Carbohydrates are essential macromolecules found in living organisms, serving as a primary energy source. Composed of carbon, hydrogen, and oxygen atoms, they are built from smaller, repeating units. Understanding these building blocks provides insight into their diverse biological roles.
Monosaccharides
Monosaccharides are the simplest carbohydrates, often called “simple sugars.” They are fundamental subunits that cannot be broken down into smaller units through hydrolysis. These molecules consist of three to seven carbon atoms and are characterized by a linear, unbranched carbon skeleton with one carbonyl group and multiple hydroxyl groups.
Examples of monosaccharides include glucose, fructose, and galactose. Glucose, a six-carbon sugar (hexose), is a primary energy source for cells, found in fruits and honey. Fructose, another hexose, is found in fruits and vegetables, and is often called fruit sugar. Galactose is found in milk as part of the disaccharide lactose. These simple sugars are water-soluble, crystalline solids, and have a sweet taste.
Disaccharides
Disaccharides are formed when two monosaccharide units join together. This linkage occurs through a process called dehydration synthesis. During this reaction, a hydroxyl group from one monosaccharide combines with a hydrogen atom from another, releasing a water molecule and forming a covalent bond called a glycosidic bond.
Examples of disaccharides include sucrose, lactose, and maltose. Sucrose, or table sugar, is formed from one glucose molecule and one fructose molecule, and is found in many fruits and vegetables. Lactose, known as milk sugar, is composed of one glucose molecule and one galactose molecule. Maltose, or malt sugar, consists of two glucose molecules linked together and is found in germinating grains. These double sugars serve as energy sources and are broken down into their individual monosaccharides during digestion.
Polysaccharides
Polysaccharides are complex carbohydrates formed from long chains of many monosaccharide units linked by glycosidic bonds. These polymers consist of hundreds or thousands of simple sugar units and play diverse roles in living organisms, primarily for energy storage and structural support. Their specific structure, whether linear or branched, dictates their function.
Starch serves as the primary energy storage polysaccharide in plants. It is composed of glucose units and is found in foods like potatoes, bread, and rice. Glycogen is the analogous storage polysaccharide in animals and humans, primarily stored in the liver and muscles. It provides a readily available source of glucose to meet energy demands, particularly during physical activity. Cellulose, another polysaccharide, provides structural support in plants, forming the main component of plant cell walls. While humans cannot digest cellulose, it functions as dietary fiber, aiding in digestive health.