We'll start off with glycolysis. However, this is not the only way to harvest energy. Since this is an introductory class, we will focus on glucose metabolism at this point. But be aware that other sugars can be used, and glycolysis can be skipped altogether. Additionally, proteins can also be fermented.
Don't become so focused on this that you overlook variations and other usable nutrients. There are entire upper-level classes that spend the entire semester looking at nothing but metabolism, but we're only devoting a couple of lectures to this topic. And glycolysis is...
often the spark for cellular respiration or fermentation. This process employs substrate-level phosphorylation, which has the benefit of being very fast. Because it occurs so rapidly, it can churn out significant amounts of ATP in a relatively short time. However, it is not very efficient. Overall, what we are going to see is that the 6-carbon glucose will be split, forming two...
three carbon pyruvates. To get to pyruvate, we will start with stage one, where we will invest an initial ATP to get everything rolling. Stage two is where we first see production.
This is where we first have NAD plus reduced to produce NADH. This is a rate limiting step. It has to happen before any ATP is made. And remember, there are only finite amounts of NAD+, so this will have to go unload quickly or everything will stop.
Next, 4 ATP are made, but remember that we had to invest 2 to get started, so our takeaway here is 2 ATP. It's not much, but this process is so fast that it can generate significant amounts quickly. Lastly, we get the 2 3-carbon pyruvates. This is the job.
between aerobic and anaerobic. It is possible to just stay here and keep doing glycolysis. If that is the option the cell goes with, it's anaerobic, because as you can tell from this brief summary, no oxygen is used here. Remember the rate limiting step? That is where recycling comes in.
We need to oxidize the NADH back to NAD+. To do this, the NADH just gives the electron back. to our three carbon sugars, the pyruvates. So ultimately, the cell is using an organic electron acceptor. Let's draw out the critical steps of glycolysis.
So I've already gotten a bit of a head start and drawn out a big cell for us. So just kind of giving some room to draw and making sure to include our cell membrane. but also including a thin PG and an outer membrane, which would make this a gram-negative cell.
The reason to draw a gram-negative cell is to drive home that idea of endosymbiosis. If you think about something like a mitochondria, those are double-membraned, and that's what we see with a gram-negative bacteria. So even though this is a single-celled organism and it doesn't have any organelles, it is still able to perform the full range of metabolism. And just to kind of make sure we appreciate that this is a living cell, we'll come over here to this pole and just draw in some DNA.
Because when we talk about Chapter 4, Later on we'll kind of have that in there. This is actually providing the instructions for what we're going to do over here with metabolism. So we're going to start off with glycolysis.
So that means we need to put in our glucose. So we're just going to draw glucose up here, six-sided sugar. And we're going to talk about...
Glycolysis. Now remember our glycolysis is going to happen in three stages and stage one is all about prep. So the only thing we see in stage one is actually that we have to invest a couple of ATP to get this process started. It's not until we get down to the split that we actually start to have our stage two.
which is our production steps. And really the most important thing here is that when we make this split, we're going to be able to load up our NAD and get NADH. And that has to happen first before anything else can occur.
And because we're breaking this bond and making two of our pyruvates, then we get two NADHs. We make four ATP here, but we had to invest those initial two, and so that leaves us with a take-home to ATP. And this happens really fast, so this is a fast generation, just a few steps we can get.
to those two ATP and we get from that split to three carbon intermediates. Now you'll hear a couple of different naming conventions with these and one is that you'll hear the term pyruvate and then you'll hear the term pyruvic acid. For our purposes these are going to be the same thing, they're not actually exactly the same. You'll notice here we still have this hydrogen and that hydrogen could be lost.
So that's a pyruvic acid, I could lose this hydrogen. Pyruvate, I've already lost the hydrogen, I'm down the hydrogen. and we've just got this negative charge on this oxygen.
So they are a little bit different. You can use whichever term you feel comfortable with when you're diagramming or talking about this process. So that's it for glycolysis.
Really fast, we generate 2 ATP through substrate level phosphorylation. It is possible to just stay here and do nothing but glycolysis, but you have to free up this NADH back to NAD because that's our rate limiting step. We can't generate any ATP before we do this reduction, so we have to have this available, and there's only small amounts of that. So we're going to have to...
recycle and get that back. But we don't have a lot of options for recycling here. We've got kind of what we're working with and the only thing that really has the space to take those electrons back is the pyruvate.
The cell can recycle and it's doing that so that it can regenerate that NAD and that's going to be done that oxidation is going to occur by reducing our pyruvate. Why a cell would do this? This is an incredibly fast process. It's not very efficient as we've kind of talked about but it's fast and sometimes you want to take speed over efficiency.
Now Why? Why take speed over efficiency? There's a lot of reasons. Do not get hung up on just oxygen availability.
