AP Biology Aerobic Respiration

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In today's lecture, we are going to be discussing aerobic respiration.0003

We will start out with the review of a comparison of aerobic versus anaerobic respiration.0008

Anaerobic respiration was discussed in detail in the previous lecture.0014

Aerobic respiration consists of glycolysis followed by the citric acid cycle and then, oxidative phosphorylation.0019

It is much more efficient than anaerobic respiration and produces approximately 36 ATPs per molecule of glucose.0028

Notice that I say approximately. The range that is given in textbooks is often 36-38.0036

In reality, it may even be much less. It could be 34, even 32.0041

Some of that depends upon conditions in the cell, but in any case, many more ATP are produced per glucose molecule metabolized compared with anaerobic respiration.0046

Recall that anaerobic respiration, which consists of glycolysis, followed by fermentation yields only a net of two ATPs compared with approximately 36 per aerobic respiration.0059

And the reason for that is in anaerobic respiration, glucose is broken down to pyruvate during glycolysis, and that energy is harnessed to make ATP; and that is as far as it goes,0082

whereas with aerobic respiration, pyruvate is further broken down so more chemical energy can be released from that original glucose molecule, and that is used to make ATP.0094

Glycolysis is the first step in both aerobic and anaerobic respiration.0109

I am going to start with the review of glycolysis, but if you are not familiar with glycolysis, I recommend that you watch the lecture on glycolysis where I go into detail.0113

This lecture assumes that you have a basic understanding of this process.0123

The overall equation for glycolysis is one molecule of glucose plus two ATPs plus two NADs form two pyruvate molecules, four ATPs, two NADHs, and two hydrogen ions are released.0127

Glucose is a 6-carbon molecule. These spheres represent carbons.0146

This is a 6-carbon molecule, and eventually, it is broken down into two 3-carbon pyruvates.0151

Recall that this initial phase of glycolysis is known as, it is the energy investment phase, and during this phase, two ATPs are used.0161

During the energy pay-off phase, four ATPs are formed.0179

Remember that for each glucose molecule, these steps, 6-10, are occurring twice.0193

On this side, I am seeing one ATP produced, but another ATP is also produced over here and again at step 10.0200

Therefore, 1, 2, 3, 4 ATPs produced. This results in a net of two ATPs.0210

In addition, two NADHs are produced - here and here - and those NADHs actually are storing energy, which is going to be released in oxidative phosphorylation, which we are going to discuss shortly.0222

Recall that some organisms are what is called obligate anaerobes. They can only perform anaerobic respirations.0240

Other organisms perform both. They are facultative anaerobes.0247

Some organisms use aerobic respiration. It is far more efficient.0253

Other organisms do not have a choice. They have to perform anaerobic respiration.0259

First step of aerobic respiration is glycolysis, and recall that this takes place in the cytoplasm.0266

Something I am going to emphasize is where each step occurs because that is something you need to know for the exam.0274

Glycolysis, this first step of aerobic respiration, takes place in the cytoplasm, and in aerobic respiration, fermentation does not occur.0281

We are left with these two pyruvate molecules per glucose, what happens to them.0291

Well, the next step is the conversion of pyruvate to acetyl-CoA.0296

In aerobic respiration, the two pyruvate molecules that were formed through glycolysis are transported into the mitochondria, where they are converted to acetyl-CoA.0303

This conversion of pyruvate to acetyl-CoA, which is the second step of aerobic respiration, occurs in the mitochondria, and it is in a specific area of the mitochondria known as the matrix.0313

Mitochondrial matrix is the location of the conversion of pyruvate to acetyl coenzyme A.0329

The third step in aerobic respiration is the citric acid cycle, and the substrate for the citric cycle is not pyruvate. It is actually acetyl-CoA.0340

What happens is two pyruvates per glucose are converted. They are transported into the mitochondrial matrix and decarboxylated.0351

