Glycolysis - Online Tutor, Practice Problems & Exam Prep

Glycolysis is the initial step of cellular respiration, breaking down one glucose molecule (6 carbons) into two pyruvate molecules (3 carbons each). This process occurs in two phases: the energy investment phase, which consumes 2 ATP, and the energy harvest phase, producing 4 ATP and 2 NADH, resulting in a net gain of 2 ATP. Glycolysis takes place in the cytoplasm and does not require oxygen, making it unique among cellular respiration stages. The pyruvates produced are then transported to the mitochondria for further processing.

Glycolysis

In this video, we're going to introduce glycolysis. Glycolysis is the very first step of cellular respiration, and it involves the breakdown of a single glucose molecule into 2 pyruvate molecules. When we take a look at the term glycolysis, you'll find these two roots embedded: 'glyco' and 'lysis.' The root 'glyco' means sugar, hence glucose is indeed a sugar. The root 'lysis' means to break down. When you put these two roots together, 'glyco' and 'lysis,' you get glycolysis, which means breaking down a sugar, specifically glucose. Glucose is broken down into 2 pyruvate molecules. Let's take a look at our image down below here at glycolysis and notice that it starts with a single glucose molecule and it ends with 2 pyruvate molecules, just as we indicated above.

It's very important to note that glucose has a total of 6 carbon atoms. Notice that below we're representing glucose as these 6 black circles, and each of these black circles represents one of the carbon atoms found in glucose. While glucose does have other types of atoms, textbooks and professors tend to focus on the carbon atoms of glucose during discussions of cellular respiration. The glucose here starts with 6 carbon atoms. At the end of glycolysis, notice that the glucose molecule is broken apart into 2 pyruvate molecules, each with 3 carbon atoms. We have one pyruvate molecule, and here we have a second pyruvate molecule.

As we move forward with aerobic cellular respiration, all 6 of the carbon atoms originally found in glucose are going to be converted to carbon dioxide (CO₂). This will specifically happen in the second and third steps of cellular respiration. This is something to look forward to as we move forward in our course, that all of the glucose's carbon atoms will eventually end up as carbon dioxide. That carbon dioxide will ultimately be exhaled or breathed out into the environment.

Glycolysis is the only step of aerobic cellular respiration that occurs outside of the mitochondria in the cell's cytoplasm. This is very important to note. Glycolysis is also the only stage of cellular respiration that does not require oxygen, a key feature to remember about glycolysis. Glycolysis can occur in both the presence and the absence of oxygen, which is not the case with all other stages of cellular respiration. Notice in our image below, we have the process of glycolysis here in this box, which takes a single glucose molecule with 6 carbon atoms and breaks it up into 2 pyruvate molecules, each with 3 carbon atoms. The process of glycolysis is occurring on the outside of the mitochondria.

Glycolysis is the only stage of aerobic cellular respiration that occurs outside of the mitochondria. All other stages occur inside of the mitochondria. Once again, glycolysis is the only stage that does not require oxygen. Below, note that we're mentioning that glycolysis occurs outside the mitochondria, specifically in the cell's cytoplasm. The location of glycolysis is an important feature to remember because glycolysis occurs outside of the mitochondria in the cell's cytoplasm, and all other stages occur inside the mitochondria.

This concludes our brief introduction to glycolysis, and we will continue to talk more about glycolysis as we move forward in our course. I'll see you all in our next video.

Where does the first stage of aerobic cellular respiration take place within a cell?

a) Mitochondrial matrix.

b) Inner mitochondrial membrane.

c) Intermembrane space.

d) Cytoplasm.

• A. Mitochondrial matrix.
• B. Inner mitochondrial membrane.
• C. Intermembrane space.
• D. Cytoplasm.

