The main difference between glycolysis and Krebs cycle is: Glycolysis is the first step involved in the process of respiration and occurs in the cytoplasm of the cell. While Krebs Cycle is the second process of respiration which occurs in the mitochondria of the cell. Both are the process involved in respiration with the aim of fulfilling the energy requirement of the body.
So Glycolysis is defined as the chain of the reactions, for the conversion of glucose (or glycogen) into pyruvate lactate and thus producing ATP. On the other hand, Kreb cycle or citric acid cycle involves the oxidation of acetyl CoA into CO2 and H2O.
Respiration is the important process of all the living being, where oxygen is utilised and carbon dioxide is released from the body. During this process, energy is released, which is used to perform various functions of the body. Apart from the above two mechanisms, there are various other mechanisms of respiration like Electron transport system, pentose phosphate pathway, anaerobic breakdown of pyruvic acid, and terminal oxidation.
In the provided content we will discuss the general difference between two most important mechanisms of respiration which are glycolysis and Krebs cycle.
Content: Glycolysis Vs Krebs Cycle
Comparison Chart
| Basis for Comparison | Glycolysis | Krebs cycle |
|---|---|---|
| Starts with | Break down of glucose into pyruvate. | Oxidize Pyruvate into CO2. |
| Also known as | EMP (Embden-Meyerhof-Parnas Pathway or Cytolplasmic pathway). | TCA (tricaboxylic acid) cycle, Mitochondrial respiration. |
| Role of Carbon dioxide | No carbon dioxide is evolved in glycolysis. | Carbon dioxide is evolved in Krebs cycle. |
| Site of occurrence | Inside the cytoplasm. | Occurs inside the mitochondria (cytosol in prokaryotes) |
| It can occur as | Aerobically (i.e. in presence of oxygen) or anaerobically (i.e. in absence of oxygen). | It occurs aerobically (presence of oxygen). |
| Degradation of molecule | A glucose molecule is degraded into two molecules of organic substances, pyruvate. | Degradation of pyruvate is completely into inorganic substances which are CO2 and H2O. |
| Consumption of ATP | It consumes 2 ATP molecules, for the phosphorylation. | It does not consume ATP. |
| Net gain | Two molecules of ATP and two molecules of NADH, for every molecule of glucose is broken down. | Six molecules of NADH2, 2 molecules of FADH2 for every two acetyl CoA enzyme. |
| Number of ATP produced | The net gain of ATP is 8 (including NADH). | The net gain of ATP is 24. |
| Oxidative phosphorylation | No role of oxidative phosphorylation. | Vital role of oxidative phosphorylation, and oxaloacetate is considered to play a catalytic role. |
| Step in the process of respiration | Glucose is broken into pyruvate, and hence glycolysis is said as the first step of respiration. | Krebs cycle is the second step of respiration. |
| Type of pathway | It is the straight or linear pathway. | It is a circular pathway. |
Definition of Glycolysis
Glycolysis is also known as ‘Embden-Meyerhof-Parnas Pathway‘. It is a unique pathway occurring aerobically as well anaerobically, without the involvement of molecular oxygen. It is the major pathway for glucose metabolism and occurs in the cytosol of all the cells. The basic concept of this process is that the one molecule of glucose gets partially oxidized into two moles of pyruvate, enhanced by the presence of enzymes.
Glycolysis is a process that occurs in 10 simple steps. In this cycle first seven steps reactions of glycolysis occur in the cytoplasmic organelles called as glycosome. While the other three reactions like hexokinase, phosphofructokinase, and pyruvate kinase are the irreversible one.
The whole cycle is divided into two phases, the first five steps are known as preparatory phase and the another is known as payoff phase. In the first five steps of this pathway, phosphorylation of glucose occurs twice and is converted to fructose 1,6 -biphosphate, so we can say that here energy is consumed due to phosphorylation and ATP is the phosphoryl group donor.
Further now fructose 1,6 -biphosphate gets splits to yield two 2,3-carbon molecules. Dihydroxyacetone phosphate, which is one of the product is converted into glyceraldehydes 3-phophate. This gives two molecules of glyceraldehyde 3-phopsphate, which are further processed to five-step payoff phase.
Pay-off phase is the energy gain phase of glycolysis, and it yields ATP and NADH in the last step. Firstly, glyceraldehyde 3-phosphate is oxidized with NAD+ as the electron acceptor (to form NADH) and an inorganic phosphate is incorporated to give a high energy molecule as 1,3 -biphosphoglycerate. Subsequently, high-energy phosphate on carbon one is donated to ADP to convert into ATP. This production of ATP is called substrate-level phosphorylation.
Thus the energy yield from the glycolysis is 2 ATP and 2 NADH, from one molecule of glucose.
Steps involved in glycolysis:
Step 1: This first step is called as phosphorylation, it is an irreversible reaction leads by an enzyme called as hexokinase. This enzyme is found in all types of cells. In this step, Glucose is phosphorylated by ATP to form a sugar-phosphate molecule. The negative charge present on the phosphate prevents the passage of the sugar phosphate through the plasma membrane and thus engaging glucose inside the cell.
Step 2: This step is called Isomerization, in this a reversible rearrangement of the chemical structure moves the carbonyl oxygen from carbon 1 to carbon 2, forming a ketose from an aldose sugar.
