Glycogenolysis is a quick and easy way to move glucose into the blood when your body has an urgent need, but there's another way to get more glucose into your blood that's effective but requires energy. This process is called gluconeogenesis . Do you see the word 'neo?' That means new. So, if we dissect this term, we see that it means 'glucose-new-creation,' or in other words, the creation of new glucose from molecules that are not carbohydrates, such as proteins and lipids. This process will often take place after a night's sleep, which is a period of time your body is not taking in food or when you have used up available glycogen stores.
Fermentation is less efficient at using the energy from glucose: only 2 ATP are produced per glucose, compared to the 38 ATP per glucose nominally produced by aerobic respiration. This is because the waste products of fermentation still contain chemical potential energy that can be released by oxidation. Ethanol , for example, can be burned in an internal combustion engine like gasoline. Glycolytic ATP, however, is created more quickly. For prokaryotes to continue a rapid growth rate when they are shifted from an aerobic environment to an anaerobic environment, they must increase the rate of the glycolytic reactions. For multicellular organisms, during short bursts of strenuous activity, muscle cells use fermentation to supplement the ATP production from the slower aerobic respiration, so fermentation may be used by a cell even before the oxygen levels are depleted, as is the case in sports that do not require athletes to pace themselves, such as sprinting .