Carbohydrates are one of three classes of primary metabolites and thus natural products. They are polyhydroxylated hydrocarbons and their derivatives. Colloquially, they are known as sugars. This tag may be applied to all questions that concern carbohydrate species in solution, reaction mechanisms and structures.
The name carbohydrate suggests a structural formula of $\ce{C}_m\ce{(H2O)}_n$. They were historically considered to be hydrates of carbon; a view supported by their reaction with concentrated sulphuric acid which yields carbon. However, not all carbohydrates abide by this general formula.
Since carbohydrates usually contain a carbonyl group along with hydroxy groups, they show a rich isomer chemistry in aquaeous solution. Glucose, for example, can switch between $\unicode[Times]{x3B1}$-pyranose, $\unicode[Times]{x3B1}$-furanose, $\unicode[Times]{x3B2}$-pyranose, $\unicode[Times]{x3B2}$-furanose, open-chain and other minority forms. $64\,\%$ of glucose in solution is $\unicode[Times]{x3B2}$-pyranose, $35\,\%$ is in $\unicode[Times]{x3B1}$-pyranose.
Most carbohydrates are natural products and are considered primary metabolites; i.e. every organism is capable of producing them in one form or another. Colloquially, they are also known as sugars due to the sweet taste of some very abundant carbohydrate species such as glucose and saccharose. Since carbohydrates are the short-term cellular energy source, and since ribose and deoxyribose are part of the DNA, carbohydrates can be considered essential for all life forms.
Per definition, carbohydrates must contain at least three carbon atoms, which means that the two simplest carbohydrates are 1,3-dihydroxyacetone and glycerinaldehyde. There is no strict upper limit. Also, carbohydrates are not limited to the elements $\ce{C, H}$ and $\ce{O}$; e.g. glucosamine, in which an amino group has replaced a hydroxy group, is also a carbohydrate derivative and within the scope of this tag.
Simple building blocks, known as monosaccharides can be combined to form di- and polysaccharides such as lactose, saccharose, amylose or cellulose. Complex polysaccharides are often found in glycoproteins which in turn are often used as cellular receptors.
