Glycolysis is a series of 10 reactions that begins the process of cellular respiration. Throughout the reactions a molecule of glucose gets split into two molecules of pyruvate. In organisms that use aerobic cellular respiration, this pyruvate will then either continue on to the mitochondria to complete aerobic cellular respiration, or if there is not enough oxygen present it will continue through the reactions of fermentation.
To start glycolysis, 2 ATP are needed to get the reactions going. Throughout the process 4 ATP are produced, resulting in a net gain of 2 ATP. This is not much energy, hence the need for aerobic cellular respiration in many organisms to produce much larger amounts of ATP.
As for electron carrier molecules, during glycolysis 2 NADH molecules are produced from 2 NAD+ molecules. These NADH molecules carry their electrons to the electron transport chain which occurs towards the end of aerobic cellular respiration. The electrons they drop off are used as energy to create an electrochemical gradient for the production of ATP.
A side note about fermentation. Although glycolysis doesn't produce much energy (only 2 ATP), it is some energy. If the organism can only do fermentation because there is not enough oxygen present, it needs to convert the NADH back into NAD+ because NAD+ is a reactant for glycolysis. So, the purpose of fermentation is to do that conversion so that glycolysis can continue to produce at least some energy for the cell. There are various byproducts of fermentation depending on the organism. In humans, the byproduct is lactic acid, while in yeast, the byproduct is ethanol and carbon dioxide. Regardless of the byproduct, the purpose of fermentation is the same, to convert the electron carrier NADH back to NAD+ to keep glycolysis going.
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