In muscle cells during exercise, does lactic acid fermentation and aeorobic respiration occur at the same time, and does this mean the cell makes more or less ATP during this time?

The cell can't completely lack oxygen, which means that some of pyruvate will move into the mitochondrion, however, lactic acid is also produced which means that anaerobic respiration occurs. Is this reasoning correct or is there some other mechanism that I am skipping that doesn't allow these processes to simultaneously occur?

Also, the NAD+ used for glycolysis is regenerated and net 2 ATP is releaseD in glycolysis, but the NADH doesn't move into the mitochondrion, so if pyruvate moves into the mitochondrion at the end of glycolysis, then there will be less NADH in the mitochondrion, which makes sense because there is too little oxygen and too many electrons in the ETC.

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    $\begingroup$ You keep posting questions in which you provide no support for your statements, no context to your qustions and do not respond to requests for clarification. Please sort out the questions you have already asked before posting any more. $\endgroup$
    – David
    Commented Jan 20, 2021 at 20:06
  • $\begingroup$ @David In this case I believe the question is self evident and clear to understand, what part of it do you need more information about? $\endgroup$
    – ten1o
    Commented Jan 20, 2021 at 20:16
  • $\begingroup$ By posting one is asking others to spend time and effort providing an answer which will be of value to the "library of detailed answers" that this site aims to build. The poster therefore has a duty to write a good question, one that shows research (including previous questions here), is specific and explains the context in which the question is asked. If clarification is requested it should be provided. One should only "answer well-asked" questions. Until you respond to the queries to your ΔG question, I see no reason to consider others. $\endgroup$
    – David
    Commented Jan 21, 2021 at 16:20

1 Answer 1


What happens at rest is that the muscle relies on blood glucose for energy supply. This involves glycolysis, Krebs' cycle and the ETC. In short, glucose is split into two trioses. The two trioses are then completely oxidatively decarboxylated. This produces reduced coenzymes, carbon dioxide and ATP. The reduced coenzymes are reoxidised by the ETC which takes the hydrogens, splits them into protons, that leave into the matrix, and electrons that jump between electron carriers, the last of which is oxygen which unites with protons too to form water. The jumping of electrons generates energy. The energy is used to pump protons from the matrix to the intermembrane space to a certain level. Once this level is reached, electron transport and proton pumping are stopped by ATP synthase and the already pumped protons diffuse through ATP synthase to the matrix. The energy that was used to pump the protons is then used to make ATP.

What happens during excercise is that energy consumption is faster so we need to generate energy faster. We use three mechanisms to do so.

  • Phosphagen system
  • Aerobic system
  • Glycogen-lactic-acid system

The phosphagen system is the one we use first. It relies on the phosphocreatine store. Phosphocreatine can donate its phosphate radical to ADP forming ATP and creatine. This system is short-lived.

When the excercise becomes prolonged, we rely on the aerobic system to maintain higher energy supply than at rest. It's essentially the same as what we do at rest with the exception that we start breaking down glycogen stores to free more glucose. The aerobic system can operate indefinitely; as long as there are enough oxygen and glucose.

If we need more energy than what the aerobic system can provide (more than oxygen supply allows), we start employing the glycogen-lactic-acid system. This system involves glycogen breakdown and glucose splitting into the trioses with no net oxidation and no decarboxylation. This is because the trioses that glucose is split to are oxidised producing reduced coenzymes, but the coenzymes can't be reoxidized by the ETC because there's not enough oxygen supply, so the trioses take back the hydrogens so the coenzymes can be used for another cycle. ATP is only produced at the substrate level. The resulting non-oxidised trioses are lactic acid molecules. Lactic acid, being an acid, causes fatigue, which is gradual reduction in the ability of the muscle to do work.

So to answer your question, yes they happen at the same time.

You can also check this answer for a more in-depth explanation.


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