Carbohydrates are molecules composed of carbon, hydrogen, and oxygen that serve as the primary source of energy for the body. These compounds are broadly defined as polyhydroxy aldehydes or ketones, or substances that produce these structures upon breakdown. Carbohydrates are categorized based fundamentally on the number of sugar units, or saccharides, they contain. This structural distinction dictates how quickly they are digested and absorbed, which directly impacts their biological role.
Classification by Single Unit Sugars (Monosaccharides)
Monosaccharides represent the most basic form of carbohydrate, consisting of just a single sugar unit. The small molecular size of these simple sugars means they cannot be broken down further through hydrolysis. This single-unit structure allows for rapid absorption directly into the bloodstream from the small intestine. They are the building blocks used to construct all larger, more complex carbohydrates.
The most well-known monosaccharide is glucose, a six-carbon sugar that functions as the body’s main source of fuel. Fructose, often called fruit sugar, is another six-carbon monosaccharide that is naturally abundant in honey and various fruits. The third important monosaccharide is galactose, which is rarely found alone in food but is a component of milk sugar. Glucose and galactose are absorbed into the intestinal cells through a process called active transport, while fructose is absorbed via a more passive method.
Classification by Double Unit Sugars (Disaccharides)
Disaccharides are formed when two monosaccharide units are chemically linked together. This connection occurs through a process called condensation, which creates a glycosidic bond and releases a water molecule. Because their molecules are larger than those of monosaccharides, disaccharides must first be broken down before they can be absorbed into the body. This breakdown process, called hydrolysis, requires specific enzymes to split the glycosidic bond.
A common example is sucrose, or table sugar, which is a combination of one glucose unit and one fructose unit. Lactose, the sugar found in milk, is a disaccharide made up of glucose and galactose. The third major disaccharide is maltose, or malt sugar, which consists of two glucose units and is often a product of starch digestion. Enzymes in the small intestine are responsible for hydrolyzing these double sugars into their constituent single units.
Classification by Complex Chains (Polysaccharides)
Polysaccharides are large polymers made up of long chains of monosaccharide units, often ranging from hundreds to thousands of units. This extensive, complex structure means they require a much longer time for digestion compared to simpler sugars, or they may not be digestible at all. The monosaccharide units are linked together by glycosidic bonds, forming structures that can be either linear or highly branched. The three most abundant types—starch, glycogen, and cellulose—are primarily composed of repeating glucose units.
Starch, the primary energy storage form in plants, is a mixture of two glucose polymers: amylose and amylopectin. Amylose is a long, unbranched chain, while amylopectin features a highly branched structure. Glycogen is the corresponding energy storage form in animals and is even more highly branched than amylopectin. Cellulose, which forms the rigid structure of plant cell walls, is a linear and unbranched polymer of glucose units.
Functional Differentiation of Polysaccharides
The distinct structures of polysaccharides directly determine their differing biological functions, which fall into the categories of energy storage and structural support. Starch and glycogen serve as energy reserves that can be accessed when an organism requires fuel. Starch is stored in plants in places like seeds and tubers and can be broken down to nourish the plant or an animal that consumes it. Glycogen is the animal equivalent, stored mainly in the liver and muscle tissues, where its high degree of branching provides many points for rapid enzymatic breakdown.
In contrast, cellulose, commonly known as fiber, serves a structural role in the plant world. The linear arrangement of its glucose units allows cellulose chains to form strong fibers that provide rigidity to the plant cell wall. Humans lack the necessary enzymes to break the specific type of glycosidic bond in cellulose. This non-digestible nature means that cellulose passes through the human digestive tract largely intact, contributing to the bulk that aids in the regulation of digestion.