Glycolysis is the conversion of the six-carbon sugar molecule glucose to two molecules of the three-carbon compound pyruvate and a little bit of energy in the form of ATP (adenosine triphosphate) and NADH (an "electron carrier" molecule). It occurs in all cells, both prokaryotic (i.e., those generally lacking in the capacity for aerobic respiration) and eukaryotic (i.e., those that have organelles and make use of cellular respiration in its entirety).
The pyruvate formed in glycolysis, a process that itself requires no oxygen, proceeds in eukaryotes to the mitochondria for aerobic respiration, the first step of which is the conversion of pyruvate to acetyl CoA (acetyl coenzyme A).
But if no oxygen is present or the cell lacks ways to perform aerobic respiration (as do those of most prokaryotes), pyruvate becomes something else. In anaerobic respiration, what do the two molecules of pyruvate get converted to?
Glycolysis: The Source of Pyruvate
Glycolysis is the conversion of one molecule of glucose, C6H12O6, to two molecules of pyruvate, C3H4O3, with some ATP, hydrogen ions and NADH generated along the way with the help of ATP and NADH precursors:
C6H12O6 + 2 NAD + 2 ADP + 2 Pi → 2 C3H4O3 + 2 NADH + 2 H+ + 2 ATP
Here Pi stands for "inorganic phosphate," or a free phosphate group not attached to a carbon-bearing molecule. ADP is adenosine diphosphate, which differs from ADP by, as you might have guessed, a single free phosphate group.
Pyruvate Processing in Eukaryotes
Just as it is under anaerobic conditions, the final product of glycolysis under aerobic conditions is pyruvate. What happens to pyruvate under aerobic conditions, and only under aerobic conditions, is aerobic respiration (initiated by the bridge reaction preceding the Krebs cycle). Under anaerobic conditions, what happens to pyruvate is its conversion to lactate to help keep glycolysis chugging along upstream.
Before looking closely at the fate of pyruvate under anaerobic conditions, it is worth looking at what happens to this fascinating molecule under the normal conditions you yourself typically experience – right now, for example.
Pyruvate Oxidation: The Bridge Reaction
The bridge reaction, also called the transition reaction, takes place in the mitochondria of eukaryotes and involves the decarboxylation of pyruvate to form acetate, a two-carbon molecule. A molecule of coenzyme A is added to the acetate to form acetyl coenzyme A, or acetyl CoA. This molecule then enters the Krebs cycle.
At this point, carbon dioxide is excreted as a waste product. No energy is required nor is any harvested in the form of ATP or NADH.
Aerobic Respiration After Pyruvate
Aerobic respiration completes the process of cellular respiration and includes the Krebs cycle and the electron transport chain, both in the mitochondria.
The Krebs cycle sees acetyl CoA blended with a four-carbon molecule called oxaloacetate, the product of which is sequentially reduced again to oxaloacetate; a little ATP and lots of electron carriers result.
The electron transport chain uses the energy in the electrons in those aforementioned carriers to produce a great deal of ATP, with oxygen required as the final electron acceptor to keep the whole process from backing up far upstream, at glycolysis.
Fermentation: Lactic Acid
When aerobic respiration is not an option (as in prokaryotes) or the aerobic system is exhausted because the electron transport chain has been saturated (as in high-intensity, or anaerobic, exercise in human muscle), glycolysis can no longer continue, because there is no longer a source of NAD_ to keep it going.
Your cells have a workaround for this. Pyruvate can be converted to lactic acid, or lactate, to generate enough NAD+ to keep glycolysis going for a while.
C3H4O3 + NADH → NAD+ + C3H5O3
This is the genesis of the notorious "lactic acid burn" you feel during intense muscular exercise, like lifting weights or an all-out set of sprints.