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

54 Chapter 2: Cell Chemistry and Bioenergetics
1
raised brick
has potential
energy due
to pull of
gravity
potential energy due to position kinetic energy heat energy
+
falling brick has
kinetic energy
heat is released
when brick hits
the floor
Figure 2–17 Some interconversions
between different forms of energy.
All energy forms are, in principle,
interconvertible. In all these processes the
total amount of energy is conserved. Thus,
for example, from the height and weight
of the brick in (1), we can predict exactly
how much heat will be released when it hits
the floor. In (2), note that the large amount
of chemical-bond energy released when
water is formed is initially converted to
very rapid thermal motions in the two new
water molecules; but collisions with other
molecules almost instantaneously spread
this kinetic energy evenly throughout the
surroundings (heat transfer), making the
new molecules indistinguishable from all
the rest.
two hydrogen
gas molecules
oxygen gas
molecule
rapid vibrations and
rotations of two newly
formed water molecules
heat dispersed to
surroundings
2
chemical-bond energy in H 2 and O 2
rapid molecular
motions in H 2 O
heat energy
battery
+
–
fan
motor
+
–
wires
fan
3
chemical-bond energy electrical energy kinetic energy
sunlight
chlorophyll
molecule
chlorophyll molecule
in excited state
photosynthesis
4
electromagnetic (light) energy
high-energy electrons
chemical-bond energy
in order that distinguishes the metabolism of a cell from the wasteful burning of
fuel in a fire. Later, we illustrate how this coupling occurs. For now, it is sufficient
to recognize that a direct linkage of the “controlled burning” of food molecules to
the generation of biological order is required for cells to create and maintain an
island of order in a universe tending toward chaos.
Cells Obtain Energy by the Oxidation of Organic Molecules
MBoC6 m2.39/2.17
All animal and plant cells are powered by energy stored in the chemical bonds
of organic molecules, whether they are sugars that a plant has photosynthesized
as food for itself or the mixture of large and small molecules that an animal has
eaten. Organisms must extract this energy in usable form to live, grow, and reproduce.
In both plants and animals, energy is extracted from food molecules by a
process of gradual oxidation, or controlled burning.
The Earth’s atmosphere contains a great deal of oxygen, and in the presence of
oxygen the most energetically stable form of carbon is CO 2 and that of hydrogen