They each lose one CO₂, and this CO₂ is the same CO₂ that we breathe out.0363

We breathe in oxygen, which is necessary to aerobic respiration and a by-product of aerobic respiration, CO₂, which we exhale out.0370

CO₂s are released, and you are going to see CO₂ released in later steps of aerobic respiration as well.0381

Two coenzyme As are needed, and these are actually added to form acetyl-CoA.0388

And you can see the structure here, the sulfur containing CoA group, and this bond right here is high energy.0398

It is unstable, and cleaving that actually releases energy.0406

Again, a lot of what we are talking about in cellular respiration is the transfer and storage of energy, the energy that started out in the glucose molecule.0410

Now, glucose molecule is partly broken down the pyruvate.0421

There is still energy stored in these bonds, and now, it is stored here in the acetyl-CoA; and it is later harnessed to make ATP.0423

You do not need to know each step of this reaction, but it is a three-step reaction; and it requires two NAD because we have two pyruvates here, and this is going to be reduced to NADH.0431

The NAD⁺ is being reduced, and the pyruvates are being oxidized, so this is yet another redox reaction.0446

Again, this concept was covered in the previous lecture.0452

You definitely need to understand electron carriers such as NAD⁺ and redox reactions.0457

You can learn that information in the previous lecture on glycolysis and anaerobic respiration.0464

Two pyruvate molecules are oxidized, and CoA groups are added.0469

The result is two acetyl-CoAs plus two CO₂ molecules that are released, two NADHs formed and two hydrogen ions released.0475

Another important thing on the AP exam, an important factor to note is in addition to the location of the steps of cellular respiration, you need to keep track of energy formation.0487

In glycolysis, we have a net of two ATPs, and there were also two NADHs; and energy is stored in this electron carrier.0501

Now, in the process of forming acetyl-CoA, - I will just call this acetyl-CoA but it is the process of forming acetyl-CoA - we have generated two NADHs.0515

So far, we have two ATPs and four NADHs.0530

Before we continue on to the citric acid cycle, we are going to stop and talk a little bit about the structure of the mitochondria.0537

The mitochondria is the site of energy production in the cell, and recall from the lecture on cell structure that it is surrounded by a double membrane.0547

According to the endosymbiosis theory, mitochondria were formed when early anaerobic prokaryotes engulfed early aerobic prokaryotes.0565

This double membrane is the result of that symbiotic relationship, where one bacteria engulfed the other one.0581

Mitochondria also have their own circular DNA, which is further evidence to support this theory of endosymbiosis.0589

Right now, what we are going to focus on, though, is structure.0598

Recall that glycolysis takes place in the cytoplasm, then, the pyruvate molecules are transported into the mitochondria, and this space in the center is called the mitochondrial matrix.0606

The citric acid cycle also takes place in the matrix.0621

Conversion of pyruvate to acetyl-CoA takes place here and then, the citric acid cycle or Krebs cycle as it is sometimes known.0625

There are two membranes. One of these membranes is the outer membrane.0645

The second membrane is called the inner membrane, and this space between the two membranes is known as the intermembrane space.0654

Later steps of aerobic respiration involve this intermembrane space and the inner membrane, where certain enzymes are located, protein complexes are located.0671

And we will later learn that there is a proton gradient formed between the intermembrane space and the mitochondrial matrix.0685

There is a differential in hydrogen ion concentration created there.0692

We will revisit this later, but for right now, you just need to know the basic structure of the mitochondria, there is a matrix.0695

There is an outer membrane, an inner membrane and an intermembrane space.0704

These infoldings increase the surface area of the mitochondrial intermembrane, and they are called cristae.0708

There are many proteins embedded in the mitochondrial inner membrane that are important for aerobic respiration, and that is why there needs to be a large surface area to this membrane.0719

We went through glycolysis. We went through the conversion of pyruvate to acetyl-CoA, and now, still in the mitochondrial matrix, the citric acid cycle will take place.0733

