Oxidative Phosphorylation?

Question

Can someone explain the following items in the context of oxidative phosphorylation, along with explaining the process?

1. Mitochondrial matrix
2. Intermembrane space
3. Inner mitochondrial membrane
4. NADH and NAD+
5. FADH₂ and FAD

Answer 1

Hi Abigail, I'll try to give you a broad overview of the concepts you mentioned above and connect them to oxidative phosphorylation.

First off, what is oxidative phosphorylation? The short answer, is it is the final step in cellular respiration where animals take oxygen and a sugar (i.e. glucose), catalyze a whole bunch of reactions, and produce water, carbon dioxide and most importantly energy in the form of a molecule called Adenosine Triphosphate (more commonly called "ATP").

More specifically, oxidative phosphorylation is the process of taking the electron carriers NADH, FADH₂ (produced in the citric acid cycle) and making a whole bunch of ATP. I'll get to those electron carriers in a minute, but first it's important to visualize where this takes place: the mitochondria.

Here is the big thing that you need to remember when thinking of the mitochondria: it has a membrane inside a membrane. Think of stuffing a plastic bag inside a balloon. The inside of plastic bag (again, inside the balloon) is called the mitochondrial matrix. The plastic bag itself would be called the inner mitochondrial membrane. The space between the inner plastic bag and the balloon is called the intermembrane space.

The process of oxidative phosphorylation takes place in the mitochondrial matrix (inside the plastic bag, remember), and in the intermembrane space (that space between the bag and the balloon).

Now back to those electron carriers: NADH, FADH₂. These molecules do exactly what their name would suggest: they carry electrons from the citric acid cycle, and deliver them to be used by oxidative phosphorylation.

Why does the oxidative phosphorylation need electron carriers you might be asking? Great question! The super quick answer is that the electrons that those carrier molecules carry are used to power protein pumps embedded in the inner mitochondrial membrane. To refer back to our bag/balloon analogy, these little protein pumps would be embedded in the plastic bag.

Finally, the last question to address is what are those pumps doing? Powered by those electrons from NADH, FADH₂, the pumps are pushing hydrogen ions (H+) from the matrix (inside of plastic bag) into the intermembrane space (between bag and balloon) to create a gradient. A gradient just means concentrating molecules into one area, so that they will naturally tend to move towards areas of lower concentration. Think of dropping a single drop of food coloring into a glass of water: the highly concentrated dye naturally spreads out so that after a couple minutes, all the water is equally colored.

So, in the case of our membrane proteins taking H+ from the matrix and pumping them into the intermembrane space, the intermembrane space becomes highly concentrated with H+. And just like the food coloring, those H+ ions want to spread out into areas of low concentration. Luckily for those ions, the inner membrane has a super important protein called ATP synthase that lets them flow back into the matrix. This brings us back to the whole reason for oxidative phosphorylation: producing ATP. That protein called ATP synthase has the incredible ability of creating ATP from a few reactants, and it does so by using that proton gradient as an energy source. All those H+ ions flowing from the intermembrane space back into the matrix power the action of ATP synthase to create most of the final energy product of cellular respiration.

I hope that helps answers your question and puts those key terms into context.