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As I understand as per literature reading and some khanacademy(https://www.youtube.com/watch?v=PtKAeihnbv0&t=315s):

Proton gradient := the gradient created when hydrogen ions moves to the intermembrane space from the mitochondrial matrix.

Mitochondrial membrane potential := the potential energy created by the proton gradient.

Does depolarization mean the loss of mitochondrial membrane potential? The lower the mitochondrial membrane potential, then the more depolarized the mitochondria?

So the way I defined proton gradient and mitochondrial potential is conceptually derived from the so-called proton-motive force used here https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2605959/ and also described here https://www.dummies.com/education/science/biology/the-proton-motive-force/ as an electrochemical force. Thus, my definition is from my rudimentary physico-chemical intuition.

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  • $\begingroup$ What does “:=“ mean, and where did you get these definitions, which I would consider poor or incorrect? Where have you encountered the term “mitochondrial polarization? To be meaningful you should provide sources and context. If you have read one thing and providing your own understanding of other terms please make this clear. $\endgroup$ – David Jul 23 at 18:20
  • $\begingroup$ Sorry, David. This is simply how understood it based from youtube videos and literature reading. So it is my personal interpretation. I posted it so people who have the expertise and who care to correct it, will. $\endgroup$ – TheLast Cipher Jul 24 at 6:48
  • $\begingroup$ Then perhaps you could modify your question to give a specific source for the use of the term so we can see whether it was used in relation to mitochondria as a whole or just the inner membrane. Also it would be better to rephrase the other sections as "I understand that the mitochondrial membrane potential is..." $\endgroup$ – David Jul 24 at 9:42
  • $\begingroup$ Hello David, I've added some of the sources. Thanks! $\endgroup$ – TheLast Cipher Jul 24 at 16:27
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    $\begingroup$ Ok. I'll see if I can find anything of use to add. $\endgroup$ – David Jul 24 at 16:44
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The inner mitochondrial membrane is important for ATP synthesis (oxidative phosphorylation) to occur. Electron transport chains are situated on the inner mitochondrial membrane where electrons traverse the chain across proteins - causing protons to traverse into the mitochondrial matrix. Creating an electrochemical proton gradient.

This is a very interesting topic and is best understood with an explanation of the JC-1 Dye.

Explanation

Mitochondrial membrane potential changes - depending on cellular events.

The dye 'JC-1' is able to indicate mitochondrial membrane potential in different cells.


If there is a negative charge inside mitochondria then the monomers express green fluorescence.

If there is a positive charge inside mitochondria then the monomers form J-aggregates and now express red fluorescence.


The ratio between red & green florescence shows the mitochondrial membrane potential.


A low or negative mitochondrial membrane potential will show more green florescence.

A high or positive mitochondrial membrane potential will show more red florescence.


Example

One event that may influence mitochondrial membrane potential is an increase in reactive oxygen species.

Reactive oxygen species are molecules that damage cells but are also important to regulate cellular activities. Some examples are Hydrogen Peroxide or Superoxide.

In this image cells are stained with JC-1. The ratio of the dye is ~1:1 at basal. As the reactive oxygen species hydrogen peroxide is added and time continues, there is more green. Showing:

A low or negative mitochondrial membrane potential in each of the cells.

Mitochondrial membrane potential

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  • $\begingroup$ Thank you very much. This is helpful. This does agree with the assumption that ROS can induce "damage" to the membrane complexes; hence a low mitochondrial membrane potential, i.e. green fluorescent? May I also know the reference of the image? Lastly, what are your thought on the definition of depolarization? $\endgroup$ – TheLast Cipher Jul 23 at 9:23
  • $\begingroup$ @TheLast Cipher "In general, harmful effects of reactive oxygen species on the cell are most often:[18] -damage of DNA or RNA -oxidations of polyunsaturated fatty acids in lipids (lipid peroxidation) -oxidations of amino acids in proteins -oxidative deactivation of specific enzymes by oxidation of co-factors" - ROS wiki page. High levels of ROS changes the gradient. Image Ref: thermofisher.com/au/en/home/life-science/cell-analysis/… $\endgroup$ – Andrew Jul 23 at 9:38
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    $\begingroup$ @TheLast Cipher Apologies, I didn't have enough room. A depolarized mitochondrial membrane potential is an indicator of increased levels of ROS. Ref: Thermofisher.com link above. $\endgroup$ – Andrew Jul 23 at 9:40
  • $\begingroup$ @Andrew — I have no interest in you or "everything you do", indeed I am generally not aware that I have commented on people's posts previously unless they have very distinctive names. My only interest is to improve answers (and questions) to make them more useful to the community. As you are new here, you may not realize that comments are primarily for asking for clarification or making suggestions for improving answers. That was the point of my remark. Let me reword it. "I suggest you could improve your answer by distinguishing between the potentials of the two mitochondrial membranes". $\endgroup$ – David Jul 24 at 9:38

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