Monosaccharides are simple sugars and the most basic units of carbohydrates.

Monosaccharides have the general formula C_nH₂O_n and are typically made up of three to seven carbons. The position of the carbonyl (C = O) group classifies the sugars into aldoses and ketoses, as shown below. If the sugar has an aldehyde group (the functional group with the structure R-CHO), meaning that the carbonyl group (C=O) is at the end of the carbon chain, it is known as an aldose. If it has a ketone group (the functional group with the structure RC(=O)R’), meaning that the carbonyl group is internal to the chain so that there are other carbons on both sides of it, it is known as a ketose.

Monosaccharides are also classified based on the number of carbons in the backbone. Trioses, pentoses, and hexoses have three, five, and six carbon backbones, respectively.

Common Monosaccharides include glucose, galactose, and fructose. Glucose (C₆H₁₂O₆) is a common monosaccharide and an essential source of energy. During cellular respiration, glucose releases energy that helps make adenosine triphosphate (ATP). Plants synthesize glucose using carbon dioxide and water, and glucose, in turn, is used for energy requirements for the plant.

Galactose (a milk sugar) and fructose (found in fruit) are other common monosaccharides. Although glucose, galactose, and fructose all have the same chemical formula (C₆H₁₂O₆), they differ structurally and stereochemically, which makes them different molecules. They are all isomers of one another, or isomeric monosaccharides. Glucose and galactose are aldoses, and fructose is a ketose.

Monosaccharides can exist as a linear chain or as ring-shaped molecules; in aqueous solutions, they are found in the ring forms. Glucose in a ring form can have two different arrangements of the hydroxyl group (OH) around the anomeric carbon (this carbon becomes asymmetric in the process of ring formation). If the hydroxyl group is on the opposite side from the ${\text{CH₂}}^{}\text{OH group}$in the sugar, it is in the α form. If it is on the same side as the ${\text{CH₂}}^{}\text{OH group}$, it is in the β form.

Practice Questions

 Khan Academy   

The structure of monosaccharides

Hemiacetal formation of carbohydrates

MCAT Official Prep (AAMC)

Key Points

• Monosaccharides are the building blocks of carbohydrates and can be classified as aldehydes or ketones.

• Monosaccharides are also classified by the number of carbons in the backbone.

• Glucose, galactose, and fructose are monosaccharide isomers, which means they all have the same chemical formula but differ structurally and stereochemically.

• Monosaccharides can exist as a linear chain or as ring-shaped molecules, and are classified based on the position of their carbonyl.

• Monosaccharides are in the α form when the hydroxyl group is below carbon number 1, while they are in the β form when the hydroxyl group is above the plane.

Key Terms

Monosaccharides: The basic unit of carbohydrates that cannot be hydrolyzed to simpler chemical compounds with the general formula (CH₂O)n.

Aldehyde (aldose): A compound that contains the functional group -CHO, where the carbonyl center is bonded to a hydrogen and a side group R.

Ketone (ketose): A compound that contains the functional group -RC(=O)R’, where the carbonyl center is bonded to two side groups R and R’.

Adenosine triphosphate (ATP): An organic compound that provides energy to cells.

Stereochemistry: The branch of chemistry concerned with the three-dimensional arrangement of atoms and molecules and the effect of this on chemical reactions.

Isomer: Any of two or more compounds with the same molecular formula but with a different structure.

Anomeric carbon: A carbon derived from the carbonyl carbon (the aldehyde or ketone group) of the open-chain form of the carbohydrate molecule.