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

MBoC6 e3.03/2.12
CATALYSIS AND THE USE OF ENERGY BY CELLS
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These forces are of four types: electrostatic attractions, hydrogen bonds, van der
Waals attractions, and an interaction between nonpolar groups caused by their
hydrophobic expulsion from water. The same set of weak forces governs the specific
binding of other molecules to macromolecules, making possible the myriad associations
between biological molecules that produce the structure and the chemistry of
a cell.
Catalysis and the Use of Energy by Cells
One property of living things above all makes them seem almost miraculously different
from nonliving matter: they create and maintain order, in a universe that is
tending always to greater disorder (Figure 2–12). To create this order, the cells in
a living organism must perform a never-ending stream of chemical reactions. In
some of these reactions, small organic molecules—amino acids, sugars, nucleotides,
and lipids—are being taken apart or modified to supply the many other
small molecules that the cell requires. In other reactions, small molecules are
being used to construct an enormously diverse range of proteins, nucleic acids,
and other macromolecules that endow living systems with all of their most distinctive
properties. Each cell can be viewed as a tiny chemical factory, performing
many millions of reactions every second.
Cell Metabolism Is Organized by Enzymes
The chemical reactions that a cell carries out would normally occur only at much
higher temperatures than those existing inside cells. For this reason, each reaction
requires a specific boost in chemical reactivity. This requirement is crucial,
because it allows the cell to control its chemistry. The control is exerted through
specialized biological catalysts. These are almost always proteins called enzymes,
although RNA catalysts also exist, called ribozymes. Each enzyme accelerates, or
catalyzes, just one of the many possible kinds of reactions that a particular molecule
might undergo. Enzyme-catalyzed reactions are connected in series, so
that the product of one reaction becomes the starting material, or substrate, for
the next (Figure 2–13). Long linear reaction pathways are in turn linked to one
another, forming a maze of interconnected reactions that enable the cell to survive,
grow, and reproduce.
Two opposing streams of chemical reactions occur in cells: (1) the catabolic
pathways break down foodstuffs into smaller molecules, thereby generating both
a useful form of energy for the cell and some of the small molecules that the cell
needs as building blocks, and (2) the anabolic, or biosynthetic, pathways use the
(A) (B) (C) (D) (E)
20 nm
50 nm
10 µm
0.5 mm
20 mm
Figure 2–12 Biological structures are highly ordered. Well-defined, ornate, and beautiful spatial patterns can be found at every level of
organization in living organisms. In order of increasing size: (A) protein molecules in the coat of a virus (a parasite that, although not technically alive,
contains the same types of molecules as those found in living cells); (B) the regular array of microtubules seen in a cross section of a sperm tail;
(C) surface contours of a pollen grain (a single cell); (D) cross section of a fern stem, showing the patterned arrangement of cells; and (E) a spiral
arrangement of leaves in a succulent plant. (A, courtesy of Robert Grant, Stéphane Crainic, and James M. Hogle; B, courtesy of Lewis Tilney;
C, courtesy of Colin MacFarlane and Chris Jeffree; D, courtesy of Jim Haseloff.)