How can the breaking of ATP release energy?
We have already seen that chemical energy is released when chemical bonds form.
In biology, we learn that ATP is the energy currency of living things. It is used to drive the processes on the metabolism. Energy is released when ATP breaks down to produce ADP + ortophosphate.
So, are we being told that in biology the breaking of a bond releases energy? Is this the opposite of what we learn in chemistry? That is a very good question to ask your teachers.
ATP molecules: the 3 phosphates are seen on the right
The answer:
It is not the actual breakdown of ATP that releases energy. Actually it costs some energy to break that bond, but the processes that follow are the ones which release the energy.
The breakage of ATP is a hydrolysis reaction, e.g., it is promoted by water.When ATP is formed , the reaction is a condensation, e.g., a water molecule is eliminated.
Following hydrolysis, the phosphate group that is liberated reacts with water and forms ortophosphate (receives the -OH group). The remaining H from that water molecule takes the place of the liberated phosphate, so that ADP is formed. That means that 2 new bonds have been formed. But that is not the whole history. There are also other factors.
1) Getting rid of one phosphate group is energetically favourable because there is a large electrostatic repulsion between phosphate groups in ATP (see picture above) because they are very close together, and all have various negative charges on the oxygens.
2) The entropy of the system is increased. Entropy is a measure of the disorganisation of systems, and spontaneous processes tend to increase entropy. Chemical reactions whose products are more disorganised than the reagents tend to occur easily (but this is not the only condition, there are also enthalpy considerations). The hydrolysis of ATP increases disorganisation because you start with one piece and end up with two.
3) Chemical reactions are also favourable when they produce highly stable products. the ortophosphate group produced from the hydrolysis of ATP is highly stable because it presents various resonance strucures. That means that the double bond can move to different positions in the molecule, causing rearrangements of charge. Stability is gained in this process. This is explained by quantum mechanics.
::::look at resonance in other molecules (external link)
So this are some of the answers to the question. It is a tricky question. I hope I could convince you!
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