Monomers and Polymers

Since carbon can form long chains, some organic molecules are gigantic macromolecules with molecular weights that range from the thousands (as in starch and proteins) to the millions (as in DNA). Most macromolecules are polymers14—molecules made of a repetitive series of identical or similar subunits called monomers (MON-oh-murs). Starch, for example, is a polymer of about 3,000 glucose monomers. In starch, the monomers are identical, while in other polymers they have a basic structural similarity but differ in detail. DNA, for example, is made of 4 different kinds of monomers (nucleotides), and proteins are made of 20 kinds (amino acids).

The joining of monomers to form a polymer is called polymerization. Living cells achieve this by means of a reaction called dehydration synthesis (condensation) (fig. 2.15a). A hydroxyl (— OH) group is removed from one monomer and a hydrogen (—H) from another, producing water as a by-product. The two monomers become joined by a covalent bond, forming a dimer. This is repeated for each monomer added to the chain, potentially leading to a chain long enough to be considered a polymer.

The opposite of dehydration synthesis is hydrolysis15 (fig. 2.15b). In hydrolysis, a water molecule ionizes into OH~ and H+. A covalent bond linking one monomer to another is broken, the OH~ is added to one monomer, and the H+ is added to the other one. All digestion consists of hydrolysis reactions.

Name and Symbol

Structure

Occurs in

Hydroxyl (- OH)

H

Sugars, alcohols

Methyl (- CH3)

H

Fats, oils, steroids, amino acids

Carboxyl (— COOH)

<

Amino acids, sugars, proteins

Amino (- NH2)

Amino acids, proteins

Phosphate (-H2PO4)

H

Nucleic acids, ATP

Figure 2.14 Functional Groups of Organic Molecules.

Figure 2.14 Functional Groups of Organic Molecules.

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