Glycolysis vs. Krebs Cycle

Glycolysis and Krebs cycle are processes of respiration that provide energy to the body.  In respiration, oxygen is utilized, and carbon dioxide is released from the body along with the formation of energy. This energy is used by the body to perform various functions. Respiration also consists of many other mechanisms; the electron transport system, the pentose phosphate pathway, the anaerobic breakdown of pyruvic acid, and terminal oxidation.

The difference between these two energies generating processes is that glycolysis is the first step of the respiration process. In contrast, the Krebs cycle is the second step of the respiration process.  Glycolysis occurs in the cytoplasm, whereas the Krebs cycle occurs in the mitochondria of the cell. In glycolysis, glucose is converted into pyruvate lactate to produce ATP through a chain of reactions, whereas in the Krebs cycle, acetyl CoA is oxidized into CO2 and H2O.

Comparison Table

Basis for ComparisonGlycolysisKrebs Cycle
DefinitionThe breakdown of glucose by the enzymes releasing energy and two pyruvic acids is called glycolysis.The sequence of reactions by which living cells generate energy in the process of aerobic respiration is called the Krebs cycle.
Others nameEmbden-Meyerhof Parnas pathway or Cytoplasmic pathwayCitric acid cycle or Tricarboxylic acid cycle or Mitochondrial respiration
Role of carbon dioxideCarbon dioxide is not evolvedCarbon dioxide is evolved
Site of occurrenceInside the cytoplasmOutside the mitochondria
Degradation of moleculeA glucose molecule form two molecules of pyruvatePyruvate is degraded into carbon dioxide and water molecules
Consumption of ATP2 ATP molecules for the phosphorylationDoes not consume ATP
Net gainTwo molecules of ATP, two molecules of NADH for each molecule of glucose broken downSix molecules of NADH2, two molecules of FADH2 for every two acetyl CoA enzyme
Number of ATP producedThe net gain of ATP is 8The net gain of ATP is 24
Oxidative phosphorylationoxidative phosphorylation plays no roleAn important role of oxidative phosphorylation and oxaloacetate plays a catalytic role
Type of pathwayStraight or linear pathwayCyclic pathway

What is Glycolysis?

Glycolysis is a unique pathway occurring aerobically as well as anaerobically without the involvement of molecular oxygen. It is a major pathway for glucose metabolism. In this process, one molecule of glucose gets oxidized into two molecules of pyruvate in the presence of enzymes.

Glycolysis occurs in ten steps in a cycle. The first seven reactions of glycolysis occur in the cytoplasmic organelles called glycosomes. The other three reactions are irreversible.

The whole process of glycolysis can be divided into two phases, the first five steps are called the preparatory phase, and the other five steps are called the payoff phase. In the preparatory phase, phosphorylation of glucose occurs twice and is converted to fructose 1,6-biphosphate, so energy is consumed due to phosphorylation, and ATP is the phosphoryl group donor.

Fructose 1,6-biphosphate gets splits to yield two 2,3 carbon molecules. Dihydroxyacetone phosphate is one of the products that is converted into glyceraldehyde’s 3-phosphate. This gives two molecules of glyceraldehyde 3-phosphate, which is further processed to five payoff phases.

The pay-off phase is the energy gain phase of glycolysis and yields ATP and NADPH in the last phase.

Steps of Glycolysis

Phosphorylation: This is an irreversible reaction that is carried out by an enzyme called hexokinase. In this, glucose is phosphorylated by ATP to form a sugar-phosphate molecule. The negative charge on the phosphate hinders the passage of the sugar-phosphate via the plasma membrane and thus engaging glucose inside the cell.

Isomerization: In this step, the reversible arrangement of the chemical structure moves the carbonyl oxygen from carbon 1 to carbon 2, forming a ketose from an aldose sugar.

Phosphorylation: In this, the new hydroxyl group on carbon one is phosphorylated by ATP for the formation of two three-carbon sugar phosphates. This is the regulation of the enzyme phosphofructokinase, which checks the entry of sugars into glycolysis.

Cleavage Reaction: In these two, three-carbon molecules are produced by cleaving the six carbons of sugar. Only the glyceraldehyde 3-phosphate can proceed through glycolysis.

Isomerization: In this step, dihydroxyacetone phosphate is isomerized to form glyceraldehyde 3-phosphate.

Energy generation phase: This is the energy generation phase. So, the two molecules of glyceraldehyde 3-phosphate undergo oxidation. Iodoacetate hinders the function of the enzyme glyceraldehyde-3-phosphate dehydrogenase by reacting with the -SH group.

Step 7: ATP is formed in this step from the high energy phosphate group that was formed in the previous step

Step 8: Then, the phosphate ester linkage in 3-phosphoglycerate is moved from carbon 3 to form 2-phosphoglycerate.

Step 9: Enol phosphate linkage is formed with the removal of water from 2-phosphoglycerate.

Step 10: ATP is formed with the transfer of ADP to the high-energy phosphate group.

What is Krebs Cycle?

In the Krebs cycle, pyruvic acid is oxidized into carbon dioxide and water. When the acetyl group enters the cycle as acetyl CoA, carbon dioxide is produced. This cycle begins with the formation of citric acid, so it is named a citric acid cycle. This cycle also contains three carboxylic groups so also named a tricarboxylic acid cycle (TCA).

Steps of Krebs Cycle

Krebs’s cycle consists of seven steps.

Step 1: In this step, citrate is formed when acetyl CoA adds its two-carbon acetyl group to oxaloacetate

Step 2: In this step, citrate is transformed into isocitrate by the removal of one water molecule and adding the other molecule.

Step 3: NAD+ is converted to NA after reduction when the isocitrate is oxidized and loses a carbon dioxide molecule.

Step 4: Carbon dioxide is lost and the resulted product undergoes oxidation and NAD+ is reduced to NADH. The remaining molecule is attached to coenzyme A through an unstable bond. Alpha-ketoglutarate dehydrogenase catalyzes the reaction.

Step 5:  GTP is formed by the displacement of CoA by a phosphate group and transferred to GDP.

Step 6: FADH2 and oxidizing succinate are generated when two hydrogens are shifted to FAD.

Step 7: The substrate is oxidized and NAD+ is reduced to NADH and oxaloacetate is regenerated.

Key Differences between Glycolysis and Krebs Cycle

  1. Glycolysis starts with the breakdown of glucose into pyruvate, whereas the Krebs cycle starts with the oxidizing pyruvate into carbon dioxide.
  2. Glycolysis occurs inside the cytoplasm whereas the Krebs cycle occurs inside the mitochondria.
  3. Glycolysis produces two molecules of ATP and two molecules of NADH from each glucose molecule. In contrast, the Krebs cycle produces six molecules of NADPH2, two molecules of FADH2 for every two acetyl CoA enzymes.
  4. Glycolysis produces eight molecules of ATP, whereas the Krebs cycle produces 24 molecules of ATP.
  5. Glycolysis can be both aerobic or anaerobic, whereas the Krebs cycle is only aerobic.
  6. Glycolysis is the first step of respiration, whereas the Krebs cycle is the second step of respiration for the production of ATP.
  7. Glycolysis is a linear pathway, whereas the Krebs cycle is a circular pathway.
  8. In glycolysis, end products are organic substances (pyruvate), whereas, in the Krebs cycle, end products are inorganic substances (CO2 and H2O).


In conclusion, glycolysis and the Krebs cycle are processes of respiration and produce energy. Both consist of several steps and differ from each other in many aspects.

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