Complex Carbohydrates (College Board AP® Biology): Study Guide

Structure & function in carbohydrates

• Carbohydrates are biological molecules made of monomer units called monosaccharides

• Complex carbohydrates form when monosaccharides join together via covalent bonds to form polysaccharide polymers

  • Complex carbohydrates can be either linear or branched

• The type of monomer and the nature of any covalent bonds determine the properties and functions of a carbohydrate

Forming a polysaccharide

• Carbohydrate monomers can join together via dehydration synthesis reactions; during this process:

  • a new covalent bond forms between two monomers, holding the carbohydrate together

  • a molecule of water is produced

• This process can repeat many times to form a polysaccharide

Example: cellulose

• Cellulose is a structural polysaccharide of glucose found in plant cell walls

• Cellulose consists of long chains of the monomer β-glucose, joined together by 1,4 glycosidic bonds

• To form the 1,4 glycosidic bonds, alternate β-glucose molecules must be rotated through 180°

• Due to the inversion of the β-glucose molecules, many hydrogen bonds form between cellulose chains, giving cellulose its structural strength

Example: starch and glycogen

• Starch and glycogen are storage polysaccharides

  • Glycogen is the storage polysaccharide of animals and fungi

  • Starch is the storage polysaccharide of plants

• Both starch and glycogen are polymers of α-glucose , in which monosaccharides are joined by either 1,4 glycosidic bonds or 1, 6 glycosidic bonds

  • The presence of 1,6 glycosidic bonds means that starch and glycogen can be highly branched molecules

• Both starch and glycogen:

  • can be compact

    • This means that large quantities can be stored in a small space

  • are insoluble

    • The molecules do not dissolve in cell cytoplasm, so they have no effect on the water potential of cells

  • exist in branched forms (glycogen is always branched while starch can be either branched or unbranched)

    • The branching means that there are more free ends where glucose molecules can either be added or removed

    • Hydrolysis and dehydration synthesis reactions can occur more rapidly, so the rate of storage or release of glucose can suit the energy demands of the cell