Lesson Video: Glycolysis | Nagwa Lesson Video: Glycolysis | Nagwa

Lesson Video: Glycolysis Biology

In this video, we will learn how to describe the process of glycolysis and recall the products made.

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Video Transcript

In this video, we will learn how to describe the process of glycolysis. We will recall the products and the reactants of glycolysis and describe the intermediate reactions that occur as these reactants are converted into the products.

Glycolysis is the term used to describe the first stage of cellular respiration. Cellular respiration is incredibly important for organisms. It is the process by which sugars are broken down to release energy. We can then use this energy to power important metabolic reactions in our cells and to carry out essential processes like digestion, breathing, and movement. You may recall the overall chemical equation for cellular respiration. Glucose plus oxygen react to form carbon dioxide and water. In this process, energy is released in the form of ATP. We are going to take a look at this general equation in a bit more detail and understand all the biochemical reactions that take place.

Glycolysis takes place in the cytoplasm of cells. The cytoplasm is the jelly-like fluid that fills the inside of the cell and contains most of the cell’s organelles. Glycolysis does not require oxygen, but the following stages of cellular respiration do. If oxygen is not present, glycolysis is followed by a process called fermentation, which does not produce as much ATP as cellular respiration. So let’s get started and have a look at the reactions of glycolysis in more detail.

Here, we can see the overall sequence of chemical reactions that occur during glycolysis, but this looks quite complicated. So let’s go through it step by step. The primary reactant of glycolysis is glucose. A glucose molecule has a ring-shaped structure. But in our simplified diagrams, let’s just use a chain of carbon atoms. Glucose is a monosaccharide. Saccharide is a word part meaning sugar, and mono- is a word part meaning one. This refers to the fact that glucose is made up of a single sugar unit. One molecule of glucose has six carbon atoms, so we refer to it as a six-carbon sugar. We get the glucose needed for glycolysis from our diet. Foods that contain carbohydrates, like pasta, potatoes, and bread, are usually a good source of glucose.

In the first stage of glycolysis, this molecule of glucose will undergo a process known as phosphorylation. Phosphorylation refers to a reaction in which phosphate groups are added to a molecule. In glycolysis, glucose is sequentially phosphorylated by two molecules of ATP. ATP stands for adenosine triphosphate. You may recall that the word part tri- means three, so triphosphate means three phosphate groups. When a molecule of ATP donates its phosphate group to a different molecule with the help of an enzyme, it is converted from ATP to ADP. ADP stands for adenosine diphosphate. Di- is a word part that means two, so we know that ADP only has two phosphate groups.

To form ADP, the bond between the second and third phosphate group in ATP breaks. When this inorganic phosphate is released from ATP, it forms a new bond with the glucose molecule. The formation of this new chemical bond between the phosphate group and the glucose molecule releases energy. The amount of energy released in making this new bond is greater than what was required to break the bond in the ATP molecule. With the help of enzymes, glucose is phosphorylated by two molecules of ATP in turn. This means that glucose gains two phosphate groups, as shown in the diagram by the Ps.

The compound that is formed is sometimes known as phosphorylated glucose, but more accurately called fructose-1,6-biphosphate. Bi- is another word part that means two. One and six refer to the carbon atoms where these phosphate groups are attached. Fructose is another monosaccharide. It is very similar to glucose, as it also contains six carbon atoms, but has a slightly different structure. Glucose is converted into fructose by the action of an enzyme. So now we have our new sugar fructose-1,6-bisphosphate.

Let’s take a look at the next set of reactions that occur in glycolysis. Fructose-1,6-bisphosphate, the sugar we have created from phosphorylating and converting glucose, is a six-carbon sugar. Next, this six-carbon compound is split into two three-carbon compounds, finally a nice, simple process. These three-carbon compounds have a few different names. This can depend on what country you are learning in. We are going to use the name glyceraldehyde-3-phosphate or G3P. But you may also see them called phosphoglyceraldehyde or PGAL or triose phosphate or TP. So don’t get confused. They are all referring to the same molecule.

We are nearly at the end of our glycolysis reactions. So let’s take a look at the final steps. Remember that from one molecule of fructose-1,6-bisphosphate, two molecules of G3P are produced. So, for the next set of reactions, remember that they will all happen twice. Next, our molecule of G3P is converted into a molecule of pyruvate. Pyruvate or pyruvic acid is an important chemical compound, which is known as an intermediate. We’ll see why this is shortly.

