Glycolysis Explained In 10 Easy Steps

The glycolytic pathway is one of the most important pathways in biology. It converts glucose into pyruvate, which then gets converted into acetyl coenzyme A (acetyl CoA) by the citric acid cycle.

This process releases energy from glucose and produces carbon dioxide and water.

Glycolysis Explained In 10 Easy Steps

Understanding glycolysis is an important part of having a solid foundation in and understanding of aerobic and anaerobic cellular respiration, but getting your head around the concepts can be a challenge. 

To help, we have put together everything you need to know about glycolysis, including the simple steps you need to understand in order to master the topic.

How Does The Body Use Energy?

Before we move to look more closely at the process of glycolysis specifically, it is useful to have a general overview of the way that our bodies use energy, and the processes that are part of this.

The body needs energy to function. Energy comes from food which contains carbohydrates, fat, protein, vitamins, and minerals.

Carbohydrates provide energy which is stored in the liver and muscles, and fat and protein also provide energy – fat is stored in adipose tissue, while protein is found in muscle tissues.

Vitamins and minerals provide energy and are found in foods.

Energy is used throughout the body. For example, when we move our arms, we use energy.

We need energy to breathe. We need energy to digest food. We need energy to think. We even need energy to sleep!

Energy is measured in calories. One calorie equals 4.2 kilocalories. 

Calories are usually broken down into three parts: carbohydrate, protein, and fat.

Carbohydrate

Carbohydrates are sugars and starches. Carbohydrates are found in fruits, vegetables, dairy products, bread, pasta, rice, beans, potatoes, corn, and other grains.

When you eat carbohydrates, they turn into glucose in your blood. Glucose is the primary source of fuel for most of your cells.

Protein

Protein is made up of amino acids. Amino acids are the building blocks of all living things. There are 20 different types of amino acids. Your body uses some of these amino acids to make new proteins. 

Proteins are important because they carry out many functions within your body. Some examples of proteins include antibodies, hormones, enzymes, and neurotransmitters.

Fat

Fat is stored in adipose tissues. Adipose tissues are located under the skin, on the stomach, and between organs. Most of the fat in your body is stored in adipose.

When you eat carbohydrates, your body breaks down the carbs into glucose. Glucose enters the bloodstream.

Glucose travels to the liver where it is converted to glycogen. Glycogen stores excess glucose until the body needs more energy.

When the amount of glycogen gets low, the body starts breaking down the glycogen into glucose. This process is called gluconeogenesis.

Glucose then travels back to the liver where it’s converted to triglycerides. Triglycerides are fatty substances that store energy.

Glucose

Glucose is a simple sugar that cannot be stored by the body. It must enter the bloodstream or get burned as energy.

If there isn’t enough glucose available, the body will break down fats and proteins to create glucose.

Glucose is also used to make ATP (adenosine triphosphate).

ATP is an essential molecule that powers every single cell in the body. ATP is produced from glucose through a series of chemical reactions.

ATP can only be created using oxygen and glucose. Without enough oxygen, the body cannot produce enough ATP. Without enough ATP, the body doesn’t have enough energy.

The body uses glucose to power its cells. The brain, heart, and muscles require large amounts of energy.

These cells don’t store much glucose. Instead, they rely on glucose entering their cells via the capillaries.

The rest of the body has smaller amounts of glucose-using cells. These cells store glucose so they have enough energy for daily activities.

If glucose levels drop too low, the body begins producing ketones.

Ketones are molecules that come from fat. Ketones are not good for the body. They cause acidosis which damages cells.

What About Ketones?

Ketones are formed when the body burns fat instead of glucose. Fatty acids are converted into acetyl CoA. Acetyl CoA then goes through several steps before becoming acetoacetyl CoA.

Finally, acetoacetyl CoAs become ketone bodies. Ketone bodies are toxic to the body.

