Organization of the Body
Forms of Energy
is the form stored in the bonds of chemical
substances. When chemical reactions occur that rearrange
the atoms of the chemicals in a certain way, the potential
energy is unleashed and becomes kinetic energy, or energy
For example, some of the energy in the foods you eat is
eventually converted into the kinetic energy of your moving
arm. However, food fuels cannot be used to energize body
activities directly. Instead, some of the food energy is cap-
tured temporarily in the bonds of a chemical called
fāt). Later, ATP’s
bonds are broken and the stored energy is released as needed
to do cellular work. Chemical energy in the form of ATP is
the most useful form of energy in living systems because it is
used to run almost all functional processes.
results from the movement of charged par-
ticles. In your home, electrical energy is found in the ﬂow of
electrons along the household wiring. In your body, electri-
cal currents are generated when charged particles called
move along or across cell membranes. ±e nervous system
uses electrical currents, called
, to transmit
messages from one part of the body to another. Electrical
currents traveling across the heart stimulate it to contract
(beat) and pump blood. (±is is why a strong electrical shock,
which interferes with such currents, can cause death.)
involved in mov-
ing matter. When you ride a bicycle, your legs provide the
mechanical energy that moves the pedals.
ik), is energy that travels in waves. ±ese waves, which
vary in length, are collectively called the
. ±ey include visible light, infrared waves, radio waves,
ultraviolet waves, and X rays. Light energy, which stimulates
the retinas of our eyes, is important in vision. Ultraviolet
waves cause sunburn, but they also stimulate your body to
make vitamin D.
Energy Form Conversions
With few exceptions, energy is easily converted from one form
to another. For example, the chemical energy (in gasoline) that
powers the motor of a speedboat is converted into the mechani-
cal energy of the whirling propeller that makes the boat skim
across the water.
Energy conversions are quite ineﬃcient. Some of the initial
energy supply is always “lost” to the environment as heat. (It is
not really lost because energy cannot be created or destroyed,
but that portion given oﬀ as heat is at least partly
is easy to demonstrate this principle. Electrical energy is con-
verted into light energy in a lightbulb. But if you touch a lit
bulb, you will soon discover that some of the electrical energy is
producing heat instead.
Likewise, all energy conversions in the body liberate heat.
±is heat helps to maintain our relatively high body tempera-
ture, which inﬂuences body functioning. For example, when
Matter is the “stuﬀ” of the universe. More precisely,
anything that occupies space and has mass. With some excep-
tions, it can be seen, smelled, and felt.
For all practical purposes, we can consider mass to be the
same as weight. However, this statement is not quite accurate.
of an object is equal to the actual amount of matter
in the object, and it remains constant wherever the object is. In
contrast, weight varies with gravity. So while your mass is the
same at sea level and on a mountaintop, you weigh just slightly
less on that mountaintop. ±e science of chemistry studies the
nature of matter, especially how its building blocks are put to-
gether and interact.
States of Matter
Matter exists in
. Examples of
each state are found in the human body. Solids, like bones and
teeth, have a deﬁnite shape and volume. Liquids such as blood
plasma have a deﬁnite volume, but they conform to the shape of
their container. Gases have neither a deﬁnite shape nor a deﬁ-
nite volume. ±e air we breathe is a gas.
Compared with matter, energy is less tangible. It has no mass,
does not take up space, and we can measure it only by its eﬀects
is deﬁned as the capacity to do work, or to
put matter into motion. ±e greater the work done, the more
energy is used doing it. A baseball player who has just hit the
ball over the fence uses much more energy than a batter who
bunts the ball back to the pitcher.
Kinetic Versus Potential Energy
Energy exists in two forms, or work capacities, and each can be
transformed to the other.
ik) is energy in
action. We see evidence of kinetic energy in the constant move-
ment of the tiniest particles of matter (atoms) as well as in larger
objects (a bouncing ball). Kinetic energy does work by moving ob-
jects, which in turn can do work by moving or pushing on other
objects. For example, a push on a swinging door sets it into motion.
is stored energy, that is, inactive energy that
, or capability, to do work but is not presently
doing so. ±e batteries in an unused toy have potential energy,
as does water conﬁned behind a dam. Your leg muscles have
potential energy when you sit still on the couch. When potential
energy is released, it becomes kinetic energy and so is capable
of doing work. For example, dammed water becomes a rushing
torrent when the dam is opened, and that rushing torrent can
move a turbine at a hydroelectric plant, or charge a battery.
Actually, energy is a topic of physics, but matter and energy
are inseparable. Matter is the substance, and energy is the mover
of the substance. All living things are composed of matter and
they all require energy to grow and function. ±e release and
use of energy by living systems gives us the elusive quality we
call life. Now let’s consider the forms of energy used by the body
as it does its work.