Glucose(C6H12O6, also known as D-glucose, dextrose, and blood sugar) is a six carbon aldose (hexose) monosaccharide, the main source of energy in most animals and plants. It can be found in almost all plants, because it is the product of photosynthesis. In humans, it is the major source of energy via the formation of adenosine tri-phosphate (ATP) in a process called oxidative phosphorylation, the end point of mammalian cellular respiration. The reaction can be summarized as follows:
C6H12O6 + O2 -> CO2 + H20
Absorption in humans
Glucose is metabolized aerobically by nearly all cells (a notable exception is the erythrocyte - Red Blood Cell - which process glucose via glycolysis) for the formation of ATP. Upon ingesting glucose, it is absorbed into the blood stream by the small intestine, carried to the liver via the portal vein where a portion is stored as glycogen or transformed into triacylglycerols (triglycerides) and packaged with specific proteins generating Very Low Density Lipoproteins (VLDLs). These lipid-protein particles then re-enter the circulation where they are hydrolyzed by the endothelial bound enzyme lipoprotein lipase (LPL) into Intermediate and Low Density Lipoproteins (LDLs), ultimately yielding free fatty acids and monoglycerides. These molecules are absorbed by adipocytes (fat cells) where they are reassembled into fats and stored as energy reserves. In addition to the liver, glucose is also stored as glycogen in skeletal muscle cells where it can be broken down at a later time in a process called glycogenolysis. This liberates free glucose for metabolism by all cells.
Pathology involving glucose
When there is insufficient glucose in your body, a person may start experiencing the symptoms of hypoglycemia. Because the lack of glucose inhibits your cells from operating properly, you will feel fatigued, light headed, and nauseous. As a reflex action, the sympathetic nervous system is triggered into releasing epinephrine from the adrenal medulla. Epinephrine increases gluconeogenesis, lipolysis and glycogenolysis. Ciculating ephinephrine may also result in tremors, nervousness, and anxiety. In extreme cases, people can go into comas as the principal source of energy for the brain is glucose.
A common disorder called diabetes mellitus is caused by a defect in normal glucose homeostasis. This occurs when one's body does not generate enough insulin due to autoimmune destruction of insulin secreting Beta-cells in the pancreas (termed Type I Diabetes Mellitus), or because body cells have been rendered "insulin resistant" (Type II Diabetes Mellitus). The latter is precipitated by obesity or certain genetic factors that cause dysfunctional cell signaling when insulin binds to its plasma membrane receptor.
Glucose is used a lot in cooking, especially baking and making various types of sweets. It is used so commonly in baking because it is readily fermented, or fed on, by yeast. It is used to make a lot of candies and frostings because it doesn't crystallize easily.
The hydrogen substituents of a glucose ring may bond to the hydroxyl substituants of another glucose molecule to form long chains of glucose and create water as a byproduct. These can often be in the form of starch or cellulose. Starch is a storage molecule used by plants to store glucose. It can be easily digested by animals because the bonds between the individual glucose molecules are relatively easy to break by adding water. Cellulose has a different and more stable chemical structure, and as such, requires much more energy to break down. Because of its strength and stability, cellulose is used by plants to form the tough outer walls of its cells. Herbivores form symbiotic relationships with bacteria to break down cellulose and provide energy to the animal consuming the cellulose.