Phases of Glycolysis

In this video, we're going to introduce the phases of glycolysis. It turns out that glycolysis actually consists of a series of a total of 10 chemical reactions. These 10 chemical reactions that make up glycolysis can actually be grouped into 2 phases. In this video we're not really going to focus on the 10 reactions so much, but we are going to focus on how these 10 reactions are grouped into 2 phases, and we're really going to be focusing on the 2 phases of glycolysis. The first phase of glycolysis is called the energy investment phase, and the second phase of glycolysis is called the energy harvest phase. Notice that the energy investment phase up above is color coordinated to the energy investment phase down below, and the energy harvest phase is color coordinated to the energy harvest phase down below. Notice in this image down below, really what we're showing you is glycolysis. Notice that it's starting with a single glucose molecule that has 6 carbon atoms, and it breaks down that glucose into 2 pyruvate molecules each with 3 carbon atoms. This whole green box represents glycolysis, but once again glycolysis occurs in 2 phases, the energy investment phase and the energy harvest phase. The energy investment phase, as its name implies with the investment, is going to require an investment of energy or an input of energy by using 2 ATP molecules. This is a little bit ironic because if you recall from our previous lesson videos, the whole point of aerobic cellular respiration is to make a lot of ATP. But the very first stage of aerobic cellular respiration, glycolysis, the very first thing that happens is that it does not make ATP, but it uses ATP. However, it turns out that this is just an investment of the ATP because by inputting 2 ATP molecules the cell is able to transition into the second phase of glycolysis which is the energy harvesting phase. As its name implies it's going to harvest energy or produce energy, specifically by forming 2 NADH molecules and 4 ATP molecules. If we take a look at our image down below at glycolysis, notice that the first phase of glycolysis, the energy investment phase, is actually going to use 2 ATPs and that is using some of the cell's energy. However, this is just an energy investment because after the cell utilizes those 2 ATPs, it can transition into the next phase here which is the energy harvest phase. The energy harvest phase notice produces 2 electron carriers, specifically 2 NADH molecules, and it also produces a total of 4 ATP molecules. Now, if we're looking at the net products from 1 single glucose molecule, then what we'll see is that not only are 2 pyruvates formed and 2 NADH are formed, but there's also a net of 2 ATP molecules that are formed. But wait a second, Jason. I thought you said that there were 4 ATP molecules that were made, not 2 ATP molecules. Well, what we need to remember is that 2 ATP molecules were being used up in the energy investment phase. So because 2 ATP molecules were used up, of the 4 ATP molecules that are being made, 2 of them are just making up for the 2 that were invested. And so really there's just a net of 2 ATP molecules made. If we take a look at this part of our image down below right here, we're reminding you that the energy harvesting phase does produce a total of 4 ATPs, as we indicated right here. However, we can't forget that the energy investment phase burns or uses 2 ATPs. So we have to subtract off those 2 ATPs from the total. And so that gives us a total net of 2 ATP molecules produced during glycolysis, 2 net ATPs made. An easy way to remember that there are 2 net ATP made during glycolysis is to remember that the net products from 1 glucose molecule are just double. You get not only 2 pyruvates but also 2 NADH, and also 2 net ATP molecules. The 2 pyruvates that are formed at the end of glycolysis end up getting transported to the mitochondrial matrix, basically getting transported to the mitochondria so that the next step of cellular respiration can take place. Notice that here glycolysis occurs once again on the outside of the mitochondria and it ends up producing 2 NADH, a net of 2 ATPs, and 2 pyruvates. These 2 pyruvates are going to make their way to the mitochondria and into the mitochondrial matrix. This here concludes our introduction to the phases of glycolysis, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

Remembering Phases of Glycolysis

In this video, what we have is a silly memory tool for helping you with remembering the phases of glycolysis. And so when it comes to the phases of glycolysis, you can remember that Gary invests to harvest his profits and travel to Mexico. And so if you can remember that Gary invests to harvest his profits and travel to Mexico, then hopefully that can help you remember these phases of glycolysis. The "G" in Gary here can help you remember that the process of glycolysis starts with a glucose molecule which has 6 carbons, represented by these 6 black circles that you see here.

Then the "I" in "invest" is going to stand for the "I" in the investment phase, which is the very first phase of glycolysis where the cell needs to invest 2 ATP molecules in order to get the process of glycolysis started. Then the "H" in "harvest" is going to stand for the "H" in the harvest phase of glycolysis, which is the second phase of glycolysis where the cell can double the amount of energy that it invested and so it's able to produce 4 ATPs and a total net of 2 ATPs since it had to invest 2 ATPs to begin with.

Then the "P" in "profits" is going to stand for the "P" in pyruvate because this is ultimately what the process of glycolysis ends with, the formation of two 3-carbon pyruvate molecules. So you can see those two 3-carbon pyruvates highlighted here. And then, once these pyruvates have been formed, the process of glycolysis is over. So you can see here that the process of glycolysis is indicated by this green box. And what needs to happen is these 2 pyruvates that were formed, they need to travel to the mitochondria. And so that is where this "Mexico" comes into place. So traveling to Mexico represents traveling, the pyruvates traveling to the mitochondria. And so that is where the second stage of cellular respiration will take place.

Notice here what we have in this image is a mitochondrion that is chilling on the beach in Mexico, sipping on some pina coladas or something like that. And so hopefully, this little memory tool here, Gary Invest to harvest his profits and travel to Mexico, can help you with remembering the phases of glycolysis. That being said, we'll be able to apply some of these concepts moving forward. So I'll see you all in our next video.