You will frequently also hear this referred to as the Krebs cycle, which is an older name, but it is often still called that. These are the same thing.0744

Again, you do not need to memorize each structure.0754

The same with glycolysis, you need to understand where this occurs - this is the third step of aerobic respiration - and this occurs in the mitochondrial matrix.0757

You need to know where it occurs, and you need to understand the process; but not every structure is even written in here because you do not need to memorize those.0767

We started out with a glucose molecule, and that was broken down to two pyruvate molecules.0778

And these are both going to enter the citric acid cycle, but for simplicity, we are just going to focus on one; but remember this is occurring with the second pyruvate molecule as well.0787

The pyruvate molecule, first, was converted to acetyl-CoA, so what you are going to end up with is acetyl-CoA.0804

That is what is actually entering the citric acid cycle- not pyruvate.0813

Both of these acetyl-CoAs are going to enter the citric acid cycle.0817

And what this is, is a series of reactions, and many of these are redox reactions; and eventually, the pyruvate is going to be completely broken down, and CO₂s will be released.0823

Through redox reaction, the energy from the breakdown of this molecule cleaving these bonds is going to be used to reduce NAD to NADH and FAD to FADH₂.0837

And energy is going to be stored in that form which then, can be harnessed in a later step until, at last, ATP is produced.0851

Think of this as a way to store the energy in another form, a way to create NADH and FADH₂, which is going to go on into the electron transport chain.0859

Recall that NAD is reduced to NADH, and here, we did not talk about this much before but like NAD, this is an electron carrier; and its reduced form is actually FADH₂.0872

Starting with the first step, oxaloacetate is a 4-carbon molecule, so we see four carbons.0892

And here, the acetyl-CoA is a 2-carbon molecule, and it enters the citric acid cycle and combines with oxaloacetate; and water is needed to form citric acid.0901

This is the first step of the citric acid cycle, formation of citric acid, hence the name.0915

Another good thing to focus on as you are studying for the exam is carbons.0924

I have emphasized that- counting the carbons.0929

We start out with pyruvate, which is a 3-carbon molecule.0931

One carbon was lost as CO₂, and remember that CO₂ is what we exhale out.0935

In aerobic respiration, oxygen is consumed. CO₂ is generated, and we will see that in different steps.0941

This is three carbons. One is lost.0951

We are down to two carbons. It is combining with a 4-carbon molecule to form 1, 2, 3, 4, 5, 6-carbon molecule citric acid.0955

In the next step, citric acid is converted to its isomer, isocitrate, so that is a pretty straightforward step.0966

Now, we have a redox reaction, which we are going to see several of these in the citric acid cycle.0975

The conversion of isocitrate to alpha-ketoglutarate involves oxidation.0982

Isocitrate is oxidized to form alpha-ketoglutarate.0988

In the process, NAD⁺ is reduced to NADH, so here, we have the formation of NADH.0995

A CO₂ is also released.1004

In the fourth step, alpha-ketoglutarate is converted to succinyl-CoA. We already talked about this CoA group before.1010

CoA is added to alpha-ketoglutarate, and this bond is a high energy bond just as this bond here in acetyl-CoA is a high energy bond.1018

CoA is added. Succinyl-CoA is formed, and NAD is reduced to NADH, so a second redox reaction.1033

Now, we get some energy production.1049

This high energy bond is cleaved, and that is used to form GTP.1051

We have not talked about GTP yet, but it is very similar to ATP and its function in the cell.1057

Remember that ATP is the energy currency of the cell, so as GTP.1063

GDP plus phosphate forms GTP.1068

GTP can, then, be used to form ATP, so GTP can transfer its phosphate group to ADP to form ATP.1083

That is why you can really just look at this as an ATP equivalent.1105

Often, this GTP is produced, and then, it just goes on to transfer the phosphate group to an ADP, so this is an ATP equivalent.1109

When we calculate the energy formed from aerobic respiration, we will often just count this as an ATP since it is equivalent, and this, again, is substrate level phosphorylation.1119

