AP Biology Glycolysis and Anaerobic Respiration

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In today's lesson, we are going to continue on with cellular energetics with the discussion of cellular respiration starting out with glycolysis and anaerobic respiration.0002

We are going to begin with an overview of cellular respiration.0014

First of all, during cellular respiration, energy stored in the chemical bonds of organic compounds such as glucose is released and used to make ATP.0019

Through this process, nutrients such as glucose are broken down, and the energy is released to make ATP.0030

Recall from the previous lesson that ATP is the energy currency of the cell.0037

It provides the direct energy source for most work in the cell such as active transport, production of macromolecules such as protein and the repair and growth of cells.0042

Just to review, here is the structure of ATP- adenosine triphosphate.0056

It contains three phosphate groups, and these bonds between the phosphate groups are high energy especially the bond between the terminal and the middle phosphate group.0061

When that is cleaved, energy can be released and used to power the cell.0074

The reason that there is a high amount of energy in these bonds is because you will notice that the phosphate groups are negatively charged.0080

Yet, they are held close together by these bonds.0087

Negative charges do not want to be closed together. They want to repel each other.0090

Yet, they are being forced to be near each other, and that creates potential energy.0094

The cleavage of that bond, it releases that energy.0099

Recall that the hydrolysis of ATP to create ADP plus phosphate has a ΔG of approximately -7.3kcal/mol.0102

This amount of energy would be released and could be used by the cell.0119

The opposite reaction, ADP plus phosphate, so the phosphorylation of ADP to create ATP, would require the input of 7.3kcal/mol.0124

And that energy, in order to phosphorylate ADP, can be found in the bonds of glucose that by breaking down glucose, that energy can be harnessed to form ATP.0140

And that is what cellular respiration is all about.0157

Notice that cellular respiration can either be aerobic or anaerobic.0160

Aerobic meaning requiring an oxygen. Anaerobic is when cellular respiration occurs without the presence of oxygen.0166

Today, we are going to focus on anaerobic respiration. In a separate lecture, we will focus on aerobic respiration.0173

Now, I mentioned that cellular respiration involves the breakdown of organic compounds such as glucose.0180

Well, where is this glucose come from?0186

In photosynthetic organism such as plants, glucose is actually produced, is generated by harnessing light energy.0189

Carbon dioxide and water are used along with light energy to produce glucose, which is later broken down via cellular respiration.0199

In organisms such as animals that are non-photosynthetic, the organic compounds come from ingesting other organisms such as plants and other animals.0208

One way or another, the organism needs to obtain glucose either by ingesting or by forming it through photosynthesis, and then, that is broken down via cellular respiration.0219

Now, in these two types of cellular respiration, aerobic and anaerobic, ATP is created for both. However, aerobic respiration is much more efficient.0231

With anaerobic respiration, only a net of two ATP molecules can be formed per glucose molecule that is metabolized.0242

Contrast that with aerobic respiration.0257

With aerobic respiration, there is a range, and you will hear different numbers in different books; but approximately 36-38 ATPs can be formed through aerobic respiration.0259

You will see that aerobic respiration is far more efficient than anaerobic respiration.0273

The first step for both of these processes is the same. It is glycolysis.0278

That is what we are going to focus on right now.0284

OK, we are going to begin by an overview of what glycolysis is, and then I am going to go into the steps of the process.0292

First of all, looking at the word glycolysis, gly meaning sugar and lysis meaning split, so glycolysis is the splitting of sugar.0299

You see here, that there is one glucose molecule that enters the glycolysis cycle, and it has 6 carbons.0314

Each of these spheres represents a carbon- 1, 2, 3, 4, 5, 6.0322

By the end of glycolysis, there will be two 3-carbon pyruvate molecules.0328

These two sides are mere images of each other, but just to keep the picture clean and clear, only one side is labelled.0335

But whatever is happening here on this side, is also happening, reflected across on the other side.0341

Looking at what has happened, one molecule of glucose is split to form two molecules of pyruvate.0347

The net production of ATP is two ATP molecules.0354

Looking at this overall equation, there is an input of one glucose molecule and two ATP molecules as well as two NAD⁺s.0359

