Important points
- Glucose is a monosaccharide.
- The chemical formula is C6H12O6 (hexose sugar).
- It is an aldohexose, i.e. a six-carbon sugar with an aldehyde group.
- It is a type of micromolecule (molecular weight between 18 and about 800 daltons).
- It is the main product of photosynthesis and the most common energy source for living organisms.
- The breakdown of glucose into pyruvic acid to release energy is called glycolysis.
- The formation of glucose from non-carbohydrate precursors is called gluconeogenesis.
- Glucose is also called dextrose or grape sugar, and the glucose present in blood is called blood sugar.
Formation in photosynthesis:
6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O
α-glucose and β-glucose
In water, glucose mostly exists as a six-membered ring (the pyranose form). When the ring closes, the –OH group on carbon 1 can point in two directions, giving two forms of glucose: α-glucose and β-glucose.
| Feature | α-glucose | β-glucose |
|---|---|---|
| –OH on carbon 1 | Below the plane of the ring | Above the plane of the ring |
| Polymers formed | Starch and glycogen | Cellulose |
| Glycosidic bonds | α-1,4 (and α-1,6 at branch points) | β-1,4 |
| Digested by humans | Yes | No |
Carbohydrates made from α-glucose
- Maltose – a disaccharide made of two α-glucose units joined by an α-1,4 glycosidic bond.
- Sucrose – a disaccharide made of one α-glucose and one β-fructose unit.
- Starch – the storage polysaccharide of plants. It has two parts: amylose (unbranched, α-1,4 bonds) and amylopectin (branched, with α-1,6 bonds at the branch points).
- Glycogen – the storage polysaccharide of animals, found mainly in the liver and muscles. It is more highly branched than amylopectin.
Starch forms helical secondary structures that can hold iodine, giving a blue colour. Cellulose, made of β-glucose, does not form these helices, so it cannot hold iodine. In a polysaccharide chain, the right end is called the reducing end and the left end the non-reducing end.
References
- NCERT Biology Textbook for Class XI, Chapter: Biomolecules.
- Nelson, D. L. and Cox, M. M. Lehninger Principles of Biochemistry, Chapter: Carbohydrates and Glycobiology.