Substrate level phosphorylation is a concept that we discussed in glycolysis.1131

Frequently, we would talk about ADP combining with inorganic phosphate to form ATP, and you could talk about GDP that same way.1142

However, remember that another source of phosphate can be a phosphate group that is on an organic compound.1154

When the source of the phosphate group is an organic compound, that is substrate level phosphorylation, and that is what is happening here.1162

Alright, we are up to step 5.1170

The results of this substrate level phosphorylation, end of step 5, is the production of succinate from succinyl-CoA.1178

Step 6 is another redox reaction. Succinate is oxidized to fumarate, and in the process, the electron carrier FAD is reduced to form FADH₂.1187

FAD accepts two protons and two electrons to form FADH₂.1200

Remember that NAD actually only accepts two electrons and one proton that neutralize its charge, so what you end up with is NADH + H⁺.1205

Here, we have FAD being reduced to FADH₂ and the production of fumarate.1218

Next, water is added to fumarate to form malate- step 7.1224

Finally, another redox reaction in which malate is oxidized to form oxaloacetate and NAD is reduced to form NADH plus a hydrogen ion.1232

We are back to regenerating oxaloacetate, and the next acetyl-CoA can come in; and the Krebs cycle will continue on again.1245

Let's look at what happened.1257

Acetyl-CoA entered the cycle, and we lost two CO₂s.1259

We start about with pyruvate, which has 3-carbon molecules.1267

One was lost here. Two was lost here, and three was lost here, so pyruvate has been completely broken down.1270

We started out with this 4-carbon molecule. Oxaloacetate added a 2-carbon molecule acetyl-CoA.1280

Two carbons were lost leaving us with a 4-carbon molecule again, so the cycle can continue.1288

You should note that the carbons that enter the cycle in the form of acetyl-CoA are not literally the same carbons that are released.1295

These two carbons enter, and they are incorporated into citric acid and isocitrate; and a carbon comes off, but it is not like this exact carbon came in, and then, it is the one that came out.1304

What is going to happen is acetyl-CoA, these carbons are incorporated.1316

And then, in a later turn of the cycle, those will leave, but when you are accounting for the carbons, you can look at it as two came in, two went out.1320

Alright, what else happened here?1330

Let's look at per glucose molecule.1338

Per glucose molecule broken down on this cycle is going to occur twice, one for each acetyl-CoA that is produced.1340

Per glucose molecule, what has been created is going to be two GTPs, one here, then, the second acetyl-CoA enters another one so two GTPs.1350

In addition, NADH was produced. How many?1362

Well, for each acetyl-CoA, one NADH, two NADH, three NADHs, and this is going to occur twice, so that is six NADHs.1367

We also had the FAD reduced to FADH₂. That is going to happen twice, so we end up with two FADH₂.1380

So, to sum up of the energy production and the NADH and FADH₂ production, because eventually1394

this NADH and FADH₂ are going to be used in oxidative phosphorylation to create ATP, in glycolysis, we got two NADHs and two ATPs.1401

These were via substrate level phosphorylation.1414

Acetyl-CoA production, which occurred in the mitochondrial matrix, so this is cytoplasm in the matrix of the mitochondria,1417

acetyl-CoA production resulted in two NADHs and no ATPs, the citric acid cycle, also in the matrix.1428

Six NADHs produced per, this is per glucose molecule metabolized.1436

Six NADHs, two FADH₂s were produced, and the equivalent of two ATPs by substrate level phosphorylation.1444

The total we have: four ATP equivalence, ten NADHs and two FADH₂s, and this is all going to go into the electron transport chain.1453

That is the next step of aerobic respiration.1465

Aerobic respiration consists of glycolysis, conversion of pyruvate to acetyl-CoA, the citric acid cycle and finally, oxidative phosphorylation.1468

Oxidative phosphorylation has two parts: the electron transport chain/ETC and chemiosmosis.1479

