Glycolysis: Study with Video Lessons, Practice Problems & Examples

Glycolysis consists of two phases: Phase A, which consumes energy, and Phase B, which produces energy. Phase A includes five reactions converting glucose into two Glyceraldehyde 3 Phosphate (G3P) molecules, utilizing ATP and enzymes like hexokinase and phosphofructokinase. Phase B transforms G3P into pyruvate, generating 2 NADH and 4 ATP through reactions involving enzymes such as glyceraldehyde 3 phosphate dehydrogenase and pyruvate kinase. This metabolic pathway is crucial for cellular respiration and energy production.

Glycolysis Concept 1

Hey, everyone. So remember, glycolysis is a sequence of 10 biochemical reactions split between phases a and b. Phase a is the energy-consuming phase. Phase b is the energy-producing phase. Now, for phase a, consisting of its first five reactions, it results in the conversion of 1 glucose into 2 Glyceraldehyde 3-Phosphate molecules, which we abbreviate as G3P. Reactions 1-3 are irreversible and each consumes 1 ATP. If we take a look here, we have glucose. We have to consume 1 ATP in order to add a phosphate group to our glucose, transforming it into glucose 6-phosphate. This can then isomerize into fructose 6-phosphate. This reaction 2 is reversible. Next, we need to add another phosphate group, so that means another ATP has to be consumed, doing this as our 2nd inorganic phosphate, transforming fructose 6-phosphate into fructose 1,6-bisphosphate. Bis meaning 2 phosphate groups. From here, we go into reaction 4, which can split into 2 different ways. We could create dihydroxyacetone phosphate or here we can make 2 glyceraldehydes 3-phosphates. And here, reaction 5, they can go between each other. It's a reversible reaction. Now, this encompasses our phase a of glycolysis. What's in blue would be our phase b, which we'll talk about later on. For now, let's just continue to talk about these different reactions in phase a of glycolysis.

Glycolysis Concept 2

In this video, we're going to take a look at reactions 1 and 2 of phase A of glycolysis. Now, reaction 1 is phosphorylation. Here the enzyme Hexokinase catalyzes the phosphorylation of glucose. It uses ATP as a source of energy and phosphate. Remember that a kinase is an enzyme that transfers a phosphate group from ATP to some specified molecule. We have glucose; we're using our hexokinase. It's going to transfer a phosphate group from ATP, transforming it into ADP, and then the hydrogen that was part of glucose here is transformed into our phosphate group in the form of PO₃^2-. So in that way, we go from glucose to glucose 6-phosphate.

Now, here, reaction 2 is isomerization. The enzyme phosphoglucoisomerase isomerizes glucose 6-phosphate to fructose 6-phosphate. They are just isomers of each other. So here, if we take a look, the number of carbons, hydrogens, oxygens, and phosphates remains the same. So we just have the changing of glucose 6-phosphate to fructose 6-phosphate through an isomerase. Remember, an isomerase is just a way of going between these different isomer forms. The same molecular formula overall, it's just different connections. Right? So that represents reactions 1 and 2 of Phase A of Glycolysis.

Glycolysis Concept 3

In this video, we're going to take a look at reaction 3, a phase of glycolysis. Now, like reaction 1, reaction 3 also consumes an ATP molecule. That's because reaction 3 is also a phosphorylation. Here though, we're dealing with our substrate in the form of fructose 6-phosphate initially instead of glucose. And because of that, our enzyme is going to be slightly different. Here for reaction 3, the enzyme we're using is phosphofructokinase. So it's still a kinase, which means it's still being utilized to transfer a phosphate group from ATP to our fructose 6-phosphate molecule. So it's going to catalyze the phosphorylation of fructose 6-phosphate. Again, because it's a kinase, it uses ATP as a source of energy and a phosphate group. So here we have our fructose 6-phosphate, and it's this hydrogen in question that's going to be replaced by a phosphate group. Again, we're utilizing our phosphofructokinase to do it, and now we're going to have our PO₃²⁻ here as well. And in that way, we've just created fructose 1,6-bisphosphate for reaction 3, a phase of glycolysis.

