17.2: Energy Systems

Learning Objectives

• Distinguish between the major energy pathways in the human body.

Introduction

Generating energy for life-sustaining and health-promoting processes is a dynamic process. Humans obtain energy through consuming a healthy, well-balanced diet. The ingested macronutrients —carbohydrates, fats, and proteins— are ultimately broken down in order to release the stored energy. Please recall: 1 g of carbohydrates or protein provides 4 kcal, whereas 1 g of fat provides 9 kcal.1

The energy can be used in different processes, such as building muscle mass, repairing muscle damage, active transport of many substances across cell membranes (e.g., sodium and potassium), as well as for mechanical energy, muscle action and, consequently, human movement. However, for all these processes to occur, the chemical energy released must be stored in the form of adenosine triphosphate (ATP).

Inside of the ATP molecule, the bonds that link the two phosphates together represent high-energy bonds because they release considerable useable energy: approximately 7.3 Kcal per molecule of ATP.2 In the specific case of muscle contraction, the myosin head contains a binding site for the molecule of ATP; the adenosine triphosphatase (ATPase), located here, splits the ATP to yield adenosine diphosphate (ADP), inorganic phosphate (Pi), and most importantly, energy.2 Therefore, ATP is the first energy source of mechanical muscle contraction processes.

However, cells can store only limited amounts of ATP: the human body only stores about 80-100 g of ATP at any given time under normal resting conditions.2 This implies that the body must be constantly generating new ATP to provide energy for all cellular ATP-dependent processes, including muscle contraction.

The human body can generate ATP through three different metabolic pathways -- depending on the immediate energy need and oxygen availability, the body produces ATP through:

1. ATP-CPr pathway
2. the glycolytic pathway
3. the oxidative system

From the three energy metabolic pathways, the first two can function in the absence of oxygen, and therefore are considered part of the anaerobic metabolism.
The last system requires the presence of oxygen and is therefore considered part of aerobic metabolism. The oxidative system comprises both carbohydrates and fat oxidation.2

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Phosphagen System

Although the most economical way for our bodies is to produce ATP under aerobic conditions through glycolysis or fat oxidation, these processes take time and require oxygen. So, when 'time' is of the essence and oxygen might be in short supply (e.g., you are attempting to escape a lion attack or are running a sprint), the body can use the ATP-Phosphocreatine (PCr) system to quickly restore at least some ATP from ADP.

The ATP-PCr pathway is an anaerobic energy system in which the generation of ATP is coupled with the exergonic (energy-releasing) breakdown of phosphocreatine stored in muscle cells. The breakdown frees inorganic phosphate, which then combines with ADP to form ATP. The ATP-PCr system is the quickest source of ATP for muscle actions. Athletes in power events lasting up to 10 s (e.g. 100 m sprints) derive most of their ATP from this system.4

Figure 9.2.1. In the ATP-PCr system, the energy liberated from the hydrolysis of phosphocreatine (PCr) is used to synthesized adenosine triphosphate (ATP) via the binding of inorganic phosphate (Pi) to adenosine phosphate (ADP).5

In order to regenerate ATP from ADP a phosphate source is required. As illustrated in Figure 9.2.1, the phosphate produced from the initial breakdown of ATP can be a source. Another source of phosphate is creatine phosphate (PCr), also called phosphocreatine. PCr is a high-energy compound formed in muscle cells when creatine (a nitrogenous organic acid found in foods and produced in the body) combines with phosphate. This phosphate can be released from creatine phosphate and added to ADP to form ATP. In addition, when the phosphate bond is broken, energy is released, which provides the fuel needed to restore ATP. These two sources of phosphate, ATP and PCr, can provide enough ATP to sustain a sprint for about 10 seconds.6

What happens next?

As the available ATP dwindles and creatine phosphate in the muscle cell is exhausted, the body relies on anaerobic and aerobic metabolism to produce the needed ATP. Although anaerobic metabolism produces more ATP per minute than aerobic metabolism, it is very limited in its use (it only provides about 1–1.5 minutes of maximal activity).6

Activities that primarily involve anaerobic metabolism are high-intensity, short-duration activities such as sprinting or heavy weightlifting. When the demand for ATP is greater than the rate at which metabolism can produce it, the activity slows down or may stop completely. This is one reason why individuals who lift weights have to rest between sets; it gives the body time to form more ATP to be used for the next set of lifts.

Aerobic metabolism produces less ATP per minute than anaerobic metabolism, but it can continue indefinitely. Low-intensity, long-duration activities, such as walking or slow jogging, primarily involve aerobic metabolism. The transformations that convert energy stored in carbohydrates, proteins, and fats into ATP are tightly integrated and somewhat complex. To learn this material, we break down these integrated processes by stages.6

References & Attributions

1. Gropper SS, Smith JL, Groff JL. (2016) Advanced nutrition and human metabolism (7th ed.). Cengage.
2. Sousa, A., Ribeiro, J. & Figueiredo, P. (2019). Physiological Demands in Sports Practice. 10.1007/978-3-030-10433-7_4.
3. Blake, J. S., Munoz, K. D., & Volpe, S. (2019). Nutrition: From Science to You (4th ed.). Pearson.
4. ATP-PCr system in The Oxford Dictionary of Sports Science & Medicine (3)
5. Sousa, A., Ribeiro, J. & Figueiredo, P. (2019). Physiological Demands in Sports Practice. 10.1007/978-3-030-10433-7_4.
6. Blake, J. S., Munoz, K. D., & Volpe, S. (2019). Nutrition: From Science to You (4th ed.). Pearson.

Additional contributions made by Claudia Kelley as part of the ASCCC 2020 OERI.

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