What Is Glycolysis? Where Does It Take Place? Pathway And Products

If you’ve taken part in a high school science class, then you’ve probably heard the term glycolysis.

Along with ATP, the creatine-phosphate system, and aerobic activity, glycolysis is another majorly important energy pathway that is responsible for many bodily functions.

Today, you’ll learn about this pathway and how it impacts your daily life.

You’ll also find out where it takes place, and methods to improve this pathway.

Glycolysis: What Is It?

Glycolytic metabolism or glycolysis is the process of breaking down glucose into pyruvate and releasing energy.

This is done by enzymes called hexokinase and phosphofructokinase. These two enzymes are found in all cells except red blood cells and certain types of cancerous cells.

The first step in glycolysis is when glucose enters the cell through specific channels on the membrane.

Glucose can enter either passively or actively. Passive entry occurs when glucose molecules diffuse across the membrane due to their size.

Active entry occurs when glucose molecules bind to proteins on the outside of the cell. 

Once inside the cell, glucose will be broken down into its component parts, which include a molecule of water, carbon dioxide, and three molecules of adenosine triphosphate (ATP).

The next step is catalyzed by hexokinase. Hexokinase is an enzyme that binds to glucose and converts it into glucose 6 phosphate.

This product is known as glucose 6 phosphate because there are six phosphate groups attached to glucose. In order for hexokinase to do this, it needs a magnesium ion to activate it. 

Magnesium ions are present in the cytoplasm and mitochondria of most cells.

However, they are not present in the nucleus. Therefore, hexokinase cannot perform its function unless it is activated by a magnesium ion.

Once hexokinase is bound to glucose, it will break the bond between the glucose and magnesium ions.

This releases the magnesium ion from the hexokinase and allows the hexokinase to now bind to another molecule of glucose. 

This process continues until all of the glucose molecules have been converted into glucose 6 phosphate. At this point, the hexokinase is ready to start the next cycle.

Once the hexokinase has completed one round of binding to glucose, it will release a molecule of ADP. The ADP will leave the hexokinase and attach itself to an enzyme called ATP synthase.

ATP synthase is responsible for creating ATP from ADP and inorganic phosphate.

During this process, ATP synthase uses the energy released from the hexokinase to create a molecule of ATP.

This entire process is repeated multiple times until all of the glucose is converted into pyruvate. Pyruvate is a simple organic acid made up of 3 carbons and 1 oxygen atom.

Pyruvate leaves the hexokinase and attaches itself to lactate dehydrogenase. Lactate dehydrogenase is an enzyme that breaks down pyruvate into lactic acid.

Lactic acid is what causes muscle fatigue during exercise.

Lactic acid is a waste product created during glycolysis. When lactic acid builds up, muscles begin to tire.

As lactic acid accumulates, more and more glucose must be used to produce enough ATP to keep the body’s systems running. 

Eventually, the body will use up all of its available glucose stores and then stop producing any more ATP.

This is why athletes who train often experience muscle soreness after exercising.

There are two ways that the body can dispose of lactic acid:

1. Lactic Acid Can Be Converted Into Acetyl CoA
2. Lactic Acid Can Go To The Liver For Removal

In the first scenario, lactic acid is converted back into glucose and pyruvic acid.

This process requires the help of several enzymes including phosphofructokinase, glucokinase, pyruvate kinase, and lactate dehydrogenase (LDH).

These enzymes work together to convert lactic acid into glucose and pyruvate.

In the second scenario, lactic acid travels to the liver where it is removed.

This process takes place through a series of reactions involving pyruvate carboxylase, malic enzyme, NAD-dependent malic enzyme, fumarase, and finally succinate dehydrogenase.

Once again, these enzymes work together to remove lactic acid from the bloodstream.

The final step in glycolysis is conversion of pyruvate into acetyl coenzyme A.

This process is catalyzed by pyruvate dehydrogenase. This entire process takes place inside the mitochondria.

How Does Exercise Affect Glycolysis?