Glycolysis Example 1

Which of the following statements is incorrect about ATP in glycolysis reactions 1 and 3? Alright. ATP provides the inorganic phosphate for phosphorylation reactions. This is true. Remember, both of these steps are irreversible, both of them consume an ATP molecule so that we can do phosphorylation. Hydrolysis of the high-energy PO bond in ATP provides energy to carry out phosphorylation. Yes, that is true. Remember, hydrolysis is one of those types of biochemical reactions that helps to produce energy. Here, we're using hydrolysis to cut that high-energy bond in order to provide the energy necessary for phosphorylation. Energy produced in reactions 1 and 3 is used to synthesize ATP from ADP. No. This is incorrect. Remember, we're using an ATP molecule in reactions 1 and 3. We are going to use a kinase to transfer a phosphate group from these ATP to their specified molecules. In turn, that changes ATP into ADP. So option C is incorrect. Now here, kinases in reactions 1 and 3 use ATP as the coenzyme. That is true. They are using ATP as a coenzyme. Remember, kinases again are just enzymes they're utilizing to transfer a phosphate group from the ATP to our specified molecule. In this case, the ATP is needed for these irreversible reactions and they serve as coenzymes. So here, this is true. The only statement that's provided here that’s incorrect would be option C.

Glycolysis Concept 4

Now in reaction 4 of phasic glycolysis, we say the enzyme aldolase cleaves the middle carbon-carbon bond of the fructose ring. And we're going to say this results in the formation of 2 triose phosphates. So here we have our fructose 1,6-bisphosphate. We utilize aldolase as our enzyme. It's going to cut here, so it cuts right in the middle. That's going to create our 2 triose phosphates. So here we're making dihydroxyacetone phosphate, and then here we're making glyceraldehyde-3-phosphate. So these would be the 2 molecules that we're creating from our fructose 1,6-bisphosphate.

Glycolysis Concept 5

Reaction 5 of Phase A of glycolysis represents the final reaction for Phase A of glycolysis. In it, it represents an isomerization reaction. So here, we're going to say dihydroxyacetone phosphate, which we abbreviate as DHAP, is isomerized to Glyceraldehyde 3-phosphate, which we abbreviate as G3P. Now, this is catalyzed by the enzyme triosephosphate isomerase, and because we're dealing with isomerization, it would be an isomerase. So, if we take a look here, we have dihydroxyacetone phosphate that isomerizes into Glyceraldehyde 3-phosphate. Since it's a reversible reaction, it can go the opposite way as well, where we're going from G3P to DHAP.

Now, here, dihydroxyacetone phosphate contains an acetone, which is a ketone, meaning that this carbon in the middle would have to be a ketone carbon. And then here, Glyceraldehyde, as the name suggests, contains an aldehyde. Remember, an aldehyde is a carbonyl group, so C double bond O, connected to an H. This explains how we are going between these two different molecules because it's an isomerization reaction; we use an isomerase to do it. Remember, this represents the final reaction, Reaction 5 of Phase A of our glycolysis.

Glycolysis Example 2

Which one of the following statements is incorrect about Glycolysis Phase A? Alright. So phosphorylation reactions 1 and 3 are catalyzed by kinases. This is true. Kinases are the enzymes that we use for transferring a phosphate group from ATP to our specified molecule.

Bond cleavage in reaction 4 produces ATP. No. Remember, phase A deals with the consumption of energy, not the production of energy. And then here, if we're talking about bond cleavage, we're really talking about hydrolysis. Hydrolysis is what can lead to the production of energy. But that's not what's happening here. So this is incorrect.

Monocleben in reaction 4 produces 2 Triose Phosphates. This is true. We create Dihydroxyacetone Phosphate and Glyceraldehyde 3 Phosphate, which can isomerize between each other.

Isomerization of DHAP, which is our Dihydroxyacetone Phosphate, and G3P, which is our Glyceraldehyde 3 Phosphate, is catalyzed by Triose Phosphate Isomerase. Now, here, this is an isomerization reaction, so the enzyme of choice would have to be an isomerase. This is a true statement. So out of our choices, the only one that is incorrect would have to be option B.

Classify each one of the following reactions as phosphorylation (P), isomerization (I), or neither (N).

a) ____ Conversion of glucose into glucose-6-phosphate.

b) ____ Conversion of glucose-6-phosphate into fructose-6-phosphate.

c) ____ Conversion of DHAP into G3P.

d) ____ Cleavage of fructose-1,6-bisphosphate into DHAP and G3P.

Here it says, classify each of the following reactions as phosphorylation with a P, isomerization with an I, or neither with N. So, conversion of glucose into glucose-6-phosphate. Remember, this represents reaction 1 of Phase A of glycolysis. It's accomplished by hexokinase and it is a phosphorylation. Next, conversion of glucose-6-phosphate into fructose-6-phosphate. Now, both of these are monosaccharides with 6 carbons. All they are are isomers of one another, glucose and fructose. So, to go between these isomers, we'd have to utilize an isomerase and this would be an example of an isomerization reaction. So this would be I. Conversion of DHAP, which is dihydroxyacetone phosphate, into G3P, which is glyceraldehyde 3-phosphate. Now remember, going between these two things represents an isomerization because they're isomers of one another. So here, this would also be I. And then finally, cleavage of fructose 1,6-bisphosphate into DHAP and G3P. Here, this is an example of bond cleavage, which is not phosphorylation or an isomerization reaction. So, it would be neither. So this would be N. So we'd say we have PIIN as a designation for each of the statements within this question.

