Carbohydrates are biomolecules composed of carbon, hydrogen, and oxygen atoms, often with a hydrogen-to-oxygen ratio of 2:1, similar to water. This gives them their name, “carbon hydrates.” They serve as a primary energy source for cells and provide structural support in biological systems.
Monosaccharides: The Simple Sugars
Monosaccharides are the simplest carbohydrates, serving as building blocks for more complex sugar molecules. Their general chemical formula is Cn(H2O)n, indicating an equal number of carbon atoms and water units. These single sugar units contain three to seven carbon atoms.
Monosaccharides exist in two main structural forms: a linear chain or a ring shape. The ring structure is the predominant form in aqueous solutions and in their solid state. Glucose, a six-carbon sugar (C6H12O6), is a widely distributed example, serving as an important energy source in humans.
Fructose, a monosaccharide found in fruits, has the same chemical formula as glucose (C6H12O6) but differs in atomic arrangement. Galactose is a component of lactose, the sugar found in milk. These simple sugars are highly soluble in water due to multiple hydroxyl groups.
From Simple to Complex: Disaccharides and Polysaccharides
Monosaccharides link to form larger carbohydrate molecules through a condensation reaction, which involves the removal of a water molecule. This linkage between two monosaccharide units is known as a glycosidic bond. These bonds can form between various carbon atoms, contributing to the diversity of carbohydrate structures.
When two monosaccharides join via a glycosidic bond, they form a disaccharide, also known as a double sugar. Examples include sucrose, lactose, and maltose, all having the general formula C12H22O11. Sucrose (table sugar) forms from one glucose and one fructose molecule, found in sugar cane and sugar beets. Lactose, the sugar in milk, is composed of a galactose molecule linked to a glucose molecule. Maltose forms from two glucose units and is derived from starch.
Polysaccharides are large, complex carbohydrates made of many monosaccharide units linked by glycosidic bonds. These long chains can be linear or highly branched, and their molecular weight can be substantial. Starch, glycogen, and cellulose are three major polysaccharides, all composed of glucose monomers.
Starch, found in plants, consists of amylose (a linear chain of alpha-1,4 linkages) and amylopectin (a branched chain with both alpha-1,4 and alpha-1,6 linkages). Glycogen, the energy storage molecule in animals, is similar to amylopectin but more highly branched, with alpha-1,4 and alpha-1,6 bonds creating branching points. Cellulose, a structural component of plant cell walls, is a linear polymer of beta-glucose units linked by beta-1,4 bonds.
How Molecular Structure Shapes Function
The specific arrangement of atoms and types of bonds within carbohydrate molecules directly influence their biological roles. The helical, compact structure of starch, formed by alpha-glucose monomers linked by alpha-1,4 glycosidic bonds, makes it an effective energy storage molecule in plants. These alpha linkages are easily broken by enzymes, allowing for efficient breakdown into glucose for energy. Glycogen, with its denser and more branched structure due to more frequent alpha-1,6 glycosidic linkages, allows for rapid mobilization of glucose, suiting the active energy demands of animals.
In contrast, cellulose’s linear structure, formed by beta-glucose monomers and beta-1,4 glycosidic linkages, results in strong, rigid fibers. These linear chains form numerous hydrogen bonds with other cellulose strands, creating robust fibrils that provide structural support to plant cell walls. Humans lack the enzymes to break down these beta-glycosidic linkages, meaning cellulose is indigestible and functions as dietary fiber rather than an energy source. The distinct molecular shapes and bonding patterns of these polysaccharides dictate whether they serve as readily available energy reserves or provide durable structural components.