Difference Between Glycolysis and Krebs Cycle is that Glycolysis or glycolysis is a metabolic pathway that serves as an initial step for carbohydrate catabolism in living beings while The Krebs cycle, or citric acid cycle, generates most of the electron carriers (energy) that will be connected in the electron transport chain (CTE) in the last part of the cellular respiration of eukaryotic cells.
What is glycolysis?
Glycolysis or glycolysis is a metabolic pathway that serves as an initial step for carbohydrate catabolism in living beings. It consists mainly of the breakdown of glucose molecules by oxidation of the glucose molecule, thus obtaining amounts of chemical energy usable by the cells.
This series of processes can occur in the presence or absence of oxygen, and occurs in the cytosol of cells, as an initial part of cellular respiration. In the case of plants, it is part of the Calvin cycle.
The reaction rate of glycolysis is so high that it was always difficult to study. It was formally discovered in 1940 by Otto Meyerhoff and a few years later by Luis Leloir, although all this thanks to previous work of the late nineteenth century.
This metabolic route is usually named through the surnames of the greatest contributors to its discovery: the Embden-Meyerhoff-Parnas route. On the other hand, the word “glycolysis” comes from the Greek glycos , “sugar,” and lysis, “break.”
Phases of glycolysis
Glycolysis is studied in two distinct phases, which are:
First phase: Energy expenditure
In this first stage, the glucose molecule is transformed into two of glyceraldehyde, a low energy efficiency molecule. Two biochemical energy units ( ATP, Adenosine Triphosphate) are consumed for this. However, in the next phase, the energy obtained will be doubled thanks to this initial investment.
Thus, phosphoric acids are obtained from ATP, which provide phosphate groups with glucose, composing new and unstable sugar. This sugar soon divides and results in two similar molecules, phosphated and with three carbons .
Despite having the same structure, one of them is different, so it is additionally treated with enzymes to make it identical to the other, thus obtaining two identical compounds. All this occurs in a chain of five-step reactions.
Second phase: obtaining energy
The glyceraldehyde of the first phase becomes the second in a compound of high biochemical energy. To do this, it is coupled with new phosphate groups, after losing two protons and electrons.
Thus, these intermediate sugars are subjected to a process of change that gradually releases their phosphates, to obtain four ATP molecules (double the amount invested in the previous step) and two pyruvate molecules, which will continue their cycle on your own, glycolysis is over. This second phase of reactions consists of five more steps
The main functions of glycolysis are simple: obtaining the necessary biochemical energy for the different cellular processes. Thanks to the ATP obtained from the breakdown of glucose, numerous life forms get the energy to survive or to fire much more complex chemical processes.
Therefore, glycolysis usually acts as a trigger or biochemical detonator for other major mechanisms, such as the Calvin cycle or the Krebs cycle. Both eukaryotes and prokaryotes are glycolysis practitioners.
Importance of glycolysis
Glycolysis is a very important process in the field of biochemistry. On the one hand, it has great evolutionary importance since it is the base reaction for the increasingly complex life and for the support of cell life. On the other hand, his study reveals details about the various existing metabolic pathways and about other aspects of the life of our cells.
For example, recent studies at universities in Spain and the University Hospital of Salamanca detected links between neuronal survival in the brain and the increase in glycolysis to which neurons can be subjected. This could be key to understanding diseases such as Parkinson’s disease or Alzheimer’s disease.
Glycolysis and gluconeogenesis
If glycolysis is the metabolic pathway that breaks the glucose molecule for energy, gluconeogenesis is a metabolic path that goes the opposite way: the construction of a glucose molecule from non-glycidic precursors, that is, not linked at all With the sugars.
This process is almost exclusive to the liver (90%) and the kidneys (10%) and takes advantage of resources such as amino acids, lactate, pyruvate, glycerol, and any carboxylic acid as a carbon source. In the absence of glucose, such as fasting, they allow the body to remain stable and functioning during a prudential period, while glycogen stores in the liver last.
What is the Krebs Cycle:
It is also known as the citric acid cycle because it is a chain of oxidation, reduction, and transformation of citrate.
