What is glucose?

Updated: Jul 22, 2021
  • Author: Setu K Patolia, MD, MPH; Chief Editor: Sridevi Devaraj, PhD, DABCC, FAACC, FRSC, CCRP  more...
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Glucose is a monosaccharide and is the primary metabolite for energy production in the body. Complex carbohydrates are ultimately broken down in the digestive system into glucose and other monosaccharides, such as fructose or galactose, prior to absorption in the small intestine; of note, insulin is not required for the uptake of glucose by the intestinal cells. Glucose is transported into the cells by an active, energy-requiring process that involves a specific transport protein and requires a concurrent uptake of sodium ions.

In the blood circulation, the concentration of glucose is tightly regulated by hormones such as insulin, cortisol, and glucagon, which regulate glucose entry into cells and affect various metabolic processes such as glycolysis, gluconeogenesis, and glycogenolysis. Glucose enters cells via facilitated transport, mediated by glucose transporter (GLUT) receptors. These receptors demonstrate tissue specificity; for example, GLUT-4 receptors are present in muscle and adipose tissues and require insulin for glucose transport. GLUT-1 receptors transport glucose across the blood-brain barrier. The rare genetic disease GLUT-1 receptor deficiency manifests with a variety of neurologic deficits such as refractory epilepsy, developmental delay, and dyskinesia. [6]

The main biochemical reaction employing glucose as its substrate is glycolysis, which, used by all tissues for the breakdown of glucose, provides energy in the form of adenosine triphosphate (ATP) and produces intermediates for other metabolic pathways. Since virtually all sugars are ultimately convertible to glucose, glycolysis serves as the hub of carbohydrate metabolism. In cells with mitochondria and an adequate supply of oxygen, pyruvate emerges as the end product of glycolysis via a 10-reaction series known as aerobic glycolysis. Pyruvate is subsequently converted through oxidative decarboxylation into acetyl coenzyme A (acetyl-CoA), the major fuel for the citric acid cycle. Alternatively, glucose undergoes anaerobic glycolysis to form lactate. [6]

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