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What product of cellular respiration creates an acid when in solution with water?

Topic question really, haven't been able to find a direct answer.

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    $\begingroup$ A lot of them do, like malate, and citrate $\endgroup$ – TanMath Oct 23 '15 at 1:43
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    $\begingroup$ So does carbon dioxide :) $\endgroup$ – akaDrHouse Oct 23 '15 at 1:55
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    $\begingroup$ @TanMath, the presence of organic acids like malate and citrate in central metabolism is not in itself acidifying. These species typically a produced in deprotonated forms and do not release protons. Acidification only occurs if there is a net release of protons during metabolism. $\endgroup$ – Roland Oct 24 '15 at 9:03
  • $\begingroup$ They are the cation of the acid. An acid is not just protons... $\endgroup$ – Technetium Oct 24 '15 at 9:39
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    $\begingroup$ Well, they are anions, but they are conjugate bases to the acid, yes. The point is that there is no acidification unless protons are released, and the existence of intermediates like malate and citrate does not imply proton release. Acidification is by definition release of protons. $\endgroup$ – Roland Oct 24 '15 at 10:35
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Cellular respiration itself, defined as the activity of the respiratory chain complexes I -- V (including ATP synthase), is not acidifying. On the contrary, free protons are captured during ATP synthesis, so cellular respiration acts to raise pH. But then ATP synthesis is always matched with ATP breakdown in reactions that utilize energy (muscle contraction, for example), and ATP breakdown liberates the captured protons again. So the complete cycle of ATP turnover is pH neutral. (For more information, see for examplle http://www.ncbi.nlm.nih.gov/pubmed/6298937)

The activity of the TCA cycle, on the other hand, produces CO$_2$ when oxidizing carbohydrates, and CO$_2$ dissolved in water is slightly acidic (pK$_a$ = 6.3) and so tends to lower pH. This is the main acidifying effect of oxidative metabolism. In animals, the CO$_2$ generated is of course released via the lungs, so the amount of CO$_2$ dissolved in body water is (more or less) constant, and therefore pH is stable.

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  • $\begingroup$ Nice answer! :-) $\endgroup$ – inf3rno Oct 27 '15 at 1:57
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CO2 is likely the answer you're looking for. This is especially true if this is in response to a 100 or 200 level biology course question. Based on the framing of the question, I would assume this is the case.

Though, as others have pointed out, there are other products that would qualify.

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In tissue and cell culture I recall that after the vascular smooth muscle cell line i was working with had used up the compounds in the buffer the supernatant started to turn acidic due to presence of lactic acic.

The cells aerobicaly produce lactate which most likely converts to lactic acid with the H2O and other byproducts ( such as a H+ )present in the supernatant.

CO2 respiration can also form carbonic acid (H2CO3) with water. These are just a couple of examples.

A proton (which forms H3O+ with water) or one of many anions that could be produced by a cell or present in an aqueous buffer, could create an acid when in solution with water.

Take into account there are many different cellular types and types of respiration.

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  • $\begingroup$ Glycolytic metabolism is indeed overall acidifying, but the question was regarding oxidative metabolism. Also, the source of protons during glycolytic metabolism is not lactate production, but ATP hydrolysis. $\endgroup$ – Roland Oct 24 '15 at 9:06
  • $\begingroup$ Lactate is produced and the proton source is ATP hydrolysis. Two different things but the by products resulting from the processes form an acidic solution . The H+ does come from the ATP hydrolysis but the cation of the acid does not. $\endgroup$ – Technetium Oct 24 '15 at 9:34
  • $\begingroup$ I also mentioned the CO2 + H2O → H2CO3 in my answer. $\endgroup$ – Technetium Oct 24 '15 at 9:38

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