Section 3.3: Functional Groups and Dehydration Reaction

Learning Objectives

• Identifying examples of functional groups
• Describe the roles of functional groups in synthesizing polymers

Biologically Significant Functional Groups

In addition to containing carbon atoms, biomolecules also contain functional groups — groups of atoms within molecules that are categorized by their specific chemical composition and the chemical reactions they perform, regardless of the molecule in which the group is found. Some of the most common functional groups are listed in Figure \(\PageIndex{1}\). In the formulas, the symbol R stands for “residue” and represents the remainder of the molecule. R might symbolize just a single hydrogen atom or it may represent a group of many atoms. Notice that some functional groups are relatively simple, consisting of just one or two atoms, while some comprise two of these simpler functional groups. For example, a carbonyl group is a functional group composed of a carbon atom double bonded to an oxygen atom: C=O. It is present in several classes of organic compounds as part of larger functional groups such as ketones, aldehydes, carboxylic acids, and amides. In ketones, the carbonyl is present as an internal group, whereas in aldehydes it is a terminal group.

Figure \(\PageIndex{1}\): Functional groups.

Macromolecule Synthesis: Dehydration Reaction

Carbon chains form the skeletons of most organic molecules. Functional groups combine with the chain to form biomolecules. Because these biomolecules are typically large, we call them macromolecules. Many biologically relevant macromolecules are formed by linking together a great number of identical, or very similar, smaller organic molecules. The smaller molecules act as building blocks and are called monomers, and the macromolecules that result from their linkage are called polymers. Cells and cell structures include four main groups of carbon-containing macromolecules: polysaccharides, proteins, lipids, and nucleic acids. The first three groups of molecules will be studied throughout this chapter. The biochemistry of nucleic acids will be discussed in Biochemistry of the Genome.

Of the many possible ways that monomers may be combined to yield polymers, one common approach encountered in the formation of biological macromolecules is dehydration synthesis. In this chemical reaction, monomer molecules bind end to end in a process that results in the formation of water molecules as a byproduct:

\[\text{H—monomer—OH} + \text{H—monomer—OH} ⟶ \text{H—monomer—monomer—OH} + \ce{H2O}\]

Figure \(\PageIndex{2}\) shows dehydration synthesis of glucose binding together to form maltose and a water molecule. Table \(\PageIndex{1}\) summarizes macromolecules and some of their functions.

Figure \(\PageIndex{2}\): In this dehydration synthesis reaction, two molecules of glucose are linked together to form maltose. In the process, a water molecule is formed.

Table \(\PageIndex{1}\): Functions of Macromolecules

Macromolecule	Functions
Carbohydrates	Energy storage, receptors, food, structural role in plants, fungal cell walls, exoskeletons of insects
Lipids	Energy storage, membrane structure, insulation, hormones, pigments
Nucleic acids	Storage and transfer of genetic information
Proteins	Enzymes, structure, receptors, transport, structural role in the cytoskeleton of a cell and the extracellular matrix

Query \(\PageIndex{1}\)

 

 

 

 

 

 

 

 

 

Exercise \(\PageIndex{4}\)

What is the byproduct of a dehydration synthesis reaction?

Key Concepts and Summary

• Functional groups confer specific chemical properties to molecules bearing them. Common functional groups in biomolecules are hydroxyl, methyl, carbonyl, carboxyl, amino, phosphate, and sulfhydryl.
• Macromolecules are polymers assembled from individual units, the monomers, which bind together like building blocks. Many biologically significant macromolecules are formed by dehydration synthesis, a process in which monomers bind together by combining their functional groups and generating water molecules as byproducts.