Lesson Explainer: Glycolysis Biology

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

All living organisms, from microscopic bacteria to giant towering trees, require energy. As humans, our bodies are hugely complex. We have around 37 trillion cells and a range of specialized tissues, organs, and organ systems that all work together to form a functioning human body. Each of these cells, tissues, and organs requires energy to perform its basic functions on a day-to-day basis. But where do they get this energy from?

As heterotrophs, we consume other organisms to provide our bodies with nutrition. A key nutritional group that we must eat regularly are carbohydrates. Carbohydrates are large biological molecules composed of many smaller sugars joined together. For example, starch is a carbohydrate found in food sources like potatoes and pasta and is composed of many repeating units of the sugar glucose.

Key Term: Carbohydrates

Carbohydrates are molecules made of carbon, hydrogen, and oxygen only, which are typically broken down to release energy in animal cells.

Autotrophic organisms are those that make their own food, such as plants. These organisms carry out specialized biological processes, such as photosynthesis, to make their own glucose.

Cellular respiration is the process by which sugars, such as glucose, are broken down to release energy that can be utilized by the cell. This energy is released in the form of molecules known as ATP.

Definition: Cellular Respiration

Cellular respiration is a process in living organisms in which carbon-containing compounds, such as glucose, are broken down to release energy in the form of ATP.

Occasionally, you may hear the mechanism of breathing in and out being referred to as respiration. It is important to note that this is not the same as cellular respiration. Gas exchange is the process of taking in and releasing carbon dioxide from the lungs, whereas cellular respiration is the process of releasing energy from carbohydrates and other biological molecules.

Cellular respiration can be divided into four main sequential stages: glycolysis, link reaction, Krebs cycle (also referred to as the citric acid cycle), and oxidative phosphorylation (also referred to as the electron transport chain).

Cellular respiration can occur in the presence of oxygen (aerobically) or in the absence of oxygen (anaerobically). Glycolysis is the first stage that occurs in both aerobic and anaerobic respiration, and the reactions of glycolysis take place in the cytoplasm of the cell, which is indicated in Figure 2.

Key Term: Aerobic

Aerobic means “with or in the presence of oxygen.”

Key Term: Anaerobic

Anaerobic means “without or in the absence of oxygen.”

Example 1: Defining Anaerobic in the Reactions of Cellular Respiration

Why is glycolysis considered an anaerobic reaction?

  1. Because it does not require oxygen
  2. Because oxygen is not produced
  3. Because oxygen is a key reactant
  4. Because carbon dioxide is produced

Answer

To answer this question, let’s have a look at the key words.

Glycolysis is the first stage of cellular respiration. Cellular respiration is a crucial process in all living organisms. Respiration takes carbon-containing compounds found in cells and breaks them down, releasing energy in the process. During glycolysis, a molecule of glucose is broken down into two 3-carbon compounds. Glycolysis takes place in the cytoplasm, which is the jelly-like substance that fills the cell.

The term anaerobic means “without oxygen.” If a reaction is said to be anaerobic, this means it can proceed when oxygen is not present. This is because oxygen is not a key reactant. If a reaction is said to be aerobic, meaning “with oxygen,” this indicates that oxygen is a key reactant and must be present for the reaction to proceed.

Looking at our options, we should be able to see that the correct answer is A. Glycolysis is considered an anaerobic reaction because it does not require oxygen.

The overall process of glycolysis is outlined in the diagram provided in Figure 3. During glycolysis, a molecule of glucose is broken down into two molecules of pyruvate (or pyruvic acid) through a series of reactions.

Key Word: Pyruvate (Pyruvic Acid)

Pyruvate is a three-carbon compound formed by the phosphorylation and breakdown of glucose and is a product of glycolysis.

Let’s take a look at these reactions in more detail.

In the first set of reactions, the six-carbon glucose molecule is phosphorylated. This means that phosphate groups are added to the glucose molecule by the coenzyme ATP. This occurs twice, so two molecules of ATP are used. This reaction is shown in Figure 4. The product is fructose 1,6-bisphosphate, a phosphorylated sugar.

