The building blocks of carbohydrates are monosaccharides, which are simple sugars consisting of single sugar units. Common examples include glucose, fructose, and galactose. These monosaccharides can link together to form larger carbohydrate structures, such as disaccharides (e.g., sucrose) and polysaccharides (e.g., starch, glycogen, cellulose). Carbohydrates serve as a primary energy source for living organisms, playing vital roles in metabolism, energy storage, and structural functions within cells.