It is often helpful to think of an organic molecule as a carbon-based core with other special parts attached. Each of these special parts is really a group of atoms called a functional group and has definite chemical properties. A hydroxyl group (-OH), for example, is a functional group. The most important functional groups are illustrated in Figure 7. These groups are important because most chemical reactions that occur within organisms involve the transfer of a functional group from one molecule to another. Other frequent chemical reactions involve the breaking of carbon-carbon bonds.
Figure 7 The clinical building blocks of life, functional groups. Six of the most important functional groups involved in chemical reactions.
Some of the molecules of living things are simple organic molecules, often having only one functional group. Other far larger molecules, called macromolecules, often have many functional groups and contain thousands of atoms. There are four major groups of biologically important macromolecules: complex carbohydrates, lipids, proteins, and nucleic acids. The four major classes of biologically important macromolecules are presented in Table 2.
Although these classes of macromolecules are each composed of different building blocks, the process by which their building blocks are put together is the same. All except the lipids are polymers, macromolecules that are built by forming covalent bonds between similar building blocks, or monomers, to form long chains.
Figure 8 Dehydration synthesis.
Dehydration synthesis is a process by which monomers (and the building blocks of lipids) are put together. During dehydration synthesis, one molecule of water is removed (dehydration) from each two monomers that are joined (synthesis). One monomer loses its hydroxyl group (-OH), and the other loses an atom of hydrogen (H). Having lost electrons they were sharing in covalent bonding, both monomers bond covalently with one another as shown in Figure 8. (Also see Figures 10, 13, and 15.) The process of dehydration synthesis uses energy, which is stored in the bond that is made, and takes place with the help of special molecules called enzymes.
Polymers are disassembled in an opposite process called hydrolysis (see Figure 15). During hydrolysis, the bonds are broken between monomers with the addition of water (and in the presence of enzymes). In fact, the term hydrolysis literally means "to break apart" (lysis) "by means of water" (hydro). The hydroxyl group of a water molecule bonds to one monomer, and the hydrogen atom bonds to its neighbor. The energy held in the bond is released.