Carbohydrate

Carbohydrates are a broad class of biomolecules composed primarily of carbon, hydrogen and oxygen, generally following an approximate 2:1 ratio of hydrogen to oxygen atoms, similar to that found in water. Although this ratio is common, many biological carbohydrates deviate from the simple empirical formula due to structural modifications such as acetylation, sulfation, or deoxygenation. In biochemistry the term carbohydrate is effectively synonymous with saccharide and refers to sugars, starches and cellulose, all of which play essential structural and metabolic roles in living organisms.

General Features and Classification

Carbohydrates occur naturally in a wide range of forms and can be classified into monosaccharides, disaccharides, oligosaccharides and polysaccharides according to their degree of polymerisation. Monosaccharides and disaccharides, often collectively described as sugars, typically bear the suffix “-ose”. Common examples include glucose, fructose, ribose, sucrose and lactose, the disaccharide found in animal milk consisting of galactose and glucose linked by a β-1,4 glycosidic bond.
Monosaccharides are the fundamental units from which more complex carbohydrates are built. Their general structure takes the form of an aldehyde or ketone possessing several hydroxyl groups. They can cyclise to form ring structures through intramolecular hemiacetal formation. Variations in the position of the carbonyl group (aldoses versus ketoses), the number of carbon atoms (trioses, tetroses, pentoses, hexoses, etc.) and their stereochemistry allow for hundreds of structural possibilities in nature. Modified monosaccharides, including deoxysugars, uronic acids and N-acetyl derivatives, greatly expand this diversity.
Disaccharides consist of two monosaccharides linked by an acetal (glycosidic) bond, while oligosaccharides generally include between three and ten monosaccharide units. Polysaccharides, composed of long chains of repeating sugar residues, perform vital roles in energy storage and structural integrity. Starch and glycogen serve as primary storage polymers in plants and animals respectively, whereas cellulose and chitin provide mechanical strength in plant cell walls and the exoskeletons of arthropods and fungi.

Biological Importance

Carbohydrates are essential to the metabolism and physiology of organisms. As energy sources, they fuel cellular respiration through molecules such as glucose. Storage polysaccharides provide reserves that can be mobilised when needed. Structurally, they form the basis of cell walls and extracellular matrices.
Several carbohydrate derivatives are central to genetic and biochemical processes.

• Ribose, a five-carbon sugar, is a key component of coenzymes such as ATP, FAD and NAD⁺ and forms the backbone of RNA.
• Deoxyribose, a modified pentose, constitutes the structural framework of DNA.

Carbohydrate-based molecules also participate in cell recognition, immunity, fertilisation, blood clotting and developmental signalling. Glycoproteins and glycolipids, for example, contain oligosaccharide chains that determine cell–cell interactions and antigenicity.
Dietary carbohydrates form a major component of human nutrition.

• Starch predominates in cereals (wheat, rice, maize), potatoes and foods derived from cereal flours.
• Sugars occur naturally in fruits, honey, milk and vegetables, while sucrose from sugarcane or sugar beet is widely used as a sweetener.
• Dietary fibre, largely composed of cellulose and other polysaccharides resistant to digestive enzymes, contributes to digestive health. Insoluble fibre aids bowel movement, whereas soluble fibre is fermented by intestinal microbiota to produce short-chain fatty acids that have various physiological benefits.

Terminology and Nutritional Use

In scientific contexts several overlapping terms such as sugar, saccharide, glucide and hydrate of carbon are used, though distinctions can be important in biochemistry. In everyday food science the word carbohydrate frequently refers to starch-rich foods—such as bread, pasta and cereals—or to sweet foods containing simple sugars. This informal usage can be ambiguous because it conflates chemical structure with nutritional behaviour. Dietary fibre, though chemically a carbohydrate, is often treated separately owing to its negligible contribution to caloric intake.

Historical Background

Humans have interacted with carbohydrate-rich plants for millennia, with evidence of sugarcane cultivation in Papua New Guinea dating back around 10,000 years. The scientific study of carbohydrates, however, developed much later. The term carbohydrate was proposed in the mid-19th century by Carl Schmidt, reflecting the empirical formula that early chemists used to describe these compounds. In 1856 Claude Bernard discovered glycogen, the principal carbohydrate reserve in animals, marking an important advance in physiological chemistry.

Structural Principles

Classically, carbohydrates were defined by the formula CₘH₂ₙOₙ, but this definition proved too restrictive and chemically misleading. Modern biochemistry recognises carbohydrates as polyhydroxy aldehydes, ketones, alcohols, acids and their derivatives, often containing additional functional groups. Many biologically relevant carbohydrates do not match the simple formula, and some molecules that do (such as formaldehyde) are not carbohydrates at all.
Monosaccharides typically follow the general structural motif H–(CHOH)ₓ–C(=O)–(CHOH)ᵧ–H, where x and y depend on chain length. They commonly exist in equilibrium between open-chain and cyclic forms. When monosaccharides join through glycosidic linkages, they form the wide array of oligosaccharides and polysaccharides found across living systems. Modifications such as deoxygenation (as in deoxyribose), acetylation (as in chitin) or the addition of sulfate groups (as in glycosaminoglycans) yield specialised molecules with unique biological properties.

Division and Classification

Carbohydrates may be divided into principal groups according to polymerisation:

• Sugars: simple carbohydrates including mono- and disaccharides.
• Oligosaccharides: short chains of monosaccharide units.
• Polysaccharides: long polymeric carbohydrates with structural or storage functions.

Monosaccharides are further classified by their carbonyl group (aldoses or ketoses), the number of carbon atoms (triose, tetrose, pentose, hexose, etc.) and their stereochemistry. An important feature of cyclic monosaccharides is the existence of anomers, structural isomers differing in configuration at the anomeric carbon. For example, α- and β-D-glucose differ in the orientation of the hydroxyl group on this carbon, giving rise to distinct chemical behaviours.