The end result of this is using the energy harnessed or is to harness the energy from NADH and FADH₂ to release that energy to form ATP.1498

We will talk about how that happens in chemiosmosis right now focusing on the first part, which is the electron transport chain.1512

What is the electron transport chain?1521

Well, it is a group of protein complexes that are located in the mitochondrial inner membrane.1523

We talked about the anatomy of a mitochondria before, and these protein complexes are located there.1529

These four complexes - 1, 2, 3, 4 - are the electron transport chain.1538

Right here, this is ATP synthase, an enzyme also embedded in the mitochondrial inner membrane, and it becomes important in chemiosmosis.1545

It is actually the enzyme that catalyze the reaction ADP plus phosphate to form ATP.1556

Now, you see why all those infoldings were necessary, the cristae, because the mitochondrial inner membrane has to have1562

large surface area because it is embedded with thousands of these enzymes and protein complexes.1569

These are multiprotein complexes, and what they contain are electron carriers.1577

What happens is the NADH and FADH₂, created in earlier steps of aerobic respiration, enter this chain, and they pass their electrons to the carrier.1584

That carrier becomes reduced, and the NADH here is oxidized back to NADH⁺.1601

NADH enters, transfers electrons to this first carrier and becomes reduced - excuse me - becomes oxidized. It becomes oxidized.1608

In the process, that carrier is going to be reduced.1619

This carrier will, then, transfer those electrons to the next carrier.1626

That carrier will be reduced. This one will be oxidized back and so on.1632

Looking at these carriers, there are various types of carrier molecules.1639

You do not have to memorize them, but you should be familiar with them. For example, one type that is common is cytochromes.1642

This is actually the most common type of carrier molecules in the electron transport chain.1649

These contain heme, and the heme molecule is able to accept and release electrons.1654

Hemoglobin contains a different type of heme and hemoglobins found in red blood cells.1664

The heme is also a carrier group, but red blood cells carry oxygen, so their job is to carry oxygen.1671

The heme group in cytochromes, their job is to accept and then, release electrons, so it is an electron carrier.1677

Another one, another example, actually the first electron carrier here, is FMN, and this is a flavoprotein. The full name is flavin mononucleotide.1687

Cytochromes, flavoproteins and various other electron carriers are used.1706

One important thing to note is that each electron carrier is more electronegative than the previous one.1714

And this is very important because the electrons are passed from NADH to the first group of carriers, second, third, fourth, and then, finally, the final electron carrier is oxygen.1719

Each one of these is more electronegative than the next. Therefore, there is a decrease in free energy along the electron transport chain.1739

As electrons are passed along the electron transport chain, they are going towards a more electronegative atom, which is what they want to do, so the free energy is dropping.1759

That free energy is harnessed to make ATP.1769

Finally, the energy that was contained in glucose in the beginning and was passed along1775

and was stored in NADH and FADH₂, it is finally being released now and can be used to phosphorylate ADP.1781

These electrons are passed along this electron transport chain via a series of redox reaction.1791

Notice that FADH₂ enters the cycle, the electron transport chain at this second complex.1801

So, here is complex 1, 2, 3 and 4.1810

FADH₂ skips the first complex. Because of that, less energy is released.1814

The electrons are passed a fewer times. Less energy is released.1822

Therefore, for each NADH that goes to the electron transport chain, approximately three ATPs are produced.1826

There is enough energy released to produce three ATPs.1835

Since FADH₂ enters up the second complex, only approximately two ATPs are produced.1839

Now, let’s look at the final electron acceptor, oxygen.1847

Oxygen is a very electronegative atom, so electrons are strongly attracted to it.1851

Here is the aerobic in aerobic respiration.1858

This is why oxygen is necessary for aerobic respiration.1862

The electrons are pulled towards that very electronegative oxygen atom, and in addition to oxygen, two hydrogen ions are picked up, and along with these two electrons, use to form water.1866

