Glycolysis, also known as the Embden-Meyerhof-Parnas pathway, is the process through which the composition of glucose is modified to extract enough energy to fuel the cells.
Glycolysis is a process that allows cellular respiration and metabolism through the breakdown of glucose.
In this article, we are going to find out more about this fascinating process. Let's get started then, shall we? Now, understanding glycolysis is all about asking the right questions!
What is Glycolysis?
Translated - the term glycolysis means sweet (“glyk”) splitting (“lysis”).
It is a series of reactions that occur within the cytoplasm of a cell allowing the extraction of energy from glucose.
Interestingly, this system is found in almost all living organisms as it is the foundation of aerobic and anaerobic cellular respiration.
Glycolysis is, therefore, the first stage of the metabolic process of energy production in living beings.
In the case of human beings, some tissues which primarily obtain energy via aerobic metabolism are also able to achieve the same via anaerobic glycolysis in case of oxygen deficiency.
This might happen, for example, if the striated muscle is subjected to an extended amount of strain. Striated muscles tend to adapt, allowing the body to meet its specific needs using anaerobic glycolysis for energy production.
The end product of this process is lactic acid. And the accumulation of lactic acid in striated muscles is the reason why you experience cramps following a heavy work-out session!
However, certain muscles, for instance, cardiac muscle fibers, are not able to adapt to an oxygen-deficient environment and thus are unable to cope under anaerobic conditions.
How does Glycolysis work?
As mentioned before, glycolysis consists of a series of reactions, 10 in total, thanks to which glucose is transformed into two molecules of a three-carbon compound (phosphoglyceraldehyde), and subsequently into two pyruvate molecules.
Glucose is an organic molecule that can be found in the blood as a result of the transformation of carbohydrates into sugars.
Thanks to the process of glycolysis, glucose is converted into pyruvic acid.
On a lighter note, you need to spend money make more money in business, right? Think of glycolysis as an energy business that occurs within the cells. Your cells expend energy to gain more energy.
The glycolytic pathway can thus be divided into two phases:
- The preparatory phase (commonly called the “investment phase”) includes the first five steps of the Glycolysis process. The cell invests 2 ATP in adding a phosphate group to the substrate (phosphorylation), by means of two kinases (enzymes that transfer a phosphate group from ATP to a molecule).
- The energy recovery phase (more commonly called the “payoff phase”) includes the remaining steps of the Glycolysis process. During this phase, a cell produces 4ATP and 2NADH. In this phase, we start from the 2 glyceraldehyde-3-phosphate (G3P) molecules produced at the end of the previous phase.
NAD+ is reduced to NADH as it is used in the oxidation passage of the substrate (G3P) by the enzyme dehydrogenase: as a matter of fact, the whole process of Glycolysis consists of an oxide-reduction. The reduced factors can be used in mitochondria to synthesize additional ATP in the internal membrane, in the presence of oxygen, or in the external membrane.
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The two pyruvate molecules enter the citric acid cycle to give additional energy with further oxidations in the presence of an aerobic environment.
Alternatively, the molecules pyruvate can be can undergo fermentation in an anaerobic environment to give rise to ethanol (alcohol fermentation) and lactic acid (lactic acid fermentation).
Wisdom from Grand Wizard Zeo
“Please note that:
1NADH produces 2.5 ATPs via the ETC, and 1FADH produces 1.5 ATPs via the ETC.
The energy spent in aerobic glycolysis is 2ATPs, while the energy gained is 4ATPs and 2NADH with a byproduct of 2pyruvate molecules - thus, the net ATP generated is 7 ATPs.
- 2ATPs + 4ATPs + 2.5 x 2 ATPs = 7ATPs
However, in anaerobic glycolysis, the NADH produced is utilized to convert pyruvate to lactate giving rise to a net yield of only 2ATPs.
In aerobic metabolism, each pyruvate molecule enters the Krebs cycle giving rise to an Acetyl-CoA molecule (generating 2NADH in the process). This reaction is called the link reaction as it links the glycolytic pathway to the Krebs cycle. Total ATPs generated from the link reaction are 5 ATPs.
Furthermore, the 1 acetyl-CoA molecule generates 1ATP, 3NADH, H+, and 1FADH. Thus, it’ll give a yield of:
1 + 3 x 2.5 + 1.5 = 10 ATPs
2 cycles of TCA occur for 1 glucose molecule as 2 pyruvate molecules are generated, bringing the total number of ATPs generated to 20 ATPs.
Joining the dots:
Glycolysis + Link reaction + 2 x TCA = 7 + 5 + 20 = 32ATPs.
Now, as mentioned before, glycolysis occurs in the cytosol. For the NADH produced in glycolysis to form 2.5 ATPs, it needs to get into the mitochondria. The transport of NADH into the mitochondria requires 1 ATP molecule per NADH molecule.
Therefore, 2 ATPs are lost in bringing the NADH from the glycolytic pathway into the mitochondria.
Thus, the net energy generated from 1 molecule of glucose is 30 ATPs.
Old calculation
Do note that however, some follow the old conversion rates that state:
1 NADH → 3 ATPs, and 1 FADH → 2 ATPs
Considering this, the calculation then becomes:
Glycolysis (8 ATPs) + Link reaction (6ATPs) + 2 x TCA (24ATPs) - Glycolytic NADH transport into mitochondria (2ATPs) = 36 ATPs.”
References:
- Harper’s Illustrated Biochemistry 31st Edition - Page no: 157 - 159; Figure 17-2
- https://link.springer.com/referenceworkentry/10.1007%2F978-3-642-11274-4_503
- https://www.ncbi.nlm.nih.gov/books/NBK482303/
- https://www.ncbi.nlm.nih.gov/books/NBK21624/
- https://www.ncbi.nlm.nih.gov/books/NBK21163/
- https://www.ncbi.nlm.nih.gov/books/NBK26894/
Author’s footnote
The glycolytic pathway and its energetics have been tested time and again in multiple competitive examinations.
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