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This tutorial presents glycolysis at a level appropriate for most undergraduate biology classes and the MCAT exam.

Click to view the video tutorial: Glycolysis.

Glycolysis is a metabolic pathway in which glucose is degraded anaerobically by cytosolic enzymes to produce two smaller pyruvate molecules and ATP.

The net product of glycolysis for the catabolism of a single glucose molecule is 2 ATP, 2 NADH and 2 pyruvate molecules.

1. Two three carbon pyruvate molecules are produced by splitting one six carbon glucose molecule.

2. Hexokinase uses ATP to phosphorylate glucose, making glucose 6-phosphate. Phosphofructokinase uses ATP to phosphorylate fructose 6-phosphate, making fructose 1,6-bisphosphate. Thus two molecules of ATP are consumed early in the glycolytic pathway. Because early glycolytic reactions use ATP, early glycolysis is sometimes called the energy investment phase of glycolysis. Later, during the energy production phase four molecules of ATP are produced. Since two molecules of ATP are invested and four molecules of ATP are produced, the net ATP production for glycolysis is two ATP.

-2 ATP + 4 ATP = 2 ATP

3. When the cytosolic concentration of ATP is high ATP binds to an allosteric site on phosphofructokinase inhibiting it’s ability to phoshorylate fructose 6-phosphate to fructose 1,6-bisphosphate. Without fructose 1,6-bisphosphate reactions that occur later in the glycolytic pathway can not occur and the rate of glycolysis slows.

4. Glycolysis is an anaerobic process, meaning that it does not require oxygen. Obligate anaerobes (organisms that die in the presence of oxygen) may use glycolysis and fermentation to produce ATP.

5. Catabolic pathways break down large molecules into smaller molecules. The energy released by breaking bonds may be harvested by the cell to do work or it may dissipate as heat. Anabolic pathways construct large molecules from smaller molecules. Anabolic pathways require energy and are usually powered by ATP. During glycolysis enzymes in the cytoplasm convert a single molecule of glucose into two smaller pyruvate molecules. The energy released by breaking bonds is used to make ATP. Thus the glycolytic pathway is also a catabolic pathway.

adenosine triphosphate (ATP)
adenosine diphosphate (ADP)
adenosine monophosphate (AMP)
fructose 6-phosphate
fructose 1,6-bisphosphate
glucose 6-phosphate
nicotinamide adenine dinucleotide (NAD+ / NADH)

RELATED TOPICS (Note: These links will go active as videos appear online.)
the citric acid cycle
electron transport and oxidative phosphorylation

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3. Campell, N., Reece, J. B., Taylor, M., & Simon, E. (2008). Biology: Concepts and Connections (5th ed.). San Francisco: Benjamin Cummings.
4. Fox, S. I. (2002). Human Physiology. (7th ed.). New York: McGraw-Hill.