How Energy-Containing Molecules Support Physical Activity

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  • 0:03 ATP
  • 1:16 Anaerobic Metabolism
  • 5:06 Aerobic Metabolism
  • 6:35 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Physical activity requires energy. Adenosine triphosphate (ATP) is the form of energy your body likes to use. In order to keep enjoying your physical activity, your body must continually make ATP. Learn how this is done both with and without oxygen.


Whether you step out of your house for a leisurely walk around the neighborhood, join in with your friends for a backyard volleyball game or run a marathon, you are engaged in physical activity. 'Physical activity' is a broad term that is defined as any movement of your muscles that requires energy. The raw energy that allows you to be active comes from eating the three energy-yielding nutrients - carbohydrates, proteins and fats - along with some potential energy reserves stored in your body. But the raw energy you get from these sources is not in a form that is usable by your body, so before your body can benefit from these energy sources, their energy must be converted into the form your body likes - namely adenosine triphosphate, which is better known as ATP. ATP is the form of energy your body uses to perform bodily functions. ATP is a high-energy-containing molecule, and it might help if you think of ATP as your body's fuel, much like gas is the fuel used by your car. In this lesson you will learn how your body gets ATP from the available raw energy sources so you have fuel to meet your energy needs during periods of physical activity.

Anaerobic Metabolism

The energy-containing molecule ATP can be made by your body in the presence of oxygen or in the absence of oxygen. We use the term anaerobic metabolism to describe energy creation in the absence of oxygen. This is a fairly easy fact to remember if you recall that the prefix 'an' means 'without' and the word 'aerobic' refers to 'oxygen,' so anaerobic metabolism allows your body to generate usable ATP without oxygen.

This is handy when you need a sudden burst of intense exercise or for the first few minutes of exercise when your body is still catching on that you upped your physical activity level. For example, when you lace up your jogging shoes and head out the door, your muscles move from a relatively sedentary state to an active state. This increase in activity requires more energy, but your heart and lungs have not yet had time to increase the amount of oxygen available.

Fortunately, your muscles have a small reserve of ATP in storage and you don't need any oxygen to retrieve this energy. Of course, the key word here is 'small' because this reserve of ATP is used up after only a few seconds of activity. Yet you do not crash because you have another energy-containing molecule stored in your muscles that can be broken down easily to make ATP called creatine phosphate (CP).

Did you notice that the word 'phosphate' is contained in both 'adenosine triphosphate' and 'creatine phosphate'? That's an important observation. In fact, you might want to think of creatine phosphate as a phosphate donor that gives broken-down ATP new life. You see, when your body needs energy it breaks down ATP. This process involves the ATP molecule giving away one of its three phosphate groups (notice that it's adenosine triphosphate, which means it has three phosphate groups). After the ATP molecules from your muscle break down, creatine phosphate jumps to the rescue and donates its phosphate group. This regenerates ATP, allowing your activity to continue.

Great system! But, there's a catch; you don't have much creatine phosphate in storage, so it isn't more than about 15 seconds until your muscles are empty. If your physical activity continues, your muscles must get more energy, but your heart rate and breathing have still not had enough time to boost oxygen levels. So your muscles must make more ATP anaerobically. This stage of anaerobic metabolism makes ATP very quickly, but it can only use glucose to make the molecules. Glucose is simple sugar that your body gets from breaking down the carbohydrates that you eat. You also have some stored glucose in your muscles called glycogen that can be used to make ATP in this way.

It's amazing how your body keeps coming up with quick ways to supply you with energy for your physical activity, but I hate to say that this system also has its drawbacks. First of all, this is a fairly inefficient method of energy production, as every molecule of glucose produces only two molecules of ATP. Secondly, the process results in lactic acid production. You can think of lactic acid as an end product of anaerobic metabolism, and if you don't get oxygen soon, it will also end your exercise session. This is because high levels of lactic acid can interfere with muscle contractions and produce muscle pain. This lactic acid build up limits your energy system, requiring you to either stop or greatly reduce your intensity.

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