Ketones are dangerous because they damage the mitochondria. Mitochondria are tiny structures inside cells that generate energy. Ketones destroy the mitochondria.

Mitochondria are responsible for generating most of the energy needed by the body. When the mitochondria are damaged, the body loses energy.

The body produces insulin to regulate blood sugar levels. Insulin helps the body use glucose. Insulin works by binding with receptors on the surface of cells.

Insulin binds to the receptor and causes the cell to open up.

Once the cell opens, glucose passes into the cell. Inside the cell, glucose is turned into glycogen. Glycogens are long chains of glucose molecules.

Insulin also increases the production of enzymes that convert free fatty acids into triglycerides.

Triglycerides (also known as Tg) are another form of energy storage. The body converts them into ketones.

When the body needs more energy, the pancreas releases insulin.

Insulin attaches to the receptors on the surface of the muscle cells. Muscle cells take up the glucose and turn it into glycogen.

Once the glycogen stores are full, the body stops taking in glucose. The glycogen is broken down into glucose and released into the bloodstream.

What Is Cellular Respiration?

What Is Cellular Respiration?

Cellular respiration is the chemical reaction that takes place within living organisms. During cellular respiration, oxygen combines with food molecules to form water and release energy.

This process is also known as oxidation. The food molecules that undergo this process are called substrates.

During cellular respiration, electrons are transferred through an electron transport chain.

Electrons are derived from food molecules during glycolysis and, as we have mentioned, glycolysis is the breakdown of glucose into pyruvate.

Why Is Cellular Respiration Important?

Our bodies need energy to function properly, and part of this involves cellular respiration.

In short, when our bodies use carbohydrates for energy, they produce CO2. Carbon dioxide is toxic to plants, but it is essential for humans.

Carbon Dioxide

In order to stay alive, our bodies must remove carbon dioxide from our bodies. Our lungs do this by breathing.

During cellular respiration, the enzyme cytochrome c oxidases transfers electrons to oxygen molecules.

Oxygen molecules combine to form water and release the energy needed to sustain life.

How Does Glycolysis Work?

During glycolysis, glucose breaks apart into two smaller molecules: pyruvic acid and dihydro-pyruvic acid.

Pyruvic Acid

Pyruvic acid is formed when one molecule of glucose is split into two molecules of pyruvate.

The resulting product is 2-phosphoglyceric acid (PGA). PGA is a precursor to lactic acid. Lactic acid is what gives milk its sour taste.

Dihydrouracil

Dihydro-pyruvic acid is formed when one molecule of glucose and fructose is broken down into two molecules of pyruvate. The resulting products are 1,3-diphosphoglycerate and 3-phosphoglycerate.

1,3-diphosphoglycerate

1,3-pyro phosphoglycerate is formed when one molecule is split into three molecules of pyruvates.

The resulting product is phosphoenolpyruvate. Phosphoenolpyruvate is used to make ATP.

Phosphoenolpyruvate

3-phosphoglyceraldehyde is formed when one molecule splits into three molecules of pyruvate. The resulting products are dihydroxyacetone phosphate and glyceraldehyde-3-phosphate.

Dihydroxyacetone phosphate

Glyceraldehyde-3 phosphate. The final product of glycolysis is lactic acid.

Lactate is a waste product that can be converted back into pyruvate for future rounds of glycolysis.

Pyruvate Kinases

Pyruvate kinases are enzymes that catalyze the conversion of pyruvate and phospho-enolpyruvate (PEP).

The conversion of pyruvates into PEP is important because it allows cells to generate more ATP.

What Is Glycolysis?

Glycolysis is the metabolic pathway where glucose (a simple sugar) is broken down into two molecules of pyruvate and then converted into lactic acid.

This happens inside our cells every time we eat food.

The glycolytic pathway is also known as the Embden Meyerhof Parnas Pathway. It was discovered by German biochemist Hans Krebs in the 1920s.

He found out that when he added glucose to yeast cells, they would break down the glucose and produce energy.