Key Term: Fructose 1,6-Bisphosphate (Fructose 1,6-Diphosphate)

Fructose 1,6-bisphosphate is a six-carbon sugar formed by the phosphorylation and conversion of glucose.

ATP stands for adenosine triphosphate, and the prefix tri- means “three.” So, the term triphosphate means one molecule of ATP with three phosphate groups. After an ATP molecule donates a phosphate group to the glucose, it forms ADP, or adenosine diphosphate. The prefix di- indicates that this molecule has only two phosphates now.

Key Term: ATP (Adenosine Triphosphate)

ATP, or adenosine triphosphate, is the molecule that carries chemical energy in living organisms.

Example 2: Describing the First Major Reaction in Glycolysis

Which of the following best describes the first major reaction that takes place in glycolysis?

  1. A glucose molecule is converted to reduced glucose by the addition of two hydrogen ions, catalyzed by the oxidation of reduced NAD.
  2. A phosphorylated fructose molecule is split into two 3-carbon molecules, known as glyceraldehyde 3-phosphate.
  3. A glucose molecule reacts with oxygen to form an unstable 6-carbon compound, which then breaks down into two molecules of glyceraldehyde 3-phosphate, catalyzed by the hydrolysis of ATP.
  4. A glucose molecule is converted to phosphorylated fructose by the addition of two phosphate molecules, catalyzed by the hydrolysis of ATP.

Answer

Glycolysis is the first step in a series of biochemical pathways that make up cellular respiration. Glycolysis is responsible for taking a molecule of glucose and converting it into products that can be used in later stages of cellular respiration.

When a glucose molecule undergoes glycolysis, one of the first things that happens is that it is phosphorylated. This means it has phosphate groups added to it. But how does this happen?

ATP is an energy-carrying molecule found in the cells of living organisms. When ATP is broken down using water (hydrolyzed), a phosphate group is released from the molecule. During glycolysis, two molecules of ATP donate a phosphate group to the glucose molecule. This reaction is summarized in the diagram below.

As we can see, the compound formed is a six-carbon sugar with two phosphate groups. The official name of this compound is fructose 1,6-bisphosphate.

Looking back at our options, we can eliminate option A, as NAD is not involved at this stage and glucose is converted using the addition of phosphate groups, not hydrogen ions. Option B is also incorrect, as at this stage the six-carbon compound has not been split into two 3-carbon compounds. Option C is also incorrect for the same reason, and although the hydrolysis of ATP is involved, oxygen is not.

Therefore, our only correct option is D. The first major reaction in glycolysis is that a glucose molecule is converted to phosphorylated fructose by the addition of two phosphate molecules, catalyzed by the hydrolysis of ATP.

This molecule of phosphorylated sugar is then split into two 3-carbon molecules. These three-carbon molecules have a few different names, but all of them refer to the same molecule. We are going to use the term glyceraldehyde 3-phosphate here (and the acronym G3P), but you may also see them referred to as phosphoglyceraldehyde (PGAL) or triose phosphate (TP).

Key Term: Glyceraldehyde 3-Phosphate/G3P (Phosphoglyceraldehyde/PGAL, Triose Phosphate/TP)

Glyceraldehyde 3-phosphate (known by alternative names) is a three-carbon compound formed by the splitting of fructose 1,6-bisphosphate during glycolysis.

The two molecules of G3P are then converted into the final products of glycolysis, two molecules of the three-carbon compound pyruvate.

To do this, each of the G3P molecules has a hydrogen removed. This process is carried out by the hydrogen-carrier coenzyme NAD. NAD accepts this hydrogen and forms reduced NAD, or NADH. Using the energy from this reaction, another phosphate group is attached to the G3P molecule.

Key Term: NAD+

NAD is a coenzyme that acts as an electron acceptor that readily forms reduced NAD (NADH) by accepting a hydrogen ion and electrons.

