Unit 3
Simple and Complex Carbohydrates

Overview

A simple sugar is an n-carbon, polyhydroxyl chain (or ring) which also contains an aldehyde or ketone functional group. The value of n ranges from 3 to about 9, but five-carbon sugars (pentoses) and six-carbon sugars (hexoses) are the most common in biological systems.

While amino acids are linked by peptide bonds, simple sugars are linked by ether bonds; and just as amino acids link to form proteins, simple six-carbon sugars link to form polysaccharides. The six-carbon sugars are the only ones that serve as building blocks for polysaccharides. Simple sugars are partially reduced molecules, and are the major biochemical fuel. [Eventually C, H, and O atoms from the donuts you eat end up as the CO2 and H20 you expend.] Besides acting as biochemical fuel, sugars are also found linked to proteins (glycoproteins) and lipids (glycolipids), thereby increasing the water solubility of these molecules. The pattern of sugars linked to individual glycoproteins in cell membranes contributes to immunological identity. In this unit, we explore the structure and chemistry of simple and complex sugars. Later units are devoted to other aspects of the biochemistry of sugars: in Unit 9, we discuss how sugars are metabolized; and in Unit 11, we show that ribose (a pentose) is an important component of DNA and RNA.

Unit 3 is divided into five lessons:

  1. Structure of Monosaccharides
  2. Formation of Polysaccharides
  3. Structure of Polysaccharides
  4. Glycoproteins, Proteoglycans, and Ionic Polysaccharides
  5. Bacterial Cell Walls

Objectives

After completing this unit, you should be able to

  1. describe the chemical structures of a simple carbohydrate (sugar) and a complex carbohydrate (polysaccharide).
  2. explain how polysaccharides are synthesized from simple six-carbon sugars.
  3. describe the roles of structural polysaccharides and storage polysaccharides, and explain how slight differences in their chemical structures enable them to fulfill their different roles.
  4. define “glycoprotein,” and explain the roles glycoproteins play in vivo.
  5. describe a proteoglycan, and explain its function.
  6. describe the structure of bacterial cell walls.
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Glossary

aldose sugar containing an aldehyde functional group
anomeric carbon the carbon atom in a cyclic monosaccharide which, in the linear monosaccharide, holds the aldehyde or ketone functional group
β (1 → 4) or
α (1 → 4)
-linked sugars		 refers to (a) the geometry of the glycosidic bond (α = opposite side of the sugar ring from the free CH2OH; β = same side), and (b) which carbon on ring A is linked to which carbon on ring B. [Note that (1 → 4) is only an example, other carbons may be linked; for example, (2 → 5) or 1 → 3)]
carbohydrate generic name for simple and complex sugars; chemically carbohydrates are polyhydroxyl aldehydes or polyhydroxyl ketones
D-sugar the stereoisomeric form of monosaccharide found most commonly in vivo
furanose monosaccharide whose structure is a five-membered ring
glycosidic bond ether bond joining two monosaccharides
glycosylation addition of a sugar group (usually glucose) to another molecule; the effect is to increase the second molecule’s water solubility
hemiacetal or hemiketal cyclic conformation of simple carbohydrates; formed by reaction of the aldehyde (or ketone) and one of the hydroxyls on the carbohydrate
hypertonic having a concentration of dissolved particles or molecules higher than that of 0.1 M saline (see “isotonic”); when a hypertonic and an isotonic solution are separated by a membrane, water flows into the hypertonic solution to equalize the concentration of dissolved molecules
isotonic having a concentration of dissolved particles or molecules equal to that of 0.1 M saline
ketose sugar containing a ketone functional group
monosaccharide basic unit of carbohydrates
polysaccharide carbohydrate formed from covalently linked monosaccharides
pyranose monosaccharide in a six-membered ring form
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