When you perform physical activity such as walking or jogging, your muscles need additional fuel to function properly.

In order to provide this extra fuel, the body increases the rate at which it converts glycogen into glucose. Glycogen is stored in the muscles.

When the body needs extra energy, it breaks down the glycogen into glucose and stores the glucose in the bloodstream.

Glucose is the main source of energy for the brain and other organs.

During exercise, there is a constant flow of glucose into the bloodstream. This means that the amount of glucose in the bloodstream remains relatively stable.

However, if the body does not have sufficient amounts of glycogen stored in the muscles, the rate of glucose production will slow down.

This is when the body begins to break down fat for energy instead of glycogen.

Fatty acids are broken down into acetyl coenzyme A and oxaloacetate. Oxaloacetate is converted into citrate by aconitase.

Citrate is then transported into the mitochondria, where it is oxidized to form carbon dioxide and water.

This entire process is known as fatty acid oxidation. The end result is the formation of acetyl coenzyme. Acetyl coenzyme is an important building block in the Krebs cycle.

Energy-Requiring Phase (Energy Investment Phase)

After the initial burst of energy has been expended, the body enters what is called the “energy requiring phase”.

During this time, the body uses the glucose and fatty acids that were produced earlier to generate ATP.

ATP is the molecule that provides the cell with the energy necessary to carry out its functions. When ATP levels drop, cells begin to die.

ATP Production Process

To produce ATP, the body must use oxygen to combine ADP and phosphate groups into ATP. The chemical reaction can be written as follows:

ADP + Pi → ATP + H₂O

To make this happen, the body must take up oxygen and release hydrogen ions.

The oxygen molecules travel across the outer membrane of the mitochondria while the hydrogen ions move back into the cytoplasm.

In addition to using oxygen to create ATP, the body also produces free radicals during this process.

Free radicals are highly reactive molecules that damage DNA and cause cellular death.

Free radicals are damaging to cells because they contain unpaired electrons. These electrons are unstable and tend to react with other molecules.

For example, free radicals may attack proteins causing them to become damaged. They may also attach themselves to DNA and cause mutations.

The good news is that antioxidants help prevent these harmful effects. Antioxidants neutralize free radicals before they can do any harm.

Energy-Releasing Phase (Energy Payoff Phase)

In the energy-releasing phase, glyceraldehyde-3-phosphate molecules will be converted, without a phosphate group, into three-carbon sugars. This is known as pyruvate.

Pyruvate is then converted into lactic acid. Lactic acid is a waste product of muscle contraction.

In fact, one third of all the oxygen consumed by the body during exercise is used to convert lactate into CO2 and water. Lactate is released from the muscles into the blood.

Why Do You Need Glycolysis? What Are Its Benefits?

Glycolysis is required for the following reasons:

1) Energy Production

Glycolysis provides the energy needed for muscular contraction. During exercise, the body uses oxygen to create adenosine triphosphate (ATP).

ATP is the primary source of energy used by cells to carry out various chemical processes.

2) Storage Of Carbohydrates

Glucono delta lactone (GDL), also called gluconolactone, is a sugar alcohol produced during the fermentation of carbohydrates.

When ingested, GDL causes the release of insulin in the body. Insulin promotes the storage of glucose in muscle and adipose tissue.

3) Metabolism Of Fatty Acids

Fatty acids can be metabolized either directly or indirectly. Direct metabolism involves breaking down fatty acids into ketones and free radicals.

Indirect metabolism occurs when fatty acids are converted into triglycerides and stored in the liver.

4) Synthesis Of Lipid Bodies

Lipid bodies are small spherical structures found within cells. They contain many different types of lipids including phospholipids, neutral fats, and sterols.

Lipid bodies are involved in several cellular functions including membrane synthesis, protein trafficking, and signal transduction.

5) Transport Of Cholesterol

Cholesterol is necessary for cell growth and repair.

It helps maintain the integrity of membranes. Cholesterol is synthesized from acetyl CoA and mevalonate. Mevalonate is derived from pyruvate and malonyl-CoA.

