<P> Glucose is a ubiquitous fuel in biology . It is used as an energy source in organisms, from bacteria to humans, through either aerobic respiration, anaerobic respiration (in bacteria), or fermentation . Glucose is the human body's key source of energy, through aerobic respiration, providing about 3.75 kilocalories (16 kilojoules) of food energy per gram . Breakdown of carbohydrates (e.g., starch) yields mono - and disaccharides, most of which is glucose . Through glycolysis and later in the reactions of the citric acid cycle and oxidative phosphorylation, glucose is oxidized to eventually form carbon dioxide and water, yielding energy mostly in the form of ATP . The insulin reaction, and other mechanisms, regulate the concentration of glucose in the blood . The physiological caloric value of glucose, depending on the source, is 16.2 kilojoules per gram and 15.7 kJ / g (3.74 kcal / g), respectively . The high availability of carbohydrates from plant biomass has led to a variety of methods during evolution, especially in microorganisms, to utilize the energy and carbon storage glucose . Differences exist in which end product can no longer be used for energy production . The presence of individual genes, and their gene products, the enzymes, determine which reactions are possible . The metabolic pathway of glycolysis is used by almost all living beings . An essential difference in the use of glycolysis is the recovery of NADPH as a reductant for anabolism that would otherwise have to be generated indirectly . </P> <P> Glucose supplies almost all the energy for the brain, so its availability influences psychological processes . When glucose is low, psychological processes requiring mental effort (e.g., self - control, effortful decision - making) are impaired . In the brain, which is dependent on glucose as the major source of energy, the glucose concentration is usually 4 to 6 mM (5 mM equals 90 mg / dL), but decreases to 2 to 3 mM when fasting . Confusion occurs below 1 mM and coma at lower levels . </P> <P> The glucose in the blood is called blood sugar . Blood sugar levels are regulated by glucose - binding nerve cells in the hypothalamus . In addition, glucose in the brain binds to glucose receptors of the reward system in the nucleus accumbens . The binding of glucose to the sweet receptor on the tongue induces a release of various hormones of energy metabolism, either through glucose or through other sugars, leading to an increased cellular uptake and lower blood sugar levels . Artificial sweeteners do not lower blood sugar levels . </P> <P> The blood sugar content of a healthy person in the short - time fasting state, e.g. after overnight fasting, is about 70 to 100 mg / dl of blood (4 to 5.5 mM). In blood plasma, the measured values are about 10 - 15% higher . In addition, the values in the arterial blood are higher than the concentrations in the venous blood since glucose is absorbed into the tissue during the passage of the capillary bed . Also in the capillary blood, which is often used for blood sugar determination, the values are sometimes higher than in the venous blood . The glucose content of the blood is regulated by the hormones insulin, incretin and glucagon . Insulin lowers the glucose level, glucagon increases it . Furthermore, the hormones adrenaline, thyroxine, glucocorticoids, somatotropin and adrenocorticotropin lead to an increase in the glucose level . In addition, there is also a hormone - independent regulation, which is referred to as glucose autoregulation . After food intake the blood sugar concentration increases . Values over 180 mg / dl in venous whole blood are pathological and are termed hyperglycemia, values below 40 mg / dl are termed hypoglycaemia . When needed, glucose is released into the bloodstream by glucose - 6 - phosphatase from glucose - 6 - phosphate originating from liver and kidney glycogen, thereby regulating the homeostasis of blood glucose concentration . In ruminants, the blood glucose concentration is lower (60 mg / dL in cattle and 40 mg / dL in sheep), because the carbohydrates are converted more by their gut flora into short - chain fatty acids . </P>

Glucose is derived from the breakdown of what biological molecule