For a molecule of G3P to become a molecule of pyruvate, two things need to happen. It needs to lose a hydrogen and a phosphate group. Let’s go through this in a bit more detail. NAD+ is a coenzyme. A coenzyme is a nonprotein compound or molecule that helps enzymes carry out biochemical reactions. When NAD+ is converted into NADH, it gains a hydrogen ion and two electrons from G3P. We say that NAD+ is reduced because it has gained electrons. You may therefore see NADH referred to as reduced NAD+. This reaction is coupled with the next one. Using the energy from the reduction of NAD+, G3P gains another phosphate group.

This new compound doesn’t last long though. Following this, the molecule loses both of its phosphate groups. And these phosphate groups are gained by two molecules of ADP. And there we have it. Through a series of biochemical reactions, our reactant glucose is converted into two molecules of our product pyruvate. You might be wondering why. Why have we looked at all of these complex reactions and not really ended up with much?

Firstly, glycolysis itself produces two molecules of ATP. Actually, the gross product is four molecules of ATP. But remember, we used two molecules of ATP in the first stage. So the net amount of ATP produced that can be used elsewhere is two molecules. This is important because ATP is an energy-carrying molecule for all of our cells. It is a small molecule, and it can be easily broken down by breaking the bond between the end and middle phosphate groups. When a phosphate group released from ATP bonds with another molecule, the reaction releases a net amount of energy. This energy can be used for pretty much all of our important life processes. This includes moving, breathing, digesting, and, you guessed it, more cellular respiration. But two molecules of ATP doesn’t sound like much, right? So why is glycolysis so important?

Glycolysis is actually the start of a series of stages in the process of cellular respiration. Glycolysis in humans rarely happens on its own. It is usually followed by reactions called the link reaction, the Krebs or citric acid cycle, and oxidative phosphorylation. Combined, all of these reactions produce a large amount of ATP for us. You may remember that we refer to pyruvate as an intermediate compound. The next stage of cellular respiration, the link reaction, cannot happen without pyruvate. So glycolysis is incredibly important for kicking off all of these subsequent reactions.

Now that we have learned about glycolysis, let’s try a practice question.

What is the net yield of ATP for one glucose molecule undergoing glycolysis?

Glycolysis is the first stage of cellular respiration. Glycolysis takes place in the cytoplasm of cells of nearly all living organisms, and it happens whether oxygen is present or not. In glycolysis, a molecule of glucose undergoes a series of biochemical reactions to form two molecules of pyruvate or pyruvic acid. In the first set of reactions in glycolysis, two molecules of ATP are actually used to convert glucose into the phosphorylated sugar fructose-1,6-bisphosphate. So, currently, the yield of ATP is minus two.

Next, the six-carbon fructose-1,6-bisphosphate is split into two three-carbon compounds. This reaction does not use any ATP, but it also does not produce any ATP. So our current yield of ATP still stands at minus two. Finally, the two three-carbon compounds need to be converted into our final product, pyruvate. In this reaction, the three-carbon compounds donate a hydrogen ion and two electrons to a coenzyme called NAD+ to form reduced NAD or NADH. This reaction is coupled to another reaction.

Using the energy from the reduction of NAD+, G3P gains another phosphate group. This new compound doesn’t last long though. Following this, G3P loses both of its phosphate groups. These phosphate groups are gained by two molecules of ADP to form ATP. For each molecule of pyruvate formed, two molecules of ATP are produced. Because there are two molecules of pyruvate, four molecules of ATP are produced in total. So our yield of ATP before this stage was minus two, and here we have produced four molecules of ATP.

Now, we’re ready to calculate the net yield of ATP. Minus two ATP plus four ATP gives us a net yield of two ATP. So, for each single molecule of glucose that undergoes glycolysis, the net yield of ATP is two molecules.

Let’s summarize what we’ve learned with some key points. Cellular respiration is the process by which living organisms break down glucose and other substrates to release energy. Glycolysis is the first stage of cellular respiration and does not use oxygen. The overall basic equation for glycolysis is glucose plus two ADP plus two inorganic phosphate yields two pyruvate plus two ATP. The first reactions in glycolysis use ATP to convert glucose into fructose-1,6-bisphosphate. Fructose-1,6-bisphosphate is converted into two molecules of G3P, and then G3P is converted into pyruvate. The net yield of ATP from glycolysis is two molecules.

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