We will talk in a minute about what these are and what they do.0369

But for right now, seeing that one, two ATP molecules are hydrolyzed. One glucose enters, and an NAD⁺ also enters the cycle.0372

The result is two pyruvate molecules, four ATPs.0381

One ATP, two ATP and on the other side, the same thing would be happening0386

We would end up with an ATP here and at this step, another ATP as well, so created 1, 2, 3, 4 ATP molecules- two in, four out, net production: two ATPs.0391

There is also a production of two NADHs, and this is going to become important when we talk about aerobic respiration.0410

Alright, I mentioned NAD⁺.0419

What does that do? Why is it important to glycolysis?0421

Well, one step of glycolysis, the one that involves NAD⁺, involves what is called a redox reaction.0424

Just going over some chemistry right now so that you could understand this step of glycolysis,0433

reactions, in which electrons are transferred from one molecule to another, are called redox reactions.0439

Longer name for that is oxidation reduction or reduction oxidation reaction, so shorten that- redox reaction.0446

When electrons are transferred from one atom or molecule to another, that is what is called a redox reaction.0457

The molecule that loses an electron, one or more electrons, is oxidized, so the molecule that loses an electron has been oxidized.0466

It is transferring those electrons or that electron to another molecule on the molecule that gains an electron we say is reduced.0487

Molecule loses an electron. It has been oxidized.0500

Molecule gains electrons. It is reduced.0503

This allows for the transfer of energy, and the reason is the electrons move from a molecule that is less electronegative to one that is more strongly electronegative.0509

Remember that electrons are attracted to very electronegative atoms.0521

A perfect example is oxygen. Electrons are attracted to oxygen.0526

Therefore, by moving towards a more electronegative atom, energy is released.0531

There is a decrease in free energy as electrons move from a less electronegative to a more electronegative atom.0536

This energy can be harnessed to make ATP, and we are going to see a lot more about this in aerobic respiration.0543

We should start understanding it now because glycolysis does involve a redox reaction.0550

These electrons are usually not transferred alone. They actually move along with a proton.0555

Therefore, the transfer that we are talking about here, involves one electron plus one proton, and if you think about it, that is hydrogen.0561

We can also think about redox reactions as being the transfer of hydrogen atoms.0573

We can think of it in terms of transfer of hydrogen or transfer of electrons.0578

Since oxygen is very highly electronegative, it is an excellent electron acceptor.0583

It becomes reduced. It accepts electrons.0592

Therefore, it is an oxidizing agent. It accepts electrons and becomes reduced and therefore, oxidizes the other molecule.0596

Oxygen is very electronegative and therefore, is a good oxidizing agent.0605

And again, this is going to come into play more in aerobic respiration when we see that oxygen is the final electron acceptor.0615

Alright, now, getting to NAD⁺, nicotinamide adenine dinucleotide is the full name. We usually just call it NAD.0624

NAD⁺ is a coenzyme, and we discussed coenzymes in the previous lecture; and this is a coenzyme or an organic cofactor.0633

Remember that cofactors are molecules that have specific functions that aid in catalyzing reaction.0643

They are necessary to catalyze the reaction so you have the enzyme.0653

And there may be a cofactor or coenzyme sitting in the active site of that enzyme, and they have various functions.0656

The function for a coenzyme such as this one, NAD, is to act as an electron carrier.0663

NAD is a coenzyme that acts as an electron carrier during glycolysis and also during other parts of cellular respiration.0669

Electrons are passed not directly from one molecule to another in glycolysis, but actually through NAD, the coenzyme. It acts as intermediary.0678

We are going to see a couple other coenzymes that act as electron carriers.0689

There is one NADP, which we are going to see in photosynthesis and also FAD.0694

We will talk about this more in - actually FAD - later lectures, but for right now, we are focusing on NAD.0706

In glycolysis, one step is a redox reaction, and that is the 6th step.0713

In that step, two hydrogen atoms are removed from the substrate, and one hydrogen ion is released.0719

