Sugars coat your cells and can switch proteins on and off
Carbohydrates are not just fuel in bread and pasta. Sugar chains decorate the outside of cells, help them recognise one another, guide how proteins fold, and in some cases act as an on-off switch for a protein's job. A sugar called ribose even forms the backbone of RNA.
Chemically, a carbohydrate is a sugar, a chain of sugars such as starch, or a derivative. Many simple sugars have carbon, hydrogen and oxygen in a 1:2:1 ratio, hence the old idea of hydrated carbon. They rank alongside proteins' amino acids, fats and nucleic acids as one of life's major families of molecules. The simplest units, monosaccharides like glucose, cannot be broken into smaller carbohydrates; two joined make a disaccharide such as table sugar, and long chains form polysaccharides.
Those chains do very different jobs. Starch in plants and glycogen in animals store energy, while cellulose stiffens plant walls and chitin armours insects and builds fungi. Ribose sits inside energy carriers like ATP as well as RNA, and its close cousin deoxyribose is part of DNA. When enzymes attach sugars to proteins or fats, the result is a glycoconjugate. These cluster on the outer cell membrane and in body fluids, where they matter for immunity, fertilisation, blood clotting and cell-to-cell contact. The study of them was named glycobiology by Raymond Dwek in 1988.
Not every organism can use every carbohydrate. Glucose is a nearly universal fuel, and many microbes eat it first: the bacterium E. coli switches off its lactose-digesting genes whenever glucose is available. Most creatures cannot digest cellulose or chitin at all; ruminants and termites rely on resident microorganisms to do it for them. In the human diet, simple sugars yield about 3.87 kilocalories per gram, and fibre-rich whole foods release glucose more slowly than refined ones.
The science has a long pedigree. In 1811 Constantin Kirchhoff found that boiling starch with acid produced glucose, launching the starch industry the next year. Carl Schmidt proposed the word carbohydrate in 1844, Claude Bernard discovered glycogen in animal livers in 1856, and Emil Fischer won the 1902 Nobel Prize in Chemistry partly for his work on sugars.
Source: Carbohydrate
