Essential Cell Biology 5th edition

86 CHAPTER 3 Energy, Catalysis, and Biosynthesis
atoms that animals need to construct new living matter. Some animals
obtain their food by eating other animals, others by eating plants. Plants,
by contrast, obtain their energy directly from sunlight. Thus, the energy
animals obtain by eating plants—or by eating animals that have eaten
plants—ultimately comes from the sun (Figure 3–7).
Figure 3–7 With few exceptions, the
radiant energy of sunlight sustains
all life. Trapped by plants and some
microorganisms through photosynthesis,
light from the sun is the ultimate source of
all energy for humans and other animals.
(Wheat Field Behind Saint-Paul Hospital
with a Reaper by Vincent van Gogh.
Courtesy of Museum Folkwang, Essen.)
ECB5 e3.07/3.07
QUESTION 3–1
Consider the equation
light energy + CO 2 + H 2 O →
sugars + O 2 + heat energy
Would you expect this reaction to
occur in a single step? Why must
heat be generated in the reaction?
Explain your answers.
Solar energy enters the living world through photosynthesis, a process
that converts the electromagnetic energy in sunlight into chemical-bond
energy in cells. Photosynthetic organisms—including plants, algae, and
some bacteria—use the energy they derive from sunlight to synthesize
small chemical building blocks such as sugars, amino acids, nucleotides,
and fatty acids. These small molecules in turn are converted into the
macromolecules—the proteins, nucleic acids, and polysaccharides—that
form the plant.
We describe the elegant mechanisms that underlie photosynthesis in
detail in Chapter 14. Generally speaking, the reactions of photosynthesis
take place in two stages. In the first stage, energy from sunlight is
captured and transiently stored as chemical-bond energy in specialized
molecules called activated carriers, which we discuss in more detail later
in the chapter. All of the oxygen (O 2 ) in the air we breathe is generated by
the splitting of water molecules during this first stage of photosynthesis.
In the second stage, the activated carriers are used to help drive a carbonfixation
process, in which sugars are manufactured from carbon dioxide
gas (CO 2 ). In this way, photosynthesis generates an essential source of
stored chemical-bond energy and other organic materials—for the plant
itself and for any animals that eat it. The two stages of photosynthesis are
summarized in Figure 3–8.
Cells Obtain Energy by the Oxidation of Organic
Molecules
To live, grow, and reproduce, all organisms rely on the energy stored in
the chemical bonds of organic molecules—either the sugars that a plant
has produced by photosynthesis as food for itself or the mixture of large
and small molecules that an animal has eaten. In both plants and animals,
this chemical energy is extracted from food molecules by a process
of gradual oxidation, or controlled burning.
Earth’s atmosphere is about 21% oxygen. In the presence of oxygen, the
most energetically stable form of carbon is CO 2 and that of hydrogen is
H 2 O; the oxidation of carbon-containing molecules is therefore energetically
very favorable. A cell is able to obtain energy from sugars or
other organic molecules by allowing the carbon and hydrogen atoms in
these molecules to combine with oxygen—that is, become oxidized—to
produce CO 2 and H 2 O, respectively. This complex step-wise process by
which food molecules are broken down to produce energy is known as
cell respiration.
Photosynthesis and cell respiration are complementary processes (Figure
3–9). Plants, animals, and microorganisms have existed together on this
SUN
Figure 3–8 Photosynthesis takes place
in two stages. The activated carriers
generated in the first stage, ATP and
NADPH, are described in detail later in the
chapter.
CAPTURE OF
LIGHT ENERGY
PHOTOSYNTHESIS
activated
carriers
of energy
ATP
NADPH
H 2 O O 2
H 2 O + CO 2
STAGE 1 STAGE 2
MANUFACTURE
OF SUGARS
sugar