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WEB TUTORIAL 6.1
Energy and Biology
Text Sections
Section 6.1 Energy Is Central to Life, p. 95
Introduction
Energy is defined as the capacity to do work. The first and second laws of thermodynamics govern energy. Chemical reactions can be either exergonic (energy
releasing) or endergonic (energy consuming). In biological systems, adenosine
triphosphate (ATP) is the primary energy storage molecule.
Learning Objectives
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Understand how energy is transformed from coal in a steam engine.
Discuss the energy requirements for building or breaking down complex
molecules.
Know what the most important cellular energy transfer molecule is.
Narration
Thermodynamics
Energy can be thought of as the capacity to do work or, equally, the capacity to
bring about change. The study of energy, or thermodynamics, is an important discipline in biology. In thermodynamics, two laws governing energy are utilized.
The first law of thermodynamics states that energy is never lost or gained, but is
only transformed. For example, in a steam engine, energy locked up in the chemical bonds of coal is transformed into heat energy and mechanical energy. There is
no loss of energy in this process, but energy is transformed from a more-ordered,
concentrated form (the chemical bonds of coal) to a less-ordered, more dispersed
form (heat).
The second law of thermodynamics states that entropy, which is a measure of the
amount of disorder in a system, always increases with a transfer of energy. In other
words, when energy is produced, matter changes from a more-ordered state to one
of less order. When a lump of coal is burning, ashes are produced and hot air disperses into the atmosphere. In the steam engine at the left, energy locked up in
chemical bonds of the coal is transformed into heat energy and mechanical energy.
The system obeys the laws of thermodynamics, since there is no loss of energy
(only transformation) and entropy is increasing with the transfer of the energy to
air.
Energy Storage and Release
Energy input is necessary in order to store away energy. For example, energy is
required to build up a more complex molecule (in this case glycogen) from simpler
molecules (in this case individual glucose units). Why? Because a large glycogen
molecule is a more complex, ordered chemical object than the individual glucose
molecules it is made of.
The flip side of this is that the breakdown of a larger molecule such as glycogen
into its component parts does not require a net input of energy; in fact, some energy will be released in such a reaction.
Adenosine triphosphate (ATP)
ATP, short for adenosine triphosphate, is life's most important energy transfer molecule. It stores energy in the form of chemical bonds between its phosphate groups.
When the bond between the second and outermost phosphate group is broken, the
outermost phosphate separates from ATP and energy is released. This separation
transforms ATP into adenosine diphosphate or ADP, and a molecule of inorganic
phosphate (Pi).
ADP then goes on to pick up another phosphate group, an energy-consuming reaction, becoming ATP again. The ATP is then capable of providing energy for yet
another reaction. The shuttling from ATP to ADP and back takes place constantly
in cells.
You should now be able to…
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Define “energy” in biological terms.
State the first two laws of thermodynamics.
Explain where energy is stored in a molecule of ATP.
Describe the reaction by which ATP releases energy.