Macromolecules are long chains of molecules made of thousands or even billions of atoms. Biological macromolecules can be divided into four categories: carbohydrates, nucleic acids, proteins, and lipids. These four types of macromolecules are the basic chemical building blocks for all organisms.
The framework of biological molecules consists predominantly of carbon atoms bonded to other carbon atoms or to atoms of hydrogen, oxygen, nitrogen, sulfur, or phosphorus. Molecules consisting only of carbon and hydrogen are called hydrocarbons. Hydrocarbons make good fuels because the oxidation of hydrocarbon compounds results in a net release of energy. Hydrocarbons are nonpolar. Most biological molecules produced by cells, however, also contain other atoms, and are polar. These molecules can be thought of as a C—H core to which molecular groups, called functional groups, are attached. Functional groups are small molecular entities that confer specific chemical characteristics when attached to a hydrocarbon. One common functional group is —OH, called a hydroxyl group. Organic molecules having the same molecular or empirical formula can exist in different forms called isomers; this difference may affect biological function. Many macromolecules are polymers consisting of long chains of similar subunits called monomers that are joined by dehydration reactions and are broken down by hydrolysis reactions. Nucleic acids are polymers of nucleotides and proteins are polymers of amino acids.
Carbohydrates are a loosely defined group of molecules that all contain carbon, hydrogen, and oxygen in the molar ratio 1:2:1. Sugars are among the most important energy-storage molecules. The simplest of the carbohydrates are the monosaccharides (simple sugars), which have three to six or more carbon atoms typically arranged in a ring form. The most important of the 6-carbon monosaccharides for energy storage is glucose. Fructose and galactose are isomers of glucose; all have the empirical formula C6H12O6. Disaccharides consist of two linked monosaccharides. When glucose forms a disaccharide with fructose, the resulting disaccharide is sucrose (table sugar). When glucose is linked to galactose, the resulting disaccharide is lactose. Polysaccharides are longer polymers made up of monosaccharides that have been joined through dehydration reactions. Organisms store the metabolic energy contained in monosaccharides by converting them into disaccharides; these are then linked together into insoluble polysaccharides called starches. The comparable molecule to starch in animals is glycogen. Cellulose is a structural polysaccharide; it is the chief component of plant cell walls. Chitin is the principal structural element in the external skeletons of many invertebrates.
Two main varieties of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Genetic information is stored in DNA, and short-lived copies of this are made in the form of RNA, which is then used to direct the synthesis of proteins during the process of gene expression. Unique among macromolecules, nucleic acids are able to serve as templates for producing copies of themselves. This characteristic allows genetic information to be preserved during cell division and during the reproduction of organisms. RNA carries information; it is part of the organelle responsible for protein synthesis, and it is also involved in the control of gene expression. As a carrier of information, the form of RNA called messenger RNA (mRNA) consists of transcribed single-stranded copies of portions of the DNA. These transcripts serve as blueprints specifying the amino acid sequence of proteins. Nucleic acids consist of long polymers of repeating subunits called nucleotides. Each nucleotide consists of a pentose (5-carbon sugar), a phosphate group, and an organic nitrogenous base. In DNA, the sugar is deoxyribose and in RNA it is ribose. Chains of nucleotides, polynucleotides, have polarity. These ends are referred to as 5′ (“five-prime”, PO4–) and 3′ (“three-prime”, –OH). Nucleotides have five types of nitrogenous bases. Two of these are large, double-ring molecules called purines that are each found in both DNA and RNA; the two purines are adenine (A) and guanine (G). The other three bases are single-ring molecules called pyrimidines that include cytosine (C, in both DNA and RNA), thymine (T, in DNA only), and uracil (U, in RNA only). Organisms use sequences of nucleotides in DNA to encode the information specifying the amino acid sequences of their proteins. The code of a DNA molecule consists of different combinations of the four types of nucleotides in specific sequences. DNA molecules in organisms exist as two polynucleotide chains. The base pairs in the double helix consists of a base in one chain attracted by two hydrogen bonds to a base opposite in the other chain. Adenine pairs with thymine (in DNA) or with uracil (in RNA), and cytosine pairs with guanine. The bases that participate in base-pairing are said to be complementary to each other. RNA carries information in the form of mRNA, it is part of the ribosome in the form of ribosomal RNA (rRNA), and it carries amino acids in the form of transfer RNA (tRNA). Adenosine triphosphate (ATP) is the energy currency of the cell. Cells use the energy released by breaking down food molecules to synthesize ATP. Cells can then use the energy released by ATP hydrolysis to drive energetically unfavorable chemical reactions, to power transport across membranes, and to power the movement of cells.
