READ3.1 Carbon: T he Framework of Biological MoleculesThe framework of biological molecules consists predominantly of carbon atoms bonded to other carbon atoms or to atoms of oxygen, nitrogen, sulfur, phosphorus, or hydrogen. Because carbon atoms can form up to four covalent bonds, molecules containing carbon can form straight chains, branches, or even rings, balls, tubes, and coils.Molecules consisting only of carbon and hydrogen are called hydrocarbons. Because carbon-hydrogen covalent bonds store considerable energy, hydrocarbons make good fuels. Theoretically speaking, the length of a chain of carbon atoms is unlimited. As described in the rest of this chapter, the four main types of biological molecules often consist of huge chains of carbon-containing compounds.Functional groups account for differences in molecular propertiesCarbon and hydrogen atoms both have very similar electronegativities. Electrons in C-C and C-H bonds are therefore evenly distributed, with no significantdifferences in charge over the molecular surface. For this reason, hydrocarbons are nonpolar. Most biological molecules produced by cells, however, also contain other atoms. Because these other atoms frequently have different electronegativities, molecules containing them exhibit regions of partial positive or negative charge. They are polar. These molecules can be thought of as a C-H core to which specific molecular groups, called functional groups, are attached. One such common functionalgroup is -OH, called a hydroxyl group. Functional groups have definite chemical properties that they retain no matter where they occur. Both the hydroxyl and carbonyl (C=O) groups, for example, are polar because of the electronegativity of the oxygen atoms. Other common functional groups are the acidic carboxyl (COOH), phosphate (PO4-), and the basic amino (NH2) group. Many of these functional groupscan also participate in hydrogen bonding. Hydrogen bond donors and acceptors can be predicted based on their electronegativities. Isomers have the same molecular formulas but different structuresOrganic molecules having the same molecular or empirical formula can exist in different forms called isomers If there are differences in the actual structure of their carbon skeleton, we call them structural isomers. Later you will see that glucose and fructose are structural isomers of C6H12O6. Another form of isomers, called stereoisomers, have the same carbon skeleton but differ in how the groups are attached to this skeleton are arranged in space. Enzymes in biological systems usually recognize only a single, specific stereoisomer. A subcategory of stereoisomers, called enantiomers, are actually mirror images of each other. A molecule that has mirror-imageversions is called a chiral molecule. When carbon is bound to four different molecules,this inherent asymmetry exists. Chiral compounds are characterized by their effect on polarized light. Polarized light has a single plane, and chiral molecules rotate this plane either to the right or left. We therefore call the two chiral forms D for dextrorotatory and L for levorotatory. Living systems tend to produce only a single enantiomer of the two possible forms; for example, in most organisms we find primarily D-sugars and L-amino acids. Biological macromolecules include carbohydrates, nucleic acids, proteins, and lipidsA polymer is a long molecule built by linking together a large number of small, similar chemical subunits called monomers. They are like railroad cars coupled to form a train. The nature of a polymer is determined by the monomers used to build the polymer. Here are some examples. Complex carbohydrates such as starch are polymers composed of simple ring-shaped sugars. Nucleic acids (DNA and RNA) are polymers of nucleotides, and proteins are polymers of amino acids. These long chains are built via chemical reactions termed dehydration reactions and are broken down by hydrolysis reactions. The Dehydration reactionDespite the differences between monomers of these major polymers, the basic chemistry of their synthesis is similar: To form a covalent bond between two monomers, an -OH group is removed from one monomer, and a hydrogen atom (H) is removed from the other. For example, this simple chemistry is the same for linking amino acids together to make a protein or assembling glucose units together to make starch. This reaction is also used to link fatty acids to glycerol in lipids. This chemical reaction is called condensation, or a dehydration reaction, because the removal of -OH and -H is the same as the removal of a molecule of water (H2O). The Hydrolysis reactionCells disassemble macromolecules into their constituent subunits through reactions that are the reverse of dehydration--a molecule of water is added instead of removed. In this process, called hydrolysis, a hydrogen atom is attached to one subunit and a hydroxyl group to the other, breaking a specific covalent bond in the macromolecule. When you eat a potato, which contains starch, your body breaks the starch down into glucose units by hydrolysis. The potato plant built the starch molecules originally by dehydration reactions. Monosaccharides are simple sugarsThe simplest of the carbohydrates are the monosaccharides. Simple sugars contain as few as three carbon atoms, but those that play the centralrole in energy storage have six. The empirical formula of six-carbon sugars is: C6H12O6 or (CH2O)6Six-carbon sugars can exist in a straight-chain form, but dissolved in water(an aqueous environment) they almost always form rings.Sugar isomers have structural differencesGlucose is not the only sugar with the formula C6H12O6. Both structural isomers and stereoisomers of this simple six-carbon skeleton exist in nature. Fructose is a structural isomer that differs in the position of the carbonyl carbon (C=); galactose is a stereoisomer that differs in the position of -OH and -H groups relative to the ring. Disaccharides serve as transport molecules in plants and provide nutrition in animalsMost organisms transport sugars within their bodies. In humans, the glucose that circulates in the blood does so as a simple monosaccharide. In plants and many other organisms, however, glucose is converted into a transport form before it is moved from place to place within the organism. In such a form, it is less readily metabolized during transport.Transport forms of sugars are