But be aware that all of that for catabolism was just a setup for the electron transport chain. This is the destination for all of the NADH and FADH2 that has been generated up to this point. Look at your diagrams and count up, starting at glycolysis, how many total electron carriers have been generated. been produced. These will end up at the plasma membrane where embedded proteins are set up to carry out back-to-back redox reactions.
There are several different names for these proteins but for the time being You can remember them as complex I, II, III, and IV. The first is NADH dehydrogenase, or complex I. This is the destination of all of the NADH.
Flavin mononucleotide, or complex II, is the destination of all of the FADH2. Coenzyme Q, or complex III, shuttles the electrons over to the cytochrome, or complex IV. These compounds are called polymers. These complexes carry out a series of back-to-back oxidations and reductions, as Complex I, II, and IV are proton pumps in bacteria, and this establishes a concentration gradient.
So let's put this in context of thermodynamics and energy transfer. We first have electron transfer potential by the electron carriers, and then This transfers energy to the proton motive force, which is the concentration gradient. This then transfers energy to phosphoryl transfer potential in the form of the counterclockwise spin on ATP synthase as the protons move down their gradient.
Let's go finish off our drawing of metabolism in the cell. And so sitting here in the membrane, taking up space. is my electron transport chain.
There are four major complexes here in the membrane. Complex 1, 2, 3, and 4. They have some alternative names. We're going to deal with 1, 2, 3, and 4. It's easier to remember. And my NADH is going to drop off at door number one. It's gonna go there, it's gonna go back to NAD, it's gonna be oxidized back to NAD, one gets reduced.
And when that happens, complex one is a pump. It's going to pump hydrogen ions out into my periplasm. So I'm going to start to push protons out into here and start to form a proton motive force, which we've seen before.
We saw this with our... Flagella. We used a proton motive force to spin our flagella.
Keep that in mind. Now, FADH2 is good, but it skips door one. So it's not going to be able to pump as many protons because it doesn't go to the first pump.
It drops off at door number two. and it pumps some protons there as well in bacteria now this is a difference between bacteria and eukaryotes and sometimes people get a little confused between general biology or cell biology and microbiology because for our bacteria two here is a pump in my eukaryotes three is the pump and two is just a shuttle so A little bit difference here, 2 is a pump and I pump more protons. Now those electrons can't just sit there, they have to keep on keeping on. And so 1 is going to pass its protons to 2 and it's a pump, so it's going to pump some more protons.
It's getting all nice and full of protons here. Both the electrons from 2, this sets from what was dropped off by NAD and FAD, H2. are going to go to three. Three just kind of pushes them along, passes that buck to four.
So all my electrons get to four, and at four they pump a few more protons. So I've got a nice robust proton motive force here. That is the purpose of this electron transport chain. is to pump protons and make a proton motive force.
That's what it does. Don't make any ATP from this, this electron transport chain. It's only about the oxidations. It's only about getting these protons pushed out here and getting my concentration gradient. Now, my electrons can't just hang around.
That wouldn't be very effective, so they need to go away. But four sitting here is like, who's going to take these? Who's going to take these?
And I'm going to need a really good oxidizer, something that's a really good oxidizing agent that could really strongly steal electrons away. And in this example, in aerobic respiration, we have the best oxidizer. Our electrons with a couple of protons are going to combine with oxygen.
Oxygen. is my electron acceptor. It is going to take those electrons and if I've got oxygen and two hydrogens, I'm going to produce some water. So oxygen is here to clear the way so I can keep making my proton motive force.
It's great. It's an electron acceptor. It allows me to keep on doing what I need to do. It's not what like makes the ATP.
My ATP haven't made that yet, is going to be assembled over here with my ATP synthase. And ATP synthase does what it says it's going to do. It's going to synthesize ATP.
So all these protons, they want to leave this high area of concentration. and ATP synthase is going to say, come over here guys, come over here, I'll let you down your concentration gradient. And as it allows those through, it causes this complex to spin counterclockwise, just like we saw with our flagella.
And what that happens, what is going to occur is it's going to snap my ADP with an extra phosphate and get ATP. Now the big question is how much? I said we want to do this because it's highly efficient.
This is a bigger payoff. How so? For every NADH that I deliver to door one, I'm going to generate enough of a proton motive force to get this thing to spin completely. And one complete spin is worth...
3 ATP being produced. FADH2 skipped door 1. It didn't make as big of a proton motive force for us. So it's only responsible for about two-thirds of a spin, which gives us 2 ATP per spin.
