. Glucose passes from the blood into contracting muscles via facilitated diffusion and is also produced by the breakdown of glycogen within muscle fibres. In a process called Glycolysis, 10 reactions quickly break down each glucose molecule into 2 molecules of pyruvic acid. These reactions use 2 ATP molecules but produce 4 yielding a net gain of 2 ATP. Pyruvic acid normally enters the mitochondria where a series of oxygen-requiring reactions produce ATP (Aerobic Cellular Respiration). During some activities, there is not enough oxygen and therefore anaerobic reactions convert most of the pyruvic acid to lactic acid in the cytosol. ~80% of LA produced diffuses out of skeletal muscle into the blood. Then liver cells convert some of the LA back to glucose. In addition to providing new glucose this process reduces the acidity of blood. Anaerobic cellular respiration produces enough energy for about 30-40 seconds of maximal muscle activity. The conversion of creatine phosphate (above) and glycolysis combined provide enough ATP to run a 400m race.
Aerobic (with O2) Cellular Respiration
Muscle activity that lasts longer than ½ minute depends increasingly on Aerobic cellular respiration. This is the breakdown of glucose to produce ATP in the presence of oxygen. It takes place solely in the mitochondria of the cell as mentioned above. In the presence of adequate oxygen, pyruvic acid enters the mitochondria where it is oxidized in reactions that generate ATP, CO2, H2O, and heat. This process is slower than anaerobic cellular respiration but produces much more ATP with each glucose molecule yielding 36 ATP. As long as oxygen continues to be available, aerobic cellular respiration will produce ATP continuously for an unlimited amount of time. Sufficient nutrients are required such as pyruvic acid obtained from glycolysis of glucose, fatty acids from the breakdown of triglycerides in adipose cells, and amino acids from the breakdown of protein. In activities that last longer than 10 minutes, the aerobic system described above provides 90% of the needed ATP. There are 2 sources of oxygen: oxygen that diffuses into muscle fibres from the blood and oxygen released by myoglobin within the muscle fibres.
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