Assume that the conversion of oxidized ubiquinone to reduced...

Assume that the conversion of oxidized ubiquinone to reduced ubiquinone by NADH dehydrogenase occurs on the matrix side of the inner mitochondrial membrane and that its oxidation by cytochrome c reductase occurs on the intermembrane space side of the membrane (see Figures $14-14$ and $14-23$ ). What are the consequences of this arrangement for the generation of the $\mathrm{H}^{+}$ gradient across the membrane?

Key Concepts

Electron Transport Chain (ETC)

The electron transport chain is a series of protein complexes and mobile carriers located in the inner mitochondrial membrane that facilitate the transfer of electrons derived from fuel oxidation. These redox reactions generate energy which is used to create a proton gradient across the membrane by coupling electron transfers with proton pumping.

Proton Gradient (Proton Motive Force)

The proton gradient is an electrochemical gradient established by the accumulation of protons (H+) on one side of a membrane, creating a difference in pH and electric charge. This gradient is a form of stored energy that drives the synthesis of ATP as protons flow back through ATP synthase.

Spatial Compartmentalization in Mitochondria

Spatial compartmentalization refers to the arrangement of different components of the respiratory chain in specific locations within the inner mitochondrial membrane and its adjacent compartments. This arrangement, such as the positioning of reactions on the matrix side versus the intermembrane space, is critical for ensuring that protons are effectively translocated to generate a robust gradient.

Energy Coupling Mechanisms

Energy coupling in mitochondria involves linking the exergonic transfer of electrons through redox reactions to the endergonic process of ATP synthesis. The spatial separation of redox events, with reduction occurring on one side of the membrane and oxidation on the other, optimizes the use of energy from electron transfers to pump protons and create the proton motive force necessary for ATP production.

Transcript

00:01 So in this question, we're asked about ubiquinone, which is part of the electron transport chain, and how it may or may not be contributing to the hydrogen ion ingredient that the mitochondria is producing during the electron transport chain.

00:17 So i first want to give you a little bit of orientation of what i have diagrams here.

00:24 So we can say that this large open space here is the intermembrane space.

00:36 Of the mitochondria.

00:37 So i haven't drawn the outer membrane, but we can just pretend that it's out there a little further away.

00:44 This is the inner membrane.

00:53 And these sort of circles that i've drawn are the complexes of the electron transport chain.

01:01 And this is the matrix.

01:06 This is the interior of the mitochondria.

01:08 And here we have nedh dehydrogenase, which is typically called complex 1.

01:16 This here is ubiquinone, and i'm going to abbreviate it as ub.

01:22 This is cytochrome reductase, which is sometimes called complex 3, but i'm going to call it complex 2.

01:29 Over here is cytochrome c, which i'm going to abbreviate as cetc.

01:35 And then over here we have cytokrome oxidase, which is sometimes called complex 4, but i'm going to call complex 3.

01:46 And let's go back to our question now that we have a little bit of an orientation of what's going on here.

01:53 And the question actually is focusing on ubiquinone.

01:59 So i'm going to circle that in blue here.

02:02 So the question says if ubiquinone is reduced, that means that it receives an electron.

02:11 So if it is reduced on the matrix side of the membrane and is oxidized on the intermembrane space side of the membrane, what does this mean for the hydrogen ion gradient that's being produced by the electron transport chain? so let's think about this for a little bit.

02:36 I'm going to represent electrons as green.

02:40 So first, let's say the electron is sitting here in nadd hydrogenase, and nadd hydrogenase reduces ubiquinone.

02:50 That means it's going to give an electron to ubiquiton...

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