In August 2008, at the 29th Olympic Games held in Beijing, Chinese female athlete Liu Chunhong lifted a 158 kg barbell in the clean and jerk, breaking the world record. This is equivalent to lifting a moderately heavy man with one hand. Why do human muscles have such great strength? Just as an electric motor needs electricity to operate, and electricity is mainly generated by burning coal or oil in power plants, human muscles also need a "high energy compound" ATP to drive their movements. The "fuel" for ATP production in cells is mainly glucose. Animals mainly obtain glucose by digesting starch (a polymer of glucose) in food.
You might ask, why not use fat as the main "fuel" since it has a "high calorie value" of 39 kJ/g, while glucose only has 17 kJ/g? The reason is that life on Earth originated in water, so all life on Earth uses water as the medium for chemical reactions. Although fat contains a lot of energy, its solubility in water is very low, and it is difficult and slow to transport and enter cells in the bloodstream, which cannot meet the rapid energy demand of cells. Therefore, it can only be used in situations where muscles do not perform intense contractions.
In contrast, glucose is highly soluble in water and does not require energy to enter cells. Since single-celled organisms, glucose has become the main "fuel" for organisms to obtain energy, and a highly efficient "chemical reaction chain" for oxidizing glucose has been developed. Through this "chain", glucose is oxidized into water and carbon dioxide, and the released energy is used for ATP synthesis. Its efficiency is very high, with 1 molecule of glucose able to produce 32 molecules of ATP, making its energy conversion efficiency comparable to that of car engines. The human body has also "inherited" this "reaction chain", which occurs in an organelle called the "mitochondria" within cells, so the mitochondria are called the "power plants" of cells.
However, relying solely on this supply mechanism is not safe enough. In emergencies, if a person needs their muscles to perform intense activities for a long time, the supply of glucose in the blood may not be sufficient. Some people may say, if we just increase the concentration of glucose in the blood, wouldn't it provide a more sufficient supply to cells? However, glucose not only "sweetly benefits" the human body, but also has a "bad habit" of combining with amino acids (-NH2) on the side chains of protein molecules, damaging the normal function of proteins. High concentrations of glucose in the blood can lead to a series of adverse consequences, and diabetes occurs as a result of this. Therefore, the concentration of glucose in the blood must be controlled at around 5 mmol, which is about 90 mg of glucose per liter of blood. Although the concentration of glucose in the blood cannot be increased, the human body is very "clever" and has several "back-up measures".
First, increase the blood flow in the muscles. During intense exercise, the amount of blood flowing through the muscles can be more than 20 times higher than usual. Although the concentration of glucose in the blood has not increased, the "visitation" of a large amount of blood can provide more glucose and increase the oxygen supply to the muscles, allowing glucose to be fully "burned".
Second, convert glucose into glycogen and store it as a reserve material in muscle cells and liver cells. In emergencies, liver glycogen can be converted back into glucose and supplied to muscle cells. However, muscle cells are not as "selfless" as liver cells. They only "take in and don't give out" glucose, and muscle glycogen is only for their own use. In emergencies, when the demand for muscle glycogen is high, some ATP is synthesized through the "glycolysis" step while converting it into lactic acid. Therefore, after intense exercise, people feel muscle soreness, which is caused by the conversion of muscle glycogen.This is because lactic acid accumulates in the muscles.
Thirdly, converting ATP into phosphocreatine is equivalent to exchanging RMB for "foreign currency" and storing it away, then converting it back when needed. If muscle cells "retain" these reserves, they are generally less likely to experience energy shortages.