The process of cellular respiration is one in which your body cells burn fuels -- carbohydrates, fat, and protein -- to yield energy that they use for a variety of cellular processes. Where respiration involves carbohydrates, there is a specific series of reactions that convert carbohydrates into metabolic waste products and yield energy.
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It's typical, in common parlance, for people to think of breathing when they hear the word "respiration." From a chemical standpoint, however, respiration is only tangentially related to breathing. Chemical respiration is the combination of carbohydrate -- and other energy-yielding molecules -- with oxygen in order to produce energy and the metabolic waste products carbon dioxide and oxygen. Overall, cellular respiration accounts for the majority of energy that your body extracts from nutrient molecules.
The purpose of cellular respiration where it relates to carbohydrate is to chemically burn sugar molecules -- the building blocks of carbohydrate -- and store the energy produced through those reactions in the form of the chemical energy currency molecule, ATP. Essentially, explain Drs. Mary Campbell and Shawn Farrell in their book "Biochemistry," humans produce about 30 ATP molecules per molecule of glucose, which is a specific kind of sugar. You obtain glucose from table sugar, milk sugar, and starch.
The process of carbohydrate respiration is divided into different parts, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." First, a series of reactions called glycolysis splits glucose into two molecules of a chemical called pyruvate. Another reaction converts pyruvate to a molecule called acetyl-CoA. The acetyl-CoA then enters a series of reactions called the Kreb's Cycle, which produces the waste products carbon dioxide and oxygen, and releases energy from the acetyl-CoA, eventually producing ATP.
As a result of carbohydrate respiration, your cells produce large quantities of ATP, which the cells then use for a variety of things. Explains Dr. Lauralee Sherwood in her book "Human Physiology," one of the uses of ATP in the body is in muscle contraction. It requires hundreds of thousands of ATP molecules for even a small muscle contraction. Cells also use ATP to communicate with one another--the ATP goes into establishing what's called a "resting membrane potential" that cells use like an electrical signal.
While it's possible for cells to produce ATP through respiration using fuels other than carbohydrate, carbohydrate -- and specifically glucose -- is the preferred fuel of many different cells. The muscle cells, for instance, rely on glucose, particularly during very hard efforts. The brain cells, too, rely preferentially on glucose, explains Dr. Sherwood. This explains why, if you eat an extremely low-carbohydrate diet, your muscles will feel weak, and your thinking will be fuzzy -- the muscles and brain need glucose to function optimally.