This pathway is responsible for converting carbohydrates into energy.

In humans, this occurs in the liver and muscles. When we eat foods rich in carbs, such as bread or pasta, these get broken down into glucose. 

Glucose enters the bloodstream and travels through the body until it reaches the liver. There, glycogen stores are used up, which means that the glucose gets stored in the liver. 

The liver converts the glucose into pyruvate, which is then transported throughout the rest of the body.

In the muscle tissue, the pyruvate is converted into lactate, which is released from the cell. Lactate is what gives us that “burn” after exercise.

The glycolytic pathway is the main source of energy for cells.

It converts glucose into pyruvate, which then gets converted into acetyl CoA, which enters the Krebs cycle. This process generates ATP, which we use to power our bodies.

Why Do We Need Glycolysis? What Does It Do?

Why Do We Need Glycolysis? What Does It Do?

When we eat foods containing carbohydrates, glucose is absorbed into the bloodstream. As mentioned above, this glucose will travel through the body until it hits the liver.

Once there, glycogen stores are depleted, so the glucose is converted into pyruvate. 

Pyruvate is then transported throughout the body where it is converted into acetyl-CoA. Acetyl-CoA is needed for making fatty acids, cholesterol, and other substances that make up our bodies.

In addition to producing energy, the glycolytic pathway produces ATP.

ATP is an important molecule that provides energy to all living things. Without it, we wouldn’t be able to move, think, or even breathe!

How Is Glycolysis Used By Our Cells?

Our cells use glycolysis to convert glucose into energy. If we don’t consume enough calories, our cells will start using proteins instead of glucose.

Proteins contain more than twice as much energy per gram as carbohydrate does.

However, protein takes longer to digest than carbohydrate, meaning that it doesn’t provide quick energy like glucose does.

Our cells also use glycolysis during times of stress. For example, if we are starving, our cells will begin breaking down their own proteins for energy.

They will do this because they want to survive.

If we are exercising, our cells will switch over to using fat as fuel. Fat has less energy than glucose, but it can be burned quickly.

During exercise, our bodies need a lot of energy, so they turn to fats first.

What Are Some Benefits Of Glycolysis?

There are many benefits of glycolysis! Here are just some of them:

Weight Loss

It helps you lose weight. Because your body uses less energy when you burn fat rather than carbohydrates, you may find yourself losing weight faster.

Appetite Satisfaction

Glycolysis can also help you to feel fuller. Eating foods high in carbohydrates causes you to feel hungry again soon after eating.

This is because your brain needs glucose to function properly. When you burn fat, however, you won’t feel as hungry.

Improved Health

Glycolysis makes you healthier. Burning fat releases ketones, which increase your metabolism.

Ketones help your body to produce energy efficiently. Your body burns fat at a higher rate than it normally would.

The process also boosts your immune system.

When you burn fat for energy, your body releases chemicals called cytokines. These chemicals boost your immune system by increasing its ability to fight infections.

Reduced Inflammation

It reduces inflammation. Inflammation is caused by excess amounts of free radicals in your body.

Free radicals cause damage to cells and tissues. When you burn fat instead of carbohydrates, you reduce the amount of free radicals in your bloodstream.

Energy Production

Glucose is a source of energy for most cells. It provides energy for muscle contraction and brain activity.

Glucose is stored in the liver and muscles as glycogen.

Lactic Acidosis

When too much lactate builds up in your body, you may experience symptoms such as fatigue, nausea, headache, and dizziness. If these symptoms persist, you should see a doctor.

Fasting Blood Sugar Levels

When blood sugar levels drop, the pancreas releases insulin to help bring them back up. Insulin helps move glucose out of the bloodstream and into the cells where it can be used for energy.

Key Terms

So, just how does glycolysis work?

Before we take a closer look, here are some of the key terms you will need to understand in order to have a more solid understanding of glycolysis.

