Monosaccharides from simple molecules

The evidence that in natural products, especially in primary metabolites, some structural patterns are repeated in different structures, implies that most organisms have common biosynthetic pathways. At the end of the nineteenth century when the biosynthetic pathways were not known, Heinrich Kiliani and Hermann Emil Fischer developed a simple model which could explain how stereochemically pure long-chained monosaccharides can form from simple molecules. The simplest, parent monosaccharide is aldotriose with only one chiral carbon atom. Hence, there are D- and L-aldotrioses. Since aldotriose is the product of mild oxidation of glycerol it can also be called glyceraldehyde. 

The structure of the artificial glucosides has been discussed, as it is probable that the disaccharides are formed from two molecules of monosaccharides by a condensation similar to that in the case of a monosaccharide and a simple alcohol. Such a condensation is possible as the monosaccharides are alcohols. 

There is strong experimental evidence in support of the first two points. The monosaccharides themselves being derived from such simple molecules as acetaldehyde, formaldehyde and water all of which are thought to have been present in the prebiotic earth. 

As you would expect from a molecule with so many polar —OH groups, the monosaccharides are water soluble. They are all white crystalline solids at room temperature and have differing sweetnesses. For example, fructose is about twice as sweet as glucose. There are many simple sugars in your life. We will show each sugar as an open-chain molecule, then in its more common cyclic form. (Sugars exist as an equilibrium mixture of open-chain and cyclic forms in water solutions.)... 

This chapter is organized around the four major classes of biological macromolecules. Recognize what most macromolecules have in common They are assembled from simple monomer units. Proteins are assembled from amino acids, carbohydrates are assembled from monosaccharides, and nucleic acids are assembled from nucleotides. Lipid is a catchall classification that includes fats, oils, phospholipids, waxes, steroids, and some other molecules. Organize your study into these four categories. 

Sugars are the basic units of carbohydrates, the most abundant organic molecules found in nature. Carbohydrates range from the simple sugars, or monosaccharides, such as glucose and fructose, to the polysaccharides, polymers that contain thou-... 

Like proteins, carbohydrates occur in almost bewildering varieties. Many of the most important carbohydrates are polymers— large molecules constmcted by hooking together many smaller molecules. We have seen that proteins are polymers constmcted from amino acids. The polymeric carbohydrates are constmcted from molecules called simple sugars or, more precisely, monosaccharides. Monosaccharides are aldehydes or ketones that contain several hydroxyl (—OH) substiments. An example of a monosaccharide is fmctose, with the stmcmre... 

---