Glycolysis: Videos & Practice Problems

Glycolysis is a metabolic pathway that breaks down glucose into pyruvate, producing a net gain of 2 ATP and 2 NADH. It occurs in the cytosol and does not require oxygen. The process involves 10 steps, starting with the phosphorylation of glucose by ATP, followed by key reactions catalyzed by enzymes like phosphofructokinase. Important intermediates include G3P, which leads to ATP production through substrate-level phosphorylation. The overall reaction can be summarized as: $C^6+2\mathrm{ATP}\to 2\mathrm{pyruvate}+2\mathrm{ATP}+2\mathrm{NADH}$.

Glycolysis Overview

Hi. In this video, we're going to be talking about glycolysis. In this first section, we are going to focus just on an overview of glycolysis, what enzymes are involved, and what glycolysis actually is. Then, later, we're going to get into the really more complex steps of glycolysis. Glycolysis is a pathway responsible for breaking down sugar. It occurs in the cytosol of the cell and does not require oxygen. If you're interested in what the formula looks like, here it is:

Glucose + NAD⁺ + ADP + phosphates eventually turns into pyruvate, NADH, and ATP through glycolysis.

The net result from a single glucose molecule via glycolysis is going to be 2 ATP, 2 NADH, and 2 pyruvates. This is really important to know. You will most likely get a question on this on the test or quiz. What are the end results of glycolysis? From one glucose molecule, you get 2 ATP, 2 NADH, and 2 pyruvates. Definitely know that.

In the process of glycolysis, there are 4 main enzymes that run glycolysis. Let's define what exactly these enzymes do, and then I'm going to show you an example of what that actually looks like chemically, so you get an idea of the enzymes and different chemical reactions that are happening in glycolysis. So the first is a dehydrogenase, which is responsible for oxidizing molecules by removing a proton and an electron. A kinase is going to add a phosphate group. A mutase shifts chemical groups from one position to another. These enzymes are regulated by levels of ATP. If there is an abundance of ATP, the rate of glycolysis will decrease because the cell doesn't need to be creating more ATP as it already has a lot of it.

So first, we have dehydrogenase. This is your starting material here, and what you see is it removes a hydrogen. So now you have this new group here. Then, you have a kinase, which adds a phosphate. You can see it moving from an OH to an OP. Then, you have an isomerase that is responsible for moving molecules around. This group here can change into this group here because it moves those molecules and those bonds to a different location. Then, you have a mutase, which is responsible for switching molecules around. In this molecule, you have this group with a P on the end and this group with an H, and then when a mutase comes in, it can change the position of the P and the H.

These are 4 very common enzymes that occur in glycolysis, and we're going to be mentioning them again and again. I just want to make sure you know what these enzymes are and what they do before we get into the nitty-gritty of glycolysis. So with that, let's now turn the page.

Glycolysis Steps

So now we're going to go through the exact steps of glycolysis. If you just flip through this handout, you'll see that there's a lot, and it's a lot of pages, but bear with me because I've organized this in a way that's really simple, and you're not going to need to know everything on every one of these pages. So, glycolysis has 10 steps, and we're going to go through each of those steps, and I'm going to tell you what you need to know about those steps and what you don't. The first step, I have sort of summarized, you know, what this step is about. This step, the very first step is about phosphorylation. What happens is glucose is going to be phosphorylated by ATP, and what you need to know about this step is that the first step is phosphorylation and that glucose is phosphorylated by ATP. This chemical reaction here, you're not going to need to know. The reason I've added these is because it's sometimes so easy just to write down, "Glucose is phosphorylated by ATP." Right? When you get through 10 steps, that can be hard to remember. So, I've added these images just so we can actually see what it looks like. Here, we have glucose, and we have our ATP come in, and after ATP is done, it actually adds this entire phosphate group onto glucose, and that becomes what's known as glucose-6-phosphate. The purpose of this is to actually trap glucose in the cell because of the phosphate; it can't leave. But if it doesn’t have the phosphate, it can leave, and then the cell won't ever be able to break it down. So, the first step is trapping glucose by phosphorylating it with ATP.

Now, the second step, what happens is oxygen is moved around. There is an oxygen on a carbonyl group, and it moves from carbon 1 to carbon 2, and this changes a ketose to an aldo sugar. If you're trying to think, "do I need to know all of this for all 10 steps?" No, you don't. The first step, you really need to know that's an important step. This one is not so much. No one's probably going to ever ask you about step 2 of glycolysis unless they're asking you to write down all 10 steps. So, just wanted to put it here, and what it does is the carbonyl oxygen actually moves. And when it moves, it can change the orientation in this molecule from glucose-6-phosphate to fructose-6-phosphate. Don't need to know these chemical names. Just know that, you know, this is what's happening.