6) Cellular Growth And Repair

The breakdown of proteins, nucleic acids, and carbohydrates are essential for cell growth and repair in all living organisms.

7) Cell Signaling

Cell signaling refers to the transmission of information between cells. In this way, signals may be sent from one part of the body to another.

For example, nerve impulses travel along nerves to tell your heart what is happening at the tip of your finger.

8) Cellular Communication

Cellular communication is the transfer of information between cells. It includes such things as sending messages to other parts of the body, releasing hormones, and regulating blood pressure.

9) Protein Synthesis

Protein synthesis is the production of new proteins from amino acids. Amino acids are the basic units that make up proteins. Proteins perform a wide variety of functions in the human body.

10) Mitosis

Mitosis is the division of a single cell into two daughter cells. This process takes place every time a cell divides.

Where Does Glycolysis Take Place?

Glycolysis takes place in the cytoplasm of eukaryotic cells. Eukaryotes include plants, animals, fungi, protists, and algae.

Cytoplasm contains ribosomes, mitochondria, lysosomes, and endoplasmic reticulum. The cytoplasm is filled with water.

Pathway Of Glycolysis

Glycolytic pathway consists of 10 steps:

1. Glucose + 2 ADP → Pyruvate + 2 ATP
2. Pyruvate + NADH + H+ → Lactate + CO2 + NAD+
3. Lactate + NAD+ + H+ → Succinate + NADH
4. Succinate + FADH2 + H+ → Fumarate + OAA
5. Fumarate + H2O + NH3 → Malate + NH4+
6. Malate + NAD+ + Pi → Oxaloacetate + NADH
7. Oxaloacetate + H2O + Pi → Citrate + NADPH
8. Citrate + NADPH + H+ → Isocitrate + NADP+
9. Isocitrate + 2 NADP+ + H+ → α-Ketoglutarate + NADPH + H2O
10. α-Ketoglutarate + NADPH + H + Fe2+ → Succinyl-CoA + NAD+ + 2 H2O

Products Of Glycolysis

1. Pyruvic acid – an intermediate product of glycolysis that is used by the citric acid cycle to produce energy (ATP).
2. Lactic acid – produced during anaerobic respiration.
3. Acetic acid – produced during aerobic respiration.
4. Ethanol – produced during fermentation.
5. Carbon dioxide – waste gas.
6. Water – waste liquid.
7. Ammonia – waste liquid.
8. Iron – waste metal.

What Is The Krebs Cycle?

In order to understand how glycolysis works, we need to know about the Krebs Cycle.

The Krebs Cycle is also known as the citric acid cycle or the tricarboxylic acid cycle. It is named after Hans Krebs who discovered it in 1912. 

The Krebs cycle is responsible for producing cellular energy through oxidative phosphorylation.

It is also involved in many important chemical reactions within the cell.

The Krebs cycle occurs in the mitochondrial matrix of the cell. It has four main stages:

1. Oxidation of acetyl coenzyme A (Acetyl CoA) to form oxaloacetic acid (OAA)
2. Reduction of oxaloacetic acid to form malonyl CoA
3. Dehydrogenation of malonyl CoA to form succinyl CoA
4. Decarboxylation of succinyl CoA to form succinic acid

The Krebs cycle produces energy by breaking down carbohydrates, fats, and protein.

During each stage of the cycle, carbon atoms are removed from molecules called organic compounds. These carbon atoms are then combined with oxygen to create carbon dioxide and water.

How Does Glycolysis Work In The Cell?

During glycolysis, glucose is broken down into pyruvic acid. Pyruvic acid is then converted into lactate. Glycolysis is the first step in the breakdown of glucose.

During glycolysis, pyruvic acid is reduced to lactic acid. Lactic acid is not completely oxidized to carbon dioxide.

Instead, some of the lactic acid is converted into acetic acid. This process is known as fermentation.

What Are The Byproducts Of Glycolysis?

