I've always known that the hydrolysis of ATP generates ADP, P, and energy, so I'd assume that if energy is necessary in a given process, ATP hydrolysis should occur (or another exothermic process).
What I don't understand is how this works in muscle contraction. I assume that part of my misunderstanding might be due to the lack of knowledge of how the contraction works at a molecular level, as well as how this phenomenon relates to the contraction at the macroscopic level (my arm contracting), hence I'm writing this question.
From a number of sources, including the image below (which is taken from an book in Italian, so I don't include the text; with reference to the figure, stadio means stage, by the way) and the one in this related question, I understand that the mechanical energy is released (i.e. the muscle exerts power on the external world) both when P is released when ADP is released, but definitely not when ATP is hydolised.
Now I do understand that the cycle in the image has to repeat a tremendous number of times while I simply raise my hand to say hello, for instance, but my knowledge of physics tells me that, whatever the amount of energy required to raise my hand is, the two power srokes summed up on all the elmentary cycles have to provide that energy (plus some more to overcome losses, like joint friction or whatever).
- But where is the energy that the two power strokes are exerting coming from? It looks like the energy is coming from the release of ADP and P, which makes me think that this is itself an esothermic process, otherwise how could it result in power strokes?
- And also, why is ATP needed, then?
- From the image I see that the ATP hydrolysis happens between stage 5 and stage 1; does this mean that stage 1 is a high energy state?
In the meanwhile that I write, I start to think that probably the core of the matter is that I mistakenly thought that ATP is what provides the energy for the mechanical action, whereas probably it provides the energy (between stage 5 and 1) that is stored in the myosin (stage 1), such that when the mechanical energy is required (between stage 1 and 2), the myson is able to release it (from stage 2 to stage 4). Does this make any sense?