Which one of the following compound pairs is produced by cleavage of fructose-1,6-bisphosphate?

• A. Dihydroxyacetone phosphate and glyceraldehyde-3-phosphate
• B. Glyceraldehyde-3-phosphate and glyceraldehyde-2-phosphate
• C. Glyceraldehyde-3-phosphate and CO₂
• D. Dihydroxyacetone phosphate and 3-phosphoglycerate

Glycolysis Concept 6

We now take a look at phase b of glycolysis. Phase b of glycolysis consists of its last five reactions. It converts 2 G3P molecules (Glyceraldehyde 3-Phosphates) into 2 pyruvates, extracting energy in the process. Here, it produces 2 NADH and 4 ATP molecules. We also say that reaction 10, the last reaction of phase b, is irreversible. So we've gone through phase a, all here in red, which leads us into the blue portion which is phase b. Here, we have our inorganic phosphate being incorporated to create 1,3-bisphosphoglycerate, and we have the production of NADH in a process. We created a high-energy molecule in the form of NADH.

We then have a reversible reaction 7, where our 1,3-bisphosphoglycerate changes into 3-phosphoglycerate. We've lost a phosphate group so that we can give it to ADP to convert it into ATP. So, we've just created ATP. In reaction 8, we've removed—well, we've switched the position of our phosphate group—to transform 3-phosphoglycerate into 2-phosphoglycerate. Then, due to the loss of water in reaction 9, we have the creation of phosphoenolpyruvate. Now we'll go in-depth in terms of each of these. We'll learn steps to help us memorize the different products and at which steps they are produced.

I know it's a lot, so don't feel overwhelmed because it is indeed a lot of reactions, but we're going to learn summaries on how to basically remember this massive amount of information. And then, finally, here, we have the removal of the last inorganic phosphate and the creation of ATP again to give us, at the end, pyruvate. Remember, we have 2 G3P molecules so this happens twice. So here we're making 1 NADH and then 2 ATP, and it does it again to give us another NADH and 2 more ATP. So that's how we come up with a total of 2 NADHs and 4 ATP molecules from phase b of glycolysis. Right. This just gives us an overall view of what's happening within this phase of glycolysis, and we're about to go more in-depth with each one of these reactions.

Glycolysis Concept 7

So here we take a look at reaction 6 of glycolysis. It happens twice because we have 2 G3P molecules that can be oxidized. Here, they undergo oxidation to produce 1,3-bisphosphoglycerate, which we are going to abbreviate as 1,3-BPG. Now, here it's catalyzed by the enzyme Glyceraldehyde 3-phosphate Dehydrogenase. And remember, our class of enzymes that we use for oxidation reactions are dehydrogenases. In this reaction, because it's an oxidation, NAD⁺ is going to be reduced to NADH. So here we have our glyceraldehyde 3-phosphate, and we're going to say, with the incorporation of an inorganic phosphate and the reduction of NAD⁺ to NADH, due to the utilization of our enzyme Glyceraldehyde 3-Phosphate Dehydrogenase. We create 1,3-bisphosphoglycerate. So here, what happens is this H is transformed into PO₄³⁻. And what we need to see here is that this 1,3-bisphosphoglycerate is basically going to be our molecule that eventually we want to convert into pyruvate. So, this is what we need to get to at the end, this pyruvate at the end of our glycolysis. Right. So just remember, step 6 or reaction 6 happens twice because we have 2 G3P molecules that need to be oxidized. Oxidation, our class of enzymes that we utilize are dehydrogenases. The name of the enzyme is just the substrate name followed by dehydrogenase.

Glycolysis Concept 8

Now, reaction 7 deals with a phosphate transfer. Again, it happens twice. Here we have 1,3-bisphosphoglycerate, which is 1,3-BPG. It produces 3-phosphoglycerate (3PG) by losing an inorganic phosphate group. Now, this is catalyzed by the enzyme phosphoglycerate kinase. Remember, a kinase deals with the transferring of a phosphate group. In this case, though, it transfers a phosphate group to ADP, transforming it into ATP. So here, we've just created some energy. So as we can see, 1,3-bisphosphoglycerate, it has 2 of our phosphate groups. Through the utilization of our kinase, we lose one of these inorganic phosphates. So now we just have a negative oxygen here, that phosphate group that was lost again goes to ADP transforming it into ATP. So this is what reaction 7 of phase B of glycolysis represents.