Pyruvic Acid

Pyruvic acid is a weak organic acid. It can be found in fruits such as apples, grapes, strawberries, and tomatoes. Pyruvic acid helps to remove toxins from the body.

Lactic Acid

Lactic acid is a type of organic compound. It is made when sugars are broken down in the cells.

Succinic Acid

Succinic acid is a colorless, odorless, crystalline substance. It is formed when sugar is broken down in the cells and turned into pyruvic acid and fumaric acid.

Fumaric Acid

Fumaric acid is another organic compound. It is used to make other chemicals. For example, it is used to make vitamin B12.

Glycerol

Glycerol is a three-carbon alcohol that is part of triglycerides. It is used to store fat.

What Are Some Of The Other Energy Pathways?

There are two other ways that cells use energy. They are the pentose phosphate pathway and fatty acid oxidation.

The Pentose Phosphate Pathway

The pentose phosphate pathway is one way that cells use energy. It uses five different types of molecules to produce energy.

The five molecules are ribulose 5-phosphate, xylulose 5-phosphate, glyceraldehyde 3-phosphate, dihydroxyacetone phosphate, and sedoheptulose 7-phosphate.

Fatty Acid Oxidation

Fatty acid oxidation is another way that cells use energy to produce ATP. It involves using oxygen to break down long chain fatty acids.

This results in the production of carbon dioxide and water. Carbon dioxide is released into the air while water is excreted out of the body.

Where Do We Find Glucose?

Glucose is a simple carbohydrate that comes from food. It is stored in the liver and muscles. When we eat foods high in carbohydrates, they get broken down into glucose.

Glucose is also produced by the pancreas.

Our bodies usually have enough glucose for immediate needs. However, if we do not eat or drink anything for a few hours, our blood glucose levels will drop.

If this happens, we may feel tired or hungry. To keep our blood glucose levels stable, our bodies need to release insulin. Insulin allows glucose to enter our cells so that they can use it for energy.

When we eat, our bodies must convert the food into energy. Our brains and nerves need glucose to function properly. Without glucose, these organs would stop working.

How Does Glucose Get Into Our Cells?

When our bodies need more energy, they release insulin. Insulin gets into the bloodstream and attaches itself to receptors on the surface of cells.

Once attached, insulin causes the cell’s membrane channels to open up. This allows for the passage of glucose into the cell. 

In some cases, glucose enters the cell through an enzyme called hexokinase. Hexokinase breaks down glucose into glucose 6-phosphate.

Then, the cell uses glucose 6-phosphatase to turn glucose 6-phosphate into glucose.

Final Thoughts

Energy metabolism is how your body uses the nutrients you eat to make energy. There are many pathways that cells use to create energy.

Understanding these pathways helps us understand why people gain weight and lose weight, as well as which system is used during certain exercise modalities and intensities.

Frequently Asked Questions

How Does Ketosis Work In The Human Body?

Ketones are a form of fuel that your body produces during periods of starvation.

Your body starts producing ketones when you don’t have any glucose available.

Ketones are often referred to as “ketone bodies.” Ketones are similar to glucose but cannot be used directly by the brain or nervous system.

Instead, they must first go through the liver before being converted into glucose.

The human body has two ways of breaking down fat: lipolysis and ketogenesis. Lipolysis occurs when there is no glucose available.

Fat is broken down into glycerol and free fatty acids. These products then leave the body. Ketogenesis occurs when there is no access to glucose.

Fat is broken down to acetyl CoA and beta-hydroxybutyrate (BHB). BHB is then converted into acetoacetate and acetone.

How Is ATP Created?

ATP is created by the breakdown of adenosine triphosphate (ATP) molecules. ATP is made up of three parts: phosphate, adenine, and one of each of the ribose sugars.

Phosphate is released from ATP. Adenine is removed from ATP. Ribose is split into two parts. One part becomes ADP, while the other becomes AMP.

Both ADP and AMP are removed from ATP. Finally, the phosphate group is added back onto the AMP molecule. This makes ATP again.

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