The remaining proton, as well as two electrons, are transferred to NAD⁺.0729

Let me write that out to make this much clearer.0734

Let x represent inorganic molecule, and here, we have xH₂, so This H₂ is part of this.0737

It is attached to this organic molecule. It is an organic substrate, and we have xH₂ plus our electron carrier NAD⁺.0746

Remember that a redox reaction involves the transport of electrons.0762

Here, there is a proton coming along with the two electrons.0766

Two electrons are transferred and as well as one proton. The other proton is released.0772

What we end up with is - these are partly transferred, partly released - the remainder of this organic substrate plus NADH plus a hydrogen ion.0781

Remember, two hydrogen atoms will be composed of two electrons and two protons, so let's figure out where everything went.0802

Two electrons and two protons, two electrons we are transferred to NAD.0810

One proton was transferred to the NAD, and then one proton is released as a hydrogen ion.0816

Let's look at the charge here.0822

NAD was positively charge here. It was NAD⁺.0823

Two electrons were transferred plus one proton, so we have two negative charges and one positive charge.0827

That is a net of -1 charge. This started out with a +1 charge.0835

+1 and -1 combined becomes neutral. That is why when NAD is reduced, when it accepts these electrons, it ends up with a neutral charge.0841

It ends up as NADH plus hydrogen ion is released.0855

What has happened here is that NAD⁺ has become reduced - this is the reduced form - and the organic substrate is now oxidized.0860

It has lost electrons, so it is oxidized. This has gained electrons, so it is reduced.0870

FAD, when it is reduced becomes FADH₂, and NAD⁺ becomes NADPH.0880

Notice that they are gaining electrons. They are gaining hydrogen, and that is how I know that this is the reduced form; and this is the oxidized form.0890

Alright, now, that helps you to understand one of the steps of glycolysis0900

OK, now we are going to go through glycolysis step by step, but remember, for the AP exam, there are some main facts that you have to know.0904

You should understand glycolysis, but you certainly do not have to memorize every intermediate and every enzyme.0912

The first thing you should know is that it takes place in the cytoplasm.0918

For cellular respiration, you should now where each step takes place, so glycolysis takes place in the cytoplasm.0923

Second: there is a net production of two ATPs.0929

There is also production of two NADHs, and as we discussed in the overall reaction, one glucose molecule is split to form two pyruvate molecules.0935

Looking here, each sphere represents a carbon, so this is a 6-carbon molecule. We end up with two 3-carbon pyruvates.0951

You should also know that each step is catalyzed by a specific different enzyme.0961

Again, you do not have to memorize these enzymes, but just be aware that there are enzymes involved at each step.0966

There are two general phases. One phase is the energy investment phase.0973

You can see that ATP is utilized, so ATP is broken down to ADP in two steps here in the beginning; so energy is invested.0981

Here, the second phase is the energy payoff phase, where ATP is produced.0992

Starting from the top, starting out with the glucose, one ATP is used, and a phosphate group that is released is transferred to glucose to form glucose 6-phosphate.1007

In this step, the glucose is activated. That is the first step.1019

In the second step, glucose 6-phosphate is converted to its isomer, fructose 6-phosphate.1025

No energy input is required for that step.1033

In the third step, again, energy input is required.1036

ATP is hydrolyzed to ADP, and fructose 6-phosphate is phosphorylated to form fructose 1,6-phosphate.1039

Now, this next step is where the molecule is split form a 6-carbon molecule to two 3-carbon molecules.1049

I want to talk a little bit more about what occurs in this split.1056

We have had step 1, 2, 3, and then, the splitting occurs, that is step four.1060

Fructose 1,6-phosphate is not split into two identical molecules. Instead, we have fructose 1,6-phosphate.1070

It is converted into two different 3-carbon molecules. They are isomers of each other.1083

One of them is glyceraldehyde-3-phosphate right here. The other one is dihydroxyacetone phosphate.1092

Only glyceraldehyde 3-phosphate continues on in glycolysis. You can see that here.1111

Yet, initially, this molecule is split into a molecule that cannot perceive down glycolysis.1117

