Key difference - Krebs cycle vs. glycolysis

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

Key areas covered

1. What is the Krebs Cycle (or Citric Acid Cycle or TCA Cycle) - Definition, Properties, Process 2. What is Glycolysis - Definition, Properties, Process 3. What are the Similarities Between Krebs Cycle and Glycolysis - Overview the similarities 4. What is the difference between Krebs cycle and glycolysis - main differences compared

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

What is the Krebs cycle?

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

Figure 1: Oxidative decarboxylation of pyruvate and Krebs cycle

During the Krebs cycle, the acetyl portion of Acetyl-CoA attaches to an oxaloacetate molecule to form a citrate molecule. The citrate is a molecule with six carbon atoms. This citrate is oxidized through a series of steps, releasing two molecules of carbon dioxide. First, the citric acid is converted into isocitrate and oxidized to α-ketoglutarate by reducing an NAD ⁺ molecule. The α-ketoglutarate is oxidized again 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 molecules to the fumarate creates malate. The malate is then re-oxidized to oxaloacetate by NAD ⁺ . The overall reactions of the Krebs cycle produce six NADH, two FADH ₂ and two ATP / GTP molecules per 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 in cellular respiration in all living organisms. This means that glycolysis occurs in both aerobic and anaerobic respiration. Glycolysis takes place in the cytoplasm. It is involved in breaking down glucose into two pyruvate molecules. The enzyme hexokinase adds a phosphate group to the glucose molecule, creating glucose-6-phosphate. The glucose-6-phosphate is then isomerized to fructose-6-phosphate. The fructose-6-phosphate is converted into 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 easily converted to dihydroacetone phosphate and glyceraldehyde-3-phosphate. The glyceraldehyde-3-phosphate is oxidized to 1,3-bisphosphoglycerate. A phosphate group from the 1,3-bisphosphoglycerate is transferred to ADP to produce an ATP. This creates a 3-phosphoglycerate molecule. The phosphate group of the 3-phosphoglycerate is transferred to the second carbon position of the same molecule to form a 2-phosphoglycerate molecule. The removal of a water molecule from the 2-phosphoglycerate creates the phosphoenolpyruvate (PEP). The transfer of the phosphate group from PEP to an ADP molecule creates pyruvate.

Figure 2: Glycolysis

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

Similarities Between the Krebs Cycle and Glycolysis

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

Difference Between Krebs Cycle and Glycolysis

definition

Krebs cycle: The Krebs cycle, also called 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 completely oxidized to carbon dioxide and water.

Glycolysis: Glycolysis refers to the series of chemical reactions that convert one molecule of glucose into two molecules of pyruvic acid.

step

Krebs cycle: The Krebs cycle is the second step in cellular respiration.

Glycolysis: Glycolysis is the first step in cell respiration.

Location

Krebs cycle: The Krebs cycle occurs in the mitochondria of eukaryotes.

Glycolysis: Glycolysis takes place in the cytoplasm.

Aerobic / anaerobic breathing

Krebs cycle: The Krebs cycle only occurs during aerobic breathing.

Glycolysis: Glycolysis occurs in both aerobic and anaerobic respiration .

procedure

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

Glycolysis: Glycolysis is involved in breaking down glucose into two pyruvate molecules.

Linear / cyclic

Krebs cycle: The Krebs cycle is a cyclical process.

Glycolysis: Glycolysis is a linear process.

End product

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

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

Consumption of ATP

Krebs cycle: The Krebs cycle does not use any ATP.

Glycolysis: Glycolysis uses two ATP molecules.

Net profit

Krebs cycle: The Krebs cycle produces six NADH molecules and two FADH ₂ molecules.

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

Net energy gain

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

Glycolysis: The net energy gain from glycolysis is 8 ATP molecules.

Carbon dioxide

Krebs cycle: Carbon dioxide is released during the Krebs cycle.

Glycolysis: No carbon dioxide is released during glycolysis .

Oxidative phosphorylation

Krebs cycle: The Krebs cycle is associated with oxidative phosphorylation.

Glycolysis: Glycolysis is not associated with oxidative phosphorylation.

oxygen

Krebs cycle: The Krebs cycle uses oxygen as a terminal oxidant.

Glycolysis: Glycolysis does not require oxygen.

diploma

The Krebs cycle and glycolysis are two steps in cellular respiration. The Krebs cycle only occurs during aerobic breathing. Glycolysis is common in both aerobic and anaerobic breathing. The Krebs cycle follows glycolysis. In glycolysis, two pyruvate molecules are made from one glucose molecule. These pyruvate molecules are completely oxidized to carbon dioxide and water during the Krebs cycle. The main differences between the Krebs cycle and glycolysis are the starting materials, the mechanism and the end products of each step.

Reference:

1. "Oxidative Decarboxylation & Krebs Cycle." Metabolic Processes. Hersi, Google Sites, available here . Accessed August 17, 2017. 2. Bailey, Regina. “10 Steps to Glycolysis.” ThoughtCo, Available here. Accessed August 17, 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. "Glykolyse" By WYassineMrabetTalk✉This vector graphic was created with Inkscape. - Own work (CC BY-SA 3.0) via Commons Wikimedia