Carbohydrate Classification, Properties, and Functions
Carbohydrate Classification
Monosaccharides (Oses)
Classified according to the functional group:
- Aldose
- Ketose
Based on the number of carbon atoms:
- Trioses (3)
- Tetroses
- Pentoses
- Hexoses
- Heptoses
Osids
- Holosids
- Oligosaccharides
- Polysaccharides
- Homopolysaccharides
- Heteropolysaccharides
- Heterosids
Carbohydrate Definition
Carbohydrates are compounds resulting from replacing one of the functional groups (-OH) of a polyalcohol with another functional group, either an aldehyde or a ketone.
General Characteristics of Carbohydrates
Carbohydrates, also known as “saccharides”, are organic molecules with the empirical formula (CH2O)n, resembling a combination of carbon and water. They are the most abundant compounds in the biosphere.
Monosaccharides (Oses)
Monosaccharides are the simplest carbohydrates, as they cannot be decomposed by hydrolysis into simpler monosaccharides. They are the monomers that make up other carbohydrates.
Properties
- White, sweet solids
- Highly soluble in water (a polar solvent)
- Insoluble in nonpolar solvents
- Reducing power (tendency to donate electrons)
- Aldose: A monosaccharide with an aldehyde functional group
- Ketose: A monosaccharide with a ketone functional group
Trioses are the simplest monosaccharides. Pentoses (5 carbon atoms) include ribose, which is part of RNA.
Hexoses
General formula: C6H12O6. An example is glucose, found in many fruits, honey, blood, etc. Blood glucose comes from the digestion of carbohydrates in food. When glucose is degraded, it provides the energy our body needs.
Fructose is another hexose found in most fruits along with glucose. Galactose is part of milk sugar. Pentoses with the -OH in the carbon and hexoses can join to form cycles, where the aldehyde or ketone groups interact with the alcohol group of another carbon in the same molecule.
Osids
Carbohydrates that yield monosaccharides upon hydrolysis, accompanied or not by other compounds.
Holosids
Carbohydrates that yield only monosaccharides upon hydrolysis. They are classified according to the number of monosaccharides:
- Oligosaccharides: 2 to 10 monosaccharides, hydrolyzable to yield multiple monosaccharide molecules. They can join via O-glycosidic bonds. Some are reducing oligosaccharides (tendency to donate electrons). As the number of monosaccharides increases, they lose properties like sweetness, solubility, etc. The O-glycosidic bond is established between two hydroxyl groups of monosaccharides, forming a water molecule and an oxygen bridge.
- Disaccharides: Union of two monosaccharides. Examples include:
- Sucrose: Fructose + glucose (common in plants, industrially extracted from sugarcane)
- Maltose: Glucose + glucose (from hydrolysis of starch and glycogen)
- Lactose: Galactose + glucose (sugar in mammalian milk)
- Trisaccharides: Union of three monosaccharides. Examples include:
- Raffinose: Galactose + glucose + fructose (abundant in plant sap)
- Polysaccharides: Union of more than 10 monosaccharides via glycosidic bonds, with very high molecular weight. They are not sweet, do not crystallize, are not water-soluble, are very hydrophilic, and are not reducing. Examples include:
- Starch: Homopolysaccharide, energy reserve in plants, a polymer of glucose
- Glycogen: Homopolysaccharide, energy reserve in animals (liver and muscles), a polymer of glucose
- Cellulose: Homopolysaccharide, a polymer of glucose, the most abundant polysaccharide in the biosphere
- Chitin: Homopolysaccharide with structural function, part of the exoskeleton of arthropods and cell walls of fungi
Heterosids
Compounds that, upon hydrolysis, yield monosaccharides and other substances like lipids or proteins. An example is glycoproteins, resulting from the union of a carbohydrate and a peptide chain.
Functions of Carbohydrates
- Monosaccharides: Primarily energy source, some structural
- Oligosaccharides: Mostly a source of monosaccharides
- Polysaccharides: Energy reserve, some structural function