This is shown as, you will frequently see oxygen shown as 1/2 O₂ because oxygen does not usually float around by itself as O. It is usually O₂.1885

Half of that would be one oxygen molecule.1894

In water/H₂O, we have one oxygen plus two protons plus two electrons to make up the two hydrogens.1897

The two protons come from these two hydrogen ions that are picked up.1904

And the two electrons come from the electrons that were originally in the NADH and then, passed along the electron transport chain, so you will end up with water.1909

The energy released form this is going to be used - we will talk about in a second - to phosphorylate ADP to form ATP, hence the name oxidative phosphorylation.1921

We see the oxidation reduction reactions and then, phosphorylating ADP.1931

OK, bottom line is that the electrons from NADH and FADH₂ enter the electron transport chain.1938

They are passed along carrier molecules through a series of redox reactions.1946

They are passed along to more electronegative atoms, which releases energy, and that energy is harnessed to form ATP.1951

Chemiosmosis is the second half of oxidative phosphorylation.1967

You see on here these arrows, and it is showing hydrogen ions, and it is showing them being pumped; and here is the mitochondrial matrix, and here, we have the intermembrane space.1972

Let’s look at this here. These electron transport chain complexes and this protein in orange - ATP synthase - these are embedded in the mitochondrial intermembrane.1991

This is the matrix. This green area in here is the intermembrane space.2008

Recall this from earlier, this green area is the intermembrane space.2014

What I focused on in the last slide was what is happening to these electrons, if they are being passed from carrier molecule to carrier molecule to the final electron acceptor.2024

Oxygen and water is formed. However, the electron transport chain is doing something else important.2035

It is forming a proton gradient, and chemiosmosis is a general term that refers to the process of coupling energy from a proton gradient to the synthesis of ATP.2043

Oxidative phosphorylation involves chemiosmosis. Photosynthesis involves chemiosmosis.2054

What happens is the electron transport chain picks up protons from the matrix and pumps them out, pumps in into the intermembrane space.2060

So, what you end up with is a very relatively high concentration of hydrogen ions in the intermembrane space compared with what is going on in the matrix.2074

There is a proton gradient, where there is a lot of protons in this intermembrane space compared with the matrix.2089

That requires energy. Where is this energy coming from?2101

Well, as these electrons are passed to the more electronegative atoms or molecules, energy is released, and that energy is harnessed to form a proton gradient.2105

Here, you see hydrogen ions being pumped to the intermembrane space.2123

The mitochondrial membrane is impermeable to ions.2129

Of course, these hydrogen ions want to go down their concentration gradient and equalize the concentration of protons, but they cannot because this membrane is impermeable to ions.2133

However, they can be transported through proton channels, and ATP synthase has two functions.2146

One is, it catalyzes the formation of ATP from ADP. The second is that it has a proton channel, so proton channel plus synthesizes ATP.2154

By having these two functions together in one enzyme, ATP synthase can couple these two processes.2174

As hydrogen ions flow down their gradient, they are releasing energy.2184

They are up here, where it is a very high concentration of hydrogen ions. They want to go down their concentration gradient.2191

As they do that, they release energy, and that energy is coupled to the formation of ATP.2198

For this reason, this hydrogen ion gradient is called the proton-motive force.2205

That is referring to the energy that is stored in this gradient, and it is utilized to form ATP; and it is coupled.2214

This exergonic process of hydrogen ion that is flowing down their concentration gradient is coupled to the endergonic process of the formation of ATP.2224

Now, this is happening in the mitochondria and in the mitochondrial membrane, but recall that prokaryotes do not have - so, we are talking about eukaryotes - mitochondria.2234

In prokaryotes, this gradient is set up in the plasma membrane.2246

Remember that we started out with a glucose molecule many steps ago.2252

That energy from the bonds in a glucose molecule was transferred into different forms via chemical bonds, via redox reactions and then, finally, into a proton gradient.2258

At last, the energy is used to make ATP.2273

So, really, it was just the transfer of energy from one form to another to create ATP.2278