Now, the 3rd step is really important, and the reason it's really important is that it has this enzyme that is going to be crucial in other steps. This is a second phosphorylation step. So, first step phosphorylation, 3rd step phosphorylation. What happens is that there's a hydroxyl group, which, if you remember, hydroxyl is going to be OH on carbon 1, is phosphorylated by ATP. So, the exact same way that step 1 was done. You have this fructose-6-phosphate, your ATP comes in, and now you have these extra phosphate groups. And now, you have 2 phosphate groups. The first one that was added and now the second one that was added. And this is catalyzed by this enzyme known as phosphofructokinase. Kind of a long name, but it's an important enzyme in glycolysis. And it's really important, and we're going to talk about it more in the future. So just kind of remember phosphofructokinase. Reviewing this, just, you know, step 3 is where phosphofructokinase is used. Now, the 4th step is going to be a cleavage step. So the molecule is cleaved somewhere, and this is a really, really important sugar, and you can see this here. There's this ring sugar with 6 carbons on it: 1, 2, 3, 4, 5, and 6. But the cleavage is actually going to turn that into 2, 3-carbon molecules. So what you get is this molecule here called G3P, which is really, really, really, really important. Kind of just circle it. This is a word you want to know. And then it forms this second molecule called DHAP, and that is not necessarily as important. And I will tell you about that in the next step. But they each have 3 carbons: 1, 2, 3, 4, 5, 6. So now we have these 2, 3-carbon molecules. So, this is a very important step. So, remember I said step 1 and step 3 were important because of the phosphorylation? Step 4 is really important. You're going to want to know that.

Now, step 5 is going to be the explanation of why this d...

Which of the following is not a result of glycolysis?

• A. ATP
• B. Pyruvate
• C. NADPH
• D. NADH

Answer: (C)

Phosphofructokinase is an enzyme that is able to do what?

• A. Phosphorylate a molecule using a phosphate from ATP
• B. Breakdown ATP to provide energy to continue glycolysis
• C. Synthesize ATP
• D. Produce NADH

Answer: (A)

What happens during substrate level phosphorylation?

• A. ATP is broken down when it donates a phosphate to a molecule
• B. A phosphate is removed and used to synthesize ATP
• C. A G3P molecule is phosphorylated
• D. NADH is created

Answer: (B)

Here’s what students ask on this topic:

What is glycolysis and where does it occur in the cell?

Glycolysis is a metabolic pathway that breaks down glucose into pyruvate, producing a net gain of 2 ATP and 2 NADH. It occurs in the cytosol of the cell and does not require oxygen. This process involves 10 steps, starting with the phosphorylation of glucose by ATP and involving key enzymes like phosphofructokinase. The overall reaction can be summarized as:

$C_6+2\mathrm{ATP}\to 2\mathrm{pyruvate}+2\mathrm{ATP}+2\mathrm{NADH}$

What are the end products of glycolysis?

The end products of glycolysis from one glucose molecule are 2 ATP, 2 NADH, and 2 pyruvate molecules. This net result is crucial to remember for exams and quizzes. The overall reaction can be summarized as:

$C_6+2\mathrm{ATP}\to 2\mathrm{pyruvate}+2\mathrm{ATP}+2\mathrm{NADH}$

What are the key enzymes involved in glycolysis?

Several key enzymes are involved in glycolysis, including:

• Hexokinase: Catalyzes the phosphorylation of glucose to glucose-6-phosphate.
• Phosphofructokinase: Catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate.
• Aldolase: Cleaves fructose-1,6-bisphosphate into two 3-carbon molecules, G3P and DHAP.
• Glyceraldehyde-3-phosphate dehydrogenase: Catalyzes the oxidation of G3P, producing NADH.
• Pyruvate kinase: Catalyzes the final step, converting phosphoenolpyruvate to pyruvate and generating ATP.

Why is the net gain of ATP in glycolysis only 2 ATP?

The net gain of ATP in glycolysis is only 2 ATP because, although 4 ATP molecules are produced during the process, 2 ATP molecules are consumed in the initial phosphorylation steps. Therefore, the net gain is:

$4-2=2\mathrm{ATP}$

What is the role of NAD+ in glycolysis?

NAD+ plays a crucial role in glycolysis as an electron carrier. During the oxidation of glyceraldehyde-3-phosphate (G3P) in step 6, NAD+ is reduced to NADH. This step is essential for the continuation of glycolysis and the eventual production of ATP. The reaction can be summarized as:

$\mathrm{G3P}+\mathrm{NAD}+\to \mathrm{NADH}+H+$