Glycolysis: Energy for Sprinting
Glycolysis is the predominant energy system used for all-out exercise lasting from 30 seconds to about two minutes, and is the second fastest way to resynthesize ATP. During glycolysis, carbohydrate, either in the form of blood glucose (sugar) or muscle glycogen (the stored form of glucose) is broken down through a series of chemical reactions to form pyruvate (glycogen is first broken down into glucose through a process called glycogenolysis). For every molecule of glucose broken down to pyruvate through glycolysis, two molecules of usable ATP are produced. Thus, very little energy is produced through this pathway, but the trade-off is that you get the energy quickly.
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Once pyruvate is formed, it has two fates: conversion to lactate, or conversion to a metabolic intermediary molecule called acetyl coenzyme A (acetyl CoA), which enters the mitochondria for oxidation and the production of more ATP. The first fate occurs when the demand for oxygen is greater than the supply (i.e., during anaerobic exercise). Conversely, when there is enough oxygen available to meet the muscles' needs (i.e., during aerobic exercise), pyruvate (via acetyl CoA) enters the mitochondria and goes through aerobic metabolism.
When oxygen is not supplied fast enough to meet the muscles' needs, which defines anaerobic glycolysis, a number of problems begin to arise inside the muscles. They lose their ability to contract effectively because of an increase in hydrogen ions (which causes the muscle pH to decrease, a condition called acidosis) and other metabolites (ADP, Pi and potassium ions). Acidosis and the accumulation of these other metabolites cause a number of problems inside muscles, including inhibition of specific enzymes involved in metabolism and muscle contraction, inhibition of the release of calcium (the trigger for muscle contraction) from its storage site in muscles, and interference with muscles' electrical charges, ultimately leading to a decrease in muscle-force production and exercise intensity.
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Aerobic System: Energy for the Long Haul
Since humans evolved for aerobic activities, it's not surprising that the aerobic system, which is dependent on oxygen, is the most complex of the three energy systems. The metabolic reactions that take place in the presence of oxygen are responsible for most of the cellular energy produced by the body. However, aerobic metabolism is the slowest way to resynthesize ATP. Oxygen, as the patriarch of metabolism, knows that it is worth the wait, as it controls the fate of endurance and is the sustenance of life. "I'm oxygen," it says to the muscle, with more than a hint of superiority. "I can give you a lot of ATP, but you will have to wait for it."
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The aerobic system, which includes the Krebs cycle (also called the citric acid cycle or TCA cycle) and electron transport chain, uses blood glucose, glycogen and fat as fuels to resynthesize ATP in the mitochondria of muscle cells (see Energy System Characteristics on page 3). Given its location, the aerobic system is also called mitochondrial respiration. When using carbohydrate, glucose and glycogen are first metabolized through glycolysis, with the resulting pyruvate used to form acetyl CoA, which enters the Krebs cycle. The resulting electrons from the Krebs cycle are then transported through the electron transport chain, where ATP and water are produced (a process called oxidative phosphorylation). Complete oxidation of glucose via glycolysis, Krebs cycle and electron transport chain produces 36 molecules of ATP for every molecule of glucose broken down. Thus, the aerobic system produces 18 times more ATP than does anaerobic glycolysis from each glucose molecule.