The solution for that is an enzyme, isomerase, that can catalyze the reaction to convert G3P to dihydroxyacetone phosphate, and this reaction can go either way.1125

However, since glyceraldehyde-3-phosphate is being consumed by glycolysis, the reaction gets pulled towards the formation of G3P.1138

As G3P is consumed, this enzyme catalyzes the formation of more from the isomer.1148

This is actually step 5, this isomerization, and the result is that you end up with two G3P molecules.1155

Initially, these are two different molecules.1165

Isomerase converts the other one two G3P, and that way, both of these can continue on through glycolysis.1167

Alright, that was step 4, step 5. Now, we are down to step 6, and notice that from here down, everything is going to occur twice per glucose molecule.1176

The glucose molecule will split, so the rest of these steps are going to all occur twice.1192

In this next step, inorganic phosphate is added. An inorganic phosphate is added to glyceraldehyde-3-phosphate to convert it to 1,3-bisphosphoglycerate.1198

This step is a redox reaction.1212

Notice that the electron carrier NAD⁺ is reduced to NADH.1215

In the process, glyceraldehyde-3-phosphate is oxidized, and that is going to occur over here as well; so you are going to end up with two NADHs from this step.1221

Now, finally, the energy payoff, energy has been invested. So far, two ATPs were used.1234

In step 7, 1,3-bisphosphoglycerate is converted to 3-phosphoglyceric acid.1242

In that process, it loses a phosphate group, and that phosphate group is transferred to ADP to form ATP.1252

The same thing is happening over here. I have an ATP formed.1262

At this point, we have used two ATPs, and now, two ATPs have been produced; so the energy debt has been paid. We are at 0.1265

In step 8, 2-phosphoglyceric acid or 2-phosphoglycerate is formed from 3-phosphoglyceric acid, and all that involves is movement or transfer of a phosphate group to a different location on this molecule.1275

It does not require the input of energy nor does it produce energy.1294

In step 9, a molecule of water is removed from the substrate, so 2-phosphoglyceric acid is converted to phosphoenolpyruvic acid or more simply PEP.1298

Finally, step 10, we get in the production of another ATP molecule.1312

PEP is converted to pyruvate, and in the process, ADP is phosphorylated.1318

Actually, I want to talk a little bit more about this. ADP is phosphorylated to form ATP.1329

Previously, what we talked about is that ADP and inorganic phosphate, we often talked about forming ATP.1333

That is not what happens here, and I want to talk about the difference.1342

The type of phosphorylation that occurs in glycolysis is called substrate level phosphorylation.1346

Instead of an inorganic phosphate being added to ADP to form ATP, phosphate is transferred from a substrate for example 1,3-bisphosphoglycerate.1359

The phosphate is transferred from there and to the ADP to form ATP, and therefore, it is called substrate level phosphorylation.1371

Both of the phosphorylations in glycolysis occur this way.1383

When we talk about aerobic respiration, a lot of the phosphorylation that we will see occurs by the addition of inorganic phosphate.1388

That is not substrate level phosphorylation, but you should be aware that this is slightly different.1396

The result is the same, though- the production of ATP.1400

Let's back up and look at what has happened.1404

One glucose molecule has been broken down to two pyruvate molecules.1406

In the process, two ATPs were invested. However, four ATPs were formed.1410

There is a net production of two ATPs. There is also a redox reaction in which NAD⁺ accepted electrons and was reduced to NADH.1418

That occurred twice, so we have two NADHs, and remember, this all occurred in the cytoplasm.1429

OK, now, looking again at aerobic vs. anaerobic respiration now that we are a little bit deeper into things, anaerobic respiration starts out.1437

Step 1 is glycolysis, and then, step two that we are going discuss is called fermentation.1450

Overall, anaerobic respiration is more complicated, but there is a bigger payoff.1461

The first step is, again, glycolysis. However, there are some additional steps that do not occur in anaerobic respiration1467

Instead of fermentation, the next step is conversion of the pyruvate to acetyl-CoA, so we will call that acetyl-CoA formation.1475

Then, acetyl-CoA enters the citric acid cycle, and finally, oxidative phosphorylation occurs; and this is just an overview.1490

