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Where does Glycolysis Occur? (A guide)

In this brief guide, we will answer the question “Where does Glycolysis Occur?” as well as some other questions related to Glycolysis.

Where does Glycolysis occur?

Glycolysis occurs in the Cytoplasm of cells. More specifically, Glycolysis occurs in the mitochondrion, where the citric acid cycle occurs in the mitochondrial matrix, and oxidative metabolism or Glycolysis occurs at the internal folded mitochondrial membranes (cristae).

Everyone is familiar with the adage “Mitochondria is the powerhouse of the cell”, which is something everyone learns in school but doesn’t really know what it means.

The reason why Mitochondria is known as the powerhouse of the cell is because the processes of energy production happen in the Mitochondria, which include the Citric Acid cycle, or Krebs cycle and Glycolysis occur.

These two processes are the reason any cell has energy, and these processes happen across all kinds of cells in the body because all cells need energy to function as they are supposed to, grow and interact with the other cells, and this in turn causes changes in the body and keep it running.

Glycolysis can occur in aerobic (oxygenated) and anaerobic (non-oxygenated) conditions, and in both conditions it occurs in the Mitochondria.

Glycolysis is the reason behind ketosis and other types of metabolic processes that occur in cells and it is a necessary thing that people need to be aware of how they will lose weight, how much sugars or carbohydrates to consume, how to make sure they stay as healthy as possible.

There are also many medical conditions that may disrupt the occurrence of glycolysis or make use of it to cause problems, like Cancer or Diabetes.

A reason why both the krebs cycle and glycolysis occur in the cytoplasm is because one of the end products of glycolysis, Pyruvate, is one of the necessary enzymes that is fed into the Krebs cycle and its lack can lead to fumarase deficiency.

What is Glycolysis?

Simply put, glycolysis is the process through which glucose is turned into energy.

The term glycolysis comes from glycose, an older term for glucose and lysis, which means degradation, and this name alone tells us what glycolysis does.

Glycolysis is the metabolic pathway that converts glucose into pyruvate, pyruvic acid, and a hydrogen ion, and the free energy from this process is released in this process is used to form the high-energy molecules ATP known as adenosine triphosphate and NADH, which refers to reduced nicotinamide adenine dinucleotide.

There are many sequences of reactions that occur in the process of Glycolysis and there are 10 enzyme-catalyzed reactions in all that are involved.

Most monosaccharides like fructose and galactose, can be converted to one of these intermediates and they can either be directly useful or utilized as steps in the overall reaction. 

Glycolysis is an oxygen-independent metabolic pathway, but there are instances where it can occur with limited oxygen as well, which is called Anaerobic glycolysis.

The wide occurrence of glycolysis also shows that it is an ancient metabolic pathway, which means that it must have been present in the non-evolved primitive cells as well, albeit in a less advanced way.

While in most organisms glycolysis occurs in the cytosol, or cytoplasm, there may be primitive organisms where it might occur elsewhere, but for the purposes of common knowledge only the glycolysis that occurs in Eukaryotic cells has been discussed here.

The most common type of glycolysis is the Embden–Meyerhof–Parnas (EMP) pathway, discovered by Gustav Embden, Otto Meyerhof, and Jakub Karol Parnas, but there are others that exist as well, like the Entner–Doudoroff pathway and various heterofermentative and homofermentative pathways. 

The glycolysis pathway can be separated into two phases, which contain further steps as well, and these are:

  • The Preparatory (or Investment) Phase: Here, ATP is consumed.
  • The Payoff Phase: Here ATP is produced.

Stages of Glycolysis

The stages of glycolysis are as follows, starting with the preparatory stage, and then moving on to the Payoff phase, and the first five are preparatory and the next few are Payoff.

The first five steps of glycolysis consume energy to convert the glucose into two three-carbon sugar phosphates, which is also known as G3P.

The first step is known as Phosphorylation of glucose by a family of enzymes called hexokinases to form glucose 6-phosphate, or G6P, and this is the reaction that consumes ATP.

Next, the G6P is rearranged into fructose 6-phosphate, or F6P, by glucose phosphate isomerase and this chemical is meant to enter the glycolytic pathway by phosphorylation at this point.

This reaction, catalyzed by phosphofructokinase 1, or PFK-1, is coupled with the hydrolysis of ATP and it is irreversible, or it requires a different pathway to do the reverse conversion during gluconeogenesis. Therefore, this reaction is a key regulatory point, and it is also a rate-limiting step.

The molecule in the previous reaction is then destabilized, and this allows the hexose ring to be split by aldolase into two triose sugars, namely, dihydroxyacetone phosphate, which is a ketose, and glyceraldehyde 3-phosphate, which is an aldose.

Also, there are two classes of aldolases: class I aldolases, present in animals and plants, and class II aldolases, present in fungi and bacteria; the two classes use different mechanisms in cleaving the ketose ring.

Next, the Triosephosphate isomerase rapidly interconverts dihydroxyacetone phosphate with glyceraldehyde 3-phosphate, GADP, and this molecule proceeds further into glycolysis.

After this step is completed, glycolysis enters the payoff phase, where there is a net gain of the energy-rich molecules ATP and NADH.

There are two triose sugars form the glycolysis in the preparatory phase, and each reaction in the pay-off phase occurs twice per glucose molecule. 

The total yield of this process is 2 NADH molecules and 4 ATP molecules, and there is a net gain of 2 NADH molecules and 2 ATP molecules from the glycolytic pathway per glucose.

The next step is the enzymatic transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP by phosphoglycerate kinase, which in turn forms ATP and 3-phosphoglycerate. 

When this step occurs, glycolysis is said to have reached the break-even point, where 2 molecules of ATP were consumed, and 2 new molecules have now been synthesized. 

