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 consists of two phases: the energy investment phase, which consumes 2 ATP, and the energy harvest phase, producing 4 ATP and 2 NADH. The net gain is 2 ATP. Glycolysis occurs in the cytoplasm and does not require oxygen, making it unique among cellular respiration stages. The pyruvate molecules then enter the mitochondrial matrix for further processing in aerobic respiration.

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 examine the term glycolysis, you'll find two roots embedded: glyco and lysis. The root 'glyco' means sugar, indicating that glucose is indeed a sugar, and 'lysis' means to break down. Combining these roots, glycolysis literally means the breaking down of glucose into 2 pyruvate molecules. Let's take a look at our image below, showing that it starts with a single glucose molecule and ends with 2 pyruvate molecules.

It's important to note that glucose has a total of 6 carbon atoms. In the depiction below, glucose is represented by 6 black circles, where each circle represents one carbon atom found in glucose. While glucose contains other types of atoms, textbooks and professors tend to focus mainly on the carbon atoms during discussions of cellular respiration. At the end of glycolysis, glucose is broken apart into 2 pyruvate molecules, each with 3 carbon atoms. This splitting of the glucose molecule is a key step in the process.

As we move forward with aerobic cellular respiration, all 6 carbon atoms originally in glucose are converted to carbon dioxide (CO₂). This conversion occurs specifically in the second and third steps of cellular respiration. This is a significant point as all of the glucose's carbon atoms will eventually end up as carbon dioxide, which will then be exhaled 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 an important feature to note, as glycolysis does not require oxygen, unlike other stages of cellular respiration. Glycolysis can occur both in the presence and absence of oxygen, making it unique among the stages of cellular respiration. Below, we describe the glycolysis process as taking place outside the mitochondria, specifically in the cell's cytoplasm. Understanding that glycolysis occurs distinctively outside the mitochondria is crucial, as all other stages occur inside the mitochondria.

This concludes our brief introduction to glycolysis. We will continue to discuss more about glycolysis 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.

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

Phases of Glycolysis

In this video, we're going to begin our lesson on the phases of glycolysis. The first stage of aerobic cellular respiration, which is glycolysis, consists of a series of 10 reactions. These 10 reactions that make up glycolysis are split or grouped into 2 phases, which we have numbered down below. The first phase of glycolysis is the energy investment phase, and the second phase is the energy harvest phase. In the first phase, the energy investment phase, as its name implies, it's a phase that requires an investment or an input of energy. That input of energy comes from using 2 ATP molecules. The energy investment phase ends with the production of 2 Glyceraldehyde 3-Phosphate (G3P) molecules. Glyceraldehyde 3-Phosphate, or G3P, is the intermediate molecule between the two phases of glycolysis. The energy investment phase ends with G3P, and the energy harvest phase begins with G3P.

In the energy harvest phase, as its name implies, it harvests energy, or in other words, produces energy. It produces this energy by forming 2 NADH molecules and a total of 4 ATP molecules. These 4 ATP molecules are produced during the energy harvest phase. To consider the net products produced from both the energy investment and energy harvest phase, we look at the net results from one single glucose molecule going through the entire process of glycolysis. It results in the formation of 2 pyruvate molecules, since the glucose gets split and converted into 2 pyruvate molecules. There will also be 2 NADH molecules produced, and in terms of the net amount of ATP, there are actually 2 net ATP molecules produced. This is because, although 4 ATPs are being produced in the energy harvest phase, we have to consider that we had to use 2 ATP molecules in the energy investment phase. Thus, there are only a total of 2 net ATPs produced throughout this entire process of glycolysis.

The 2 pyruvates that are formed are actually going to be transported to the mitochondrial matrix in eukaryotic organisms. The mitochondrial matrix is really just the inside of the mitochondria. This is where the next step of cellular respiration will begin. If we take a look at our image down below, we can get a better understanding of these phases of glycolysis. This entire box represents all of glycolysis, which starts with a glucose molecule with 6 carbons and ends with 2 pyruvate molecules, each with 3 carbons. Notice that this entire process of glycolysis is broken down into 2 phases. Here in red, we have the energy investment phase, and in blue, we have the energy harvest phase. Right in between the two is the molecule G3P, Glyceraldehyde 3-Phosphate.

In the energy investment phase, the first phase here, energy must be invested. 2 ATP molecules are going to be used up. They're going to be burnt. That is the energy investment phase. Those ATPs are being invested to begin the process of glycolysis. The energy investment phase ends with the production of 2 G3P molecules, as you see here. Then, the energy harvest phase over here is going to begin with those G3P molecules. Through a series of many different reactions, the energy harvest phase is going to harvest energy by producing 2 NADH and by producing a total of 4 ATPs, 2 here and 2 over here. If we consider once again the net amount of ATP molecules produced, we need to remember that in the energy harvest phase over here in blue, there is a total of 4 ATPs produced. However, in the energy investment phase here, we had to burn 2 ATPs, consume 2 ATPs, so we subtract 4 minus 2 to get a total of 2 net ATPs produced in the entire process of glycolysis from start to finish, including both the energy investment and energy harvest phase. These pyruvate molecules are then going to enter into the mitochondrial matrix in order for the next step of cellular respiration to take place.

This here concludes our brief lesson on the phases of glycolysis, and we'll be able to get some practice applying these concepts as we move forward and continue to learn more as well. 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. 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. And so 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. And 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 2 C3 carbon pyruvate molecules. So you can see those 2 C3 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. So, notice here what we have in this image is a mitochondria that is chilling on the beach in Mexico, sipping on some piña 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. And so 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 blank 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 second 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 of them 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 carbons) into two pyruvate molecules (3 carbons 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 pyruvate molecules produced then enter the mitochondrial matrix for further processing in aerobic respiration.

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 ends with the production of two Glyceraldehyde 3-Phosphate (G3P) molecules. The energy harvest phase begins with G3P and produces 4 ATP and 2 NADH molecules through a series of reactions. The net gain from glycolysis is 2 ATP, as 2 ATP were consumed in the investment phase. The process concludes with the formation of two pyruvate molecules, which then enter the mitochondrial matrix for further processing.

What is the net gain of ATP in glycolysis and how is it calculated?

The net gain of ATP in glycolysis is 2 ATP molecules. This is calculated by considering the total ATP produced and the ATP consumed during the process. In the energy harvest phase, 4 ATP molecules are produced. However, in the energy investment phase, 2 ATP molecules are consumed to initiate the process. Therefore, the net gain is calculated as follows:

$4-2=2$

Thus, the net gain of ATP from glycolysis is 2 ATP molecules.

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. Additionally, glycolysis is the initial step that breaks down glucose into two pyruvate molecules, setting the stage for further energy production in the mitochondria.

What happens to the pyruvate molecules produced in glycolysis?

The pyruvate molecules produced in glycolysis are transported into the mitochondrial matrix in eukaryotic cells. Once inside the mitochondria, these pyruvate molecules undergo further processing in the next stages of cellular respiration, specifically the Krebs cycle (also known as the citric acid cycle) and the electron transport chain. During these stages, the pyruvate molecules are further broken down, leading to the production of additional ATP, NADH, and FADH₂, and ultimately resulting in the release of carbon dioxide (CO₂) as a byproduct.