Overview
Carbohydrates are a broad class of organic molecules composed primarily of carbon, hydrogen and oxygen. Scientists often describe their simplified composition with the formula Cm(H2O)n and call individual units saccharides. In biochemistry texts they are introduced as chemical compounds built from sugar units; each unit contains the elements represented by carbon, hydrogen and oxygen. The term carbohydrate captures both small sugars and large polymers formed when sugars link together.
Classification and structure
Carbohydrates are commonly grouped by size and complexity:
- Monosaccharides: single sugar units such as glucose and fructose.
- Disaccharides: two monosaccharides joined by a glycosidic bond (for example, sucrose).
- Oligosaccharides: short chains of a few sugar units.
- Polysaccharides: long polymers such as starch, glycogen, cellulose and chitin.
The way monosaccharides connect — the position and orientation of glycosidic bonds — determines properties such as digestibility and solubility.
Biological roles and importance
Carbohydrates play several central roles in living systems. They act as immediate energy sources and as longer-term storage forms of energy in many organisms. Plants synthesize starch for energy reserves and form cellulose for structural support, while animals store glucose as glycogen. Carbohydrate structures also participate in cell recognition, signalling and as components of nucleic acids and glycoproteins.
Sources, digestion and metabolism
Dietary carbohydrates come from plants (grains, fruits, vegetables) and from animal-derived sugars in dairy. Human digestion breaks down many digestible polysaccharides and disaccharides into monosaccharides for absorption; these are then used in pathways such as glycolysis and stored or converted as needed. Some polysaccharides, notably cellulose, resist human digestive enzymes and contribute to dietary fiber.
History, uses and notable distinctions
The study of sugars and their chemistry has a long history in both chemistry and nutrition. Beyond biology, carbohydrates are important in industry: starches and cellulose feedstocks for papermaking, fermentation, and biodegradable materials. A key distinction among carbohydrates is functional — digestible energy sources versus structural or indigestible polymers — which has implications for health, ecology and material science.
For further reading on foundational definitions and chemical behavior see saccharide introductions and general biochemical resources such as educational summaries.