Glucose is an essential metabolic substrate of all mammalian cells. D-glucose is the major carbohydrate presented to the cell for energy production and many other anabolic requirements. Under normal physiological conditions, it is the sole source energy for the brain. Glucose is obtained directly from the diet. Dietary carbohydrates from which humans gain energy enter the body in different forms, such as monosaccharides, disaccharides, and polysaccharides and more so, also by the synthesis of other substrates in organs such as the liver. Glucose and other monosaccharides are transported across the intestinal wall to the hepatic portal vein and then to liver cells and other tissues. There they are converted to fatty acids, amino acids and glycogen (the liver stores excess glucose as glycogen and little glucose is normally converted to fat), or are oxidized by the various catabolic pathways of cells. In the first rate-determining step of metabolism, glucose is transported across the plasma membrane by the facilitative glucose transporters. Hexokinase then phosphorylates glucose to glucoso-6-phosphate. The product generally enters the glycolytic pathway, generating NADH, ATP, and pyruvate, or the pentose phosphate pathway. In the presence of sufficient oxygen, pyruvate can be fed into mitochondria and fully oxidized to produce more ATP. When oxygen is limited, pyruvate is disposed in the form of lactate and glycolysis becomes the main source for ATP production. Pentose phosphate pathway plays an important role in the synthesis of pentose sugars for DNA and RNA, as well as generation of NADPH for the synthesis of lipids.
