Essential Cell Biology 5th edition

52 CHAPTER 2 Chemical Components of Cells
TABLE 2–2 THE CHEMICAL COMPOSITION OF A BACTERIAL CELL
Substance
Percent of Total
Cell Weight
Approximate Number
of Types in Each Class
Water 70 1
Inorganic ions 1 20
Sugars and precursors 1 250
Amino acids and precursors 0.4 100
Nucleotides and precursors 0.4 100
Fatty acids and precursors 1 50
Other small molecules 0.2 3000
Phospholipids 2 4*
Macromolecules (nucleic acids,
proteins, and polysaccharides)
24 3000
*There are four classes of phospholipids, each of which exists in many varieties
(discussed in Chapter 4).
Sugars Are both Energy Sources and Subunits of
Polysaccharides
The simplest sugars—the monosaccharides—are compounds with the
general formula (CH 2 O) n , where n is usually 3, 4, 5, or 6. Glucose, for
example, has the formula C 6 H 12 O 6 (Figure 2–18). Because of this simple
formula, sugars, and the larger molecules made from them, are called
carbohydrates. The formula, however, does not adequately define the
molecule: the same set of carbons, hydrogens, and oxygens can be joined
together by covalent bonds in a variety of ways, creating structures with
different shapes. Thus glucose can be converted into a different sugar—
mannose or galactose—simply by switching the orientations of specific
–OH groups relative to the rest of the molecule (Panel 2–4, pp. 72–73).
In addition, each of these sugars can exist in either of two forms, called
the d-form and the l-form, which are mirror images of each other. Sets
of molecules with the same chemical formula but different structures
are called isomers, and mirror-image pairs of such molecules are called
Figure 2–18 The structure of glucose,
a monosaccharide, can be represented
in several ways. (A) A structural formula
in which the atoms are shown as chemical
symbols, linked together by solid lines
representing the covalent bonds. The
thickened lines are used to indicate the plane
of the sugar ring and to show that the –H
and –OH groups are not in the same plane as
the ring. (B) Another kind of structural formula
that shows the three-dimensional structure of
glucose in a so-called “chair configuration.”
(C) A ball-and-stick model in which the
three-dimensional arrangement of the atoms
in space is indicated. (D) A space-filling
model, which, as well as depicting the threedimensional
arrangement of the atoms, also
shows the relative sizes and surface contours
of the molecule (Movie 2.1). The atoms in (C)
and (D) are colored as in Figure 2–9: C, black;
H, white; O, red. This is the conventional
color-coding for these atoms and will be used
throughout this book.
(C)
H
C
HO
(A)
CH 2 OH
C O
H
OH H
C C
H OH
OH
C
H
H
CH 2 OH O
HO
H
H
OH
HO OH
H
H
(B)
(D)