HUMAN ENERGY SYSTEMS

TOPICS COVERED IN THIS LESSON

• (a) Nutrients and ATP—The Energy Molecule

• (b) Two Systems, Three Pathways

What Are Nutrients?

Nutrients are chemical substances obtained from food and used by the body for many different processes. They are the raw materials our bodies need to supply energy, to regulate cellular activities, and to build and repair tissues. All organisms—including humans—require nutrients to perform their life functions and to obtain the energy necessary for survival.

The Three Key Energy Nutrients

The food we take in contains three key energy nutrients that are broken down over the course of digestion:

• Carbohydrates

• Protein

• Fats

Of these three key nutrients, carbohydrates are our most important source of energy.

The Central Role of Carbohydrates in Supplying Energy

Carbohydrates are the most abundant organic substances in nature, and they are essential for human and animal life. Sugars and starches are examples of carbohydrates.

The main functions of carbohydrates are to provide materials to build cell membranes and to provide energy for use by cells.

Glucose is the usual form in which carbohydrates are assimilated by humans. Glucose is stored within skeletal muscle and within the liver as glycogen.

ATP—The Common Energy Molecule

To be usable, nutrients in the food we eat need to be reconstituted (or resynthesized) into a universal form of energy—a “free energy” that can then be used for muscle contraction and many other physiological processes.

Adenosine Triphosphate (ATP) The final form this free energy takes is adenosine triphosphate, ATP—the common energy molecule for all living things. ATP captures the chemical energy resulting from the breakdown of food and is then used to fuel the various cellular processes in our bodies.

The Release of Energy from ATP

Energy is released when a trailing phosphate atom is broken from the ATP molecule. This results in ADP (adenosine diphosphate plus energy), as in the formula below: ATP —> ADP + P + Energy

The Problem of Resynthesizing ATP

In high demand by the body, ATP energy supplies are used up very quickly. The problem becomes how to resynthesize new supplies of ATP to ensure that bodily functions continue. There are two methods for resynthesizing ATP: • anaerobic (without oxygen) and • aerobic (with oxygen).

Two Energy Systems

The anaerobic system occurs without the requirement of oxygen. It can occur in two separate metabolic pathways, one not involving the breakdown of glucose and the other involving the partial breakdown of glucose.

The aerobic system, a separate but to some extent overlapping energy system, requires oxygen. It involves many enzymes and several complex sub-pathways, and it leads to the complete breakdown of glucose. (Fats and protein also enter the cycle at this stage.)

Three Metabolic Pathways

There are two energy “systems” (anaerobic and aerobic), but there are three metabolic “pathways” by which ATP energy reserves are restored. They are:

• ATP-PC (anaerobic alactic)

• Glycolysis (anaerobic lactic)

• Cellular respiration

In the presence of oxygen, the second pathway (glycolysis) is also the beginning of the third pathway (the aerobic system).

ATP-PC (Anaerobic Alactic)

This pathway draws on processes deep within the muscle fibre itself.

• It allows for quick, intense muscle contraction.

• It is “alactic” — lactic acid is not a byproduct.

Glycolysis (Anaerobic Lactic)

This pathway involves the partial breakdown of glucose, with lactic acid a byproduct.

• It does not involve oxygen and is therefore “anaerobic.”

• It allows for longer bursts of energy.

Cellular Respiration

The aerobic system (cellular respiration) is the main source of energy during endurance events.

• It involves oxygen and the complete breakdown of glucose.

• It yields large amounts of ATP

LESSON 7.1 SUMMARY

The main ideas include:

• Nutrients in food need to be resynthesized into a universal form of “free energy,” known as ATP, that can then be used for physiological processes.

• The body has two systems for resynthesizing ATP: anerobic (without oxygen) and aerobic (with oxygen).

• Within these two energy systems, there are three main metabolic pathways by which ATP energy reserves are restored: ATP-PC (anaerobic alactic); glycolysis (anaerobic lactic); and cellular respiration (aerobic).