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Name: April
Status: educator
Grade: 9-12
Location: DE
Country: USA
Date: Spring 2010


Question:
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?"



Replies:
April,

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.

Best wishes,

Tom Collins


Hi April,

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 energy".

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,
Burr Zimmerman



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