Biochemical Engineering Fundamentals, Part 1 - Bailey J., Ollis D. 1989

The Chemical Foundations of Life
Sugars and Polysaccharides
Disaccharides and Polysaccharides

Since many simple sugars exist predominantly in cyclic form in solution, they do not undergo reactions characteristic of aldehydes or ketones. At the same time, in the D-glycopyranose ring, the hydroxyl group at C-1 is more reactive than the other hydroxyl groups. As shown in the reaction scheme below, this hydroxyl group, occupying the a-position, can react with the hydroxyl group at C-4 of another sugar, eliminating a Water molecule to form an a-1,4-glycosidic bond:

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The Condensation of two monosaccharide molecules yields a disaccharide. In addition to maltose, which is formed from two D-glucose molecules, the following Disaccharides are relatively common:

The most important food product, common sugar, is sucrose, which is found in all photosynthetic plants. Sucrose undergoes Hydrolysis more readily than other disaccharides; the resulting mixture of the Monosaccharides glucose and fructose is called invert sugar. A relatively rare but important disaccharide, lactose, is found only in milk. Since some people are lactose intolerant and therefore cannot consume milk, enzymatic processes for the hydrolysis of milk sugar are currently being developed.

Upon further polymerization of glucose, new 1,4-glycosidic bonds can form. Amylose is an unbranched polymer made of glucose residues with a molecular weight ranging from a few thousand to half a million:

Starch, the reserve food source of plants, typically contains about 20% amylose, although this value can vary quite widely. Starch granules are large enough to be easily seen under a Microscope in many plant Cells.

The amylose fraction of starch, as already mentioned, consists of water-insoluble linear polymers; however, the bulk of starch is amylopectin, also a polymer of d-glucose, which differs from amylose in its branched Structure*. On average, there is one branch point for every 25 glucose residues; chain branching occurs through The formation of a glycosidic bond between the hydroxyl at C-1 of one chain and the hydroxyl at C-6 of another chain:

As a rule, amylopectin molecules are larger than amylose molecules; their molecular weight ranges from 1 to 2 million. Amylopectin is water-soluble and can form gels by adsorbing water. Partial acid or enzymatic Hydrolysis of Starch leads to the formation of various branched fragments of amylopectin called dextrins. Dextrins are used as thickeners and in the manufacture of pastes. Of course, partial hydrolysis also yields glucose, maltose, and other relatively small sugars. In this way, corn syrup is produced from corn starch.

Polymeric granules of Glycogen, which resembles amylopectin in its degree of branching, serve as a reserve source of glucose in animal cells, especially in Liver and Muscle cells. In glycogen, the length of the linear segments between branch points is typically about 12 units; thus, glycogen molecules are even more highly branched than amylopectin molecules. The Molecular Weight of glycogen can be 5 million or more. Glycogen also Functions as a reserve energy source in some microorganisms, including enterobacteria.

* See the article: Green M. M., Blankenborn G., Hart H., "Textbook Errors. 123; Which Starch Fraction is Water-Soluble, Amylose or Amylopectin?" J. Chem. Educ., B52, 729 (1975).