The nature and uses of energy
Any time an object is not moving, it has a store of potential
energy—a capacity to do work, simply owing to its posi-
tion in space and the arrangement of its parts. If a station-
ary runner springs into action, some of the runner’s poten-
tial energy is transformed into kinetic energy, the energy
of motion.
Energy on the move does work when it imparts motion
to other things—for example, when you throw a ball. In
skeletal muscle cells in your arm, the energy currency ATP
(adenosine triphosphate, Sections 2.13, 3.8, 3.13–3.14) gave
up some of its potential energy to molecules of contractile
units and set them in motion. The combined motions in
many muscle cells resulted in the movement of whole mus-
cles. The transfer of energy from ATP also released another
form of kinetic energy called heat, or thermal energy.
The potential energy of molecules is called chemical
energy and is measured as kilocalories. A kilocalorie is
the amount of energy it takes to heat 1,000 grams of water
from 14.5°C to 15.5°C at standard pressure.
As noted in Chapter 3, cells use energy for chemical
work, to stockpile, build, rearrange, and break apart sub-
stances. They channel it into mechanical work—to move
cell structures and the whole body or parts of it. They
also channel it into electrochemical work—to move charged
substances into or out of the cytoplasm or an organelle
compartment.Laws of thermodynamics
We cannot create energy from scratch; we must first get
it from someplace else. Why? According to the first law
of thermodynamics, the total amount of energy in the
universe remains constant. More energy cannot be cre-
ated; existing energy cannot vanish or be destroyed. It can
only be converted from one form to some other form. For
instance, when you eat, your cells extract energy from food
and convert it to other forms, such as kinetic energy for
moving about.
With each metabolic conversion, some of the energy
escapes to your surroundings, as heat. Even when you
“do nothing,” your body gives off about as much heat as
a 100-watt lightbulb because of conversions in your cells.
The energy being released is transferred to atoms and mol-
ecules that make up the air, and in this way it heats up the
surroundings, as shown in Figure A.1. In general, the body
cannot recapture energy lost as heat, but the energy still
exists in the environment outside the body. Overall, there
is a one-way flow of energy in the universe.
The human body obtains its energy mainly from the
covalent bonds in organic compounds, such as glucose and
glycogen. When the compounds enter metabolic reactions,
specific bonds break or are rearranged. For example, your
cells release usable energy from glucose by breaking all
of its covalent bonds. After many steps, six molecules of
carbon dioxide and six of water remain. Compared with
glucose, these leftovers have more stable arrangements of
atoms, but chemical energy in their bonds is much less.
Why? Some energy was lost at each breakdown step lead-
ing to their formation. This is why glucose is a much better
source of usable energy than, for example, water is.
As the molecular events just described take place, some
heat is lost to the surroundings and cannot be recaptured.
Said another way, no energy conversion can ever beENERGY GAINED
BY SURROUNDINGS
(locker room air)ENERGY LOST
FROM A SYSTEM
(a human body)transfer of
body heatNET ENERGY CHANGE = 0Figure A.1 The body uses energy for life processes and
loses a portion of it as heat. (© Cengage Learning; photo: Evan Cerasoli)
Concepts in Cell Metabolism
Appendix I
Appendix i A-1Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).