This is one of the two substrate-level phosphorylation steps, and it requires ADP, this is why when the cell has plenty of ATP and little ADP, this reaction does not occur. 

Also, ATP decays relatively quickly when it is not metabolized, which also makes this an important regulatory point in the glycolytic pathway.

After this step, the Phosphoglycerate mutase isomerises 3-phosphoglycerate into 2-phosphoglycerate.

Lastly, a final substrate-level phosphorylation forms a molecule of pyruvate and a molecule of ATP by means of the enzyme pyruvate kinase and this is an additional regulatory step, similar to the phosphoglycerate kinase step.

What are the end products of Glycolysis?

The end products of Glycolysis are Pyruvate, ATP and NADH.

Pyruvate

Pyruvic acid is extremely important because it supplies energy to cells through the citric acid cycle, which is also known as the Krebs cycle when oxygen is present.

It also helps when it ferments to produce lactate when oxygen is lacking.

When there is insufficient oxygen, this acid is broken down anaerobically, and this process creates lactate in animals and ethanol in plants and microorganisms.

Pyruvate that occurs out of glycolysis is converted by the process of fermentation to lactate and the enzyme lactate dehydrogenase is used in this process.

Also, the coenzyme NADH is used along with pyruvate in lactate fermentation, and this may also create acetaldehyde with the enzyme pyruvate decarboxylase which then turns to ethanol in alcoholic fermentation.

Also, Pyruvate is sold as a weight-loss supplement, but there isn’t enough evidence of this yet.

A systematic review of six trials found a statistically significant difference in body weight with pyruvate compared to placebo but it should be noted that all of the trials had methodological weaknesses and the magnitude of the effect was small. 

Also, there were adverse effects to the use of pyruvate for weight loss, such as diarrhea, bloating, gas, and increase in low-density lipoprotein (LDL) cholesterol. 

ATP

ATP is the primary and most important end product of Glycolysis, because Adenosine triphosphate (ATP) provides energy to drive many processes in living cells.

ATP is extremely important in muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis and therefore it is no surprise that it is found in all known forms of life.

ATP is often referred to as the “molecular unit of currency” of intracellular energy transfer because of the important roles it plays in the metabolic processes of the body, and how much energy in the cells is simply due to ATP.

ATP is produced from the payoff stage of Glycolysis, and when it is consumed in metabolic processes, it converts either to adenosine diphosphate (ADP) or to adenosine monophosphate (AMP). 

Also, in addition to glycolysis, there are other processes that regenerate ATP so that the human body recycles its own body weight equivalent in ATP each day.

ATP is also extremely crucial because it is a precursor to DNA and RNA, and is used as a coenzyme.

A very important use of ATP when it is synthesized chemically outside the body is to correct heart rate or improve cardiovascular activity.

NADH

Another end product of Glycolysis is Nicotinamide adenine dinucleotide which also has several essential roles in metabolism. 

To begin with, NADH acts as a coenzyme in redox reactions, where it acts as a donor of ADP-ribose moieties in ADP-ribosylation reactions, or as a precursor of the second messenger molecule cyclic ADP-ribose.

It also acts as a substrate for bacterial DNA ligases and a group of enzymes called sirtuins that use NAD+ to remove acetyl groups from proteins. 

Also, apart from the metabolic functions it serves, NADH leads to NAD+, which is an adenine nucleotide that can be released from cells spontaneously and by regulated mechanisms, which means that it has important roles outside the cell as well.

The most significant aspect of NADH is that because cancer cells utilize increased glycolysis, and because NAD enhances glycolysis, nicotinamide phosphoribosyltransferase, which is a salvage pathway for NAD, is often amplified in cancer cells.

Also, NADH has been studied for its potential use in the therapy of neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease, but so far there have not been any leads in this regard, as seen in a placebo-controlled clinical trial where NADH for Parkinson’s failed to show any effect.

Conclusion

In this brief guide, we answered the question “Where does Glycolysis Occur?” as well as some other questions related to Glycolysis.

Glycolysis is an important process in the body and it can be the basis of many problems, both physical and mental, the most significant of which is diabetes, a common lifestyle disorder.

If you have any more questions about Glycolysis similar to Where does Glycolysis occur, please feel free to reach out to us.

Frequently Asked Questions (FAQs): Where does Glycolysis occur?

What are the steps of glycolysis?

The steps of Glycolysis include:

Hexokinase. 
Phosphoglucose Isomerase. 
Phosphofructokinase. 
Aldolase. 
Triosephosphate isomerase. 
Glyceraldehyde-3-phosphate Dehydrogenase.
Phosphoglycerate Kinase. 
Phosphoglycerate Mutase.

What does glycolysis produce?

Glycolysis produces 2 ATP, 2 NADH, and 2 pyruvate molecules.

Glycolysis is the aerobic catabolic breakdown of glucose, which produces energy in the form of ATP (Adenosine Triphosphate, NADH, and pyruvate, and this last chemical enters the citric acid cycle in turn, which produces more energy.

What is the end product of glycolysis?

The end products of the process of glycolysis are pyruvate, NADH and ATP when it occurs in aerobic settings and lactate in anaerobic settings. Pyruvate is a product of glycolysis that in turn enters the Krebs cycle which is involved in more energy production.

Citations

https://www.nature.com/scitable/content/metabolism-in-a-eukaryotic-cell-glycolysis-the-14705577/#:~:text=Glycolysis%20takes%20place%20in%20the,folded%20mitochondrial%20membranes%20(cristae).

https://www.microscopemaster.com/glycolysis.html

https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/7%3A_Cellular_Respiration/7.2%3A_Glycolysis/7.2D%3A_Outcomes_of_Glycolysis#:~:text=1%3A%20Glycolysis%20produces%202%20ATP,cycle%20to%20produce%20more%20energy.

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