Hexokinase

Hexokinase is an enzyme found inside every cell in your body. It converts glucose into glucose 6 phosphate (Glucose-6-P).

Glucose-6-Phosphate is used to create adenosine triphosphate (ATP), which is the main source of cellular energy.

Phosphofructokinase

PhosphofructoKinase is another enzyme that is found in every cell in your body, except red blood cells. It breaks down fructose-6-phosphate into fructose-1,6-bisphosphate.

Fructose-1,6-Bisphosphate then gets converted into glyceraldehyde 3-phosphate. 

The glyceraldehyde 3 phosphates get broken down into pyruvate. Pyruvate is then oxidized into acetyl coenzyme A, which produces ATP.

Aldolase

Aldolase is an enzyme that catalyzes the conversion of dihydroxyacetone phosphate into glyceraldehyde-3-phosphate and ribulose 5-phosphate.

Ribulose 5-phosphate is then converted into xylulose 5-phosphate. Xylulose 5-Phosphate is then converted into sedoheptulose 7-phosphate. 

Sedoheptulose-7-phosphate is then converted to erythrose 4-phosphate.

Erythrose 4-phosphate is then converted into glyceraldehyde 3-phosphate, which is then converted into pyruvate, which is then oxidized into Acetyl Coenzyme A, which creates ATP.

Enzymes

Enzymes are proteins that speed up chemical reactions within the human body. They are vital to all life on earth.

Enzymes are responsible for breaking down food molecules into smaller units so they can be absorbed by the body. Without enzymes, our bodies could not digest food.

The Glycolytic Pathway: The Simple Version

The Glycolytic Pathway: The Simple Version

The glycolytic pathway is a series of chemical reactions that converts glucose into pyruvate.

This process takes place in cells throughout the body, but it’s particularly important in the liver where most of our glucose metabolism occurs. 

In fact, this pathway is so critical to life that we don’t actually produce any energy from it.

Instead, all of the energy produced by the breakdown of glucose goes towards maintaining cellular function.

Now that you know what each step of glycolysis is, let’s take a quick look at how this process works. Glycolysis can be explained in ten simple steps:

Step 1: Carbohydrates are broken down into their component parts. This is done through digestion.

For example, when you eat bread or pasta, the starch granules break down into individual sugar molecules.

Step 2: Sugar molecules are transported from the digestive tract into the bloodstream.

Step 3: In the liver, these sugar molecules are stored as glycogen.

Step 4: The liver also converts the glycogen into glucose.

Step 5: The glucose travels throughout the body and gets converted into other forms of energy, such as ATP.

Step 6: If there isn’t enough glucose available in the bloodstream, the liver starts converting protein into glucose.

Step 7: Liver converts protein into glucose.

Step 8: The pancreas releases insulin to help transport the glucose into the muscle cells where it is needed.

Step 9: Muscle cells use the glucose to produce energy.

Step 10: When the muscles are no longer using the glucose, the liver converts some of it back into glycogen.

Energy-requiring Phase

In Step 1, carbohydrates are broken down into their component parts. These include monosaccharides, disaccharides, oligosaccharides, polysaccharides, and starches.

  • Monosaccharides are single sugars (glucose, galactose, mannose, etc.).
  • Disaccharides are two sugars linked together (sucrose, maltose).
  • Oligosaccharides are three or more sugars linked together (starch, lactose, raffinose, etc.).
  • Polysaccharides are long chains of sugars (glycogen, cellulose, hemicellulose, pectins, etc.)

Starches are large chains of sugars (amylose, amylopectin)

Energy-producing Phase

After being digested, carbohydrate molecules enter the bloodstream and travel to the liver. In the liver, the carbohydrate molecules are stored as glycogens.

When the liver has excess glycogen, it breaks it down into glucose. Glucose enters the bloodstream.

When the body needs energy, the liver converts some glycogen into glucose. Then, the glucose travels to the muscles and is used to make ATP.

