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-Metacelsus- t1_j11tl4y wrote

This paper is a good review of pyruvate transport into the mitochondria. Basically, pyruvate can flow across the mitochondrial outer membrane using non-selective anion channels such as VDAC1. There are two proteins, MPC1 and MPC2 (named for being mitochondrial pyruvate carriers), that form a complex that transports pyruvate through the inner membrane into the mitochondrial matrix, where the oxidation happens. The MPC complex carries a proton along with pyruvate (which is known as proton symport). Since pyruvate is negatively charged this means the overall transport process is charge-neutral across the inner membrane.

In general, compounds diffuse from regions of high concentration to regions of low concentration. Since pyruvate is consumed in the mitochondrial matrix, the concentration will be lower there, so pyruvate will diffuse inside if the proper transport proteins are present. In absence of oxygen, pyruvate won't be oxidized in the mitochondrial matrix, so this concentration gradient would be much lower (or absent entirely).

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piranhabait89 t1_j145td1 wrote

Yeah. But why?

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hercola t1_j15d1o2 wrote

Substrates flow down their concentration gradients the same way that if you pour concentrated syrup into water, it will naturally spread all throughout. The reason why it does this is that it is thermodynamically favorable to equilibrate like this.

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user280102 t1_j15vf55 wrote

This makes sense but my question is that the molecules outside the mitochondria don't "know" that the concentration within the matrix is lower. Is it that the receptor is sensitive to concentration of pyruvate within the matrix, so that its activity depends on the intra-mitochondrial concentration?

Thinking out loud here but could it be that the affinity of the receptor depends on the extra-mitochondrial matrix regardless of within? Maybe affinity is the wrong word too, more like the increased probability of pyruvate binding to the receptor the higher the concentration.

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hercola t1_j15w9bn wrote

Nothing has to know about the concentration gradient. As long as there is a way to flow, substrates will spontaneously flow.

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The_RealKeyserSoze t1_j16b7sf wrote

>“This makes sense but my question is that the molecules outside the mitochondria don't "know" that the concentration within the matrix is lower.”

They don’t need to “know”. They are randomly bouncing around, if there are more bouncing around on one side of a wall odds are more will pass through the gate to the side with less bouncing around than the other way around. And so they flow with the concentration gradient.

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