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Electron Transport System

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

The electron transport system is the stage in cellular respiration in which oxidative phosphorylation occurs and the bulk of the ATP is produced. The electron transport system creates an electron gradient inside the mitochondria along the inner membrane so that when protons re-enter the matrix through the ATP synthase, their potential energy is converted into chemical energy in the form of ATP.

The last step of aerobic respiration is known alternately as electron transport system or as oxidative phosphorylation. I tend to prefer the term electron transport system because it tells you what it does. Oxidative phosphorylation well it does tell you what it does, it uses much more complex language. Just to help you understand it, oxidative refers in the fact that you're using oxygen gas in this process, phosphorylation phosphor means you're adding phosphate to something. What are you adding phosphate to? You're adding it to adenosine diphosphate to make the energy molecule called adenosine triphosphate.

A couple of other kinds of phosphorylations you may hear about would be sub straight level of phosphorylation which is the process of making ATP using enzymes. And in photosynthesis they'll sometimes talk about photo phosphorylation. Now as I said it's the last step of aerobic respiration what it does it uses the high energy electrons that were generated by the cell doing glycolsis and the Krebs cycle putting them onto high energy electrons and then the electron transport system uses those high energy electrons to make some ATP. This happens in the cristae of the mitochondria and this uses a concept or a process known as chemiosmosis which I'll discuss more later.

Now let's take a quick look at a cell, remember all living things are made up of cells, you are made up of probably somewhere around a trillion cells. A tree, trillions if not quadrillions of cells. And a plant cell would look like this with lots of chloroplasts generating the tons of glucose that the plant needs to do its everyday life whether to pay for the energy to build the materials that it needs to stay alive. And it'll send some of that glucose over here towards the mitochondria as it goes to the cytoplasm it'll be broken down through glycolsis but it'll then enter the mitochondria. If we take a closer look at the mitochondria there's 2 major locations where processes happen. One is the inner space called the matrix the other are the folds of membrane that are collectively called the cristae.

And it's the cristae where the electron transport system happens. Now the electron transport system is this last step here, so we began with glycolsis breaking apart the glucose forming pyruvate and spitting off some high energy electrons in the form, are being carried on the electron carrier NADH. More NADH and FADH2 carrying high energy electrons are sent off to the electron transport system. And this is the final step which consumes oxygen and spits out water and generates a ton of ATP. If we take a closer look at that folded membrane called the cristae you'll find that there are several proteins and other molecules embedded within that membrane.

And what they do is they take the high energy electrons that are being carried by NADH or FADH2 and they pass them one to the next which is why collectively it's called an electron transport system. So let's follow the electrons from one NADH, what will happen is that it drops it off here on this large protein complex. This protein complex is a pump, it pumps hydrogen ions H+ ions from the matrix into this gap or that space between the inner and outer membrane of the mitochondria. So it shoves those hydrogen ions by shoving them that takes energy just like if you're pushing a car up a hill. And so the electron loses a little bit of it's energy but it still has a fair amount of energy just when electrons go through a wire from a battery to a battery powered fan those electrons are losing some of their energy as they turn that fan.

Well we send it from this carrier to that one, this too is a hydrogen ion pump and it's pumping these hydrogen ions again across the membrane. We're starting to build up a really strong concentration gradient. Lots of hydrogen ions on one side, very few on the other. This last electron, sorry the electron then gets past here to this last complex where again hydrogen ions are moved across the membrane and the used up energy electrons. So now they're very low energy they're dumped onto oxygen molecules to form water with some more of those hydrogen ions that are floating around inside of the matrix. Collectively we've wound up creating a high concentration of hydrogen ions on one side, very few on the other.

And it's more than just a concentration gradient however because remember these are hydrogen ions, they have a strong positive on charge. If we put a whole bunch of positive ones on one side that makes this side comparatively negative. All of these positives repel each other, they're attracted to this more negative side. Why is it negative? Because we've been removing a lot of its positive things, we can create upwards of a thousand times greater concentration of hydrogen ions on one side than the other. This kind of separation of charge using hydrogen ions across a membrane is called a chemiosmotic gradient, and so we're going to under go chemiosmosis in this process to create ATP. So we use this gradient, these hydrogen ions are trying desperately to get across as they're being pushed by the charges attracted by the charges being pushed by the concentration gradient and they go through the specialized channel.

On the head of this channel you'll have a group of proteins that work together to form what's called an ATP synthase, so sometimes this is called the ATP synthase channel and as the hydrogen ions push their way through this channel they physically make the ATP synthase molecules rotate and as they rotate they grab adenosine diphosphate and phosphate and slam them together to form ATP they spit that out and the hydrogen ions return back down. And that's how you make a ton of ATP through aerobic respiration. But this is also why you need oxygen, because without oxygen this pathway of electrons backs up and without the oxygen to dumps those low energy electrons on this whole process grinds to a halt and no more ATP.