Although living cells are primarily made up of water, a number of other molecules are also abundant. Gigantic molecules, called macromolecules, populate a cell and provide it with important functions for life. For example, macromolecules provide structural support, a source of stored fuel, the ability to store and retrieve genetic information, and the ability to speed biochemical reactions. Four major types of macromolecules—proteins, carbohydrates, nucleic acids, and lipids—play these important roles in the life of a cell. Here we will look at three of these macromolecules: proteins, carbohydrates, and lipids.
In terms of overall structure, all macromolecules except the lipids are considered polymers. A polymer is a chain of similar subunits, or monomers, that are linked together by covalent bonds. In proteins, the monomers are amino acids; in carbohydrates, the monomers are sugars. Lipids are a diverse group of molecules, which come in a variety of nonpolymeric forms.
Proteins are chains of amino acids linked by peptide bonds. The 20 different amino acids used to make all proteins differ only in their side chains, and the properties of these side chains account for the great diversity of protein structure and function.
Ten of the side chains are hydrophilic—either polar and uncharged or electrically charged. Seven of the side chains are hydrophobic. Three of the side chains fall into a special-case category based on their structural features, although glycine and proline generally are hydrophobic.
Collagen is an example of how a protein's amino acid sequence determines its structure and function. Amino acid sequences are encoded in the DNA of genes.
Collagen is the most abundant mammalian protein. It is the main component of skin, bones, and teeth. This protein is composed of three helical polypeptide chains that form a stiff, supercoiled cable. The type of helix in each polypeptide is unique to collagen and results from collagen's unusual amino acid composition.
A collagen helix forms largely from the influences of two types of amino acids: proline, which introduces sharp twists in the polypeptide, and glycine, which has a small side chain (H) that doesn't interfere with packing in the helix. Collagen contains many of these amino acids, but few bulky ones (e.g., phenylalanine).
Each helix contains three amino acids per turn, with glycine located at every third position. The three polypeptides in a collagen molecule associate with their glycines all facing collagen's center. Glycine is the only amino acid small enough to allow the polypeptides to pack (by hydrogen bonding) into a tight cable.
Individual collagen molecules cross link to other collagen molecules to form tough collagen fibrils. Collagen, from which gelatin is derived, is a poor source of essential amino acids—the amino acids that the body cannot manufacture and must receive from the diet.
Carbohydrates include simple sugars called monosaccharides as well as large polymers called polysaccharides. Glucose is a hexose, a sugar composed of six carbon atoms, usually found in ring form. A starch macromolecule is a polysaccharide composed of thousands of glucose units.
Glucose molecules can be added to starch by a condensation reaction. In condensation reactions, two molecules covalently bond to each other and release a water molecule. Here, the bond forms between the first carbon of one glucose and the fourth of the other, creating an α-1,4 glycosidic linkage.
Branching of new chains can also occur between carbons 1 and 6.
Different types of starches are, in fact, distinguished by the amount of branching. Amylose, or plant starch, is not highly branched.
Glycogen, by comparison, is highly branched. This polysaccharide is stored in animal livers and muscles.
Polysaccharides are forms of stored energy that can be easily hydrolyzed to yield glucose. Glucose can then be further broken down to release energy that is used in cellular activity.
A triglyceride (also called triacylglycerol) is composed of three fatty acid molecules and one glycerol molecule. The fatty acids attach to the glycerol molecule by a covalent ester bond. The long hydrocarbon chain of each fatty acid makes the triglyceride molecule nonpolar and hydrophobic.
Palmitic acid is a fatty acid with 16 carbon atoms. It is called a saturated fatty acid, because all the carbon atoms in the chain are single bonded to each other and are fully "saturated" with hydrogen atoms.
Fatty acids, such as palmitic acid, are built by adding two carbon atoms at a time to smaller fatty acid molecules.
A cell can use palmitic acid to form other fatty acids, such as oleic acid. In this process, oleic acid is formed by adding two carbon atoms to palmitic acid, and then by inserting a double bond between carbons 9 and 10. Because oleic acid has one double bond, it is considered a monounsaturated fatty acid.
Polyunsaturated fatty acids, such as linoleic acid, have two or more double bonds. The double bonds kink these molecules and prevent them from packing tightly together. The loose packing results in triglycerides that are liquid at room temperature. Mammals cannot make linoleic acid; it is required in the diet.
A glycerol molecule makes up the backbone of a triglyceride, in which glycerol's three hydroxyl (or –OH) groups form ester covalent bonds with three fatty acid molecules. The covalent bonds form by condensation reactions in which water is a byproduct.
Triglycerides can contain a mix of saturated, monounsaturated, and polyunsaturated fatty acids. They provide a concentrated store of energy.
A macromolecule's structure is intimately connected with its function. Consider, for example, a protein. This type of polymer is made up of a chain of amino acids that are strung together in a precise sequence. The amino acid sequence determines the protein's 3-dimensional shape and chemical reactivity, which, in turn, endow a protein with its specific function. Some proteins, for example, have shapes that allow them to grab molecules and speed chemical reactions. Others, such as strong cables of collagen, provide structural support to cells and tissues.
Carbohydrates include the relatively small glucose molecule and the enormous glycogen molecule, which may consist of hundreds of thousands of glucose monomers. Carbohydrates are energy-rich. Many, such as glycogen, provide energy-storage functions. Other carbohydrates, such as cellulose—a component of plant cell walls—serve primarily structural roles in a cell.
Lipids are the only macromolecules that are not polymers. Lipids are diverse in structure and function, but all have in common that they are hydrophobic—that is, they are nonpolar and do not dissolve in water.