Matrix/ intermembrane space pH question

[virtualmaster999]

Hi all!

I'm confused on this question I came across dealing with cell respiration:

During oxidative phosphorylation, the pH of the mitochondrial matrix:

• A. increases as ATP is generated
• B. decreases as oxygen enters the mitochondrion
• C. increases as the electron transport chain proceeds
• D. is always lower than the pH of the intermembrane space
• E. decreases when water is formed.

I knew that as the ETC proceeds, H+ is pumped out of matrix into IMS, so matrix pH INC while IMS pH DEC.

I picked answer E. If water is being formed, doesnt that mean that H+ is flowing back into the matrix, so pH decreases?

I can see how A is out, as it would decrease, and same with D, as it is NOT always higher. I also see how C is right, but not how B/E are not possible.

Could someone help me out with this?

Thanks in advance!!

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[virtualmaster999]

Is the H+ that reduces O2 to water coming from the matrix? So then the pH wouldnt decrease when this occurs (no H+ coming in for this portion of oxidative phosph.?)

 

[MRZ8]

I'm wondering the same thing. From the TCA cycle (in the mitochondrial matrix), the NADH and FADH2 get oxidized and lose electrons to the electron carriers (i.e cytochrome C) and become NAD+ and FAD2+, so where did those protons go? As electrons jump from carrier to carrier, protons are shuttled from the matrix to the inter membrane space. Here, I think this proton movement explains the pH increase of the matrix.

HOWEVER, there's a proton electrochemical gradient now: [H+]IMS > [H+]matrix. So H+ passes through ATP synthase, making ATP. When electrons passed through (I think cytochrome IV?), they are joined by O2 (electron acceptor) and protons from the IMS/matrix to make H20 (which is neutral and doesn't dissociate into H+ readily in the matrix).

Back to your question, the matrix pH increases because more H+ (from NADH and FADH2) are lost to the IMS than gained by ATP synthase. I hope my reasoning makes sense. Correct me if I'm wrong, anybody.

 

[virtualmaster999]

I'm wondering the same thing. From the TCA cycle (in the mitochondrial matrix), the NADH and FADH2 get oxidized and lose electrons to the electron carriers (i.e cytochrome C) and become NAD+ and FAD2+, so where did those protons go? As electrons jump from carrier to carrier, protons are shuttled from the matrix to the inter membrane space. Here, I think this proton movement explains the pH increase of the matrix.

HOWEVER, there's a proton electrochemical gradient now: [H+]IMS > [H+]matrix. So H+ passes through ATP synthase, making ATP. When electrons passed through (I think cytochrome IV?), they are joined by O2 (electron acceptor) and protons from the IMS/matrix to make H20 (which is neutral and doesn't dissociate into H+ readily in the matrix).

Back to your question, the matrix pH increases because more H+ (from NADH and FADH2) are lost to the IMS than gained by ATP synthase. I hope my reasoning makes sense. Correct me if I'm wrong, anybody.

Thanks for that! I got most of what you said, I'm just stuck on the part dealing with water. So you're saying the H+ to reduce O2 comes from the matrix, not the IMS going TO the matrix? is that why the pH wouldnt decrease?

 

[MRZ8]

Thanks for that! I got most of what you said, I'm just stuck on the part dealing with water. So you're saying the H+ to reduce O2 comes from the matrix, not the IMS going TO the matrix? is that why the pH wouldnt decrease?

I saw a video on this today. I think cytochrome IV takes up the O2 as well as H+ from both the inter membrane space and the matrix. But this uptake of 2H+ shouldn't influence pH as much as the massive amounts of H+ given up by NADH and FADH2 from the matrix.