postulateone

The first step is glycolysis ("sugar-splitting"). Two ATP molecules are used to attach a two phosphate groups to each side of the 6 carbon glucose molecule. Then two NAD+ come along and in a redox reaction become 2NADH. Then we input four ADP to get four ATP. The sugar is split to get two pyruvate molecules, each has three carbons. The next step is pyruvate oxidation. Two molecules of CO2 are released. We get two more NADH from two NAD+. Then acetate, two carbons, attatches to the enzyme CoA, thus acetetyl CoA. Then comes the krebs cycle. Oxeloaceate attaches to aceteyl CoA for make six carbon citrate. The citrate loses carbons over the course of the cycle which in turn give off CO2 (four molecules per glucose). Six NADH are made per glucose, two ATP, and two FADH2. Now there are no carbons.
The NADH moves electrons and H+ to the electron transport chain. NADH is oxidized to NAD+, the electrons and H+ given off are actively transported into electron transport proteins which move all the H+ into the intermembrane space. The proton motive force is what is responsible for working with this gradient to move the H+ to the ATP synthase which, as H+ move through it, turns ADP into ATP. A maximum of 38 ATP molecules can be made through this process. It is important to note that as electrons move through the ETC, they lose potential energy. At the end of the chain, they are "caught" by oxygen.
When there is an absence of oxygen, the ETC gets backed-up because no oxygen is there to collect the electrons. This equates to lower levels of NAD+ and higher levels of NADH. Fermentation is a process which exclusively makes NAD+ by breaking down the pyruvate into oxygen. When there is oxygen, at least glycolysis can continue. In animals, the byproduct of this is lactic acid and in yeast, you get ethanol.
Wow!