Carbohydrates are a fundamental class of macronutrients that serve as a primary energy source for the human body. These organic compounds are widely present in various foods and beverages, including fruits, vegetables, grains, and dairy products. The body efficiently breaks down carbohydrates to produce glucose. This glucose then fuels the cells, tissues, and organs, providing the necessary energy for daily functions.
The Fundamental Building Block
The fundamental building blocks of all carbohydrates are simple sugar units known as monosaccharides. Monosaccharides, meaning “single sugars,” are the simplest forms of sugar molecules. They cannot be broken down into smaller carbohydrate units through hydrolysis. Their general chemical structure typically features a linear and unbranched carbon skeleton, containing one carbonyl functional group (either an aldehyde or a ketone) and a hydroxyl group on each of the remaining carbon atoms. This structure allows them to exist in both straight-chain and cyclic (ring) forms, which can interconvert in solution. Monosaccharides are the basic units from which more complex carbohydrates are constructed.
How Subunits Form Larger Carbohydrates
Monosaccharides link to form more complex carbohydrates through dehydration synthesis, also known as a condensation reaction. During this reaction, a water molecule is removed as two monosaccharide units join, forming a covalent bond known as a glycosidic bond.
When two monosaccharide units combine, they form a disaccharide, meaning “two sugars.” Common examples of disaccharides include sucrose, which is table sugar formed from glucose and fructose; lactose, found in milk and composed of glucose and galactose; and maltose, created from two glucose units. These disaccharides can be broken back down into their individual monosaccharide components through hydrolysis, which adds a water molecule back into the glycosidic bond.
Beyond disaccharides, many monosaccharide units can link together to form polysaccharides, or “many sugars.” These long chains can be linear or branched, performing various functions in living organisms. Polysaccharides serve as energy storage molecules, such as starch in plants and glycogen in animals. They also provide structural support, with cellulose forming the cell walls of plants and chitin found in the exoskeletons of insects and cell walls of some fungi.
Common Examples of Carbohydrate Subunits
Glucose is a monosaccharide often referred to as “blood sugar” or “dextrose.” It is the most abundant and primary energy source for nearly all living organisms. Plants produce glucose during photosynthesis, and it is a key component in the formation of starch, glycogen, and cellulose.
Fructose, known as “fruit sugar,” is another common monosaccharide. Fructose is naturally present in fruits, honey, and certain vegetables. It is one of the three dietary monosaccharides directly absorbed by the gut, alongside glucose and galactose. Fructose is often bonded with glucose to form sucrose, or common table sugar.
Galactose is a monosaccharide that is a key component of lactose, the disaccharide found in milk. The body can convert galactose into glucose for energy.