Glycolysis Example 1

So here we have an example problem that wants us to fill in the blank here using one of these 5 potential answer options down below. And the example problem says there is an energy investment step needed to get glycolysis started requiring the use of 2 ATPs. And once again we've got these 5 potential answer options down below. And so what we need to recall is that glycolysis has 2 phases. The energy investment phase and the energy harvest phase. The energy investment phase requires an investment of 2 ATPs, and so really that's what this example problem is getting at. In order for the energy investment step to occur, it requires the use of 2 ATPs. And so the correct answer here is going to be answer option a. Now, after the 2 ATPs have been used, then the cell can transition to the 2nd phase of glycolysis, which is the energy harvest phase. And the energy harvest phase produces a total of 4 ATPs and 2 NADH. And of those 4 ATPs, because 2 ATPs got burnt, ultimately it's only a net of 2 ATPs that are made. But once again in terms of the energy that needs to be invested, it's 2 ATPs that need to be invested. So the correct answer to this example is answer option a, and that concludes this example. So I'll see you all in our next video.

Starting with one molecule of glucose, glycolysis results in the net production of which of the following sets of energy-containing products?

a) 2 NAD⁺, 2 pyruvate, and 2 ATP.

b) 2 NADH, 2 pyruvate, and 2 ATP.

c) 4 NADH, 2 pyruvate, and 4 ATP.

d) 6 CO₂, 2 pyruvate, and 2 ATP.

• A. 2 NAD⁺, 2 pyruvate, and 2 ATP.
• B. 2 NADH, 2 pyruvate, and 2 ATP.
• C. 4 NADH, 2 pyruvate, and 4 ATP.
• D. 6 CO₂, 2 pyruvate, and 2 ATP.

Which of the following is a result of glycolysis?

a) A net gain of four ATP per one glucose molecule.

b) Conversion of FAD to FADH₂.

c) Conversion of one glucose molecule to two pyruvate molecules.

d) Conversion of NADH to NAD⁺.

• A. A net gain of four ATP per one glucose molecule.
• B. Conversion of FAD to FADH₂.
• C. Conversion of one glucose molecule to two pyruvate molecules.
• D. Conversion of NADH to NAD⁺.

Here’s what students ask on this topic:

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

Glycolysis is the first step of cellular respiration, involving the breakdown of one glucose molecule (6 carbon atoms) into two pyruvate molecules (3 carbon atoms each). This process occurs in the cytoplasm of the cell and does not require oxygen, making it unique among the stages of cellular respiration. Glycolysis consists of two phases: the energy investment phase, which consumes 2 ATP, and the energy harvest phase, which produces 4 ATP and 2 NADH, resulting in a net gain of 2 ATP. The pyruvates produced are then transported to the mitochondria for further processing.

What are the two phases of glycolysis and what happens in each phase?

Glycolysis consists of two phases: the energy investment phase and the energy harvest phase. In the energy investment phase, the cell uses 2 ATP molecules to initiate the breakdown of glucose. This phase is crucial for preparing the glucose molecule for subsequent reactions. In the energy harvest phase, the cell produces 4 ATP molecules and 2 NADH molecules. Since 2 ATP were used in the investment phase, the net gain is 2 ATP. Additionally, 2 pyruvate molecules are formed, which are then transported to the mitochondria for further processing in cellular respiration.

Why is glycolysis considered unique among the stages of cellular respiration?

Glycolysis is unique among the stages of cellular respiration for several reasons. Firstly, it occurs in the cytoplasm of the cell, whereas the other stages occur inside the mitochondria. Secondly, glycolysis does not require oxygen, allowing it to occur in both aerobic and anaerobic conditions. This is in contrast to the other stages of cellular respiration, which require oxygen. Lastly, glycolysis is the only stage that involves an initial investment of ATP (2 ATP) to produce a net gain of ATP (2 ATP) and NADH (2 NADH).

What are the net products of glycolysis from one glucose molecule?

From one glucose molecule, the net products of glycolysis are 2 pyruvate molecules, 2 NADH molecules, and a net gain of 2 ATP molecules. Initially, 2 ATP molecules are consumed in the energy investment phase, but 4 ATP molecules are produced in the energy harvest phase, resulting in a net gain of 2 ATP. The 2 pyruvate molecules formed are then transported to the mitochondria for further processing in the subsequent stages of cellular respiration.

How does the energy investment phase of glycolysis differ from the energy harvest phase?

The energy investment phase and the energy harvest phase of glycolysis differ in their roles and outcomes. In the energy investment phase, the cell uses 2 ATP molecules to phosphorylate glucose, preparing it for further breakdown. This phase is essential for initiating the glycolytic pathway. In contrast, the energy harvest phase produces energy by generating 4 ATP molecules and 2 NADH molecules. Since 2 ATP were used in the investment phase, the net gain is 2 ATP. Additionally, 2 pyruvate molecules are formed, which are then transported to the mitochondria for further processing.