Carbohydrates: Structure, Classification, and Properties

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Carbohydrates

Definition and General Properties

Carbohydrates are essential biomolecules that serve as the primary source of energy for most living organisms. They are defined as polyhydroxy derivatives of aldehydes or ketones, meaning they contain multiple hydroxyl (–OH) groups along with either an aldehyde or ketone functional group.

General Formula: Most simple carbohydrates follow the formula Cn(H2O)n.
Classification: Carbohydrates are classified based on the number of sugar units and the type of carbonyl group present.

Classification of carbohydrates into monosaccharides, disaccharides, oligosaccharides, and polysaccharides

Classification of Carbohydrates

Monosaccharides

Monosaccharides are the simplest carbohydrates, consisting of a single sugar unit. They are classified by the number of carbon atoms and the type of carbonyl group (aldose or ketose).

Aldoses: Monosaccharides with an aldehyde group.
Ketoses: Monosaccharides with a ketone group.

Number of 'C' atoms	Name of the monosaccharide	Aldose	Ketose
3	Triose	Glyceraldehyde (Glyceroe)	Dihydroxyacetone
4	Tetrose	Erythrose	Erythrulose
5	Pentose	Ribose	Ribulose
6	Hexose	Glucose	Fructose
7	Heptose	Glucoheptose	Sedoheptulose

Table of monosaccharide classification by carbon number, aldose, and ketose

Structures of Common Aldoses

Aldoses are monosaccharides with an aldehyde group at one end. Their structures vary based on the arrangement of hydroxyl groups along the carbon chain.

Structures of common aldoses

Structures of Common Ketoses

Ketoses are monosaccharides with a ketone group, typically at the second carbon atom. Their diversity arises from the number of carbons and the arrangement of hydroxyl groups.

Structures of common ketoses

Disaccharides

Disaccharides are carbohydrates composed of two monosaccharide units joined by a glycosidic bond. Common examples include maltose, sucrose, and lactose.

Maltose: Composed of two glucose units.
Sucrose: Composed of glucose and fructose.
Lactose: Composed of glucose and galactose.

Maltose structure

Oligosaccharides and Polysaccharides

Oligosaccharides contain 3–10 monosaccharide units, while polysaccharides are large polymers with more than ten sugar units. Polysaccharides serve as storage (e.g., starch, glycogen) or structural (e.g., cellulose) molecules in cells.

Starch: Storage polysaccharide in plants, composed of amylose and amylopectin.
Glycogen: Storage polysaccharide in animals, highly branched.
Cellulose: Structural polysaccharide in plant cell walls, composed of β-glucose units.

Amylose and amylopectin structures Glycogen structure Cellulose structure

Properties and Reactions of Monosaccharides (Glucose & Fructose)

Isomerism in Monosaccharides

Monosaccharides can exist as structural isomers (same formula, different structure) and stereoisomers (same structure, different spatial arrangement). D- and L- isomers refer to the orientation of the hydroxyl group on the asymmetric carbon farthest from the carbonyl group.

D- and L- isomers of glucose

Oxidation Reactions

Monosaccharides like glucose can undergo oxidation to form various acids:

Glucuronic acid: Formed by oxidation of the primary alcohol group at C-6 of glucose.
Glucaric acid (aldaric acid): Formed by oxidation of both the aldehyde and the primary alcohol group.

Oxidation of D-glucose to D-glucuronic acid

Ring Structure of Glucose

Glucose commonly exists in a cyclic (ring) form in solution, resulting from the reaction between the aldehyde group and a hydroxyl group to form a hemiacetal.

Ring structure of glucose

Example: The conversion of glucose from its linear to cyclic form is essential for its biological function and recognition by enzymes.

Additional info: Carbohydrates play critical roles in energy storage, structural integrity, and cell signaling. Their diversity in structure allows for a wide range of biological functions.