Carbohydrates or saccharides are the most abundant of the four major classes of biomolecules. They fill numerous roles in living things, such as the storage and transport of energy (e.g., starch, glycogen) and structural components (e.g., cellulose in plants and chitin in arthropods). In addition, carbohydrates and their derivatives play major roles in the working process of the immune system, fertilization, pathogenesis, blood clotting, and development.

Foods high in carbohydrates include breads, pastas, beans, potatoes, bran, rice, and cereals. Most such foods are high in starch. Carbohydrates require less water to digest than proteins or fats and are the most common source of energy in living things. Proteins and fat are necessary building components for body tissue and cells, and are also a source of energy for most organisms.