Remember that for each NADH, the result is going to be approximately three ATP formed.2285

Well, for each FADH₂, there will be approximately two ATP formed.2295

Let's go back and summarize what is going on with the energy.2302

When we last talked about the summary of what was going on with the energy, it was after the citric acid cycle but before oxidative phosphorylation.2309

At that point, glycolysis had produced two NADHs, and here, this ATP, I have broken it down.2317

This is ATP produced by substrate level phosphorylation.2326

OK, that is this first column, two ATPs from glycolysis from substrate level phosphorylation.2334

Then, acetyl-CoA was produced, two NADHs, no ATPs, then, the citric acid cycle produced six NADHs per, this is per glucose molecule and two FADH₂s.2340

There were also two GTPs, ATP equivalence, produced via substrate level phosphorylation.2355

Prior to oxidative phosphorylation, these processes had produced four ATPs.2362

Now, all this stored energy from these NADHs and FADH₂s was released to form ATPs.2368

This second column shows ATPs produced from NADH.2375

This third column shows ATPs produced from FADH₂ going through the electron transport chain.2383

As I mentioned, each NADH that goes through the ETC yields enough energy for about three ATPs. Each FADH₂ yields enough energy for about two ATPs.2394

Let's see what is going on here. The first one is a little bit complicated.2410

I have two NADHs. If each of those produces three ATPs, I would expect six ATPs, but there is only four.2414

And the reason is that the NADH from glycolysis needs to be transported into the mitochondria.2423

Remember that the electron transport chain is in the mitochondria, but the NADH from glycolysis is still sitting out there in the cytoplasm.2440

The transport takes...this uses one ATP per NADH transported.2448

Although these NADHs enter the electron transport chain, and three ATPs come out per NADH, one ATP was used per NADH to get it in the mitochondria in first place.2456

This is really the net production, and this is also the net ATP.2471

This is the net production of ATP that results from NADH because we have to subtract off that energy investment.2476

OK, acetyl-CoA production occurs in the mitochondrial matrix, so the NADH is already sitting there where it needs to be.2484

Two NADH molecules times three yields 6 ATPs.2491

Citric acid cycle occurs in the mitochondrial matrix, so six NADHs times three ATPs. That is eighteen ATPs produced.2497

FADH₂, two of those times two ATPs produces four ATPs.2507

When we add all this up, 4 + 28 + 4, we get 36 molecules of ATP produced.2514

You might hear the number 38. 38 would be, if you were to say OK, these NADHs produced six molecules of ATP.2523

And you did not subtract off the fact that two ATPs were invested to transport this into the matrix.2533

That is why you may hear different numbers.2539

Also, different cell types use different transport molecules for this, and some of those use more ATPs. Some use less, so there is not one exact number.2542

The range maybe 34 - 38. It may be even less in cells that are less sufficient, but common number is 36 compared with only two ATPs produced by glycolysis.2551

One thing to keep in mind is just the flow of energy as you look at the entire process of aerobic respiration.2564

We started out with glucose. Energy was stored in the chemical bonds of glucose.2573

It was, then, transferred to NAD, which became NADH and FADH₂ through electron transfer.2578

The energy was stored there. Then, these electrons were passed along the electron transport chain, where they lost free energy2588

And a proton gradient was created, so the proton-motive force or the process of chemiosmosis.2598

Finally, that energy was used to generate ATP.2605

Ultimately, the energy in the chemical bonds of glucose was used to make ATP, which is the energy currency of the cell.2608

Reviewing with some examples, example one: in anaerobic respiration glycolysis is followed by fermentation.2618

Why isn't fermentation necessary when a cell is undergoing aerobic respiration?2627

Remember the purpose of fermentation? Well, fermentation regenerates NAD.2633

NADH enters fermentation and is oxidized back to NAD⁺, which is used for glycolysis.2647