Citrate or citric acid is a six-carbon structure that completes the cycle regenerating into oxaloacetate. Oxaloacetate is the molecule necessary to produce citric acid again.
The Krebs cycle is only possible thanks to the glucose molecule produced by the Calvin cycle or the dark phase of photosynthesis.
Glucose, through glycolysis, will generate the two pyruvates that will be produced, in what is considered as the preparatory phase of the Krebs cycle, acetyl-CoA, necessary to obtain citrate or citric acid.
The reactions of the Krebs cycle occur in the inner membrane of the mitochondria, in the intermembrane space that is located between the cristas and the outer membrane.
This cycle requires enzymatic catalysis to function, that is, it needs the help of enzymes so that the molecules can react with each other and it is considered a cycle because there is a reuse of the molecules.
Krebs cycle steps
The beginning of the Krebs cycle is considered in some books from the transformation of glucose generated by glycolysis into two pyruvates.
In spite of this, if we consider the reuse of a molecule to designate a cycle since it is the regenerated four-carbon oxaloacetate molecule, we will consider the phase before it as preparatory.
In the preparatory phase, the glucose obtained from glycolysis will be separated to create two three-carbon pyruvates, also producing an ATP and a NADH per pyruvate.
Each pyruvate will oxidize by transforming into a two-carbon acetyl-CoA molecule and generating a NADH of NAD +.
The Krebs cycle runs each cycle twice simultaneously through the two acetyl-CoA coenzymes generated by the two pyruvates mentioned above.
Each cycle is divided into nine steps where the most relevant catalyst enzymes for the regulation of the necessary energy balance will be detailed:
The two-carbon acetyl-CoA molecule binds to the four-carbon oxaloacetate molecule.
Free CoA group.
It produces six-carbon citrate (citric acid).
The second and third step
The six-carbon citrate molecule is converted into an isocitrate isomer, first removing a water molecule to, in the next step, incorporate it again.
Free water molecule.
It produces isocitrate and H2O isomer.
The six-carbon isocitrate molecule oxidizes into α-ketoglutarate.
Liberate CO 2 (a carbon molecule).
It produces five-carbon α-ketoglutarate and NADH + NADH.
Relevant enzyme: isocitrate dehydrogenase.
The five-carbon α-ketoglutarate molecule is oxidized to obtain succinyl-CoA.
Release CO 2 (a carbon molecule).
It produces four-carbon succinyl-CoA.
Relevant enzyme: α-ketoglutarate dehydrogenase.
The four-carbon succinyl-CoA molecule replaces its CoA group with a phosphate group producing succinate.
It produces four-carbon succinate and ATP of ADP or GTP of GDP.
The four-carbon succinate molecule oxidizes to form fumarate.
It produces four-carbon fumarate and FDA FADH2.
Enzyme: allows FADH2 to transfer its electrons directly to the electron transport chain.
The four-carbon fumarate molecule is added to the malate molecule.
Release H 2 O.
Produces four carbon malate.
The four-carbon malate molecule is oxidized by regenerating the oxaloacetate molecule.
Produces: four carbon oxaloacetate and NADH NADH.
Krebs cycle products
The Krebs cycle produces the vast majority of theoretical ATPs generated by cellular respiration.
The Krebs cycle from the combination of the four-carbon oxalacetate or oxalacetic acid molecule with the two-carbon acetyl-CoA coenzyme to produce citric acid or six-carbon citrate will be considered.
In this sense, each Krebs cycle produces 3 NADH of 3 NADH +, 1 ATP of 1 ADP, and 1 FADH2 of 1 FAD.
As the cycle occurs twice simultaneously due to the two acetyl-CoA coenzymes product of the previous phase called pyruvate oxidation, it must be multiplied by two, which results in:
- 6 NADH that will generate 18 ATP
- 2 ATP
- 2 FADH2 that will generate 4 ATP
The previous sum gives us 24 of the 38 theoretical ATPs that result from cellular respiration.
The remaining ATP will be obtained from glycolysis and oxidation of pyruvate.
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