Carbohydrates are macromolecules containing carbon, hydrogen, and oxygen and are represented by the stoichiometric formula (CH2O)n, where n is the number of carbons in the molecule.
The basic structure of a carbohydrate is a chain of carbon atoms, numbered from the most oxidized end, which contains a carbonyl carbon (C=O) in the form of either an aldehyde or ketone, as shown in the figure below. Based on this, carbohydrates can be referred to by whether they are an aldose or ketose, the number of carbons in the chain (for example, a pentose has five carbons), or by putting the concepts together (for example, an aldopentose is a five-carbon carbohydrate with an aldehyde functional group). Carbohydrate chains come in different lengths, and biologically important carbohydrates belong to three categories: monosaccharides, disaccharides, and polysaccharides.
Carbohydrates are generally referred to by their common names. Examples are glucose, sucrose, galactose, and ribose. They can also exist as single monomers (monosaccharides) or can be chained together by glycosidic linkages in pairs of two (disaccharides), short chains (oligosaccharides), or long chains (polysaccharides).
Monosaccharides undergo cyclization from their straight-chain forms (represented as two-dimensional Fischer projections) to create cyclic sugar molecules (represented as three-dimensional Haworth forms). Depending on the number of carbons in the sugar, monosaccharides also may be known as trioses (three carbons), pentoses (five carbons), and or hexoses (six carbons). When a monosaccharide ring forms, the anomeric carbon can take on either an α- (down) or β-conformation (up).
In addition, 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). The chiral carbon determines the absolute configuration, or three-dimensional organization, of a molecule. Epimers and anomers are types of stereoisomers of carbohydrates that differ in the position at a single carbon atom.
Practice Questions
Khan Academy
The structure of monosaccharides
MCAT Official Prep (AAMC)
Section Bank C/P Section Passage 10 Question 76
Practice Exam 1 C/P Section Passage 8 Question 40
Section Bank B/B Section Question 14
Section Bank B/B Section Passage 11 Question 85
Key Points
• Carbohydrates are named and classified by the number of carbon atoms, the functional groups on the molecule, and the stereochemistry of the sugar.
• The absolute configuration for a carbohydrate is assigned based on the last chiral carbon in the chain.
• Carbohydrates undergo cyclization from their straight-chain forms (Fischer projection) to create cyclic sugar molecules (Haworth form).
Key Terms
Macrmolecules: Large molecules made up of many monomeric units linked through covalent bonds; these include carbohydrates, lipids, proteins, and nucleic acids.
Stoichiometric formula: Based on the law of conservation of mass, describes how the amount of products for a given chemical reaction can be determined based on the amount of reactants.
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’.
Monosaccharides: The basic unit of carbohydrates that cannot be hydrolyzed to simpler chemical compounds with the general formula (CH2O)n.
Disaccharides: A carbohydrate formed when two monosaccharides are connected by a glycosidic linkage.
Polysaccharides: A long carbohydrate chain formed by many monosaccharides connected by glycosidic linkages.
Glycosidic linkage: A type of covalent bond that joins a carbohydrate molecule to another group.
Oligosaccharides: A carbohydrate chain formed by several monosaccharides connected by glycosidic linkages.
Fischer projection: A two-dimensional representation of a molecule that maintains information about its absolute configuration.
Haworth projection: A simple three-dimensional representation of a monosaccharide.
Anomeric carbon: A carbon derived from the carbonyl carbon (the ketone or aldehyde functional group) of the open-chain form of the carbohydrate molecule.
Chiral: A geometric property of molecules that indicates that it cannot be superimposed over its mirror image through any series of rotations or translations.
Epimers: A type of stereoisomer that differs in configuration at a single stereogenic center (the anomeric carbon).
Anomers: A type of stereoisomer that differs in configuration at the hemiacetal or acetal carbon; they are a specific type of epimer.
Stereochemistry: The branch of chemistry concerned with the three-dimensional arrangement of atoms and molecules and the effect of this on chemical reactions.
Stereoisomers (aka optical isomers): Any of two or more compounds with identical molecular formulas and arrangements of atoms that only differ in the spatial arrangement of their atoms; they have non-superimposable mirror images.