Step 3: This is also a phosphorylation step, the new hydroxyl group on carbon 1 is phosphorylated by ATP, for the formation of two three-carbon sugar phosphates. This step is regulated of the enzyme phosphofructokinase, which checks the entry of sugars into glycolysis.
Step 4: This is named as cleavage reaction. Here two three-carbon molecule is produced by cleaving the six carbon sugar. Only the glyceraldehyde 3-phosphate can proceed immediately through glycolysis.
Step 5: This is also Isomerization reaction, where the other product of step 4, dihydroxyacetone phosphate is isomerized to form glyceraldehyde 3 -phosphate.
Step 6: From this step, the energy generation phase will start. So the two molecules of glyceraldehyde 3-phosphate are oxidized. By reacting with the -SH group, Iodoacetate inhibits the function of enzyme glyceraldehyde-3-phosphate dehydrogenase.
Step 7: ATP is formed, from the high-energy phosphate group that was generated in step 6.
Step 8: The phosphate ester linkage in 3-phosphoglycerate, having free energy is moved from carbon 3 to form 2-phosphoglycerate.
Step 9: Enol phosphate linkage is created with the removal of water from 2-phosphoglycerate. Enolase (enzyme catalyzing this step) is inhibited by fluoride.
Step 10: Forms ATP, with the transfer of ADP to the high energy phosphate group, generated in step 9.
Definition of Krebs Cycle
This cycle occurs in the matrix of mitochondria (cytosol in prokaryotes). The net result is the production of CO2 when the acetyl group entering the cycle as Acetyl CoA. In this, the oxidation of pyruvic acid into carbon dioxide and water occurs.
Krebs cycle was discovered by H.A Krebs (a German-born biochemist) in the year 1936. As the cycle begins with the formation of citric acid, it is called citric acid cycle. The cycle also contains three carboxylic groups (COOH), hence also called as a tricarboxylic acid cycle (TCA cycle).
Steps involved in Krebs cycle:
Step 1: Citrate is produced in this step when Acetyl CoA adds its two-carbon acetyl group to oxaloacetate.
Step 2: Citrate is converted to its isocitrate (an, an isomer of citrate), by the removal of one water molecule and adding the another.
Step 3: NAD+ is reduced to NA when isocitrate is oxidized and loses a CO2 molecule.
Step 4: CO2 is lost again, the resulting compound is oxidized and NAD+ is reduced to NADH. The remaining molecule gets attached to coenzyme A through an unstable bond. Alpha-ketoglutarate dehydrogenase catalyzes the reaction.
Step 5: GTP is generated by the displacement of CoA by a phosphate group and transferred to GDP.
Step 6: In this step, FADH2 and oxidizing succinate are formed, when two hydrogens are transferred to FAD.
Step 7: The substrate gets oxidized and NAD+ is reduced to NADH and oxaloacetate is regenerated.
Key Difference Between Glycolysis and Krebs Cycle
- Glycolysis is also known as EMP (Embden-Meyerhof-Parnas Pathway or Cytoplasmic pathway) starts with the breakdown of glucose into pyruvate; Krebs cycle is also known as TCA (tricarboxylic acid) cycle. Mitochondrial respiration starts oxidizing pyruvate into CO2.
- Net gain of the whole cycle is two molecules of ATP and two molecules of NADH, for every molecule of glucose which is broken down, while in Krebs cycle six molecules of NADH2, 2 molecules of FADH2 for every two acetyl-CoA enzymes.
- Total number of ATP produced is 8 and in Krebs cycle, total ATP is 24.
- No carbon dioxide is evolved in glycolysis while in Krebs cycle carbon dioxide is evolved.
- The site of occurrence of glycolysis is inside the cytoplasm; Krebs cycle occurs inside the mitochondria (cytosol in prokaryotes).
- Glycolysis can occur in the presence of oxygen i.e aerobic or in absence of oxygen i.e.anaerobic; Krebs cycle occurs aerobically.
- A glucose molecule is degraded into two molecules of an organic substance, pyruvate in glycolysis, while degradation of pyruvate is completely into inorganic substances which are CO2 and H2O.
- In Glycolysis 2 ATP molecules are consumed for the phosphorylation while Kreb cycle there is no consumption of ATP.
- No role of oxidative phosphorylation in glycolysis; there is a major role of oxidative phosphorylation as well as oxaloacetate is considered to play a catalytic role in Krebs cycle.
- As in glycolysis, glucose is broken into pyruvate, and hence glycolysis is said as the first step of respiration; Krebs cycle is the second step of respiration for the production of ATP.
- Glycolysis is a straight or linear pathway; while Krebs cycle is a circular pathway.
Conclusion
Both the pathways produce energy for the cell, where Glycolysis is the breakdown of a molecule of glucose to yield two molecules of pyruvate, whereas Kreb cycle is the process where acetyl CoA, produces citrate by adding its carbon acetyl group to oxaloacetate. Glycolysis is essential for the brain which depends on glucose for energy.
Kreb cycle is an important metabolic pathway in supplying energy to the body, about 65-70% of the ATP is synthesized in Krebs cycle. Citric acid cycle or Krebs cycle is the final oxidative pathway which connects almost all the individual metabolic pathway.
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