Glucose is a simple sugar, or monosaccharide, with the molecular formula C₆H₁₂O₆ and a molar mass of approximately 180.16 g/mol. It belongs to the carbohydrate family and is a major fuel for living organisms. Its naturally predominant form, D-glucose, is also called dextrose. Glucose circulates in blood, participates in metabolism, and supplies the repeating units of several important storage and structural carbohydrates. (pubchem.ncbi.nlm.nih.gov)
Chemical structure and stereochemistry
Glucose is classified as an aldohexose: its open-chain form contains six carbon atoms, an aldehyde group at carbon 1, and five hydroxyl groups. Its formula alone does not uniquely identify it; other sugars have the same formula but different arrangements of atoms or functional groups. This illustrates isomerism, which helps explain why sugars with identical elemental compositions can have different chemical and biological properties. (openstax.org)
In aqueous solution, glucose exists predominantly in cyclic forms rather than as an open chain. The hydroxyl group on carbon 5 can react with the aldehyde group on carbon 1, forming a six-membered hemiacetal ring containing five carbon atoms and one oxygen atom. This form is called glucopyranose. Ring formation creates a new stereochemical distinction at carbon 1, the anomeric carbon. The resulting α and β forms are anomers, differing in the orientation of the anomeric hydroxyl group. (assets.openstax.org)
The α and β forms interconvert through the open-chain form. Their changing proportions cause the optical rotation of a freshly prepared solution to change until equilibrium is reached, a process called mutarotation. The D designation describes stereochemical configuration, not the direction of optical rotation; D- and L-glucose are mirror-image enantiomers. (ncstate.pressbooks.pub)
Chemical behavior
Glucose is a reducing sugar because its cyclic hemiacetal can open to expose an aldehyde group. Under suitable conditions, glucose reduces silver ions in Tollens’ reagent or copper ions in Benedict’s reagent while itself undergoing oxidation. These reactions historically provided qualitative tests for reducing sugars, but they are not specific to glucose. Oxidation of its aldehyde group can produce gluconic acid. (openstax.org)
The anomeric carbon also participates in forming glycosidic bonds, which connect sugar units. The position and stereochemical configuration of these bonds strongly influence the properties of the resulting carbohydrate. Thus, substances built entirely from glucose can differ substantially in shape, function, and susceptibility to digestion. (openstax.org)
Occurrence and larger carbohydrates
Free glucose occurs in fruits, berries, vegetables, and honey. Much of the glucose available to organisms is instead incorporated into larger carbohydrates. Starch is a plant storage carbohydrate consisting of amylose and amylopectin. Amylose mainly contains α(1→4) linkages, whereas amylopectin also has α(1→6) branch points. Glycogen, an important storage carbohydrate in animals, has a related but more highly branched structure. (fao.org)
Cellulose consists of glucose units joined by β(1→4) linkages and forms a major structural component of plant cell walls. Humans can digest starch but lack the enzymes needed to hydrolyze cellulose directly. The contrast shows that nutritional availability depends not simply on the constituent sugar but also on how its units are connected. (openstax.org)
In photosynthesis, carbon dioxide is incorporated into organic compounds. The Calvin cycle produces three-carbon sugar phosphates, notably glyceraldehyde-3-phosphate, that can subsequently contribute to glucose and other carbohydrate synthesis. Glucose is therefore often used in simplified overall photosynthesis equations, although it is not the immediate product released by the cycle. (openstax.org)
Cellular utilization
Glycolysis converts one glucose molecule into two molecules of pyruvate. It consumes two molecules of adenosine triphosphate (ATP) and produces four, giving a net yield of two ATP, together with two NADH. The pathway does not directly require oxygen. An early phosphorylation step converts glucose to glucose-6-phosphate, helping retain it within the cell and directing it into subsequent reactions. (openstax.org)
During aerobic cellular respiration, pyruvate can undergo further oxidation through acetyl-CoA and the citric acid cycle, with additional ATP production associated with oxidative phosphorylation. Alternatively, fermentation regenerates the oxidized electron carrier needed to sustain glycolysis. Mature mammalian red blood cells lack mitochondria and rely on glycolysis for ATP production. Glucose intermediates also supply material for biosynthetic pathways rather than being used exclusively as fuel. (ncbi.nlm.nih.gov)
Blood glucose and regulation
Blood glucose is controlled by coordinated processes of production, uptake, utilization, and storage. Insulin, secreted by pancreatic β cells, promotes glucose uptake in muscle and adipose tissue, supports glycogen synthesis, and suppresses hepatic glucose production. In these insulin-sensitive tissues, increased availability of GLUT4 transporters at the cell surface facilitates uptake. Glucagon supports glucose availability during fasting by stimulating hepatic glycogen breakdown and gluconeogenesis. These mechanisms contribute to metabolic homeostasis. (ncbi.nlm.nih.gov)
Glucose measurements are used in evaluating diabetes mellitus. Fasting and random plasma glucose tests measure concentrations at particular times, while an oral glucose tolerance test measures responses to a glucose load. The A1C test instead measures glucose attachment to hemoglobin and reflects average blood glucose over approximately three months. Conditions affecting red blood cells or hemoglobin can alter its interpretation. (niddk.nih.gov)
Production and applications
Commercial glucose is produced through hydrolysis of starch, followed by purification and, for crystalline dextrose, crystallization. Glucose syrup is not necessarily pure glucose: it can contain other saccharides remaining from starch conversion. Dextrose and glucose syrups are used in confectionery, baked goods, preserves, and other foods to provide sweetness and modify crystallization or texture. Glucose also supplies fermentable carbohydrate for yeast and other microorganisms in industrial fermentation. (fao.org)