Chapter Summary

• Macromolecules are polymers constructed by the formation of covalent bonds between smaller molecules called monomers. Macromolecules in living organisms include polysaccharides, proteins, and nucleic acids. Organizationally, large lipid structures may also be considered like macromolecules.

• Functional groups are small groups of atoms that are consistently found together in a variety of different macromolecules. Functional groups have particular chemical properties that they confer on any larger molecule of which they are a part. Review Figure 3.1, Activity 3.1

• Structural, cis-trans, and optical isomers have the same kinds and numbers of atoms but differ in their structures and properties. Review Figure 3.2

• The many functions of macromolecules are directly related to their three-dimensional shapes, which in turn result from the sequences and chemical properties of their monomers.

• Monomers are joined by condensation reactions, which release a molecule of water for each bond formed. Hydrolysis reactions use water to break polymers into monomers. Review Figure 3.4

• The functions of proteins include support, protection (e.g., skin surface), catalysis, transport, defense, regulation, movement, signaling, and storage. Review Table 3.1

• Proteins consist of one or more polypeptide chains, which are polymers of amino acids. Four atoms or groups are attached to a central carbon atom: a hydrogen atom, an amino group, a carboxyl group, and a variable R group. The particular properties of each amino acid depend on its side chain, or R group, which may be charged, polar, or hydrophobic. Review Table 3.2, Activity 3.2

• Peptide linkages, also called peptide bonds, covalently link amino acids into polypeptide chains. These bonds form by condensation reactions between the carboxyl and amino groups. Review Figure 3.6

• The primary structure of a protein is the sequence of amino acids in the chain. This chain is folded into a secondary structure, which in different parts of the protein may form an α helix or a β pleated sheet. Review Focus: Key Figure 3.7A–C

• Disulfide bridges and noncovalent interactions between amino acids cause polypeptide chains to fold into three-dimensional tertiary structures. These bridges as well as noncovalent interactions allow multiple polypeptide chains to form quaternary structures. Review Figure Focus: Key Figure 3.7C and D

• Heat, alterations in pH, and certain chemicals can all result in a protein becoming denatured. This involves the loss of tertiary and/or econdary structure as well as biological function. Review Figure 3.10

• The specific shape and structure of a protein allow it to bind noncovalently to other molecules. In addition, amino acids may be modified by the covalent bonding of chemical groups to their side chains. Such binding may result in a protein changing its shape. Review Figures 3.12, 3.13

• Chaperone proteins enhance correct protein folding and prevent inappropriate binding to other molecules. Review Figure 3.14

• Carbohydrates contain carbon bonded to hydrogen and oxygen atoms and have the general formula (C₁H₂O₁)_n.

• Monosaccharides are the monomers that make up larger carbohydrates. Hexoses such as glucose are six-carbon monosaccharides; pentoses have five carbons. Review Figure 3.16, Activity 3.3

• Glycosidic linkages, which have either an α or a β orientation in space, are covalent bonds between monosaccharides. Two linked monosaccharides are called a disaccharide; larger units are oligosaccharides and polysaccharides. Review Figure 3.17

• Starch is a polymer of glucose that stores energy in plants, and glycogen is an analogous polymer in animals. They can be easily broken down to release stored energy. Review Figure 3.18

• Cellulose is a very stable glucose polymer and is the principal structural component of plant cell walls.

• Lipids are hydrocarbons that are insoluble in water because of their many nonpolar covalent bonds. They play roles in energy storage, membrane structure, light harvesting, intercellular communication, and protection.

• Fats and oils are triglycerides. A triglyceride is composed of three fatty acids covalently bonded to a molecule of glycerol by ester linkages. Review Figure 3.20

• A saturated fatty acid has a hydrocarbon chain with no double bonds. These molecules can pack together tightly. The hydrocarbon chain of an unsaturated fatty acid has one or more double bonds that bend the chain, preventing close packing. Review Figure 3.21

• A phospholipid has a hydrophobic hydrocarbon “tail” and a hydrophilic phosphate “head”; that is, it is amphipathic. In water, the interactions of the tails and heads of phospholipids generate a phospholipid bilayer. The heads are directed outward, where they interact with the surrounding water. The tails are packed together in the interior of the bilayer, away from water. Review Figure 3.22

• Other lipids include vitamins A, D, E, and K, steroids, and plant pigments such as carotenoids.

• See Activity 3.4 to review macromolecule structures.