Carbohydrates are biological molecules that serve as a primary source of energy and structural components for living organisms. They are built from smaller, repeating units called monomers. This basic building block links together to form the larger chain, determining the chemical properties and biological function of the entire structure.
Defining the Monomer
The monomer of a carbohydrate is a monosaccharide. These molecules are composed of carbon, hydrogen, and oxygen atoms, often following the general chemical formula \(\text{C}_n\text{H}_{2n}\text{O}_n\). They have a carbon backbone containing between three and seven carbon atoms, with hydroxyl (\(\text{-OH}\)) groups attached to most carbons. One carbon atom features a carbonyl group (a double bond to an oxygen atom), classifying the sugar as either an aldose or a ketose.
Although monosaccharides can exist as linear chains, the predominant form in an aqueous solution is a ring structure. This cyclization occurs when the carbonyl group reacts with a hydroxyl group within the same molecule, forming a stable ring of five or six atoms. For example, glucose mainly forms a six-membered ring (pyranose), while fructose often forms a five-membered ring (furanose).
Key Examples of Monosaccharides
The most biologically relevant monosaccharides are the hexoses, which are six-carbon sugars sharing the chemical formula \(\text{C}_6\text{H}_{12}\text{O}_6\). These molecules—glucose, fructose, and galactose—are structural isomers, meaning they have the same atoms arranged differently, giving them unique properties. Glucose is the most well-known, often called blood sugar, and functions as the primary energy source for cells. Its structure, featuring an aldehyde functional group, allows it to be efficiently metabolized through cellular respiration.
Fructose is commonly found in fruits and honey. Structurally, it is a ketose, meaning its carbonyl group is located internally, which influences its metabolism. Galactose is a component of lactose, the sugar found in milk. Smaller monosaccharides, such as ribose and deoxyribose, are also important, acting as structural components of ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), respectively.
Connecting Monomers to Complex Carbohydrates
Monosaccharides link together to form larger carbohydrate structures. The chemical reaction that joins two single sugar units is known as dehydration synthesis, or a condensation reaction. During this process, a molecule of water (\(\text{H}_2\text{O}\)) is released. The remaining oxygen atom acts as a bridge, forming a covalent connection called a glycosidic bond between the two sugars.
When two monosaccharides join, they form a disaccharide, such as sucrose (table sugar), which is a glucose molecule bonded to a fructose molecule. When many monomers are linked together, the resulting structures are called polysaccharides. These polymers serve two main functions: energy storage and structural support. Starch and glycogen are examples of storage polysaccharides, while cellulose forms the rigid structural material in plant cell walls.