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Cells produce energy by breaking down macromolecules. This is a biochemical process called respiration, which involves the conversion of “food energy” into chemical energy in the form of adenosine triphosphate (ATP). The first step of this intricate and tightly regulated process is called glycolysis. 

There is much to learn about this complex process.

 What is it? How does it work? Where does glycolysis occur? What is the end product of glycolysis? Is it aerobic or anaerobic? 

We answer all of your questions concerning glycolysis.

What is Glycolysis?

The word “glycolysis” is derived from two Greek words: “Glykos,” which means sweet and “Lysis” which means splitting or to split. Combining the two terms together, you get a term that describes the metabolic breakdown of glucose to release energy.

For many organisms, biochemical reactions (e.g. muscle contraction) require energy in the form of adenosine triphosphate (ATP). During this process, the primary source of energy (aka glucose) must be broken down through a series of subsequent processes to release chemical energy.

Apart from ATP, the metabolic pathways release two molecules of pyruvate (a three-carbon molecule) and nicotinamide adenine dinucleotide (NADH).

Where Does Glycolysis Occur?

So where does glycolysis take place? What part of the cell does it occur in?

Since glycolysis is the first phase of cellular respiration, it is metabolism in a eukaryotic cell. Glycolysis takes place in the cytoplasm. In the cell, it takes place in the cytosol, which is the fluid containing the cell’s organelles. The next step of cellular respiration occurs in the mitochondria.

How Does Glycolysis Work?

White and red cells
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As mentioned, glycolysis occurs in the cytosol of the cell and it can occur both anaerobically (without the presence of oxygen) and aerobically (without the presence of oxygen). The presence of oxygen determines the type of process that will occur post-glycolysis, as we will discuss later in this article.

The process of glycolysis starts with the consumption of energy (which is in the form of adenosine triphosphate). This process is called the preparatory phase, which is followed by the payoff phase. This phase involves the release of energy (which is also in the form of adenosine triphosphate). The glucose breakdown results in pyruvate. During the process, the glucose is repeatedly catalyzed by its enzyme.

What are the 2 Phases of Glycolysis?

The glycolysis process is a complex one that can be separated into two broad phases: the energy-releasing phase and the energy-requiring phase.

Energy-Requiring Phase

This first step involves the glucose molecule splitting into two 3-carbon molecules (aka pyruvates). 

The steps for this process include:

  • Glucose is converted into glucose-6-phosphate using one ATP, which will eventually be converted to ADP.
  • Glucose-6-phosphate is converted into fructose-6-phosphate by phosphoglucose isomerase.
  • Fructose-6-phosphate is converted into fructose-1,6-bisphosphate is converted by phosphofructokinase. The process requires only ATP, which is converted to ADP.
  • The reaction is catalyzed by bisphosphate aldolase to convert fructose-1,6-bisphosphate into two three-carbon molecules.
  • Finally, the conversion of DHAP is catalyzed by triose phosphate isomerase. DHAP is converted to glyceraldehyde-3-phosphate.

Energy-Releasing Phase

Otherwise known as the “energy payoff phase,” this phase converts the glyceraldehyde-3-phosphate molecules to three-carbon sugars without pyruvate, the phosphate group. This reaction releases energy as the molecules gain more stability by converting two ADP to ATP and one NAD+ to NADH for each glyceraldehyde-3-phosphate molecule.

The detailed steps of this glycolysis phase are as follows:

  • A redox reaction converts glyceraldehyde-3-phosphate into bisphosphoglycerate, which releases an H+ ion from NAD+ to NADH.
  • Phosphoglycerate kinase simultaneously converts ADP to ATP by converting 1,3-bisphosphoglycerate to 3-phosphoglycerate via phosphoglycerate kinase.
  • 3-phosphoglycerate converts to 2-phosphoglycerate via Phospoglycerate mutase.
  • 2-phosphoglycerate is converted to phosphoenolpyruvate (PEP) by the enolase.
  • PEP is converted to pyruvate, which simultaneously converts ADP to ATP.

Is Glycolysis Aerobic or Anaerobic?

There are two types of cellular respiration: anaerobic and aerobic. 

One occurs in the absence of oxygen (anaerobic) whereas the other occurs in the presence of oxygen (aerobic). Both begin with glycolysis, which involves splitting glucose.

Infographic of aerobic respiration and the process of glycolysis
Image Credit: VectorMine / Shutterstock.com

Glycolysis is an anaerobic process since it doesn’t need oxygen to proceed. This process produces a minimal amount of ATP. Electron transport and the Krebs cycle don’t need oxygen to proceed. In the presence of oxygen, these processes will produce more ATP than just glycolysis.

What is the End Product of Glycolysis?

Using the power of ATP and with assistance from different enzymes, glycolysis forms three products:

  1. NADH (two molecules)
  2. Pyruvate (two molecules)
  3. Adenosine triphosphate

So, what happens to the products of glycolysis? It depends on the presence of oxygen. For example, in aerobic conditions, the oxygen enables the pyruvate to enter the Krebs cycle (also known as the citric acid cycle) so it can break down into more energy. As a part of this process, oxygen must act as the final acceptor of the electrons. Oxygen is also required to oxidize NADH back into NAD+ to allow the glycolysis pathway to continue.

This process occurs in the cell’s mitochondria. In red blood cells, the glycolysis pathway becomes the primary source of energy; the Krebs cycle doesn’t occur in red blood cells due to the mitochondria’s absence.

In anaerobic conditions, it’s impossible for oxygen to serve as the final electron acceptor. If this is the case, lactate dehydrogenase converts pyruvate into lactate, which is lactic acid’s conjugate base. 

Although pyruvate is being converted to lactate, the conversion of NADH to NAD+ occurs at the same time. This process is called lactic acid fermentation, which occurs when the muscle consumes the oxygen all at once, causing the muscles to cramp. You may experience pain when this happens. The pain is due to the lactate acid accumulated in the blood.

Where Do Glycolysis Products Go?

The pyruvate enters the mitochondria to be converted into acetyl CoA. Next, this new molecule enters the aerobic respiration of the Krebs cycle. After the pyruvate’s reaction to the electron transport chain. ATP is generated from a glucose molecule during cellular respiration, which includes the two ATP from glycolysis.

Glycolysis, as complex as the process sounds, is the most crucial pathway since most of the energy used by the cells comes from glucose. It is one of the “ancient” pathways that are used by all living organisms.

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