Essential Cell Biology 5th edition

The Use of Energy by Cells
85
A
B
raised brick
has potential
energy due
to pull of
gravity
potential energy due to position kinetic energy heat energy
two hydrogen
gas molecules
chemical-bond energy in H 2 and O 2
battery
+
+
oxygen gas
molecule
–
fan
motor
wires
falling brick has
kinetic energy
rapid vibrations and
rotations of two newly
formed water molecules
rapid molecular
motions in H 2 O
(kinetic energy)
+
–
heat is released
when brick hits
the floor
heat dispersed to
surroundings
heat energy
Figure 3–6 Different forms of energy are
interconvertible, but the total amount
of energy must be conserved. (A) We
can use the height and weight of the brick
to predict exactly how much heat will be
released when it hits the floor. (B) The
large amount of chemical-bond energy
released when water (H 2 O) is formed from
H 2 and O 2 is initially converted to very
rapid thermal motions in the two new H 2 O
molecules; however, collisions with other
H 2 O molecules almost instantaneously
spread this kinetic energy evenly throughout
the surroundings (heat transfer), making
the new H 2 O molecules indistinguishable
from all the rest. (C) Cells can convert
chemical-bond energy into kinetic energy
to drive, for example, molecular motor
proteins; however, this occurs without
the intermediate conversion of chemical
energy to electrical energy that a manmade
appliance such as this fan requires.
(D) Some cells can also harvest the energy
from sunlight to form chemical bonds via
photosynthesis.
fan
C
chemical-bond energy electrical energy kinetic energy
sunlight
chlorophyll
molecule
chlorophyll molecule
in excited state
photosynthesis
D
electromagnetic (light) energy
high-energy electrons
chemical-bond energy
molecules (chemical-bond energy) is converted into the thermal motion
of molecules (heat energy). This conversion of chemical energy into heat
energy causes the universe as ECB5 a whole e3.06/3.06 to become more disordered—as
required by the second law of thermodynamics. But a cell cannot derive
any benefit from the heat energy it produces unless the heat-generating
reactions are directly linked to processes that maintain molecular order
inside the cell. It is the tight coupling of heat production to an increase
in order that distinguishes the metabolism of a cell from the wasteful
burning of fuel in a fire. Later in this chapter, we illustrate how this coupling
occurs. For the moment, it is sufficient to recognize that—by directly
linking the “burning” of food molecules to the generation of biological
order—cells are able to create and maintain an island of order in a universe
tending toward chaos.
Photosynthetic Organisms Use Sunlight to Synthesize
Organic Molecules
All animals live on energy stored in the chemical bonds of organic molecules,
which they take in as food. These food molecules also provide the