In colloquial terms, respiration refers to the process of breathing. In scientific and medical terminology, however, respiration is a series of chemical reactions taking place at the cellular level. In fact, respiration is defined as the reactions through which nutrient molecules like sugar are transformed into product molecules, which liberates energy that organisms use to maintain life. There are three distinct types of respiration.
Humans and most other multicellular organisms rely primarily upon aerobic respiration, meaning respiration that takes place in the presence of oxygen. During this cellular process, explain Drs. Reginald Garrett and Charles Grisham, sugars are first converted into the much smaller molecule pyruvate. The pyruvate molecule then reacts with an enzyme to produce acetyl Co-A, which is chemically burned in oxygen to generate carbon dioxide and water, the waste products of aerobic respiration. Another important product of aerobic respiration is adenosine triphosphate, or ATP, which is a chemical energy molecule and is used by cells to provide for their energy needs. ATP, for instance, fuels every muscle contraction. Compared to other types of respiration, aerobic respiration is the most efficient and yields the most energy.
Some organisms don’t have access to oxygen, and other organisms occasionally experience oxygen deficits. For this reason, there’s a second kind of respiration that can take place without oxygen, though it’s quite a bit less efficient than aerobic respiration. Lactic fermentation involves the conversion of sugar molecules to pyruvate, after which point no further chemical burning of the nutrient molecules takes place. Note Drs. Garrett and Grisham, the energy yield of lactic fermentation is approximately fifteen times less per sugar molecule than that of aerobic respiration. Humans use anaerobic respiration only infrequently and for short durations, such as in leg muscles during the last moments of a sprint. The pyruvate waste is converted to lactic acid, which yields no additional energy, but does result in the burning sensation in, for instance, a sprinter’s legs.
Some very small organisms, like certain bacteria, are able to produce enough energy through anaerobic respiration—meaning respiration that proceeds without oxygen—to provide for their permanent energy needs. Yeast are one example of such an organism. They employ a strategy called ethanolic fermentation, which starts very similarly to lactic fermentation with the conversion of sugar to pyruvate. Explain Drs. Mary Campbell and Shawn Farrell in their book, “Biochemistry,” however, at this point differences arise. During ethanolic fermentation, pyruvate reacts further to lose a carbon atom. The carbon is liberated in the form of the molecule carbon dioxide, and leaves behind the molecule ethanol, or drinking alcohol. While ethanolic fermentation is not very energy efficient, it nevertheless produces enough energy to satisfy the needs of yeast.
- “Biochemistry”; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- “Biochemistry”; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005