“A reciprocal function to glycolysis is gluconeogenesis, a process that takes place primarily in the liver and synthesizes glucose from non-carbohydrate molecules. The mechanisms are essentially the reverse of glycolysis, and when one function is active, the other is off – i.e. glycolysis and gluconeogenesis do not occur at the same time. The body uses glucose-6-phosphate (G-6-P) produced via this mechanism to store glycogen in the liver, kidney cortex, and sometimes in skeletal muscle. It can cause low blood sugar levels, however, because without G6P, skeletal muscles and other organs cannot release glucose from the glycogen storage. This means glucose cannot be delivered to the bloodstream, and in turn, many body organs like (most of) the liver, the kidneys, and skeletal muscle undergo gluconeogensis to get glucose for their own use.

As many organs cannot produce glucose for their own use, some cells, namely red blood cells, do not have an electron transport chain and thus rely solely on glycolytic mechanisms to produce ATP. There are also times when oxygen is not available to other cells, often due to heavy exercise, and heterotrophs must find other ways to convert glucose to energy. Under anaerobic conditions, many heterotrophs use an alternate pathway known as fermentation, a process that includes the initial phase of glucose catabolism (glycolysis) but does not proceed through the second phase (cellular respiration). There are two types of fermentation – alcohol fermentation and lactic acid fermentation – and although they occur in different organisms and produce different end products, both begin with the pyruvic acid molecules generated from glycolysis. The net reaction for glycolysis is:

Glucose + 2ADP + 2P_i + 2NAD⁺ → 2 Pyruvate + 2ATP + 2NADH + 2H⁺ + 2H₂O

Alcohol fermentation occurs in yeast and some bacteria, and proceeds as follows:

Lactic acid fermentation, which occurs in fungi, bacteria, and human muscle cells, proceeds another way:

Both fermentation processes serve primarily to regenerate carrier coenzymes, and are not ideal mechanisms for obtaining energy from glucose. ”
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