If there is not enough glucose in the bloodstream, the body uses protein to create glucose.

Protein is broken down into amino acids. Some amino acids are then converted into glucose.

Glucose is then released into the bloodstream and travels to the muscles. Once inside the muscles, glucose is used to make ATP and power the cells.

The Glycolytic Pathway: The Scientific Version

There are ten steps to the glycolysis process, and these are as follows: 

  1. The first step is called phosphorylation. This means that glucose is broken down into two smaller molecules of pyruvate by adding a phosphate group (P). In this case, the phosphate group is attached to the third carbon from the end of the sugar molecule. This reaction occurs because the enzyme hexokinase will attach the phosphate group to the third carbon from an oxygen atom on the glucose molecule.
  2. The second step is called decarboxylation. This means that the pyruvate molecule now has one less carbon than it started with. The reason for this is because the pyruvate now contains only one carboxylic acid group instead of two. This is why the whole process is called decarboxylation.
  3. The third step is called oxidation. This means that the remaining single carboxylic acid is oxidized back into a double-bonded carbonyl group. This is what makes up the backbone of our cells.
  4. The fourth step is called reduction. This means that the double bond between the two carbons is reduced back into a single bond.
  5. The fifth step is called dehydration. This means that water is removed from the system.
  6. The sixth step is called hydration. This means that water enters the system.
  7. Step seven is called acetylation. This means another phosphate group is added to the original pyruvate.
  8. The eighth step is called dephosphorylation. This means the phosphate group is removed from the pyruvate. This allows the pyruvate to be used again.
  9. The ninth step is called phosphorylating. This means that the phosphate group is put back onto the pyruvate, allowing it to start the cycle over again.
  10. The tenth step is called gluconeogenesis. This means that the newly formed pyruvate can be converted into other sugars, such as glycogen or lactose.

Gluconeogenesis

If the body does not have enough glucose, the liver will start gluconeogenesis. Gluconeogenesis is the production of glucose from noncarbohydrate sources.

The liver converts fatty acids into acetoacetate and beta-hydroxybutyrate. Acetoacetate is turned into acetone by the liver.

Beta-hydroxybutyrates are converted into acetone and carbon dioxide.

The ketones are then excreted via urine. Liver can convert protein into glucose.

When the liver cannot keep up with its demand for glucose, it begins to convert proteins into glucose.

It first turns them into free amino acids. Free amino acids are then converted to pyruvate. Pyruvate is then converted to glucose.

This process is called gluconeogenesis.

Glucagon

Glucagon is a hormone produced by the pancreas that helps regulate blood sugar levels, and it increases the rate at which glycogen is made in the liver.

Electron Transport Chain

During cellular respiration, four different types of enzymes work together to transfer electrons.

These enzymes are cytochrome c oxidase, succinate dehydrogenase, NADH dehydrogenase, and ubiquinol cytochrome C reductase.

What Are Enzymes?

Enzymes are proteins that speed up reactions inside your body.

They help you digest food, absorb nutrients, convert food into energy, and many other functions. There are thousands of different kinds of enzymes.

What Are The Parts Of An Enzyme?

Enzyme structure

There are four main parts of an enzyme:

  • A Protein Backbone
  • Binding Sites
  • Active Center
  • Allosteric Sites

Protein Backbone

Enzymes are made from long chains of amino acids. Amino acids are the building blocks of proteins. Proteins are linear chains of amino acids.

Each chain is called a polypeptide. Polypeptides are folded into specific shapes by hydrogen bonding between their side groups.

Binding Sites 

A protein binds to its substrate through the interaction of its functional groups with those of the substrate.

In general, these interactions occur between polar atoms such as oxygen and nitrogen, and nonpolar atoms such as carbon and sulfur.

Active Center

When an enzyme works, it changes shape. Its active center is where this happens.

The active center contains a group of amino acids that form bonds with the substrate. The active center is usually located near the middle of the enzyme.