Glycolysis Concept 9

Reaction 8 is an isomerization reaction that occurs twice. We have 3-phosphoglycerates, so we actually have two of them, which are 3 PG. Hence, it happens twice. It undergoes isomerization to yield 2-phosphoglycerate, which is 2 PG. This is catalyzed by the enzyme Phosphoglycerate Mutase. Mutase is simply a class of Isomerase. Remember, isomers have the same molecular formula but different connections. The mutase assists in the movement of our phosphate group, our inorganic phosphate group from position 3, carbon 3, to position 2. Here, this group is moved to here, and we're going to say that we'll produce our 2-phosphoglycerate, so the OH group comes here. Hence, we have the movement of these functional groups within our 3-phosphoglycerate to create 2-phosphoglycerate. The inorganic phosphate was connected to the oxygen on position 3, carbon 3, but now it's being moved over to this position, which is part of carbon 2. As you can see, this is a reversible reaction. So although we are moving this around, we can go backward in terms of this reaction if necessary. Again, a mutase is just a class of isomerase.

Glycolysis Example 3

Here it says, which of the following glycolysis reactions will produce an ATP molecule? Here we're going from 3-phosphoglycerate to 2-phosphoglycerate. Here, this represents an isomerization reaction where we utilize a class of isomerases called mutases. We're just moving the inorganic phosphate from one position on 3-phosphoglycerate to another position to transform it into 2-phosphoglycerate. This does not produce ATP. Glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate. Now, this represents an oxidation reaction. We have the incorporation of an inorganic phosphate group, and we are producing NADH because we are reducing NAD⁺. Although we're making a high-energy molecule in the form of NADH, we do not make ATP. Glucose to Glucose 6-phosphate. Now, this reaction is part of phase a of glycolysis and it is an energy-consuming process. We need to basically invest ATP in order to add that phosphate group to glucose. So, this is going to do the opposite. It's not going to produce ATP, it's going to consume it. So, by the process of elimination, it's d. Here we have 1,3-bisphosphoglycerate, and we're going to 3-phosphoglycerate. We're going to say that in this reaction, we're utilizing a kinase. And with this kinase, we're going to lose an inorganic phosphate from 1,3-bisphosphoglycerate, and we're going to give it to ADP, transforming it into ATP. In that way, we're making or producing an ATP molecule.

Glycolysis Concept 10

Now, reaction 9 of glycolysis represents a reversible dehydration reaction. Here it happens twice and we have 2-phosphoglycerate, which is 2PG. It undergoes dehydration to produce phosphoenolpyruvate, or PEP. Now, here it's catalyzed by the enzyme Enolase. So here we're going to say that we have our 2-phosphoglycerate, we're going to use our Enolase, and in the process, we're going to lose water. So we lose H₂O. Now, here we lose this hydrogen and this oxygen; carbons still need to maintain their 4 bonds because they're tetravalent, they want to make 4 bonds. So they have no other options but to make a double bond with each other. This transforms our 2-phosphoglycerate to phosphoenolpyruvate. Alright. So, again, reaction 9 of glycolysis represents a reversible dehydration reaction. And we're utilizing the enzyme enolase in order to do this.

Glycolysis Concept 11

Now, reaction 10 represents a phosphate transfer. It also represents the last step in glycolysis and it's also irreversible. Here, Phosphoenolpyruvate (PEP) yields pyruvate by losing its inorganic phosphate group. This process is catalyzed by the enzyme pyruvate kinase. We know it's a kinase because a phosphate group is being transferred. The ADP gains the inorganic phosphate group to produce ATP. In this reaction, we use our Pyruvate Kinase Enzyme, which helps us to transfer the inorganic phosphate to ADP, changing it to ATP. By doing this, we form a pyruvate molecule which has a carbonyl group where the inorganic phosphate group used to be, and the CH₂ group is converted into a methyl group. This pyruvate molecule represents the final molecule within this glycolysis reaction. Thus, we've gone through phase A and phase B for a total of 10 reactions to reach this final molecule.

