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> THE ENERGY
OF LIFE
THE ENERGY OF LIFE
Energy is the
capacity or ability to do work. In living things, the biological work
that requires energy includes processes such as growing, moving,
reproducing, and repairing damaged tissues.
Energy exists in
several forms: heat, radiant energy {electromagnetic radiation from the
sun), chemical energy (stored in chemical bonds of molecules),
mechanical energy, and electrical energy. Energy can exist as stored
energy—called potential energy—or as kinetic energy, the energy of
motion (Figure 3-5). You can think of potential energy as an arrow on a
drawn bow. When the string is released and the arrow shoots through the
air, the potential energy is converted to kinetic energy. Thus, energy
can change from one form to another. The study of energy and its
transformations is called thermodynamics. There are two laws about
energy that apply to all things in the universe: the first and second
laws of thermodynamics.
The First Law
of Thermodynamics
According to the
first law of thermodynamics, energy cannot be created or destroyed,
although it can be transformed from one form to another. As far as we
know, the energy present in the universe
at its
formation, approximately L5 billion years ago, equals the amount of
energy present in the universe today. This is all the energy that can
ever be present in the universe. Similarly, the energy of any object
and its surroundings is constant. An object may absorb energy from its
surroundings, or it may total energy
content of that object and its surroundings is always the same.
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As
stipulated by the first law of thermodynamics, then, living
organisms cannot create the energy they require to live.
Instead, they capture energy from the environment and use it to do
biological work. This process involves transforming energy from one
form to another. Through the process of photosynthesis, for example,
plants absorb the radiant energy of the sun and convert it into the
chemical energy contained in the bonds of food molecules.
Similarly, the chemical energy of food can be transformed into the
mechanical energy of walking, running, slithering, flying, or
swimming. |
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The Second
Law of Thermodynamics
As each energy
transformation occurs, some of the energy is changed to heat energy that
is then given off into the surroundings. This energy can never again be
used by the living organism for biological work, but because of the
first law of thermodynamics, it is not "gone;" it still exists in the
surroundings.
The second law
of thermodynamics can be stated most simply as follows: when energy is
converted from one from to another some useful energy (that is, energy
available to do work) is degraded into a lower-quality, less useful
form— usually heat that disperses into the surroundings. As a result,
the amount of useful energy available to do work in the universe
decreases over time.
Low-quality
energy is more dilute, or disorganized. Entropy is a measure of this
disorder, or randomness; organized, useful energy has low entropy,
whereas disorganized, low-quality energy has high entropy. Entropy is
continuously increasing in the universe, and at some time
Low-quality heat
that is uniformly distributed throughout the universe. When that
happens, the universe will cease to operate because no work will be
possible; everything will be at the same temperature, so there will be
no way to convert the thermal energy of the universe into useful
mechanical energy. Another way to explain the second law of
thermodynamics, then, is that entropy, or disorder, in a system tends to
increase over time.
Living things
have a high degree of organization and at first glance appear to refute
the second law of thermodynamics; that is, as living things grow and
develop, they maintain a high level of order and do not appear to become
more disorganized. However, living things are able to maintain their
degree of order over time only with the constant input of energy. That
is why plants must photosynthesize and animals must eat.
Photosynthesis and Cell Respiration
Photosynthesis
is the biological process in which light energy from the sun is captured
and transformed into the chemical energy of food. Photosynthetic
pigments such as chlorophyll (which is green and gives plants their
green color) absorb radiant energy. This energy is used to manufacture a
sugar called glucose (C6HI2O6) from carbon dioxide
(COZ) and water (H2O), with the liberation of
oxygen (O2) as a waste product:
Photosynthesis,
which is essential for life on Earth, is performed by plants, algae, and
a few bacteria. Photosynthesis provides these organisms with a ready
supply of energy (in glucose molecules) that they can use as the need
arises. The energy can also be transferred from one organism to
another—for instance, from plants to the organisms that eat plants
(Figure 3-7). Photosynthesis also produces oxygen, which is required by
living things when they break down food.
The chemical
energy that plants store in food molecules is released within cells of
plants, animals, or other living organisms through cell respiration.
In this process, food molecules such as glucose are broken down in
the presence of oxygen (and water) into carbon dioxide and water.
Cell respiration
makes the chemical energy stored in rood molecules available to
the cell for biological work. All living organisms respire to obtain
energy.
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