Carbohydrates are compounds made of carbon, hydrogen, and oxygen. They include monosaccharides like glucose and fructose, disaccharides formed from two monosaccharides like sucrose, and polysaccharides formed from long chains of monosaccharides like starch, glycogen, and cellulose. Monosaccharides can be classified as aldoses or ketoses depending on whether they contain an aldehyde or ketone group. Aldoses are reducing sugars while ketoses are non-reducing. Disaccharides and polysaccharides provide energy storage in plants and animals. Starch, glycogen, and cellulose are made of glucose monomers arranged differently, influencing their properties and uses in organisms.