key concept3.3Simple Sugars Are the Basic Structural Unit of Carbohydrates

Carbohydrates make up a large group of molecules that all have similar atomic compositions but differ greatly in size, chemical properties, and biological functions. Carbohydrates usually have the general formula (C1H2O1)n, (n stands for a number), which makes them appear as “hydrates of carbon” (a hydrate refers to water), hence their name. However, carbohydrates are not really “hydrates” because the water molecules are not intact. Rather, the linked carbon atoms are bonded with hydrogen atoms (—H) and hydroxyl groups (—OH), the components of water.

Carbohydrates have four major biochemical roles:

  1. They are a source of stored energy that can be released in a form usable by organisms.

  2. They are used to transport stored energy within complex organisms.

  3. They serve as carbon skeletons that can be rearranged to form new molecules.

  4. They form extracellular assemblies such as cell walls that provide structure to organisms.

Some carbohydrates are relatively small, with molecular weights of less than 100. Others are true macromolecules, with molecular weights in the hundreds of thousands.

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  • Carbohydrates function primarily in storing and transporting chemical energy and as sources of carbon for building new macromolecules.

  • Polysaccharides of glucose all provide energy storage and structural functions but vary in branching patterns and type of glycosidic linkages between glucose units.

There are four categories of biologically important carbohydrate defined by the number of monomers:

  1. Monosaccharides (mono, “one,” + saccharide, “sugar”), such as glucose, are simple sugars. They are the monomers from which the larger carbohydrates are constructed.

  2. Disaccharides (di, “two”) consist of two monosaccharides linked together by covalent bonds. The most familiar is sucrose, which is made up of covalently bonded glucose and fructose molecules.

  3. Oligosaccharides (oligo, “several”) are made up of several (3–20) monosaccharides.

  4. Polysaccharides (poly, “many”), such as starch, glycogen, and cellulose, are polymers made up of hundreds or thousands of monosaccharides.