Glycolysis Example 4

Which of the following enzyme catalyzes the conversion of phosphoenolpyruvate to pyruvate? Now, this represents reaction 10 of glycolysis, which is a phosphate transfer reaction. We know that when we're transferring phosphate groups, that involves our class of enzymes known as kinases. So if we take a look, we have a kinase here and we have a kinase here. This couldn't be the answer to pyruvate dehydrogenase because that deals more with oxidation, not the transferring of a phosphate group. And then pyruvate carboxylate, that's just talking about our pyruvate group with the carboxylate group that's been deprotonated, has an O negative. And remember, we're looking for the enzyme and it doesn't end with the name of an enzyme as well. Now here it's either C or D.

Now phosphoenolpyruvate carboxykinase, this deals with the carboxylation of PEP. But remember, we're not trying to add a carboxylic acid group to PEP, we're trying to lose a phosphate group from PEP in order to change it into Pyruvate. This happens not with option C as an enzyme but with option D. Pyruvate kinase is the enzyme of choice which will lead to the transferring of our phosphate group from PEP to ADP transforming it into ATP. While doing this, PEP is transformed into pyruvate. So option D would be our final answer.

Which enzyme catalyzes the oxidation of glyceraldehyde-3-phosphate in reaction 6 of glycolysis?

• A. glyceraldehyde-3-phosphate dehydrogenase
• B. glyceraldehyde-3-phosphate acyltransferase
• C. glyceraldehyde-3-phosphate oxidase
• D. triosephosphate isomerase

What is the energy output of reaction 7 of glycolysis (1,3-bisphosphoglycerate to 3-phosphoglycerate)?

• A. 2 ATP
• B. 2 NADH
• C. 1 ATP
• D. 1 NADH and 1 FADH₂

What product is formed when phosphoglycerate mutase moves the phosphate group in 3-phosphoglycerate?

• A. Dihydroxyacetone phosphate
• B. 2-Phosphoglyceraldehyde
• C. 1,3-Bisphosphoglycerate
• D. 2-Phosphoglycerate

Here’s what students ask on this topic:

What are the main phases of glycolysis?

Glycolysis consists of two main phases: Phase A and Phase B. Phase A is the energy-consuming phase, where glucose is converted into two Glyceraldehyde 3 Phosphate (G3P) molecules through five reactions. This phase utilizes ATP and enzymes like hexokinase and phosphofructokinase. Phase B is the energy-producing phase, where G3P is transformed into pyruvate, generating 2 NADH and 4 ATP molecules. This phase involves enzymes such as glyceraldehyde 3 phosphate dehydrogenase and pyruvate kinase. Together, these phases are crucial for cellular respiration and energy production.

What enzymes are involved in the first phase of glycolysis?

The first phase of glycolysis, also known as Phase A, involves several key enzymes. Hexokinase catalyzes the phosphorylation of glucose to glucose 6-phosphate. Phosphoglucoisomerase then converts glucose 6-phosphate to fructose 6-phosphate. Phosphofructokinase catalyzes the phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate. Aldolase splits fructose 1,6-bisphosphate into dihydroxyacetone phosphate (DHAP) and Glyceraldehyde 3 Phosphate (G3P). Finally, triosephosphate isomerase converts DHAP to G3P, completing Phase A.

How is ATP produced in glycolysis?

ATP is produced in glycolysis during Phase B, the energy-producing phase. This phase involves the conversion of Glyceraldehyde 3 Phosphate (G3P) into pyruvate. Specifically, ATP is generated in two steps: first, 1,3-bisphosphoglycerate donates a phosphate group to ADP to form ATP, catalyzed by phosphoglycerate kinase. Second, phosphoenolpyruvate (PEP) donates a phosphate group to ADP to form ATP, catalyzed by pyruvate kinase. Since each G3P molecule goes through these steps, and there are two G3P molecules, a total of 4 ATP molecules are produced.

What is the role of NAD+ in glycolysis?

NAD+ plays a crucial role in glycolysis as an electron carrier. During the sixth reaction of glycolysis, Glyceraldehyde 3 Phosphate (G3P) is oxidized to 1,3-bisphosphoglycerate. This reaction is catalyzed by the enzyme glyceraldehyde 3 phosphate dehydrogenase. In this process, NAD+ is reduced to NADH by accepting electrons. This reduction is essential for the continuation of glycolysis, as it helps in the production of ATP and ensures the proper flow of electrons within the cell's metabolic pathways.

What is the final product of glycolysis?

The final product of glycolysis is pyruvate. During the last step of glycolysis, phosphoenolpyruvate (PEP) is converted into pyruvate by the enzyme pyruvate kinase. This reaction also produces ATP from ADP. Since glycolysis starts with one glucose molecule and produces two molecules of Glyceraldehyde 3 Phosphate (G3P), which are then converted into two pyruvate molecules, the overall process results in the formation of two pyruvate molecules per glucose molecule.