Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

43
Cell Chemistry and
Bioenergetics
chapter
2
It is at first sight difficult to accept the idea that living creatures are merely chemical
systems. Their incredible diversity of form, their seemingly purposeful behavior,
and their ability to grow and reproduce all seem to set them apart from the
world of solids, liquids, and gases that chemistry normally describes. Indeed,
until the nineteenth century animals were believed to contain a Vital Force—an
“animus”—that was responsible for their distinctive properties.
We now know that there is nothing in living organisms that disobeys chemical
or physical laws. However, the chemistry of life is indeed special. First, it is based
overwhelmingly on carbon compounds, the study of which is known as organic
chemistry. Second, cells are 70% water, and life depends largely on chemical reactions
that take place in aqueous solution. Third, and most important, cell chemistry
is enormously complex: even the simplest cell is vastly more complicated
in its chemistry than any other chemical system known. In particular, although
cells contain a variety of small carbon-containing molecules, most of the carbon
atoms present are incorporated into enormous polymeric molecules—chains of
chemical subunits linked end-to-end. It is the unique properties of these macromolecules
that enable cells and organisms to grow and reproduce—as well as to
do all the other things that are characteristic of life.
The Chemical Components of a Cell
Living organisms are made of only a small selection of the 92 naturally occurring
elements, four of which—carbon (C), hydrogen (H), nitrogen (N), and oxygen
(O)—make up 96.5% of an organism’s weight (Figure 2–1). The atoms of these elements
are linked together by covalent bonds to form molecules (see Panel 2–1, pp.
90–91). Because covalent bonds are typically 100 times stronger than the thermal
energies within a cell, they resist being pulled apart by thermal motions, and they
are normally broken only during specific chemical reactions with other atoms and
molecules. Two different molecules can be held together by noncovalent bonds,
atomic number
1
H
1
Li
11
Na
23
19
K
39
Rb
Cs
Fr
atomic weight
Be
12
Mg
24
20
Ca
40
Sr
Ba
Ra
Sc
Y
La
Ac
Ti
Zr
Hf
Rf
23
V
51
Nb
Ta
Db
24
Cr
52
42
Mo
96
25
Mn
55
26
Fe
56
27
Co
59
28
Ni
59
29
Cu
64
30
Zn
65
5
B
11
Ga
Tc Ru Rh Pd Ag Cd In Sn Sb
W Re Os Ir Pt Au Hg Tl Pb Bi Po
Al
6
C
12
14
Si
28
7
N
14
15
P
31
8
O
16
16
S
32
34
He
Ne
Ar
Ge As Se Br Kr
79
Te
9
F
19
17
Cl
35
53
I
127
At
Xe
Rn
In This Chapter
The Chemical Components
of a Cell
CATALYSIS AND THE USE OF
ENERGY BY CELLS
HOW CELLS OBTAIN ENERGY
FROM FOOD
Figure 2–1 The main elements in cells,
highlighted in the periodic table. When
ordered by their atomic number and
arranged in this manner, elements fall
into vertical columns that show similar
properties. Atoms in the same vertical
column must gain (or lose) the same
number of electrons to attain a filled outer
shell, and they thus behave similarly in
bond or ion formation. Thus, for example,
Mg and Ca tend to give away the two
electrons in their outer shells. C, N, and O
occur in the same horizontal row, and tend
to complete their second shells by sharing
electrons.
The four elements highlighted in red
constitute 99% of the total number of
atoms present in the human body. An
additional seven elements, highlighted in
blue, together represent about 0.9% of
the total. The elements shown in green are
required in trace amounts by humans. It
remains unclear whether those elements
shown in yellow are essential in humans.
The chemistry of life, it seems, is therefore
predominantly the chemistry of lighter
elements. The atomic weights shown here
are those of the most common isotope of
each element.