<P> Even if there is no oxygen present, glycolysis can continue to generate ATP . However, for glycolysis to continue to produce ATP, there must be NAD+ present, which is responsible for oxidizing glucose . This is achieved by recycling NADH back to NAD+ . When NAD+ is reduced to NADH, the electrons from NADH are eventually transferred to a separate organic molecule, transforming NADH back to NAD+ . This process of renewing the supply of NAD+ is called fermentation, which falls into two categories . </P> <P> In alcohol fermentation, when a glucose molecule is oxidized, ethanol (ethyl alcohol) and carbon dioxide are byproducts . The organic molecule that is responsible for renewing the NAD+ supply in this type of fermentation is the pyruvate from glycolysis . Each pyruvate releases a carbon dioxide molecule, turning into acetaldehyde . The acetaldehyde is then reduced by the NADH produced from glycolysis, forming the alcohol waste product, ethanol, and forming NAD+, thereby replenishing its supply for glycolysis to continue producing ATP . </P> <P> In lactic acid fermentation, each pyruvate molecule is directly reduced by NADH . The only byproduct from this type of fermentation is lactate . Lactic acid fermentation is used by human muscle cells as a means of generating ATP during strenuous exercise where oxygen consumption is higher than the supplied oxygen . As this process progresses, the surplus of lactate is brought to the liver, which converts it back to pyruvate . </P> <P> If oxygen is present, then following glycolysis, the two pyruvate molecules are brought into the mitochondrion itself to go through the Krebs cycle . In this cycle, the pyruvate molecules from glycolysis are further broken down to harness the remaining energy . Each pyruvate goes through a series of reactions that converts it to acetyl coenzyme A. From here, only the acetyl group participates in the Krebs cycle--in which it goes through a series of redox reactions, catalyzed by enzymes, to further harness the energy from the acetyl group . The energy from the acetyl group, in the form of electrons, is used to reduce NAD+ and FAD to NADH and FADH, respectively . NADH and FADH contain the stored energy harnessed from the initial glucose molecule and is used in the electron transport chain where the bulk of the ATP is produced . </P>

Which type of carbohydrate plays an important role in producing atp