Carbohydrate

A carbohydrate is a biomolecule consisting of carbon (C), hydrogen (H), and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula Cₘ(H₂O)ₙ (where m could be different from n). However, not all carbohydrates conform to this precise stoichiometric definition (e.g., uronic acids, deoxy-sugars), and numerous chemicals that do conform to it are not carbohydrates (e.g., formaldehyde, acetic acid). Carbohydrates are technically hydrates of carbon; structurally they are more accurately viewed as polyhydroxy aldehydes and ketones.

Carbohydrates perform numerous roles in living organisms. Polysaccharides serve as energy stores (e.g., starch and glycogen) and as structural components (e.g., cellulose in plants, chitin in arthropods). The 5-carbon monosaccharide ribose is an important component of coenzymes (e.g., ATP, FAD, and NAD) and the backbone of the genetic molecule known as RNA. The related deoxyribose is a component of DNA. Saccharides and their derivatives include many other important biomolecules that play key roles in the immune system, fertilization, preventing pathogenesis, blood clotting, and development.

Etymology

The term "carbohydrate" derives from the French hydrate de carbone, meaning "hydrate of carbon". The name was originally used for compounds with the general formula Cₙ(H₂O)ₘ. In modern biochemistry, the term is applied to sweet or starchy foods, but it encompasses a broad class of organic compounds. The scientific nomenclature of carbohydrates is complex, but common names like "sugar", "starch", and "cellulose" are widely used. The word "saccharide" comes from the Greek σάκχαρον (sákkharon), meaning "sugar".

Structure

Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds upon hydrolysis. They exist in open-chain (acyclic) or cyclic forms. In solution, simple sugars with four or more carbon atoms predominantly exist as cyclic hemiacetals, formed by intramolecular reaction between a carbonyl group and a hydroxyl group. For example, glucose forms a six-membered pyranose ring, while fructose can form a five-membered furanose ring. The ring closure creates an anomeric carbon, giving rise to α and β configurations. This stereochemistry is critical in determining the properties of disaccharides and polysaccharides.

Monosaccharides can be linked together by glycosidic bonds to form larger structures. The bond forms between the hemiacetal group of one sugar and the hydroxyl group of another, resulting in acetal (or ketal) formation. The orientation and position of these bonds dictate the structure and digestibility of the resulting oligo- or polysaccharide.

Classification

Carbohydrates are traditionally classified into four major groups based on their degree of polymerization: monosaccharides, disaccharides, oligosaccharides, and polysaccharides.

Monosaccharides

Monosaccharides are the simplest carbohydrates, often called simple sugars. They are the building blocks of disaccharides and polysaccharides. They generally have 3 to 7 carbon atoms. Common examples include glucose (grape sugar, dextrose), fructose (fruit sugar), and galactose. Based on the functional group, they are categorized as aldoses (containing an aldehyde group) or ketoses (containing a ketone group). The number of carbon atoms is denoted by prefixes: triose (3C), tetrose (4C), pentose (5C), hexose (6C), heptose (7C). Glucose, a hexose, is the primary energy source for cells.

Disaccharides

Disaccharides consist of two monosaccharides joined by a glycosidic bond. Sucrose (table sugar) is composed of glucose and fructose. Lactose (milk sugar) comprises glucose and galactose. Maltose, derived from starch digestion, contains two glucose units. These disaccharides are hydrolyzed to their constituent monosaccharides by specific enzymes, such as sucrase, lactase, and maltase, respectively.

Oligosaccharides

Oligosaccharides contain 3 to 10 monosaccharide units. They are often found attached to proteins (glycoproteins) or lipids (glycolipids) on cell surfaces, where they function in cell recognition and signaling. Examples include raffinose (a trisaccharide) and stachyose (a tetrasaccharide), found in legumes. Human digestive enzymes cannot break down many oligosaccharides; instead, gut microbiota ferment them, often producing gas.

Polysaccharides

Polysaccharides are polymers of more than 10 monosaccharides and can be linear or branched. They are classified into storage polysaccharides and structural polysaccharides.

• Starch is the primary energy storage polysaccharide in plants. It consists of two components: amylose (linear α-1,4-linked glucose) and amylopectin (branched with α-1,6 linkages).
• Glycogen is the animal storage polysaccharide, structurally similar to amylopectin but more extensively branched, stored mainly in liver and muscle cells.
• Cellulose is a structural polysaccharide composed of β-1,4-linked glucose chains, forming rigid fibers in plant cell walls. Humans lack cellulase and cannot digest it, though it serves as dietary fiber.
• Chitin is a structural polysaccharide in arthropod exoskeletons and fungal cell walls, composed of N-acetylglucosamine units.
• Heparin, a glycosaminoglycan, acts as an anticoagulant in blood.

