Before discussing what atoms make up carbohydrates?, I would like to discuss what is carbohydrates? Carbohydrates are the one of the four macromolecules of the biological system.
There are four macromolecules in the biological system and these are Proteins, Carbohydrates, Lipids and Nucleic Acids. Except for lipids, all other three macromolecules exist in polymeric form. Carbohydrates or hydrates of carbon are the organic compounds that are composed mainly of carbon, hydrogen and oxygen in the ratio of 1:2:1 (stoichiometric ratio). Let’s take the example of glucose. The molecular formula of glucose is C6H12O6 that can also be written as C6(H2O)6 that reveals carbon and water are in the same proportion (hydrates of carbon).
Carbohydrates are simply polyhydric aldehydes (e.g. glucose and galactose) or ketones (e.g. fructose). Carbohydrates can be classified in many ways based on their structure and composition. To be considered as carbohydrate, there must be at least three carbon and the simplest carbohydrate is glyceraldehyde (aldotriose). Based on the number of carbon atoms, carbohydrates can be trioses, tetroses, pentoses or hexoses. In the same ways, based on the presence of the type of carbonyl group, carbohydrates can be aldoses or ketoses.
As I have mentioned before that, carbohydrates are mainly composed of carbon, hydrogen and oxygen atoms (e.g. glucose, fructose, galactose, mannose, lactose, and many more), but there are some types of carbohydrates that contain sulfur, phosphorous, and nitrogen in addition to the carbon, hydrogen, and oxygen.
So, what atoms makeup carbohydrates? The main components of carbohydrates are Carbon, Hydrogen, and Nitrogen while minor components are Sulfur, Phosphorous, and Nitrogen. To make you clear about the occurrence of these minor atoms, I will give you some examples.
Carbohydrates containing sulfur, nitrogen, and phosphorous are glycosaminoglycans or GAGs (e.g. keratan sulfate, chondroitin sulfate, hyaluronic acid, etc.). These carbohydrates are also called mucopolysaccharides. However, they don’t function as reserved carbohydrates (that provide energy) but they have some special properties. Glycosaminoglycans contain repeating units of disaccharides and these disaccharide units are composed of amino sugars (e.g. N-acetylglucosamine or N-acetylgalactosamine) and sugar acids (e.g. Glucuronic acid and iduronic acid).
Glycosaminoglycans are often phosphorylated and sulfated and these modifications are found either at amino-N or hydroxy-O atoms of the sugar. I hope, the answer is clear now. So, not all carbohydrates are purely composed of C, H, and O atoms only, but there are some carbohydrates that contain S, N, and P atoms. In the same way, there are many other types of carbohydrates that contain these extra elements. One such example is glycophosphatidylinositol (GPI) that contains C, H, O, P, and N within its structure. I hope you are clear now about what atoms make up carbohydrates.