Quick question about mitochondria and intermembrane space

[PhaseShift11]

Hey guys,

So I have two statements and a conclusion but one of them does not seem to work out.

1) The intermembrane space is acidic due to the protons being pumped there from the matrix. True or false?
2) The intermembrane space is connected to the cytoplasm through porins in the outer membrane. True or false?

If both of those are true, how can the intermembrane space be acidic while the cytoplasm is not? I got a question wrong on a practice passage regarding this because I assumed the pH of both would be the same.

Thanks!

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[muhali3]

Well, isn't the gradient for the hydrogen ions to fall back through ATP synthase to the matrix...

So I don't think a significant amount would diffuse out to the cytoplasm.

This is just an assumption though, I'm not the best at cellular respiration.

 

[tn4596]

Ye, I think the proton quickly fall back into the inner membrane to generate ATP. So in a sense It could be acidic for a brief amount of time when the proton gradient is not balanced. Think about it, if all the proton escape into the cytosol in order for it to be acidic, ATPsynthase would not be able to make any ATP.
The intermembrane space should be similar to the cytosol due to the porin in the outer membrane. So that is true...
Also, the cytosol is more volumous compared to the IMP, so even if a few proton manage to escape from the mitochrondria...which is prolly impossible, it would not affect the pH at all.

 

[Curse]

http://www.cytochemistry.net/cell-biology/mitochondria_import.htm

Long story short, lets consider hydrogen ions to be "special" that the outer membrane does not let it enter/leave easily due to its charge?

"The protons can get out but they only have one passage to escape. This is a passage created by a protein enzyme called ATP synthetase"
http://homepages.ius.edu/gkirchne/glycolysis.htm

 

[Majik]

Hey guys,

So I have two statements and a conclusion but one of them does not seem to work out.

1) The intermembrane space is acidic due to the protons being pumped there from the matrix. True or false?
2) The intermembrane space is connected to the cytoplasm through porins in the outer membrane. True or false?

If both of those are true, how can the intermembrane space be acidic while the cytoplasm is not? I got a question wrong on a practice passage regarding this because I assumed the pH of both would be the same.

Thanks!

What a great question. First of all, both of those statements are true. Protons are pumped by the Electron Transport Chain by high energy carrier molecules (NADH and FADH2) into the Intermembrane Space of the Mitochondria. These protons can exit the porins located on the outer membrane of the mitochondria into the cytoplasm. More useful however is utilizing this proton gradient in converting ADP + Pi (phosphate) into the higher energy ATP molecule.

With regard to your second question, the cytosol is certainly not acidic. While its true protons can escape into the cytosol, there is no notable drop in pH because there is tons more water by comparison that outnumber the protons enter greatly. This would alter the concentration immensely (the protons become diluted in water).

Think of it this way, would adding 1x10^-18M concentration of HCl to a cup of water have any notable effect on the pH of that water? Well, the pH of neutral water is 7. Adding 1x10^-18 Hydrogen Ions (remember HCl dissociates completely; strong acid), would cause no notable change in the pH of water. On the other hand, adding 10M HCl WOULD DRASTICALLY alter the pH. The pH would change from 7 to -1! Hopefully this clears things up 🙂

 

[PhaseShift11]

Thanks for the help so far.

So as I understand it, H+ will generally remain in the intermembrane space due to its charge/its usefulness in ATP synthase. Even if some protons escaped, they would not cause a change in pH similar to how a drop of acid will not change the pH of the ocean.

That makes sense - can I ask then, what "content" is similar between the cytoplasm and the intermembrane space then? What do those porins in the outer membrane connecting them actually do? This may be nit-picking I suppose, so for the most part my question has been answered.

Thanks.

 

[MD Odyssey]

Thanks for the help so far.

So as I understand it, H+ will generally remain in the intermembrane space due to its charge/its usefulness in ATP synthase. Even if some protons escaped, they would not cause a change in pH similar to how a drop of acid will not change the pH of the ocean.

That makes sense - can I ask then, what "content" is similar between the cytoplasm and the intermembrane space then? What do those porins in the outer membrane connecting them actually do? This may be nit-picking I suppose, so for the most part my question has been answered.

Thanks.

I would imagine that there are all sorts of proteins and transporters embedded in the membrane of the mitochondria. Pyruvic acid, acetyl-CoA, and a bunch of other things have to get through it in order to get into the matrix of the mitochondria. The way that I think about it (which may not be altogether correct) is that if something is used in a biochemical process inside the mitochondria, there has to be some sort of transporter to get it in there. I don't see how something like pyruvic acid gets into the matrix of the mitochondria without a special transporter allowing it through the lipid bilayer.

 

[MT Headed]

2) The intermembrane space is connected to the cytoplasm through porins in the outer membrane. True or false?

If both of those are true, how can the intermembrane space be acidic while the cytoplasm is not?

I think you are assuming that a porin will allow protons to leak out of the mitochondrial intermembrane space.

I don't think that is a valid assumption.

 

[PhaseShift11]

Ah, I see. I assumed it would because when I searched it up I saw (from wiki):

"It contains numerous integral proteins called porins, which contain a relatively large internal channel (about 2-3 nm) that is permeable to all molecules of 5000 daltons or less."

I suppose you're right, that wasn't a fair assumption to make, but I saw that it didn't seem to say charged molecules could not cross. Also H+ is a super super small molecule so I figured it's not like I'm talking about massive glucose molecules moving in and out with ease.

However I understand now, and it makes good sense.

Thanks all!