Difference Between Glycolysis and Kreb’s Cycle

The differences between glycolysis and Kreb’s cycle are mainly due to the following factors like:

  • The occurrence of the process
  • Type of respiration
  • Net energy production i.e. ATP

The occurrence of the Process: Glycolysis occurs in the cytoplasm whereas Kreb’s cycle occurs inside the mitochondria.

Type of Respiration: Glycolysis can carry out both aerobic and anaerobic kind of respiration whereas Kreb’s cycle only carries out aerobic respiration.

Net energy production: The net production of ATP in glycolysis is 8 molecules whereas in Kreb’s cycle the net production is 12 molecules.

Content: Glycolysis Vs Kreb’s Cycle

  1. Comparison Chart
  2. Definition
  3. Process
  4. Net Production
  5. Key Differences
  6. Conclusion

Comparison Chart

PropertiesGlycolysisKreb’s cycle
Alternative nameEMP pathway (Embden, Meyerhof & Parnas pathway)TCA cycle (Tri-Carboxylic Acid cycle)
SubstrateGlucosePyruvate
End productPyruvateOxaloacetate
OccurrenceCytoplasmMitochondria
Respiration stageFirst stage of cellular respirationSecond stage of cellular respiration
Flow of pathwayLinearCyclic
Oxidative phosphorylationDo not involved in oxidative phosphorylationInvolved in oxidative phosphorylation
Respiration typeCytoplasmic respirationMitochondrial respiration
ATP inputConsumes 2molecules of ATPDo not consumes ATP
ATP outputProduces 8molecules of ATPProduces 24molecules of ATP
Number of enzymes neededTenEight
Oxygen requirementCan occur in both presence and absence of oxygenOccurs in presence of oxygen
CO2 evolvementAbsentPresent

Definition of Glycolysis Vs Kreb’s Cycle

Glycolysis: It defines as the lysis or degradation of a simple carbohydrate i.e. Glucose into chemical compound Pyruvate which is the anion of the pyruvic acid.
It was discovered by Embden, Meyerhof and Parnas that’s why glycolysis also refers to the EMP pathway, which is named after the scientists. The glycolysis process occurs in a cell cytosol i.e. the extramitochondrial soluble fraction. It is the linear pathway, which involves sequential chemical modification, each by different enzymes.

Kreb’s cycle: It defines as the oxidation of key intermediate that is Pyruvate formed by the glycolysis process to the end product Oxaloacetate.
It was discovered by Szent Gyorgyi and Hans Krebs that’s why it also refers to Szent-Gyorgyi- Krebs cycle. The Kreb’s cycle occurs inside the mitochondria. It is the cyclic pathway, where the cycle continues in the same pattern.

Process

The process of Glycolysis

It divides into two phases namely preparatory or investment phase and pay-off phase.


Preparatory Phase

It involves the conversion or phosphorylation of Glucose into Glyceraldehyde 3-Phosphate. During this phase, ATP is consumed by the cell. Thus, the preparatory phase involves the following step:

preparatory phase

  • Uptake of glucose by the cells and its phosphorylation: The glucose is uptaken by the cell and phosphorylates into glucose 6-phosphate. This reaction consumes the 1molecule of ATP. The reaction is catalyzed by an enzyme “Hexokinase”.
  • Conversion of glucose 6-phosphate: In this conversion of glucose 6-phosphate to fructose 6-phosphate by the enzyme “Phosphohexose isomerase”.
  • Conversion of fructose 6-phosphate: In this, conversion of fructose 6-phosphate to fructose 1, 6-biphosphate by the enzyme “Phosphofructokinase-1”. This reaction consumes another molecule of ATP.
  • The transition of fructose 1, 6-biphosphate: In this, splitting of fructose 1, 6-phosphate occurs which forms two molecules of triose phosphates. One is glyceraldehyde 3-phosphate (Aldotriose) and other is Dihydroxyacetone phosphate (Ketotriose). This reaction catalyzes by an enzyme “Aldolase”. The reaction of Glyceraldehyde 3-phosphate and Dihydroxyacetone phosphate is the reversible reaction, catalyzes by an enzyme “Triosephosphate isomerase”.

Pay-off phase

It involves the conversion or oxidation of Glyceraldehyde 3-Phosphate to pyruvate. During this phase, ATP is released by the cell. Thus, the pay off phase involves the following step:

pay off phase

  • Oxidation of Glyceraldehyde 3-phosphate: In this reaction, Glyceraldehyde 3-phosphate oxidizes into 1, 3-Biphosphoglycerate. Here two molecules of NADH+ releases. This reaction catalyzes by an enzyme “Glyceraldehyde 3-phosphate dehydrogenase”.
  • Conversion of 1, 3-Biphosphoglycerate: In this reaction, 1, 3-Biphosphoglycerate converts into 3-Phosphoglycerate by forming two molecules of ATP. This reaction catalyzes by an enzyme “Phosphoglycerate kinase”.
  • The transition of 3-Phosphoglycerate: In this reaction, 3-Biphosphoglycerate converts into 2-Phosphoglycerate catalyzes by an enzyme “Phosphoglycerate mutase”.
  • Degradation of 2-Phosphoglycerate: In this reaction, 2-Phosphoglycerate converts into Phosphoenol pyruvate by releasing two molecules of H2O, catalyzes by an enzyme “Enolase”.
  • Degradation of Phosphoenolpyruvate: In this reaction, Phosphoenolpyruvate converts into Pyruvate by releasing two molecules of ATP, catalyzes by an enzyme “Pyruvate kinase”.

