Carbohydrates are a broad class of organic compounds that includes sugars, sugar chains, and chemically modified sugar derivatives. Most contain carbon, hydrogen, and oxygen, although some also contain nitrogen, sulfur, or other elements. They range from small molecules such as glucose to large structural and storage polymers. In living organisms, carbohydrates supply fuel, store chemical energy, support biological structures, and participate in interactions between cells and their surroundings. The chemical definition is broader than the everyday use of “carbohydrate” to describe starchy or sugary foods. (goldbook.iupac.org)
Chemical definition and structure
The name originally referred to compounds with the formula Cₙ(H₂O)ₙ, suggesting “hydrates of carbon.” Many simple sugars fit this formula, but it is neither a universal carbohydrate formula nor a sufficient definition. Modern terminology also includes sugar alcohols, sugar acids, deoxy sugars, amino sugars, and their derivatives. These compounds are structurally related to sugars rather than literal combinations of carbon and water. (goldbook.iupac.org)
A monosaccharide typically has several hydroxyl groups and, in its open-chain form, an aldehyde or ketone group. Sugars with an aldehyde are called aldoses; those with a ketone are ketoses. Classification also reflects carbon number: trioses have three carbons, pentoses five, and hexoses six. Glucose is an aldohexose, whereas fructose is a ketohexose. (openstax.org)
The three-dimensional arrangement of these groups matters. Chirality gives rise to stereoisomers, including the D and L configurations. These labels describe configuration, not the direction in which a sugar rotates polarized light. In solution, many monosaccharides predominantly form rings. Ring formation creates an anomeric center, allowing α and β forms that can interconvert through ring opening and closure. Such differences influence both chemical behavior and biological recognition. (ncbi.nlm.nih.gov)
Classification and glycosidic linkages
Carbohydrates are commonly classified by the number of sugar units they contain:
- Monosaccharides are single sugar units, including glucose, fructose, galactose, and ribose.
- Disaccharides contain two units. Sucrose combines glucose and fructose; lactose combines glucose and galactose; maltose contains two glucose units.
- Oligosaccharides contain relatively short chains of sugar residues.
- Polysaccharides contain longer chains, which may be linear or branched. The boundary between oligosaccharides and polysaccharides is conventional rather than chemically sharp. (openstax.org)
Sugar units are connected by glycosidic bonds. A linkage is specified by the participating carbon positions and the configuration of the anomeric carbon. Consequently, identical sugar building blocks can produce very different substances. Cleavage of these bonds by hydrolysis breaks larger carbohydrates into smaller units. In biological synthesis, specialized enzymes commonly transfer sugar residues from activated donors to growing chains. (ncbi.nlm.nih.gov)
Storage and structural functions
Starch is a major carbohydrate reserve in plants. It contains amylose, composed mainly of α(1→4)-linked glucose chains, and amylopectin, which also has α(1→6) branch points. Glycogen, an important reserve in animals, has similar linkages but is more extensively branched. Its organization provides numerous chain ends at which glucose residues can be added or removed. (ncbi.nlm.nih.gov)
Cellulose also consists of glucose, but its β(1→4) linkages produce extended chains that associate into strong fibers. It is a major component of plant cell walls. The contrast between cellulose and starch illustrates how linkage geometry changes a polymer’s properties without changing its basic sugar building block. Chitin, composed of modified glucose residues containing nitrogen, provides structural support in arthropod exoskeletons and fungal cell walls. (ncbi.nlm.nih.gov)
Carbohydrates also occur within other essential biomolecules. Ribose is the sugar component of RNA, while deoxyribose forms part of DNA. These sugars contribute to the repeating backbone of nucleic acids rather than serving primarily as energy stores. (ncbi.nlm.nih.gov)
Production and metabolism
Through photosynthesis, plants and other photosynthetic organisms use light energy to produce organic compounds from carbon dioxide. The resulting three-carbon sugar phosphates can be used to synthesize glucose, sucrose, starch, and other carbohydrates. These products supply both metabolic fuel and raw materials for growth. (openstax.org)
Carbohydrate breakdown is central to metabolism. In glycolysis, a six-carbon glucose molecule is converted into two three-carbon pyruvate molecules, with a net production of two ATP molecules and reduced electron carriers. Glycolysis does not directly require oxygen. Subsequent pathways determine whether glucose-derived carbon undergoes further oxidation or enters fermentation and other metabolic processes. Carbohydrate intermediates also provide starting materials for the synthesis of other cellular constituents. (openstax.org)
Digestion and nutritional terminology
During human digestion, enzymes break digestible carbohydrates into absorbable simple sugars. Breakdown begins in the mouth and continues in the small intestine, where most nutrient absorption occurs. Absorbed sugars enter the circulation and are transported to tissues for use or storage. (niddk.nih.gov)
In food-energy calculations, available carbohydrate is commonly assigned approximately 4 kilocalories, or 17 kilojoules, per gram. This factor does not apply uniformly to every carbohydrate. Dietary fiber includes carbohydrate components that resist digestion in the small intestine; some undergo microbial fermentation in the colon. Their energy contribution therefore differs from that of readily digested sugars and starch. “Simple” and “complex” describe broad structural categories, not complete nutritional assessments of foods. (fao.org)
Cell surfaces and molecular recognition
Sugar chains, often called glycans, can be attached to proteins or lipids. This attachment, termed glycosylation, creates structures with roles beyond fuel storage. Glycans can influence protein stability and function, contribute to extracellular organization, and provide recognition sites for binding proteins. (ncbi.nlm.nih.gov)
At the cell surface, differences in sugar sequence, branching, and chemical modification generate diverse recognition patterns. These participate in cell–cell interactions and interactions with microorganisms. Some microbial adhesion proteins and toxins bind particular host glycans. A glycan’s function therefore depends not only on its chemical structure but also on its molecular surroundings, tissue location, and biological context. (ncbi.nlm.nih.gov)