Allosteric Sites

Allosteric sites allow enzymes to work faster or slower than normal. When an allosteric site interacts with another part of the same protein, it affects the shape of the enzyme.

How Do Enzymes Work?

How Do Enzymes Work?

Enzymes change the chemical state of substrates. This process is called catalysis. Catalysts are chemicals that increase the rate at which a reaction occurs.

Enzymes bind to the substrate. This binding causes the substrate to change its shape.

The enzyme then uses its active center to break apart the bond between the substrate and itself.

It does this by changing the shape of the enzyme so that it can fit around the substrate. This results in the release of energy.

Enzymes use energy to break apart their substrates. Types of energetic reactions include:

Enzymatic Reactions

Enzymes cause chemical reactions to happen much faster than they would on their own.

Benzylation Reaction

Enzymes change the chemical composition of a substrate.

Hydrolysis Reaction

Enzymes break down a substance into smaller pieces.

Oxidoreduction Reaction

Enzymes change the oxidation state of a compound.

Reduction Reaction

Enzymes reduce the amount of electrons in a molecule.

Substrate Activation

Activation means that the substrate becomes easier to react with.

Inactivation

Inactivation means that the substrate becomes harder to react with.

Catalytic Cycle

The catalytic cycle is how an enzyme converts a substrate into products.

Proton Transfer Reaction

An enzyme transfers a proton from one place to another.

Electron Transfer Reaction

An enzyme moves an electron from one place to another place.

Intermediate Products

After the enzyme has broken apart the substrate, it produces intermediate products. These intermediate products are not the final product. They are used for other purposes.

Products

The final products are what the enzyme makes when it breaks down a substrate.

What Is The Link Between Enzymes And Glycolysis?

The link between glycolysis and enzymes is that both processes involve the breaking down of carbohydrates. Both processes also require ATP (adenosine triphosphate) and NADH (nicotinamide adenine dinucleotide).

However, while glycolysis requires a catalyst to convert glucose to pyruvate, enzymes do not. Instead, enzymes help glycolysis take place.

As we have mentioned, glycolysis takes place when glucose enters the cell through diffusion. Once inside the cell, glucose is converted into pyruvate.

Pyruvate is a simple sugar made up of two carbon atoms and three hydrogen atoms.

Glucose is converted into pyruvic acid by the enzyme hexokinase. Hexokinase is found in the cytoplasm of cells.

In order to make more pyruvate, the body needs oxygen. Oxygen binds to pyruvic acid and creates water. This step is known as decarboxylation.

Decarboxylation releases energy. Energy is released because the bonds holding together the molecules have been broken.

This allows the molecules to move away from each other. This movement is known as entropy.

When oxygen combines with pyruvic acid, it forms acetaldehyde. Acetaldehyde is a toxic substance. To prevent toxicity, acetaldehyde is oxidized back into pyruvic acid.

Oxidation releases energy. This process is called dehydrogenation. Dehydrogenation occurs when electrons are removed from the acetaldehyde molecule.

In order to keep the reaction going, the body must get rid of excess pyruvic acid. Excess pyruvic acid can be eliminated by converting it into lactic acid.

Lactic acid is a simple sugar made of four carbon atoms and six hydrogen atoms. Lactic acid is produced during aerobic respiration. 

When lactic acid is formed, it causes the pH level to drop. This lowers the number of protons present in the solution.

A decrease in the number of protons results in a lower concentration of H+ ions. 

Lowering the concentration of H+ ions decreases the activity of the enzyme lactate dehydrogenase. Lactate dehydrogenase helps convert lactic acid into pyruvate again.

After all of these steps, the result is pyruvate. It is now ready to enter the citric acid cycle.

Final Thoughts

Glycolysis is an essential process in the body, and one that must be seen in the wider context of the systems and operations of the body.

By understanding glycolysis, you will  have a better understanding of the way your body operates and functions on a daily basis.

Jennifer Dawkins