The process of Kreb’s Cycle

In Kreb’s cycle, the process is in cyclic form and it does not involve preparatory and pay off phase, unlike glycolysis. It simply involves oxidation of pyruvate (Key intermediate of glycolysis) to Acetyl CoA which initiates the Kreb’s cycle. The steps of Kreb’s cycle includes the following steps:

krebs cycle

  1. Condensation of Acetyl CoA: In this, Acetyl CoA forms by the end product of the glycolytic pathway that is Pyruvate by releasing one molecule of NADH+. Acetyl CoA further oxidizes to form citrate by releasing CoA-SH.
  2. Isomerization of citrate: In this, Citrate isomerizes into D-Isocitrate, during which an intermediate product forms that are Cis- Aconitase where both the reaction catalyzes by the enzyme “Aconitase”. This reaction produces two molecules of H2O.
  3. Oxidative decarboxylation of Isocitrate: In this reaction, Isocitrate converts into α-Ketoglutarate, catalyzes by an enzyme “Isocitrate dehydrogenase” by releasing one molecule of CO2 and another molecule of NADH+.  
  4. Oxidative decarboxylation of α-Ketoglutarate: In this, α-Ketoglutarate converts into Succinyl CoA.  Here one molecule of CO2 and another molecule of NADH+ releases. This reaction catalyzes by an enzyme “α-Ketoglutarate dehydrogenase”.
  5. Conversion of Succinyl CoA: During this reaction, Succinyl CoA converts into Succinate catalyzes by an enzyme “Succinate thiokinase” by releasing one molecule of GDP.
  6. Oxidation of Succinate: In this Succinate converts into Fumarate catalyzes by an enzyme “Succinate dehydrogenase” by releasing one molecule of FADH2.
  7. Hydration of Fumarate: In this Fumarate converts into Malate catalyzes by an enzyme “Fumarase” by releasing one molecule of H2O.
  8. Oxidation of malate: In this Malate converts into Malate catalyzes by an enzyme “Malate dehydrogenase” by releasing one molecule of NADH+.

Net Production

In glycolysis

The net production of the glycolytic pathway is two molecules each of ADP, NADH+, H2O and four molecules of ATP.

Therefore, as from two molecules of NADH+, forms six molecules of ATP plus four molecules of ATP gives a total of 8molecules of ATP.

StepPhaseEnzymeATP investedATP released Net production
1Preparatory phaseHexokinase1ATP-1
3Phosphofructokinase1ATP-1
6Pay-off phaseGlyceraldehyde 3-P dehydrogenase6ATP (by two molecules of NADH+)+6
7Phosphoglycerate kinase2ATP+2
10Pyruvate kinase2ATP+2
Total10-2= 8 ATP molecules per glycolysis pathway

In Kreb’s cycle

The net production of the glycolytic pathway is three molecules each of NADH+, H2O and one molecule of GDP and QH2.

Therefore, three molecules of NADH+ forms nine molecules of ATP.  One molecule of GDP gives one molecule of ATP. And one molecule of FADH2 gives two molecules of ATP i.e. a total of 12 molecules of ATP per TCA-cycle.

StepEnzymeSource of ATP Net production
3Isocitrate dehydrogenase1 NADH+3 ATP
4α-Ketoglutarate dehydrogenase1 NADH+3 ATP
5Succinate thiokinase1 GDP1 ATP
6Succinate dehydrogenase1 FADH22 ATP
8Malate dehydrogenase1 NADH+3 ATP
Total12 ATP molecules per TCA cycle

Key Differences Between Glycolysis and Kreb’s Cycle

  1. The alternative name for glycolysis is the EMP pathway named after the scientists Embden, Meyerhof & Parnas pathway) whereas Kreb’s cycle also refers to the TCA cycle (Tri-Carboxylic Acid cycle).
  2. Glycolysis starts with substrate glucose which forms an end product i.e. Pyruvate. In the Krebs cycle, the substrate is pyruvate which forms an end product oxaloacetate.
  3. The occurrence of glycolysis is in the cell cytosol or cytoplasm and hence the respiration type is cytoplasmic. And, the occurrence of the Krebs cycle is inside the mitochondria and hence the respiration type is cytoplasmic.
  4. The flow of pathway is linear in glycolysis whereas cyclic in the Krebs cycle.
  5. Glycolysis is the first stage of cellular respiration whereas the Krebs cycle is the second stage of cellular respiration.
  6. The net production in Glycolysis and Kreb’s cycle is 8  and 12 molecules of ATP per cycle.

Conclusion

We have studied the main differences in the properties of both Glycolysis and Kreb’s cycle, where they differ in the site of occurrence, the stage of respiration, respiration type, the flow of pathway, net production of ATP etc. The difference in both the processes where each modification of a chemical compound is catalyzed by different enzymes.

The glycolysis pathway completes by the association of 10 enzymes. The TCA cycle completes the association of 8 enzymes. One of the other difference is in the net production of both the cycle where glycolysis produces 8 ATP molecules and the TCA cycle produces 12 ATP molecules.

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