Carbohydrates can be named based on the length of the carbon chain, the functional groups present on the sugar, and the stereochemistry of the sugar.
To be classified as a carbohydrate, a molecule must have at least a 3-carbon backbone, an aldehyde or ketone group, and at least two hydroxyl groups.
Simple carbohydrates such as glucose, lactose, or dextrose, end with an “-ose”. Trioses are the simplest monosaccharides containing three carbon atoms. Carbohydrates with four, five, and six carbon atoms are called tetroses, pentoses, and hexoses, respectively. Carbohydrates that contain an aldehyde group as their most oxidized functional group are called aldoses, and those with a ketone group are called ketoses. Thus, a six-carbon sugar with an aldehyde group is an aldohexose, while a five-carbon sugar with a ketone group is a ketopentose. Consequently, furanose and pyranose are collective terms for sugars that have a five- and six-membered ring structure, respectively. For example, fructofuranose is a fructose in a five-membered ring while glucopyranose is a glucose in a six-membered ring is called.
The presence of chiral carbons in carbohydrates determines a molecule’s stereochemistry as either the D or L configuration. The D configuration is naturally occurring in metabolism (similar to L-amino acids).
While the official nomenclature can be handy for identifying carbohydrate structures, common names are also frequently used when referring to sugars, such as glucose, fructose, galactose, and mannose.
Carbohydrate molecules can exist as single monomers (monosaccharides), or chained together by glycosidic linkage in pairs of two (disaccharides), short chains (oligosaccharides), or long chains (polysaccharides).
Practice Questions
Khan Academy
The structure of monosaccharides
Hemiacetal formation of carbohydrates
MCAT Official Prep (AAMC)
Practice Exam 1 B/B Section Passage 5 Question 27
Key Points
• A carbohydrate must have at least a three-carbon backbone, an aldehyde or ketone group, and at least two hydroxyl groups.
• Carbohydrates with aldehyde and ketone group are called aldoses and ketones, respectively.
• Common names are also used when referring to sugars such as glucose, fructose, galactose, and mannose.
• The presence of chiral carbons in carbohydrates determines a molecule’s stereochemistry as either the D or L configuration.
• Carbohydrate molecules can exist as monosaccharides, or be chained together by glycosidic linkages into disaccharides, oligosaccharides, or polysaccharides.
Key Terms
Functional groups: Specific groupings of atoms in a molecule that provide characteristic chemical reactions to the molecule.
Stereochemistry: The branch of chemistry concerned with the three-dimensional arrangement of atoms and molecules and the effect of this on chemical reactions.
Aldehyde: A compound that contains the functional group -CHO, where the carbonyl center is bonded to a hydrogen and a side group R.
Ketone: A compound that contains the functional group -RC(=O)R’, where the carbonyl center is bonded to two side groups R and R’.
Chiral: A geometric property of molecules that indicates that it cannot be superimposed over its mirror image through any series of rotations or translations.
Monosaccharides: The basic unit of carbohydrates that cannot be hydrolyzed to simpler chemical compounds with the general formula (CH2O)n.
Glycosidic linkage: A type of covalent bond that joins a carbohydrate molecule to another group.
Disaccharides: A carbohydrate formed when two monosaccharides are connected by a glycosidic linkage.
Oligosaccharides: A carbohydrate chain formed by several monosaccharides connected by glycosidic linkages.
Polysaccharides: A long carbohydrate chain formed by many monosaccharides connected by glycosidic linkages.