Why doesn't that happen here? Well, it does not need to.2659

Remember in the electron transport chain, NADH and FADH₂ enter the electron transport chain, and they pass their electrons to carrier molecules.2662

In the process of doing that, they are oxidized, so they are oxidized back to NAD⁺ and FAD.2675

Fermentation is not necessary for aerobic respiration because both of these are oxidized back to their original forms just by transferring their electrons to the ETC.2683

NAD⁺ and FAD are automatically regenerated and can be reused for glycolysis in the citric acid cycle.2699

Label the location for each of the following processes and structures.2710

OK, What we have here is a cell, and the mitochondria shown is very large and out of proportion because that is where most of these processes happen.2715

The first process is glycolysis, and that occurs in the cytoplasm, so I am going to write that here.2724

Glycolysis forms pyruvate from glucose, and those pyruvate molecules are transported into the mitochondria2738

And here, we have conversion of pyruvate to acetyl-CoA. That occurs in the mitochondrial matrix.2748

Remember we get glucose. It forms two pyruvates, and those are transported into the matrix.2754

Here, we get, I will write acetyl-CoA for acetyl-CoA formation.2761

Next is the citric acid cycle. This also takes place in the matrix, so I will put this here.2767

Citric acid cycle, also in the matrix.2776

Where is the electron transport chain? The electron transport chain is located in the mitochondrial inner membrane.2781

Here is the outer membrane and the inner membrane.2794

There is not a lot of room, so I will go ahead and put an arrow indicating the inner membrane.2796

ATP synthase, the enzyme that contains a proton channel and uses that energy to form ATP to phosphorylate ADP to form ATP.2801

This is also located in the mitochondrial inner membrane, again, right here.2811

And remember that a proton gradient is formed between the inner membrane and the matrix.2820

Example three: why are only approximately two ATP produced per FADH₂ compared with three ATP per NADH that go through the electron transport chain?2829

Use the figure below to explain.2842

Here, we have the electron transport chain, and we see that NADH starts right at the beginning at the first complex, so NADH enters at complex 1.2844

Electrons from NADH are transferred to this first electron carrier and then, on through carrier 2, cytochrome C and then, carrier 4 and then, to the final electron acceptor of oxygen.2862

Contrast that with FADH₂.2879

Here, we see FADH₂ enters at complex 2. Therefore, not as much energy is released.2883

The electrons are not transferred down through as many carriers.2894

Here, it is skipping that first complex, and therefore, less electron transfer.2899

Therefore, only about two ATPs are produced per FADH₂ compared with three for NADH.2910

The mitochondrial inner membrane is not permeable to hydrogen ions.2920

What would happen to the cell if it were to become permeable to hydrogen ions and why?2925

Well, recall that the electron transport chain uses energy from the transfer of those electrons to more2932

electronegative molecules that uses that energy to form a proton gradient, the proton-motive force.2942

What the electron transport chain does is it picks up hydrogen ions from the matrix and pumps them out here into this intermembrane space.2948

So, we end up with much higher concentration of hydrogen ions here compared with here.2960

ATP synthase is embedded in the mitochondrial inner membrane.2968

As these protons go down their proton gradient through ATP synthase, that energy is used to synthesize, to phosphorylate ADP and form ATP.2972

The only way that these hydrogen ions can get to the matrix is to go through a channel like ATP synthase because this membrane is impermeable to hydrogen ions.2987

Now, let's we suddenly made it permeable. Now, hydrogen ions can just go through anywhere they want.2996

They will just pass through anywhere, and this gradient will soon be gone.3001

They are not forced to go through ATP synthase. The gradient will dissipate, and that energy has just been lost and not harnessed.3006

The explanation is that the hydrogen ions could bypass the proton channel in ATP synthase, and energy from the proton-motive force would or could not be harnessed to make ATP.3014

And because of that, the cell would not have enough energy, and that could eventually be fatal to the cell.3051

That concludes this lecture on aerobic respiration here at Educator.com.3060

Thanks for visiting.3065