Some organisms do not need as much energy as others and sometimes the conditions do not allow for aerobic respiration run successfully. In these situations it may be necessary for organisms to run a process known as anaerobic (literally meaning without oxygen) respiration. Anaerobic respiration occurs when the amount of oxygen available is too low to support the process of aerobic respiration. There are two main types of anaerobic respiration, alcoholic fermentation and lactic acid fermentation. These are not the preferred method of releasing the energy from the glucose molecules, but in order to survive when energy is needed, it is the only alternative.
Alcoholic fermentation is the process of converting glucose into the molecule ethanol. This process is generally done by microscopic organisms like bacteria and certain fungi like yeast. The glucose are broken down by glycolysis releasing two ATP molecules and creating two pyruvic acid molecules. The pyruvic acid molecules then have another carbon atom removed from each, resulting in two ethanol molecules and two carbon dioxide molecules. No more ATP is produced after the initial two from the glycolysis reaction. The carbon dioxide produced as a by-product when yeast perform alcoholic fermentation on the sugar in the bread mix is the gas that causes dough to rise. The sugars in produce like barley, grapes and other grains and fruits is what allows yeast to produce the bubbles and alcohol in beer, wine and other liquors.
This process is used to create much of the ethanol that is now being used as a gasoline additive for automobiles. Corn or soybeans are generally used as the source of starch that is then broken down by enzymes into thousands of glucose molecules. These glucose molecules are then introduced to a series of bacteria that perform the alcoholic fermentation to produce the ethanol. The idea is to eventually change over all automobiles to be able to run on pure synthetic fuel (organic is a bad term since all petroleum-based products are part of the field of organic chemistry).
The process of lactic acid fermentation is what causes your muscles to “burn” when you do a workout or perform at a high level of physical activity for an extended period of time. Lactic acid fermentation occurs in animals after glycolysis if there is not enough oxygen to perform aerobic respiration. Again the only ATP that is produced occurs during the glycolysis reactions. One three-carbon molecule (pyruvic acid) is changed into another three-carbon molecule (lactic acid) and no carbon dioxide is produced like during other forms of respiration. The muscle tissue of animals is broken down by the lactic acid allowing them to rebuild stronger for the next time. The more that you workout or exercise, the more conditioned your muscles become and the less likely they will have to revert to anaerobic respiration in the future. It does take a lot of time, energy and dedication to train your body to be efficient at performing aerobic respiration, which is why most people get frustrated and stop working out.