Bonds: Lowest Energy, or Energy Storage?
Date: Spring 2010
Generally speaking, when bonds are created, energy is
released and likewise, energy is generally required to induce a bond
to break. This makes perfect sense, until I begin to work with
biological systems and read time and time again that energy is
"released" when the third phosphate is removed from ATP. "When the
bond is broken, energy is released" is the most common statement in
every biology text book out there. The number of ATP produced in
each step of Cellular Respiration is a requirement of the Advanced
placement courses, as the ATP created is generally accepted as the
energy currency of the cell. HOWEVER--since breaking bonds REQUIRES
energy, how does breaking the bond of the third phosphate group in
ATP "release energy for life processes?"
One of my all time favorite issues!!! I also have been plagued by
this chemistry-biology bonding-energy issue, being involved with
educators in both Biology and Chemistry fields and having to make
sense of the different text books. Your analysis of bond making and
breaking is correct, but the problem is that in normal chemical
systems bonds are rarely simply broken to form unbounded atoms, or
the reverse, in isolation. Nearly always some bonds (or other types
of forces) are broken whilst some are made and this complicates things.
In the case of ATP losing a phosphate this problem is exacerbated by
the fact that, apparently, the only process that happens is the
breaking of the bond in the ATP molecule to form ADP and a
phosphate. Energy is, indeed, released and the logical conclusion
seems to be that "energy is stored in this bond and this is released
when it is broken". What is being missed out here is all the other
interactions ("bonds") that are present before and after the
covalent bond in ATP is broken. To fully understand why energy is
released these all must be taken into account. There are various
interactions between the ATP molecule and the water that surrounds
it and these will differ slightly from those of the resulting ADP
formed after the reaction, but the big change during the process is
the formation of the phosphate ion and the surrounding hydration
sphere. The forming of this strong interaction between phosphate and
water is where energy is released in this process but this is often
overlooked by biology text books. It TAKES energy to break the ATP
covalent bond but MORE energy is released when the phosphate ion is
hydrated (I am oversimplifying here and ignoring other interactions).
It is ironic that students often encounter thermochemistry and
thermodynamics for the first time (albeit without using those terms)
with the ATP example in Biology. It is one of the most confusing
examples of bonding and is perfectly set up to foster misconceptions
about bond making and breaking.
The idea of a bond as a "store of energy" has major issues. The
covalent bond broken in the ATP process could be thought of like
this, but ONLY relative to other possible bonding situations. It
will always take energy to break that bond, but more energy will
generally be released when other bonds are formed. A bond is, by
definition, a minimum energy situation (like a well that the atoms
"fall" into). If one bond's minimum energy is higher than another
bond's then moving from the first situation to the second will
release energy overall, like coming out of a shallow hole and
falling into a deeper one. Would you describe the first situation as
a "store" (i.e. high point) or a "well" (low point)? It depends what
you are comparing it to. The "hole" for ATP is a shallow one (it
contains a weak bond), so it is used as a "high energy molecule"
within natural systems.
This is a great question because it really gets at a core of
chemistry: thermodynamics. It also highlights how simplifying concepts
to make them easier to understand can lead to what appear to be
contradictions. Hopefully I can clear up a little bit of confusion.
One way to make sense of this is to think of chemicals as a collection
of individual molecules and/or atoms -- think of them at the molecular
level. We can call some collection of atoms or molecules our "system".
Every chemical system has a certain amount of energy contained in it.
The amount of energy depends on the configuration of atoms and
molecules (chemical bonds) and the motion of those molecules and atoms
(thermal and/or kinetic energy). The configuration of the atoms can be
constantly changing -- such as vibrations and rotations of atoms or
molecules and chemical reactions.
Each molecular configuration (each chemical compound or atom) may have
a different energy content, which depends on how all the atoms and
molecules are arranged. Some molecules are very high in energy -- such
as the ATP you described, or long carbon chains like gasoline. Other
molecules are very low in energy, such as a nitrogen gas molecule or
some mineral crystals. It is not accurate to say that all bonds "store"
energy -- it depends on the molecule in question. Some bonds are
actually lower in energy than un-bonded systems (for example, many
crystals are lower-energy than a mixture of their respective ions).
It is more accurate to say that each molecule has a characteristic
energy that depends on its unique configuration.
When molecules change configurations, this is called a chemical
reaction. One type of molecule converts to another, and the beginning
state often has different energy contained in it than the ending
state. For each chemical reaction, you can compare the energy levels
of reactants (starting materials) and products (ending materials) and
determine if there is a net energy release or net energy absorption.
You cannot figure out the difference by counting the bonds -- it
depends on the specific nature of each specific molecule. Not all
bonds are created equal, so to speak.
There is another concept known as "activation energy" which basically
means you have to add a little energy to get a chemical reaction
going. When you chemically react some lower-energy components to form
some higher-energy components, you have to add energy. Since the end
state has more energy than the starting state, this is very intuitive.
However, when you react higher-energy components to get lower-energy
components (such as combusting fuel), you need to add a little energy
first to get the reaction started. This is known as "activation
In some special cases, there is enough energy around (such as random
thermal fluctuations) to start the reaction, but there is still a
little "uphill climb" to start. The "uphill" part refers to the
creation of some higher-energy "transition state", which is a
temporary or unstable, higher-energy configuration of atoms or
molecules. Often a transition state is formed by breaking a bond,
which requires some energy. When these transition state molecules
re-configure into the final state (for example, by forming a bond),
energy is released (compared to the higher-energy transition state,
but not necessarily compared to the reactants).
In the case of ATP, that third Phosphate group is highly energetic, so
the chemical reaction of ATP --> ADP releases energy. You do need some
activation energy to induce the bond to break, and there are enzymes
in organisms that serve to reduce the activation energy required to
make that transition. In fact, reducing activation energy is a very
important role of enzymes -- that's how they "facilitate" chemical
reactions in organisms. So this reaction goes from ATP, to a
higher-energy transition to state, to ATP + PO4 + energy.
The same reaction can happen in reverse, where ADP and a phosphate ion
are converted back to ATP. The reverse starts with ADP, PO4, and (even
more) energy (than it took previously). The reaction results in
formation of ATP, but also gives off some energy residual. The energy
residual occurs because the transition state to get to ATP is a little
higher in energy than the final ATP, and some energy is released when
the final bond is formed.
I hope this is helpful -- if you are interested in more information, I
suggest you read about "activation energy", "internal energy", and if
you are really motivated, this leads directly into thermodynamics,
including "free energy" and "entropy". And of course feel free to
ask/clarify any other items with AAS.
Hope this helps,
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Update: June 2012