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Glycolysis, the “splitting of glucose”, is the breakdown of glucose or glycogen in order to provide ATP, which occurs in the liver and in the sarcoplasm of muscle cells. Glycolysis is the energy system that predominantly provides energy for higher-intensity activities lasting 15-90 seconds and it provides a net gain of 2-3 ATP.

The initial steps of glycolysis are the additions of two phosphates to the glucose molecule at the expense of two molecules of ATP. The result is a 6 carbon sugar-diphosphate molecule and two low-energy ADP molecules. This 6 carbon sugar-diphosphate molecule is being split into two 3-carbon sugar-phosphate molecules. Each one of the 3-carbon sugar-phosphate molecules is converted through a series of reactions to pyruvate and hydrogen. So glycolysis produces ATP but also two byproducts: pyruvic acid and hydrogen ions.

Hydrogen Removal

The hydrogen build up becomes a problem because it causes an increase in the cell’s acidity (pH levels), which prevents the cell from functioning properly and hence leads to a halt in energy production. Therefore, the two hydrogen transport molecules, coenzymes NAD & FAD, are important for glycolysis to occur because hydrogen ion removal must occur so that cells can continue to provide energy; NAD accepts and transports 1 hydrogen atom (H) so that glycolysis can proceed (NAD —> NADH). NADH has to donate the hydrogen portion to reform NAD so that hydrogen transport/removal can continue.

Hydrogen Removal: Aerobic vs. Anaerobic

NADH can be reformed to NAD in two ways, with oxygen and without oxygen. When oxygen is available, the hydrogen from NADH can be transported to the mitochondria of the cell, where the mitochondria can produce ATP aerobically. If oxygen is not available to accept hydrogen in the mitochondria (anaerobic glycolysis) then pyruvic acid in the sarcoplasm can accept the hydrogen to form lactic acid and thereby reforming NADH to NAD. How the hydrogen ions are being removed depends on the presence or absence of oxygen, which is exercise intensity specific. During higher intensities oxygen is not available (anaerobic glycolysis) to transport hydrogen, during lower intensities oxygen is available (aerobic glycolysis).

Anaerobic Energy Production

During higher intensities, when oxygen is not available to accept and transport the hydrogen ions, pyruvic acid accepts the hydrogen ions to form lactic acid, which rapidly dissociate to form lactate and hydrogen. Then lactate can either be used by slow skeletal muscle cells or the heart during aerobic metabolism to form ATP or during anaerobic metabolism by the liver to produce glucose via gluconeogenesis, where new glucose is being synthesized, released into the blood stream, transported to muscle tissue, and used by muscle cells as energy. So, lactate can be used to provide immediate energy for muscle cells during aerobic metabolism by transforming lactate to pyruvate to acetyl Co A, which can enter the Krebs cycle, or lactate can be used during anaerobic metabolism to synthesize glucose/glycogen in the liver, which then can be used again my muscle cells for energy. The process by which lactate is being transported to the liver and converted into glucose/glycogen is called the Cori Cycle.

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