Difference Between Krebs Cycle and Glycolysis

Main Difference – Krebs Cycle vs Glycolysis

Krebs cycle and glycolysis are two steps in cellular respiration. Cellular respiration is the biological oxidation of the organic compound, the glucose for releasing chemical energy. This chemical energy is used as the energy source in cellular functions. The Krebs cycle comes after the glycolysis. The main difference between Krebs cycle and glycolysis is that Krebs cycle is involved in the complete oxidation of pyruvic acid into carbon dioxide and water whereas glycolysis converts glucose into two molecules of pyruvic acid. The Krebs cycle occurs inside the mitochondria in eukaryotes. Glycolysis occurs in the cytoplasm of all living organisms. The Krebs cycle is also known as the citric acid cycle or tricarboxylic acid cycle (TCA cycle). The glycolysis is also known as Embden-Meyerhof-Parnas (EMP) pathway.

Key Areas Covered

1. What is Krebs Cycle (or Citric Acid Cycle or TCA Cycle)
      – Definition, Characteristics, Process
2. What is Glycolysis
      – Definition, characteristics, Process
3. What are the Similarities Between Krebs cycle and Glycolysis
      – Outline of Common Features
4. What is the Difference Between Krebs Cycle and Glycolysis
      – Comparison of Key Differences

Key Terms: Acetyl-CoA, ATP, Cellular Respiration, Citric Acid Cycle, FADH, Glycolysis, Glucose, GTP, Krebs Cycle, NADH, Oxidative Decarboxylation, Pyruvate, TCA Cycle

What is Krebs Cycle

The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid cycle (TCA cycle), is the second step of the aerobic respiration in living organisms. During the Krebs cycle, pyruvate is completely oxidized into carbon dioxide and water. Pyruvate is produced in the glycolysis, which is the first step of cellular respiration. These pyruvates are then imported into the matrix of the mitochondria to undergo oxidative decarboxylation. During oxidative decarboxylation, pyruvate is converted to acetyl-CoA by removing a carbon dioxide molecule and oxidizing to acetic acid. Then, a coenzyme A is attached to the acetic part, forming the acetyl-CoA. This acetyl-CoA then enters the Krebs cycle.

Figure 1: Oxidative Decarboxylation of Pyruvate and Krebs Cycle

During Krebs cycle, the acetyl part of the acetyl-CoA is attached to an oxaloacetate molecule to form a citrate molecule. The citrate is a six-carbon molecule. This citrate is oxidized by a series of steps, which releases two carbon dioxide molecules from it. First, the citric acid is converted into isocitrate and oxidized to α-ketoglutarate by reducing a NAD+ molecule. The α-ketoglutarate is again oxidized to succinyl-CoA. The succinyl-CoA takes a hydroxyl group from water and forms succinate. The succinate is oxidized to fumarate by FAD. The addition of water molecule to the fumarate produces malate. The malate is then oxidized back to oxaloacetate by NAD+. The overall reactions of the Krebs cycle produce six NADH, two FADH2, and two ATP/GTP molecules per one glucose molecule. The process of oxidative decarboxylation along with the Krebs cycle is shown in figure 1.

What is Glycolysis

Glycolysis is the first step of cellular respiration in all living organisms. That means glycolysis occurs in both aerobic and anaerobic respiration. Glycolysis occurs in the cytoplasm. It is involved in the breakdown of glucose into two molecules of pyruvate. A phosphate group is added to the glucose molecule by the enzyme hexokinase, producing glucose 6-phosphate. The glucose-6-phosphate is then isomerized to fructose-6-phosphate. The fructose 6-phosphate is converted to fructose 1, 6-bisphosphate. The fructose 1, 6-bisphosphate is split into dihydroxyacetone and glyceraldehyde by the action of the enzyme aldose. Both dihydroxyacetone and glyceraldehyde are readily converted into dihydroacetone phosphate and glyceraldehyde 3-phosphate. The glyceraldehyde 3-phosphate is oxidized to 1, 3-bisphosphoglycerate. One phosphate group from the 1, 3-bisphosphoglycerate is transferred to ADP to produce an ATP. This produces a 3-phosphoglycerate molecule. The phosphate group of the 3-phosphoglycerate is transferred into the second carbon position of the same molecule to form a 2-phosphoglycerate molecule.  The removal of a water molecule from the 2-phosphoglycerate produces the phosphoenolpyruvate (PEP). The transfer of the phosphate group of PEP to an ADP molecule produces the pyruvate.

