Cellular Respiration: Glycolysis - Online Tutor, Practice Problems & Exam Prep

Glycolysis is the initial step of cellular respiration, breaking down one glucose molecule (6 carbon atoms) into two pyruvate molecules (3 carbon atoms 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 involves the breakdown of a single glucose molecule into 2 pyruvate molecules. When we look at the term glycolysis, you'll find these two roots embedded. You'll find the root glyco, which means sugar, and glucose is indeed a sugar.

Moreover, the root lysis means to break down. When you combine these two roots together, glyco and lysis, you get glycolysis, which specifically means breaking down glucose 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 ends with 2 pyruvate molecules, just as we indicated above. It's important to note that glucose has a total of 6 carbon atoms.

Notice that we're representing glucose as these 6 black circles, and each circle represents one of the carbon atoms found in glucose. Glucose does have other types of atoms as well, but textbooks and professors mainly focus on the carbon atoms of glucose. At the end of glycolysis, notice that the glucose molecule was broken apart into 2 pyruvate molecules, each with 3 carbon atoms. Here we have one pyruvate molecule and here we have a second pyruvate molecule.

As we move forward with aerobic cellular respiration, all six of the carbon atoms that were originally found in glucose are going to be converted to carbon dioxide or CO₂. This transformation specifically happens in the second and third steps of cellular respiration. All of the glucose molecule's carbon atoms will eventually end up as carbon dioxide, which is then exhaled into the environment. Note that glycolysis is the only step of aerobic cellular respiration that occurs outside of the mitochondria, in the cell's cytoplasm.

This is a crucial feature to remember about glycolysis. It can occur in both the presence and absence of oxygen, unlike all the other stages of cellular respiration which require oxygen. Notice in our image, the process of glycolysis occurs outside the mitochondria, reflecting its unique positioning in cellular respiration. It's the only stage that occurs outside the mitochondria and the only one that does not require oxygen.

Once again, glycolysis occurs outside mitochondria, specifically in the cell's cytoplasm. The location of glycolysis is an important feature because it takes place outside the mitochondria, while all other stages occur inside the mitochondria.

This concludes our brief introduction to glycolysis, and we will continue to discuss more about it as we progress 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.

Phases of Glycolysis

In this video, we're going to introduce the phases of glycolysis. It turns out that glycolysis 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. Now 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. In this image down below, what we're showing you is glycolysis. Notice that it starts with a single glucose molecule that has 6 carbon atoms and 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, requires an investment of energy or an input of energy by using 2 ATP molecules.

This is a 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 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, the energy harvesting phase is 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, the energy investment phase, uses 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 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, what we'll see is 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. 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.

Notice 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 ATP. 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 be able 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. Now these 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 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, we have 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. 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. 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 2 three-carbon pyruvate molecules. So you can see these 2 three-carbon pyruvates highlighted here. And then once these pyruvates have been formed, the process of glycolysis is over. You can see here that the process of glycolysis is indicated by this green box.

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. Traveling to Mexico represents the pyruvates traveling to the mitochondria. 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 piña coladas.

Hopefully, this little memory tool, Gary Invests 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. The example problem says, there is an energy investment step needed to get glycolysis started requiring the use of 2ATP's. And once again we've got these 5 potential answer options down below. And so what we need to recall is that glycolysis has two phases, the energy investment phase and the energy harvest phase. The energy investment phase requires an investment of 2ATPs.

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 2ATPs. And so the correct answer here is going to be answer option a. Now after the 2ATPs have been used, then the cell can transition to the second phase of glycolysis which is the energy harvest phase. And the energy harvest phase produces a total of 4ATPs and 2NADH.

And of those 4ATPs, because 2 of them got burnt, ultimately it's only a net of 2ATPs that are made. But once again in terms of the energy that needs to be invested, it's 2ATPs 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.

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⁺.

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. Unlike other stages of cellular respiration, glycolysis does not require oxygen, making it unique. The process 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?

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 setting up the subsequent reactions. The energy harvest phase follows, where the cell produces 4 ATP molecules and 2 NADH molecules. However, since 2 ATP were used in the investment phase, the net gain is 2 ATP. This phase also results in the formation of two pyruvate molecules, which are then transported to the mitochondria for further processing.

What are the net products of glycolysis?

The net products of glycolysis from one glucose molecule are 2 pyruvate molecules, 2 NADH molecules, and a net gain of 2 ATP molecules. Initially, 4 ATP molecules are produced, but since 2 ATP are consumed during the energy investment phase, the net gain is 2 ATP. These products are crucial for the subsequent stages of cellular respiration, where the pyruvate molecules are transported to the mitochondria for further processing, and the NADH molecules are used in the electron transport chain to generate more ATP.

Why is glycolysis considered an anaerobic process?

Glycolysis is considered an anaerobic process because it does not require oxygen to occur. This is in contrast to other stages of cellular respiration, such as the Krebs cycle and the electron transport chain, which do require oxygen. Glycolysis takes place in the cytoplasm of the cell and can proceed whether oxygen is present or not. This feature allows cells to generate ATP even in low-oxygen conditions, making glycolysis a critical pathway for energy production in various environments.

How does the energy investment phase of glycolysis work?

The energy investment phase of glycolysis involves the initial consumption of 2 ATP molecules to phosphorylate glucose and convert it into a more reactive form. This phase consists of several steps where glucose is ultimately split into two three-carbon molecules. The investment of ATP is necessary to destabilize the glucose molecule, making it easier to break down in subsequent reactions. Although it may seem counterintuitive to use ATP when the goal is to produce it, this investment is crucial for the efficient functioning of the entire glycolysis pathway.