Individual carbohydrate molecules can be classified by the number of carbons each contain. 3-carbon sugars are trioses; 4-carbon sugars are tetroses, 5-carbon sugars are pentoses, 6-carbon sugars are hexoses. Carbohydrates can also be classified by the functional group present on it. If an aldehyde group (-RC(=O)H) is their most oxidized functional group, then it’s an aldose. If a ketone group(-RC(=O)R’) is their most oxidized group, then it’s a ketose.
Carbohydrates can also be classified into 3 classes based on the number of sugars that make them up. monosaccharides, disaccharides, and polysaccharides. As the prefixes suggest, monosaccharide means “single sweet unit,” disaccharide means “two sweet units,” and polysaccharide means “multiple sweet units.”
Shown above is glucose, whose structure and formula is worth memorizing (along with fructose, galactose, and mannose). It is also worth memorizing that monosaccharides have the general formula C(n)H(2n)O(n). For example, ribose is C5H10O5. Generally, most carbohydrates will be shown in ring structure form (the cyclic form), but the long-chain structure is also important for many metabolic processes, where the ring is broken. In the long-chain form, the most oxidized carbon will always be labeled as the C1 carbon. When a sugar is cyclized from its long chain form to its ring form, the anomeric carbon (C1) is the new chiral center. THe same carbon was the crbonyl carbon in the long chain structure. When two monosaccharides bind together, they form a glycosidic linkage, which is a covalent bond. As shown below, there are two types of glycosidic bonds, α and β. Which one depends on the stereochemistry (ew) or orientation of the -OH (hydroxy) group relative to the C1 carbon (the anomeric carbon). If the hydroxy group is located above the plane of the molecule (cis to the free CH2OH) , and thus above the C1 carbon, then the molecule is β . If it is below the plane (trans to the free CH2OH), then the molecule is α. It is possible to shift from one anomeric form to another, through a process known as mutarotation, which has the long-chain form as an intermediate.
This method for classifying glycosidic linkages holds true for all linkages in polysaccharides. The formation of the glycosidic bond involves the condensation of H2O (water). Breaking the glycosidic bond consumes H2O, and is known as hydrolysis, which is the general term for using the addition of water to break a bond. In general, hydrolysis of polysaccharides into monosaccharide components is thermodynamically favorable. However, this hydrolysis is really slow without the presence of an enzyme to catalyze it. Generally, enzymes are named for the sugar they hydrolyze. Maltase is the enzyme that catalyzes the hydrolysis of maltose into two glucose monosaccharides. Glycogen is a common polysaccharide used to store carbohydrates in animals. Shown below, it consists of thousands of glucose molecules joined together by α-1,4 linkages in chains and α-1,6 linkages in branches.
Cellulose is similar to glycogen except it’s unbranched and only has α-1,4 linkages. It’s the main structural component of plant cell walls and is a significant source of fiber in the human diet. Starch is also similar to glycogen, with both α-1,6 linkages and α-1,4 linkages, except it’s overall less branched than glycogen and can also exist in an unbranched form. You should associate amylose and amylopectin with starch. Starches are used by plants to store energy.