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I was going over slides of energy expenditure in muscle cells. It mentions that in muscle tissue, the cell's energy charge ([ATP] / [AMP]) is the principle factor controlling glycolytic activity, and also that glycolysis’ primary role here is to provide ATP for contraction.

However, the slides also mentions that "Keep in mind that [ATP] + [ADP] + [AMP] remains relatively constant in the cell over short time frames" with no further explanation. I was under the impression that ATP was actively consumed in muscle tissue during intense activity (which is why lactic acid builds up in muscles: oxidative phosphorylation can't keep up with your rate of energy expenditure).

Thus, my question is Are my slides wrong? During muscle activity, is there a general lack or constancy of ATP?

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Adenosine triphosphate (ATP) is often thought of as the energy currency of cells. It is not "used up" per se, but energy is released from the conversion of ATP to ADP (adenosine diphosphate), and yet more can be obtained by removing another phosphate to make AMP (adenosine monophosphate).

Here's a chemical schematic of ATP, you can see the three phosphates to the left.

enter image description here

Your slides state that the amount of (ATP + ADP + AMP) remains constant in the muscle, which makes sense because ATP is not used up, but converted to ADP. So as cells use energy, they do not actually reduce the total concentration of (ATP + ADP + AMP). Energy is required to add the phosphates back onto an ADP or AMP molecule - the process is called oxidative phosphorylation in cellular respiration (although this is not the only way).

I hope I didn't misunderstand your question.

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    $\begingroup$ Ohhhhhhhhh, I think I may have just misunderstood the professor's wording in the slides. This makes total sense of course (and must be what the prof is referring to) but didn't occur to me. I was understanding that the concentration of EACH nucleotide remained constant (which is not so), and not the SUM of all three. Thanks for the epiphany :). $\endgroup$ – LanceLafontaine Aug 13 '12 at 21:26
  • $\begingroup$ The pluses stand for addition, not "and". $\endgroup$ – mgkrebbs Aug 13 '12 at 23:42
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    $\begingroup$ In fact there is even a certain balance ATP/ADP/AMP + GTP/GDP/GMP + IMP/XMP as these are all interconvertible purine nucleotides. $\endgroup$ – R Stephan Aug 14 '12 at 6:21

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