Following this, each G3P molecule has two phosphate groups removed. These phosphate groups are donated to two molecules of ADP, forming ATP.

It is important to remember that the reaction shown in Figure 5 happens for both molecules of G3P. This means that the overall net products of glycolysis are as follows:

  • two molecules of ATP (with four being made but two used in the reactions),
  • two molecules of NADH,
  • two molecules of the three-carbon compound pyruvate.

Example 3: Recalling the Products of Glycolysis

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

Answer

Glycolysis is the first stage in a series of biochemical reactions that make up cellular respiration. The overall aim of cellular respiration is to break down organic compounds, mainly carbohydrates and sugars like glucose, and release energy in the process.

This energy is stored in the form of ATP, or adenosine triphosphate. This relatively small molecule is constantly being broken down and regenerated in cells, so it provides an easily accessible and reliable source of energy.

To see how many molecules of energy are generated during glycolysis, let’s recap the reactions that make up this stage.

The overall steps that occur are as follows:

  1. conversion of glucose into a phosphorylated sugar (namely, fructose 1,6-bisphosphate),
  2. splitting of this six-carbon sugar into two 3-carbon compounds,
  3. conversion of these three-carbon compounds into molecules of pyruvate (or pyruvic acid).

In step 1, two molecules of ATP are actually used up in the conversion of glucose into fructose 1,6-bisphosphate. They are then hydrolyzed to form ADP and a phosphate group, which is donated to the glucose.

In step 2, no molecules of ATP are either used or produced.

In step 3, for each molecule of G3P that is converted into pyruvate, two molecules of ATP are formed from the phosphorylation of ADP. It is important to note, however, that for each molecule of glucose, there are two molecules of G3P, so this reaction happens twice. So, when we double this, we see that the reaction produces 4 molecules of ATP for each molecule of glucose.

However, this is not the end of the calculation. The question asks for the net yield. This means we need to take the total number of ATP molecules produced and then subtract any molecules used up in the process. We know that 4 molecules are produced but that 2 molecules are used up in the first step.

So, this means that the net yield of ATP for one molecule of glucose is 2.

Example 4: Recalling the Products of Glycolysis

What 3-carbon compound is the final product of glycolysis?

  1. Pyruvate
  2. Glucose
  3. Glyceraldehyde 3-phosphate
  4. 3-PGA
  5. NAD

Answer

Glycolysis is the first stage in a series of reactions that make up cellular respiration. The most common compound that is broken down during cellular respiration is glucose, a six-carbon compound found in many carbohydrates and readily available in our diet.

When a molecule of glucose enters the process of glycolysis, it is first phosphorylated and converted into another six-carbon sugar. This sugar is unstable and quickly breaks down into two 3-carbon compounds. These compounds are called glyceraldehyde 3-phosphate (G3P) and may also be referred to as phosphoglyceraldehyde (PGAL) or triose phosphate (TP).

However, this is not the final product. These three-carbon compounds then undergo a series of reactions to be converted into a different three-carbon compound called pyruvate, or pyruvic acid. Pyruvate is the primary reactant in the next stage of cellular respiration, the link reaction.

Therefore, the three-carbon compound that is the final product of glycolysis is pyruvate.

Glycolysis does not require the presence of oxygen to take place, so this stage of respiration can happen under aerobic and anaerobic conditions. This reaction is also universal to nearly every living organism. It occurs in the cytoplasm of bacterial, fungal, plant, and animal cells. This provides important evidence that all life on Earth evolved from a few common ancestors.

Let’s review some of the key points that we have learned about glycolysis.

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 aerobic and anaerobic respiration.
  • The overall basic equation for glycolysis is glucose 2 pyruvate molecules + 2 ATP molecules.
  • The first reactions in glycolysis use ATP to convert glucose into fructose 1,6-bisphosphate.
  • Fructose 1,6-bisphosphate is converted into G3P and then into pyruvate by the action of NAD.
  • The conversion of each molecule of G3P into pyruvate produces two molecules of ATP.

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