Basic metabolism II: carbohydrate

In health, carbohydrate accounts for about 50% of total intake of dietary energy. After digestion and absorption, it is metabolized to provide energy (4 kcal/g) or is stored in muscle and liver as glycogen. The carbohydrate stores of the body are about 400-500 g in the fed state. Six-carbon glucose molecules degraded by a series of chemical reactions to three-carbon pyruvate by the reactions of glycolysis: pyruvate can be further metabolized to lactate. These reactions occur in the cell cytoplasm, and can provide energy at high rates without the involvement of molecular oxygen, and are termed anaerobic. Pryuvate (and lactate) can be further oxidized to carbon dioxide and water by the reactions of the Krebs (tricarboxylic acid) cycle that occurs in the mitochondria and leads to the production of carbon dioxide and water. Glucose is an essential fuel for the brain and for some other cells, notably red blood cells. Because body carbohydrate reserves are limited (and because stored fatty acids cannot be converted to carbohydrate) the metabolism of carbohydrates in different tissues is tightly regulated. Some de novo synthesis of glucose is possible from non-carbohydrate sources, including glycerol and the carbon skeletons of certain amino acids.