44 Chapter 2. What’s inside cells[[Student version, December 8, 2002]]
Table 2.1:Composition of bacterial cells, by weight. [From (Albertset al.,1997)]Small molecules (74%):
Ions, other inorganic small molecules 1.2%
Sugars 1%
Fatty acids 1%
Individual amino acids 0.4%
Individual nucleotides 0.4%
Water 70%
Medium and big molecules (26%):
Protein 15%
RNA 6%
DNA 1%
Lipids 2%
Polysaccharides 2%Figure 2.16:(Structure.) Space-filling model of DPPC (dipalmitoyl phosphatidylcholine), a common phospholipid
molecule. Two “tails” (hydrocarbon chains, right) join to a “head” group (left) via phosphate and glycerol groups
(middle). Molecules like this one self-assemble into bilayer membranes (Figures 2.24 and 2.25), which in turn form
the partitions between cell compartments. Chapter 8 will discuss self-assembly. [Copyrighted figure; permission
pending.]
Figure 2.16 shows a typicalphospholipidmolecule. Phospholipids are formed by joining one or
twofatty acid chains (“tails”), via a glycerol molecule, to a phosphate and thence to a “head group.”
As described in Section 2.3.1 and Chapter 8, phospholipids self-assemble into thin membranes,
including the one surrounding every cell. Phospholipid molecules have long but informative names;
thus for example dipalmitoylphosphatidylcholine (orDPPC)consists of two (“di”)palmitic acid
chains joined by aphosphatetoacholineheadgroup. Similarly, most fats consist of three fatty
acid chains joined together by chemically bonding them to the three carbon atoms in a glycerol
molecule, to form a “triglyceride.” The joining is accomplished by a condensation reaction similar
to the one shown in Figure 2.15.