Figure 2: Glycolysis

The overall reactions of the glycolysis produce two pyruvate molecules, two NADH molecules, two ATP molecules, and two water molecules. The complete process of glycolysis is shown in figure 2. 

Similarities Between Krebs Cycle and Glycolysis

• Krebs cycle and glycolysis are two steps of cellular respiration.
• Both Krebs cycle and the glycolysis occur in the cytoplasm in prokaryotes.
• Both Krebs cycle and glycolysis are driven by enzymes.
• Both Krebs cycle and glycolysis produce NADH and ATP.

Difference Between Krebs Cycle and Glycolysis

Definition

Krebs Cycle: Krebs cycle, also known as the citric acid cycle or tricarboxylic acid cycle (TCA cycle), refers to the series of chemical reactions in which pyruvate is converted to acetyl-CoA and is completely oxidized into carbon dioxide and water.

Glycolysis: Glycolysis refers to the series of chemical reaction in which a glucose molecule is converted into two pyruvic acid molecules.

Step

Krebs Cycle: Krebs cycle is the second step of the cellular respiration.

Glycolysis: Glycolysis is the first step of the cellular respiration.

Location

Krebs Cycle: Krebs cycle occurs inside the mitochondria of eukaryotes.

Glycolysis: Glycolysis occurs in the cytoplasm.

Aerobic/Anaerobic Respiration

Krebs Cycle: The Krebs cycle only occurs in aerobic respiration.

Glycolysis: The glycolysis occurs in both aerobic and anaerobic respiration.

Process

Krebs Cycle: The Krebs cycle is involved in the complete oxidation of pyruvate into carbon dioxide and water.

Glycolysis: The glycolysis is involved in the degradation of glucose into two molecules of pyruvate.

Linear/Cyclic

Krebs Cycle: The Krebs cycle is a cyclic process.

Glycolysis: The glycolysis is a linear process.

End Product

Krebs Cycle: The end product of Krebs cycle is an inorganic carbon substance.

Glycolysis: The end product of glycolysis is an organic substance.

Consumption of ATP

Krebs Cycle: Krebs cycle consumes no ATP.

Glycolysis: Glycolysis consumes two ATP molecules.

Net Gain

Krebs Cycle: Krebs cycle produces six NADH molecules and two FADH2 molecules.

Glycolysis: Glycolysis produces two pyruvate molecules, two ATP molecules, two NADH molecules.

Net Gain of Energy

Krebs Cycle: The net gain of energy of the Krebs cycle is equal to 24 ATP molecules.

Glycolysis: The net gain of energy of the glycolysis is equal to 8 ATP molecules.

Carbon Dioxide

Krebs Cycle: Carbon dioxide is released during the process of Krebs cycle.

Glycolysis: No carbon dioxide is released during the process of glycolysis.

Oxidative Phosphorylation

Krebs Cycle: Krebs cycle is connected with the oxidative phosphorylation.

Glycolysis: Glycolysis is not connected with the oxidative phosphorylation.

Oxygen

Krebs Cycle: The Krebs cycle uses oxygen as the terminal oxidant.

Glycolysis: Glycolysis does not require oxygen.

Conclusion

Krebs cycle and glycolysis are two steps in cellular respiration. The Krebs cycle only occurs in aerobic respiration. Glycolysis is common to both aerobic and anaerobic respiration. The Krebs cycle follows glycolysis. During glycolysis, two pyruvate molecules are produced from a glucose molecule. Those pyruvate molecules are completely oxidized into carbon dioxide and water during the Krebs cycle. The main difference between Krebs cycle and glycolysis is the starting materials, mechanism, and the end products of the each step.

Reference:

1.“Oxidative Decarboxylation & Krebs Cycle.”Metabolic Processes.Hersi, Google Sites, Available here. Accessed 17 Aug. 2017.
2.Bailey, Regina. “10 Steps of Glycolysis.” ThoughtCo, Available here. Accessed 17 Aug. 2017.

Image Courtesy:

1. “Citric acid cycle noi” By Narayanese (talk) – Modified version of Image:Citricacidcycle_ball2.png. (CC BY-SA 3.0) via Commons Wikimedia
2. “Glycolysis” By WYassineMrabetTalk✉This vector image was created with Inkscape. – Own work (CC BY-SA 3.0) via Commons Wikimedia

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