Human body is a sophisticated machine. Like any other machine, human body needs fuel to provide energy for its nonstop operation, especially for exercise, which consumes at most 200 times more energy than quiet activities. We know that the food we take in every day is the fuel, but how exactly is energy generated and converted? It’s actually a long and complex process. Through digestive system and a series of metabolic processes, nutrients in food will be converted into ATP (adenosine triphosphate) - the only compound that powers our cells.
From cell development to explosive muscle contractions in the case of exercise, ATP energizes all of our bodily processes. It consists of adenosine and three phosphate groups. ATP releases energy only when it is hydrolyzed by enzymes contained in all of our cells, especially in muscle fibers. During hydrolysis, the ATP molecule releases energy to power our muscles. However, muscle fibers contain only a very limited amount of ATP. When we exercise, its power will be exhausted in about 2-3 seconds. In order to keep the exercise going, muscle fibers use several other mechanisms to produce, move, and store ATP depending on the intensity and duration of muscle activity, including free ATP and the phosphagen system, glycolysis, and oxidative phosphorylation.
First: Free ATP and the phosphagen system--explosive, fast, short time
Free ATP and the Phosphagen system is the fastest way to produce ATP. When the exercise starts, free and available ATP is rapidly depleted. Meanwhile, phosphocreatine, originally stored in muscle fibers, will be decomposed, and release energy at the same time. The released energy can help ADP to re synthesize ATP. Nevertheless, neither ATP or phosphocreatine is much stored in muscle. As a result, this system doesn’t last long but provides a great deal of energy for the first few seconds of high intensity exercise like sprint.
Second: Glycolytic system--medium power and time
This system mainly produces ATP from glucose (which comes from most of the food and fruit you eat) through a series of reactions. It takes place when ATP and phosphocreatine are about to run out. Although lasting longer than last system, glycolytic system can only hold for about 30 seconds due to the limited reserves of glucose and other involved substance.
Third: Oxidative phosphorylation--slow but long lasting
Our illustration has so far been focusing on energy that comes from food, but neglecting another elementary need of human body--oxygen. During exercise we are gasping for much air, but both ATP-PC system and glycolytic system are anaerobic system (no oxygen required). That is to say, all the oxygen we inhale is used for the slowest but most reliable and efficient energy system--oxidative phosphorylation.
Oxidative phosphorylation produces ATP with the help of oxygen and energy generated by the digestion and metabolism of carbohydrates, fat ,or protein. The whole process takes place at the internal membranes of the mitochondria. It is named oxidative phosphorylation because oxygen is required to charge the mitochondrial membrane. In addition, the Krebs cycle reaction provides energy to produce ATP from ADP and inorganic phosphate. Oxidative phosphorylation primarily use fat as its fuel, but when fat is not enough, it may use carbohydrates or amino acids instead.
The process of oxidative phosphorylation is quite time consuming due to its complexity. If the exercise we do is not so intense, like jogging or fast walking, ATP will be consumed at a slower speed, allowing more time for ATP re synthesis. Therefore, as long as we get sufficient supply of oxygen, sugar, protein and fat, the energy from oxidative phosphorylation system can sustain these exercise for a long time.
The three system mentioned above are not carved up but intertwined. While one is holding the steering wheel, the other two might be giving a little help. Together, they provide energy for all our exercise and activities.