Proteins functions can be grouped into 1) enzyme catalysts, 2) defense, 3) transport, 4) support, 5) motion, 6) regulation, and 7) storage. Proteins are linear polymers made with 20 different amino acids. Amino acids contain an amino group (–NH2) and an acidic carboxyl group (–COOH). Each amino acid has the same chemical backbone, but differs in the side, or R, group. The unique character of each amino acid is determined by the nature of the R group. The specific order of amino acids determines the protein’s structure and function. Each amino acid affects the shape of the protein differently, depending on the chemical nature of its side group. The covalent bond that links two amino acids is called a peptide bond. A protein is composed of one or more long unbranched chains. Each chain is called a polypeptide. Each kind of protein has a specific amino acid sequence. Although many different amino acids occur in nature, only 20 commonly occur in proteins. Of these 20, 8 are called essential amino acids because humans cannot synthesize them and thus must get them from their diets. The shape of a protein determines its function. In every protein studied, essentially all the internal amino acids are nonpolar ones. Polar and charged amino acids are restricted to the surface of the protein, except for the few that play key functional roles. The primary structure of a protein is its amino acid sequence. Secondary structure results from hydrogen bonds forming between nearby amino acids. The tertiary structure is the final 3-D shape of the protein. Quaternary structure (only for proteins with multiple polypeptides) is the arrangement of of the multiple polypeptides in space. Similar structures in otherwise dissimilar proteins are called motifs. Domains of proteins are functional units within a larger structure. Chaperone proteins help other proteins fold correctly. If a protein’s environment is altered, the protein may change its shape or even unfold completely; this process is called denaturation. Denatured proteins are usually biologically inactive. Any given organism usually has a tolerance range of pH, temperature, and salt concentration; within that range, its enzymes maintain the proper shape to carry out their biological function. Renaturation occurs when a small protein spontaneously refolds into its natural shape. The fact that some proteins can spontaneously renature implies that tertiary structure is strongly influenced by primary structure. For proteins with quaternary structure, the subunits may be disassociated (separated) without losing their individual tertiary structure.
Lipids are a somewhat loosely defined group of molecules with one main chemical characteristic: They are insoluble in water because they have a high proportion of nonpolar C–H bonds. Many lipids are built from a simple skeleton made up of two main kinds of molecules: fatty acids and glycerol. Fatty acids are long-chain hydrocarbons with a carboxyl group at one end. Glycerol is a 3-carbon polyalcohol. Because it contains three fatty acids, a fat molecule is commonly called a triglyceride. If all of the internal carbon atoms in a fatty acid chain are bonded to two hydrogen atoms, we call this saturated, which refers to its having the maximum hydrogen atoms possible. A fatty acid with double bonds between one or more pairs of successive carbon atoms will have fewer hydrogen atoms, and thus is said to be unsaturated. Fatty acids with one double bond are called monounsaturated, and those with more than one double bond are termed polyunsaturated. When fats are partially hydrogenated industrially, this can produce trans fats. Other kinds of lipids besides fats include terpenes, steroids, and prostaglandins. Fats are relatively more reduced than carbohydrates, and will thus release more energy upon oxidation; this makes fats much more efficient for storing chemical energy. Excess carbohydrate is converted to fat for storage. Most fats produced by animals are saturated, whereas most plant fats are unsaturated. Complex lipid molecules called phospholipids are among the most important molecules of the cell because they form the core of all biological membranes. The basic structure of a phospholipid includes glycerol, fatty acids, and a phosphate group.
—March 2021
—April 2023
Lance Jones 