Now let's look back up through here. How many total NADH and FADH2s do we have? Well, we had 6 here, but we...
digested a complete glucose. We had two back up here from my linker step. And don't forget, I had the two up here from glycolysis.
So I have a total of 10 NADHs. That means I can generate 30 ATP. And my FADH2s, I only got two of them and they don't give me as much, but I can get four. ATP. So I get a total of grand total of 34 ATP if I commit to doing the electron transport chain.
And I think we can all agree that 34 ATP might take us a little bit longer to get here, but this is a lot more than two. So oxidative phosphorylation, a little bit slower, not as fast, but way more efficient. Okay, so the terminal step of oxidation involves complex IV, handing off the electrons to oxygen, that combines this with hydrogens to form water.
In respiration, the final electron acceptor is inorganic. In aerobic respiration, this is oxygen, but But do be aware that there's also anaerobic respiration that uses sulfurs or nitrogen as electron acceptors. But as we saw, this doesn't directly make ATP. This just propagates the production of the proton motive force.
This image from your textbook gives you a close-up look at the four complexes. On the cytoplasm side, the electron carriers arrive at their specific complex, NADH at complex 1, and SADH at complex 2. FADH2 at complex 2. In bacteria, complex 1 and 2 are both hydrogen pumps. This is just a little different in eukaryotic mitochondria, where complex 3 is the pump instead of complex 2. When the electrons reach complex IV, a few more hydrogens are pumped, and for aerobic respiration, the inorganic oxygen takes the electrons, combines them with hydrogens, and produces water.
But the star of ATP production is the ATP synthase. The structure and function of this important protein wasn't fully understood until Paul Boyer's work in the 1930s. This complex of proteins has two major portions.
The F1 portion is what we find in the cytoplasm, and it is the catalytic part that physically puts the third phosphate onto ADP. The motor is the F0 portion, which is embedded in the membrane. This will be a channel for the proton motor force to move down its gradient. The ADP and the ADP2 are the same. and free inorganic phosphate will enter into one of the beta subunits on the F1 portion and will be bound loosely while on another beta subunit an ATP will be bound tightly.
A little ATP will form without a proton motive force, but this is a catalyst and it needs to go back to how it was prior. This is the role of the PMF. Since the F0 portion spins, it will be bound to the F1 portion. it will twist the F1 portion because they are connected by the gamma subunit. This releases the bound ATP, slaps the third phosphate onto the ADP, and then opens up a spot.
This cycle repeats. as it spins. For one full spin, three ATP will be assembled. This is accomplished by the counterclockwise spin of the F-naught portion.
What does this look like? It should look reminiscent of the flagella, as these two mechanisms have a shared history. Let's watch a short animation of what this looks like. So here we have an animation of our ATP synthase. And this is drawn for a mitochondria, but like we talked about, this is really similar.
So just this is the cytoplasm of a bacteria, and this would be the periplasm in that same bacteria. So embedded in the membrane, we have our motor, and up here is our catalytic portion in the cytoplasm. So we're going to track this yellow portion as it spins around counterclockwise. Here comes an ADP and a phosphate, smack, we release an ATP.
And we formed this new one, but it's bound tightly. The ADP and phosphate comes in, loosely bound, snap, because of that. Turning. counterclockwise rotation.
I've released another ATP, here comes my loosely bound, here comes my yellow, snap! There goes my third ATP and I've made a full rotation. So for one full rotation my beta subunits are going to combine three ATP for each full rotation. Let's finish up by looking at the other side of the coin of metabolism, anabolism. Why does a cell need energy?
What work are they accomplishing? It is important to understand that building up reactions are not simply the reverse of the breakdown. The delta G's and energetics of the steps helps us to understand why, energetically, we can't just reverse stream. Biosynthesis pathways take energy and produce things needed by the cell in order to maintain order. These might include sugars or other polysaccharides.
Gluconeogenesis is the pathway that builds glucose for a cell and the pentose phosphate pathway can help to build 5-carbon ribulose sugars and these are the building blocks of substances like NAD. Additionally the monomers we'll talk about in Chapter 4 that are used to build nucleotides and amino acids have to come from somewhere, these are built in biosynthetic pathways. This can be de novo, but more often, use different intermediates from glycolysis or the citric acid cycle. Remember, we focused very narrowly on these processes.
They are truly multipurpose and used in many ways by a cell. The fatty acids that make up the membrane, the defining feature of a cell, also have to be built and maintained. When it comes time to divide, a cell has to have plenty of energy available so that it can build more phospholipids and both cells get equal amounts of membrane. And some microbes will even use lipids as a form of energy storage.