The next lecture covers this in extreme detail, but just to have some context with which to compare aerobic and anaerobic respiration.1501

The first step is the same. Subsequent steps are different.1512

And the ATP production here, net of two ATPs here approximately 36-38 or maybe even less ATPs depending on the conditions in the cell.1515

But overall, many more ATPs, then, are produced by anaerobic respiration.1529

Why is there such a difference?1536

Well, in anaerobic respiration, glucose is broken down to pyruvate and that releases some of the energy stored in the chemical bonds of glucose.1538

But, the pyruvate is not completely, then, broken down.1545

Therefore, there is still energy that has not been used, whereas in aerobic respiration, the pyruvate ends up completely broken down.1549

CO₂ is released, and therefore, a much more energy can be taken from that original glucose molecule.1557

Really, anaerobic respiration only takes it part of the way.1564

When we talk about organisms that perform aerobic or anaerobic respiration, we can classify them.1569

You might hear the term obligate anaerobe, obligate or strict anaerobe- same thing.1575

An obligate or strict anaerobe does not have a choice. It can only perform cellular respiration anaerobically.1583

In fact, these organisms are often harmed by the presence of oxygen.1593

One example of an obligate anaerobe is the bacteria that causes botulism. There are other types of anaerobes that are called facultative anaerobes.1597

For example, many yeasts are facultative anaerobes. They can perform either aerobic or anaerobic respiration.1612

Obligate anaerobes do not have a choice. The only type of respiration they can do is anaerobic.1621

Facultative can do either.1626

Alright, anaerobic respiration, the first step is glycolysis. The second step is fermentation.1629

Continuing on with this overview, when cellular respiration is anaerobic, we need the second step in order to regenerate NAD⁺.1639

Because you could look at anaerobic respiration and say "OK, we did glycolysis, got the two ATP, let's just stop, why even do fermentation?".1652

Well, remember that in steps 6 of glycolysis, NAD is required, and it is reduced to NADH.1661

The cell does not just keep creating new NAD⁺ from scratch.1672

In fact, NAD⁺ forms NADH and then, plus the hydrogen ion.1676

NADH can be recycled back to NAD⁺, and then it is reused.1686

Without regenerating NAD⁺, the cell would just stop doing glycolysis.1694

There has to be a way to regenerate NAD⁺, and that is the purpose of fermentation.1700

Here is glycolysis: started out with our glucose, ended up with pyruvate, ATP and NADH + H⁺.1707

Now, how do we get this back? Well there is two ways.1716

One is alcohol fermentation. The second is called lactic fermentation.1719

Alcohol fermentation is actually a two-step process.1725

In the first step, carbon dioxide/CO₂, is removed from the pyruvate molecule to yield acetaldehyde, an intermediate.1730

You do not have to memorize all this, but it is helpful to just, kind of, know overall what happens.1746

A CO₂ is released, and acetaldehyde is formed.1751

Then, a hydrogen is added to acetaldehyde to form ethanol, so the product is ethanol.1756

Where does this hydrogen come from? Well, it comes from NAD.1763

The acetaldehyde is reduced, which oxidizes NADH back to NAD⁺.1767

Fermentation has accomplished its goal. NAD⁺ has been formed.1774

And notice I have a two in front of everything, and that is because I am discussing this as per glucose molecule.1779

For each glucose molecule, two pyruvates are generated.1786

Therefore, two pyruvates would enter the fermentation cycle yielding two NAD⁺s and two ethanols.1789

A second type of fermentation is called lactic acid fermentation. This is a one-step process, and some types of bacteria perform alcohol fermentation1800

Other types perform lactic acid fermentation.1813

Many yeast, fungi, perform lactic acid fermentation, and we actually perform this in our muscle cells as well.1816

When you are exercising hard, you use up the oxygen in your muscle cells. The muscle cells use a lot of ATP.1825

They are forced to go to anaerobic respiration, and the product here, lactic acid is one reason why when you are exercising, your muscles burn; and that is because of this production.1831