Yeah, we don't have any oxygen used in here and that's great, but this choice here is very rarely just about oxygen. So a lot of it has to do with membrane space. How much available space do we have in here? The ETC takes up a lot of space.
A lot of our gram-negative bacteria have flagella. That takes up a lot of space. If you have multiple flagella, that takes up more space.
And now you're also supposed to bring glucose or other nutrients into the cell through those transporters. That takes up more space. There's not infinite space there. So if there's just not a lot of membrane space for the electron transport chain, the cell can do this instead.
Also, enzyme availability. There is a lot of Stuff going on here with the next step, the citric acid cycle, lots of different options for that. And so sometimes those enzymes just aren't available.
And sometimes it can be the electron acceptor availability, what's around. That is what this pyruvate is acting as. It's acting as an electron acceptor.
And so... In fermentation, kind of the definition of fermentation is that we use an organic acceptor, and that's when there aren't any of the inorganic ones available. So this is our glycolysis kind of in a snapshot. These are the main steps you need to remember for this, and next we'll be adding in the citric acid cycle. In your textbook, you will find this figure for glycolysis.
Don't panic. You don't need to memorize every step in enzyme shown here at this point. You need to know the key steps that we drew out and a couple of the enzyme classes.
Note the key things we've already discussed. Each line is the stages we summarized. Stage 1, stage 2, and the optional stage 3. Notice on line 1 that phosphates are added and the molecules change shape, but nothing breaks.
There are... couple of key enzyme classes that we see here. Notice that when anything has the ending of kinases, a phosphate is involved. For instance, hexokinase is when the first ATP is used and a phosphate is moved onto the glucose.
Then again with the phosphofructokinase, when the second phosphate is added. You should remember that kinases are involved with moving phosphates and isomerases change the shape without adding or taking anything away. So here we just see a shift from glucose to fructose. The second line is the production.
Again, note that NADH has to be reduced, the G3P is oxidized, before any ATP is produced. Four ATP are produced, and after considering the two invested, that leaves us a net of two ADP. and we are left with two pyruvate or pyruvic acids.
If stage three is performed, then NADH is oxidized by donating its electron back onto pyruvate, which can form byproducts such as lactic acid or alcohol. This third stage that is an optional step of glycolysis is also referred to as fermentation. Fermentation is just a special term for using an organic electron acceptor, and in this case allows us to keep doing glycolysis. For many, many various reasons, a cell may select to stay in glycolysis.
Sure, it's not the most efficient way to make ATP, but it is speedy. Many people get hung up on the availability of oxygen, especially in the microbial world. This is rarely the reason a cell decides to do fermentation. But even in human cells, a tumor cell will tend to do fermentation. Obviously, it's got access to oxygen, but tumor cells are greedy and they don't care about efficiency.
They will choose glycolysis even in the presence of oxygen just so they can gobble up all the sugars quickly. In bacteria, many of the choices are tied to the space available in the membrane. The electron transport chain takes up a lot of space. If it comes down to transport proteins that bring the sugar in, or the more efficient ETC, many cells just decide to pack their membrane full of glucose transporters and maximize their ability to bring glucose into the cell, over the efficiency of how much energy they can get out of that glucose. There is also the question of the availability of citric acid cycle enzymes.
This cycle is participating in many other pathways. If all of its enzymes are currently being used in lipid synthesis, it might be better to just stay in glycolysis a while longer. Ultimately, the real hang-up in continuing glycolysis is NAD availability.
Remember, no ATP is made until after NAD is reduced to NADH, but there are only tiny amounts of NAD. Therefore, we have to oxidize NADH back to NAD to be able to continue doing glycolysis. The only thing available to be reduced is the pyruvate. In reducing the pyruvate, different cells will make different end products.
Yeast cells have a tendency to make ethanol or drinking alcohol. Many bacteria make lactic acids. This is why, if streptococcus mutans in your mouth that can access lots of sugar will lead to dental caries. The lactic acid degrades enamel.
Table 3.4 from your textbook shows the wide variety of byproducts that can be made when fermentation is performed. First we see the yeast that make alcohol and the streptococcus that makes lactic acid. But we also have Clostridiums that can make acetate and a host of enteric bacteria that produce a mix of acids that can really upset your stomach. Last, we see some acidic fermentation versus alcoholic pathways.
Note, it is all about making NAD available for glycolysis. Yeasts aren't making alcohol to make teenagers happy. Yeasts only make alcohol as a way to make you feel better.
to free up NAD so that it can go back and be available for glycolysis. In alcohol, on the left, the terminal carbon is cut away generating CO2 and the hydrogens go back to make ethyl alcohol. On the right, no carbons are lost, so the three carbons are still there, but when the hydrogens are added back on, it makes lactic acid.