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

CATALYSIS AND THE USE OF ENERGY BY CELLS
55
H 2 O
PHOTOSYNTHESIS
CO 2 + H 2 O
O 2
O 2 + SUGARS
PLANTS
ALGAE
SOME BACTERIA
ENERGY
OF
SUNLIGHT
CO 2
SUGARS AND
OTHER ORGANIC
MOLECULES
CELLULAR RESPIRATION
SUGARS + O 2 H 2 O + CO 2
CO 2
USEFUL
CHEMICAL-
BOND
ENERGY
MOST
LIVING
ORGANISMS
is H 2 O. A cell is therefore able to obtain energy from sugars or other organic molecules
by allowing their carbon and hydrogen atoms to combine with oxygen to
produce CO 2 and H 2 O, respectively—a MBoC6 process m2.41/2.18 called aerobic respiration.
Photosynthesis (discussed in detail in Chapter 14) and respiration are complementary
processes (Figure 2–18). This means that the transactions between
plants and animals are not all one way. Plants, animals, and microorganisms have
existed together on this planet for so long that many of them have become an
essential part of the others’ environments. The oxygen released by photosynthesis
is consumed in the combustion of organic molecules during aerobic respiration.
And some of the CO 2 molecules that are fixed today into organic molecules
by photosynthesis in a green leaf were yesterday released into the atmosphere
by the respiration of an animal—or by the respiration of a fungus or bacterium
decomposing dead organic matter. We therefore see that carbon utilization forms
a huge cycle that involves the biosphere (all of the living organisms on Earth) as a
whole (Figure 2–19). Similarly, atoms of nitrogen, phosphorus, and sulfur move
between the living and nonliving worlds in cycles that involve plants, animals,
fungi, and bacteria.
O 2
H 2 O
Figure 2–18 Photosynthesis and
respiration as complementary processes
in the living world. Photosynthesis
converts the electromagnetic energy in
sunlight into chemical-bond energy in
sugars and other organic molecules.
Plants, algae, and cyanobacteria obtain
the carbon atoms that they need for this
purpose from atmospheric CO 2 and the
hydrogen from water, releasing O 2 gas
as a by-product. The organic molecules
produced by photosynthesis in turn serve
as food for other organisms. Many of these
organisms carry out aerobic respiration,
a process that uses O 2 to form CO 2 from
the same carbon atoms that had been
taken up as CO 2 and converted into sugars
by photosynthesis. In the process, the
organisms that respire obtain the chemicalbond
energy that they need to survive.
The first cells on the Earth are
thought to have been capable of neither
photosynthesis nor respiration (discussed
in Chapter 14). However, photosynthesis
must have preceded respiration on the
Earth, since there is strong evidence that
billions of years of photosynthesis were
required before O 2 had been released in
sufficient quantity to create an atmosphere
rich in this gas. (The Earth’s atmosphere
currently contains 20% O 2 .)
Oxidation and Reduction Involve Electron Transfers
The cell does not oxidize organic molecules in one step, as occurs when organic
material is burned in a fire. Through the use of enzyme catalysts, metabolism takes
these molecules through a large number of reactions that only rarely involve the
direct addition of oxygen. Before we consider some of these reactions and their
purpose, we discuss what is meant by the process of oxidation.
CO 2 IN ATMOSPHERE AND WATER
RESPIRATION
PHOTOSYNTHESIS
ANIMALS
PLANTS, ALGAE,
BACTERIA
FOOD
CHAIN
HUMUS AND DISSOLVED
ORGANIC MATTER
SEDIMENTS AND
FOSSIL FUELS
Figure 2–19 The carbon cycle. Individual carbon atoms are incorporated into organic molecules of
the living world by the photosynthetic activity of bacteria, algae, and plants. They pass to animals,
microorganisms, and organic material in soil and oceans in cyclic paths. CO 2 is restored to the
atmosphere when organic molecules are oxidized by cells or burned by humans as fuels.
MBoC6 m2.42/2.19