OK, in lactic acid formation, again, two pyruvates enter the cycle.1843

Two NADHs are used, and the product is lactic acid along with NAD⁺, so either method regenerates NAD⁺.1851

Let's go ahead and review the concepts we have learned through some examples.1864

Example one: indicate on the figure below, during which steps of glycolysis substrate level phosphorylation occurs.1868

During which steps is energy is invested? What is the net number of ATP produced?1878

Alright, taking these one at a time, which steps of glycolysis substrate level phosphorylation occurs?1884

Substrate level phosphorylation is the formation of ATP through the transfer of a phosphate group to ADP.1892

And the origin of that phosphate group is an organic substrate rather than inorganic phosphate.1900

I want to look for ATP being generated, and I see that that happens here and here.1908

This is step 1, step 2, step 3, step 4. Step 5 is the isomerization of G3P and the other product of step 4, step 6, step 7, step 8, step 9 and step 10.1915

Substrate level phosphorylation occurs during step 7 and steps 10.1938

ATP is formed by transfer of a phosphate group from an organic substrate.1945

During which steps is energy invested? Well, energy is invested in the form of ATP.1950

ATP is hydrolyzed to ADP during two steps: step 1 and step 3.1955

Remember that this is called the energy investment phase, and this is the energy payoff phase.1963

What is the net number of ATP produced? Well, one, two are used, and this is occurring twice, so I have an ATP here, an ATP here, an ATP here and an ATP here, 1, 2, 3, 4- 4 produced.1980

I had four ATP minus two gives me a net of two ATP produced.2000

The first answer: substrate level phosphorylation during steps 7 and 10.2009

Energy is invested in steps 1 and 3, and there is a net of two ATP produced.2015

Example two: what would happen if glycolysis was not followed by fermentation in a cell that was undergoing anaerobic respiration and why?2025

Remember that the purpose of fermentation is to regenerate NAD.2035

Fermentation regenerates, and it does that by oxidizing NADH resulting in products of either lactic acid or ethanol.2041

NAD⁺ is necessary for glycolysis.2066

If no NAD⁺ will be generated, glycolysis would stop. Without NAD, glycolysis would stop.2076

They would grind to a halt once the NAD is used up.2090

If glycolysis stops, no ATP would be produced. The cell could not carry out its functions, and eventually the cell would die.2094

It would not have the energy for cellular processes.2107

Fermentation, if it did not occur, NAD would not be recycled.2111

Glycolysis would stop. No ATP will be produced, and the cell will probably die.2118

Why are many more ATP produced per glucose molecule metabolized by aerobic respiration than for anaerobic respiration?2127

Remember, there is as many as 36 more ATP molecules produced in aerobic respiration compared with anaerobic respiration. Why is that?2133

Well, much of the energy from the glucose molecule is not harnessed in anaerobic respiration.2145

In anaerobic respiration, glucose is broken down to pyruvate, and things pretty much end there.2151

Fermentation occurs, but it does not yield anymore energy.2162

Glucose is broken down to two pyruvate molecules.2170

In aerobic respiration, glucose is broken down to two pyruvate molecules, but that is followed by the citric acid cycle and oxidative phosphorylation.2172

And during those processes, the energy is still stored in the pyruvate is harnessed.2186

Pyruvate is further broken down in the citric acid cycle, and that energy is harnessed to make more ATP. This can only occur in the presence of oxygen, though.2192

Is glycolysis an exergonic reaction or an endergonic reaction? Give support for your response.2203

Remember that in an exergonic reaction, energy is released, and this occurs during the breakdown of reactants.2209

A molecule such as glucose that is being broken down, the energy stored in those chemical bonds is released.2225

An endergonic reaction is one in which energy is required.2233

It must be input for the reaction to occur, and that would be an anabolic process such as creating a macromolecule like proteins from its building blocks like amino acids.2238

Glycolysis is a breakdown, a reaction, which something is being broken down. Glucose is being broken down.2252

and therefore, it is an exergonic reaction, and we also know that because ATP is formed as a result of glycolysis because energy is being released.2261

That concludes this lecture on glycolysis and anaerobic respiration.2274

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