Carbohydrates are an important source of energy for most living organisms. The continued activity of every living cell depends on highly coordinated biochemical reactions, which are fuelled by energy generated through carbohydrate metabolism.
Pathways of carbohydrate metabolism
This is an ancient pathway found in almost all living cells. In this pathway, a small amount of energy is generated as one glucose molecule is converted into two molecules of pyruvate.
Glycogenesis and glycogenolysis
In vertebrates, glucose is converted to glycogen in a process called glycogenesis, which occurs when the body has been well fed and glucose levels are high. This glycogen can be broken back down to provide glucose during periods of fasting. This is called glycogenolysis.
Glucose can also be synthesized from noncarbohydrate precursors such as fats and proteins, in a metabolic pathway called gluconeogenesis.
The pentose phosphate pathway
This pathway converts glucose-6-phosphate (derived from glucose) to ribose-5-phosphate and other types of monosaccharides. Ribose-5-phosphate is essential in the synthesis of DNA and RNA.
Glycolysis occurs in almost every living cell and is one of the oldest of all the biochemical pathways. The enzymes and stages involved in glycolysis have been highly conserved in prokaryotes and eukaryotes.
Glycolysis is a form of anaerobic metabolism, meaning the reactions involved do not require oxygen. Also called the Embden-Meyerhof-Parnas pathway, glycolysis involves a glucose molecule being split and converted into two pyruvates. When this occurs, several carbon atoms are oxidized. A small amount of energy is generated and captured in two molecules each of ATP and NADH.
How the pyruvate is then processed depends on whether the organism is aerobic (depends on oxygen for survival) or anaerobic (does not depend on oxygen for survival). In an anaerobic organism, pyruvate may be converted to a waste substance such as ethanol, lactic acid or acetic acid and excreted. In aerobic animals, the pyruvate is oxidized to give carbon dioxide and water in a process called aerobic respiration.