Functions

Carbohydrates are central to energy metabolism, structural integrity, and cellular communication.

Energy Source and Storage

Glucose is the primary metabolic fuel for most organisms. It is oxidized via glycolysis, the citric acid cycle, and oxidative phosphorylation to produce ATP. Surplus glucose is stored as glycogen in animals and starch in plants. When energy demand rises, these polymers are broken down (glycogenolysis in animals) to release glucose.

Structural Roles

Cellulose, hemicellulose, and pectin form the structural framework of plant cell walls. Chitin provides rigidity to fungal cell walls and arthropod exoskeletons. Peptidoglycan, a carbohydrate-peptide polymer, gives shape to bacterial cell walls. Glycosaminoglycans (e.g., hyaluronic acid, chondroitin sulfate) are key components of the extracellular matrix in animals, contributing to tissue structure and hydration.

Cellular Recognition and Signaling

Oligosaccharides attached to proteins and lipids on the cell surface form the glycocalyx, which mediates cell–cell recognition, adhesion, and immune response. Blood group antigens (A, B, O) are carbohydrate structures on red blood cells. Lectins, proteins that bind specific carbohydrate moieties, are involved in pathogen recognition and cell migration.

Other Functions

Ribose and deoxyribose are pentose sugars forming the backbone of RNA and DNA, respectively. Several coenzymes (ATP, NAD⁺, FAD, coenzyme A) contain carbohydrate components. Dietary fiber (indigestible polysaccharides) promotes bowel health, regulates blood glucose levels, and lowers cholesterol.

Metabolism

Carbohydrate metabolism begins with digestion. Polysaccharides and disaccharides are hydrolyzed by enzymes (amylases, disaccharidases) in the mouth and small intestine into monosaccharides, primarily glucose, which are absorbed into the bloodstream. Insulin, secreted from pancreatic β-cells, facilitates glucose uptake into cells. Inside cells, glucose undergoes glycolysis, yielding pyruvate with a net gain of ATP. Under aerobic conditions, pyruvate enters the mitochondria and is fully oxidized to CO₂. In anaerobic conditions, lactate is produced. The liver can synthesize glucose from non-carbohydrate precursors (gluconeogenesis) and maintain blood glucose homeostasis.

Excess glucose is converted to glycogen (glycogenesis) or, when glycogen stores are full, to fatty acids via de novo lipogenesis. During fasting, glycogen is depleted, and fats and proteins become major energy sources. The pentose phosphate pathway, an alternative glucose oxidation route, produces NADPH and ribose-5-phosphate for biosynthetic reactions and nucleotide synthesis.

Nutrition

In human nutrition, carbohydrates are a major source of energy, providing about 4 kilocalories per gram. Dietary carbohydrates are broadly categorized as simple (sugars) and complex (starches and fibers). Simple carbohydrates include monosaccharides and disaccharides found in fruits, milk, and added sugars. Complex carbohydrates are found in whole grains, legumes, and vegetables. The glycemic index (GI) measures how quickly a carbohydrate-containing food raises blood glucose levels. Diets high in high-GI foods are associated with an increased risk of type 2 diabetes and cardiovascular diseases, while low-GI diets and higher fiber intake have protective effects.

Dietary fiber, though not digestible, benefits health by improving laxation, modulating gut microbiota, and reducing lipid absorption. Recommended carbohydrate intake varies, but general guidelines suggest that 45–65% of total daily calories should come from carbohydrates, primarily complex ones.

Industrial and Commercial Applications

Carbohydrates are used extensively in the food industry as sweeteners, thickeners, and stabilizers. High-fructose corn syrup, derived from corn starch, is a common sweetener. Modified starches serve as gelling agents and adhesives. Cellulose is used in paper, textiles, and as a food additive. Chitin and its deacetylated derivative chitosan have applications in water treatment, wound dressings, and biodegradable plastics. Bioethanol, a renewable fuel, is produced by fermentation of carbohydrate-rich crops like sugarcane and corn.

Analytical Methods

Detection and quantification of carbohydrates employ various techniques. The Molisch test indicates the presence of all carbohydrates. Fehling's and Benedict's tests detect reducing sugars through the reduction of copper(II) ions. Specific sugars can be identified by polarimetry, chromatography (HPLC, GC), and enzymatic assays. Glycosidic linkages are analyzed by methylation analysis and NMR spectroscopy. Mass spectrometry, often coupled with liquid chromatography, is widely used for detailed structural elucidation.