ATP is the Energy Source
Believe it or not, I rode my bike from St. Louis, MO, to Chicago, IL, when I was in college. One of my favorite parts of that trip (as well as training for the trip) was eating as much as I wanted. I ate as much of whatever I felt like eating, and I was still fit as a fiddle. Our muscles comprise a large amount of our body mass, and they require enormous amounts of energy to contract. As my muscles were contracting a lot while riding my bike, they needed a lot of energy. Even at rest, our muscles require a lot of energy. Where does the energy come from? Muscular System: Muscle Metabolism Essay.
Ultimately, energy comes from the food we eat. Muscle cells, however, don’t use sugar, fats or proteins to contract. Rather, our cells convert the energy stored in those nutrient molecules into energy stored within ATP(adenosine triphosphate). That’s the universal energy molecule for living cells. ATP, in turn, provides the energy needed for muscular contraction. Exercising muscle gobbles up billions of ATP molecules every second. In this lesson, we’ll describe how our muscle cells utilize energy from nutrients to make ATP and, therefore, contract.
ATP and Creatine Phosphate
While resting, skeletal muscle makes more ATP than it needs. As ATP is not very stable, the excess ATP transfers energy to creatine. That’s a molecule made by our muscles from amino acids. This is the reaction: ATP + Creatine -> ADP + Creatine Phosphate (CP).
So, you see, the phosphate is transferred from ATP to creatine to make creatine phosphate. As creatine phosphate, or phosphocreatine, is more stable than ATP, it provides an effective way to store energy. During contraction, the contractile protein myosin breaks down ATP producing ADP and phosphate. The energy stored in creatine phosphate is then used to recharge the ADP as follows: CP + ADP -> Creatine + ATP.
So, you see, the phosphate is now transferred back to ADP to make ATP, and the ATP can be used for contraction. These reactions are catalyzed by the enzyme we call creatine phosphokinase (or CPK), and it’s located in the muscle cell. CPK leaks into the bloodstream when muscles are damaged. For example, this happens with a myocardial infarction, or a heart attack, that results in heart muscle damage. Clinical tests are used to measure circulating levels of CPK and thus, assess the level of muscle damage. Muscular System: Muscle Metabolism Essay.
It is important to note that ATP and CP reserves are exhausted within about 15 seconds of exercise. That’s not very long. Therefore, the cell must be able to generate, or synthesize, ATP if it is to continue working.
Glycolysis
The beginning of ATP synthesis in the cell is termed glycolysis. During glycolysis, energy is released from the breakdown of sugar, and it’s used to make ATP. Specifically, glucose is broken down to pyruvic acid (or pyruvate) in the cell’s cytoplasm. Glycolysis is an anaerobicprocess, as it does not require molecular oxygen. Let me quickly note that glycolysis can occur in the presence of oxygen, it simply doesn’t need it.
In the absence of molecular oxygen, however, glycolysis is the only source of ATP. During glycolysis, each glucose molecule is broken down into two pyruvic acid molecules. This is a multi-step process, and it produces four ATP molecules. However, two ATP molecules are used early in the process of glycolysis, and that yields a net gain of only two ATP per glucose going through glycolysis. That’s not very much. Let me quickly note that pyruvic acid is converted into lactic acid in the absence of oxygen. Lactic acid, at least in part, is responsible for muscle cramping.
Because only two ATP molecules are gained for each glucose molecule, we need a lot of glucose to support anaerobic metabolism. Glycogen is a large molecule made up of a whole bunch of individual glucose molecules, and it provides a store of glucose for the muscle cells. Eventually, however, our glycogen stores run out and aerobic synthesis of ATP is needed, which we’ll talk about next. Muscular System: Muscle Metabolism Essay.