Figure 2.6. The hydrogen bond.
nitrogen or sulfur by a covalent bond gets close to a second electron-grabbing atom; then that second atom also grabs a small share of the electrons to form what is known as a hydrogen bond. The atom to which the hydrogen is covalently bonded is called the donor because it is losing some of its share of electrons; the other electron-grabbing atom is the acceptor. For a hydrogen bond to form, the donor and acceptor must be within a fixed distance of one another (typically 0.3 nm) with the hydrogen on a straight line between them.
Liquid water is so stable because the individual molecules can hydrogen bond, as illustrated in Figure 2.6a. Hydrogen bonding also plays a critical role in allowing DNA to store and replicate genetic information. Figure 2.6b shows how the base pairs (page 69) of DNA form hydrogen bonds in which hydrogen atoms are shared between nitrogen and oxygen and between nitrogen and nitrogen.
Very large molecules, or macromolecules, are central to the working of cells. Large biological molecules are polymers: they are assembled by joining together small, simpler molecules, which are therefore called monomers. Chemical technology has mimicked nature by producing many important polymers—polyethylene is a polymer of ethylene monomers. Cells make a number of macromolecules that we will introduce, together with their monomer building blocks, in this chapter.
Carbohydrates—sugars and the macromolecules built from them—have many different roles in cells and organisms.
All carbohydrates are formed from the simple sugars called monosaccharides. Figure 2.7 shows the monosaccharide called glucose. The form shown at the top has five carbons joined each to the other with an oxygen atom completing the ring. As well as the oxygen in the ring, glucose has five other oxygens, each in an -OH (hydroxyl) group. Glucose easily switches between the three forms, or isomers, illustrated in Figure 2.7. The two ring structures are stereo isomers: although they comprise the same atoms connected by the same bonds, they represent two different ways of arranging the atoms in space. The two stereo isomers, named a and j, continually interconvert in solution via the open-chain form.
Figure 2.8 shows five other monosaccharides that we will meet again in this book. Like glucose, each of these monosaccharides can adopt an open-chain form and a number of ring structures. In Figure 2.8 we show each sugar in a form that it adopts quite often. These sugars share with glucose the two characteristics of monosaccharides: they can adopt a form in which an oxygen atom completes a ring of carbons, and they have many hydroxyl groups. The generic names for monosaccharides are derived from the Greek for the number of their carbon atoms, so glucose, galactose, mannose, and fructose are hexoses (6 carbons) while ribose and ribulose are pentoses (5 carbons). Classically, a monosaccharide has the general formula C„ (H2O)„, hence the name carbohydrate. All the monosaccharides shown in Figures 2.7 and 2.8 fit this rule—the four hexoses can be written as C6(H2O)6 and the two pentoses as C5(H2O)5.
It is worth noting that although both sugars and organic acids contain an —OH group, the behavior of an —OH group that forms part of a carboxyl group is very different from one that is not next to a double-bonded oxygen. In general the —OH group in a carboxyl group will readily give up an H+ to water or other acceptors; this is not true of —OH groups
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