Essential Cell Biology 5th edition

CHAPTER THREE
3
Energy, Catalysis, and
Biosynthesis
One property above all makes living things seem almost miraculously
different from nonliving matter: they create and maintain order in a universe
that is tending always toward greater disorder. To accomplish this
remarkable feat, the cells in a living organism must continuously carry
out a never-ending stream of chemical reactions to maintain their structure,
meet their metabolic needs, and stave off unrelenting chemical
decay. In these reactions, small organic molecules—amino acids, sugars,
nucleotides, and lipids—can be taken apart or modified to supply
the many other small molecules that the cell requires. These molecules
are also used to construct an enormously diverse range of large molecules,
including the proteins, nucleic acids, and other macromolecules
that constitute most of the mass of living systems and endow them with
their distinctive properties.
THE USE OF ENERGY BY CELLS
FREE ENERGY AND CATALYSIS
ACTIVATED CARRIERS AND
BIOSYNTHESIS
Each cell can be viewed as a tiny chemical factory, performing many millions
of reactions every second. This incessant activity requires both a
source of atoms in the form of food molecules and a source of energy.
Both the atoms and the energy must come, ultimately, from the nonliving
environment. In this chapter, we discuss why cells require energy, and
how they use energy and atoms from their environment to create and
maintain the molecular order that makes life possible.
Most of the chemical reactions that cells perform would normally occur
only at temperatures that are much higher than those inside a cell. Each
reaction therefore requires a major boost in chemical reactivity to enable
it to proceed rapidly within the cell. This boost is provided by a large
set of specialized proteins called enzymes, each of which accelerates, or
catalyzes, just one of the many possible reactions that a particular