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Muscle Metabolism: Synthesis of ATP

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  • 0:01 ATP is an energy source
  • 1:32 ATP and Creatine Phosphate
  • 3:30 Glycolysis
  • 5:21 Aerobic metabolism
  • 8:06 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

Did you know our muscles use billions of molecules every second to contract? This lesson describes the different metabolic processes by which ATP is generated by the muscle cell. Examples are utilized to define glycolysis, the citric acid cycle and oxidative phosphorylation.

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?

ATP (adenosine triphosphate) provides the energy for muscular contraction.
adenosine triphosphate

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.

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 process of breaking down sugar used to make ATP is called glycolysis.
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 anaerobic process, 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.

Aerobic Metabolism

Aerobic metabolism simply refers to the synthesis of ATP utilizing oxygen. This accounts for about 95% of the ATP used under resting conditions.

Aerobic metabolism occurs within mitochondria of the muscle cells. When energy demands are relatively low and oxygen is readily available, the main job of glycolysis is to supply the mitochondria with pyruvic acid that the mitochondria will then use to make ATP. However, when energy demand is increasing and oxygen supply is limited - for example, at the beginning of exercise - glycolysis alone provides the ATP that we need for contraction.

With prolonged activity, the cardiovascular system will catch up and deliver enough oxygen so the cell can switch over to aerobic metabolism. That's a good thing because our glycogen stores are not unlimited. Furthermore, aerobic metabolism is more efficient than anaerobic metabolism. If you remember, in anaerobic metabolism only two molecules of ATP are gained per glucose, whereas with aerobic metabolism, each molecule provides enough energy for 34 molecules of ATP. That's a pretty good deal!

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