Difference Between Krebs Cycle and Glycolysis

Krebs Cycle vs Glycolysis

Krebs’s cycle and glycolysis are two cellular pathways producing energy for cells. Both are involved in catabolism and occur in different cellular locations using different enzymatic reactions in order to convert different starting material for different products yielding different amounts of ATP. In aerobic respiration, glycolysis is followed by Krebs’s cycle and, in anaerobic respiration, glycolysis takes place alone.

Krebs’s Cycle

Krebs’s cycle also known as citric acid cycle occurs in the mitochondrion. This organelle is present only in eukaryotes. This is the second step of glucose catabolism in eukaryotes and does not occur in prokaryotes like bacteria. Krebs’s cycle uses the product of glycolysis (Pyruvic acid) as starting material, but that cannot directly enter Krebs’s cycle. First of all Pyruvic acid molecules are converted to Acetyl Co-A and energy released is used to convert NAD to NADH. Inside the mitochondrion Acetyl, Co-A (2 Carbon molecule) is captured by Oxaloacetic acid (4 Carbon) and makes Citric acid (6C). This substrate then undergoes a series of enzyme-driven reactions and gets converted to oxaloacetic acid starting material. This is why we call it a cycle. During the many steps of the cycle, high energy electrons are released. These reduce NAD to NADH. FAD also acts as an electron acceptor and become FADH2. The cycle also forms ATP and releases CO2. For calculation sake, if we consider a glucose molecule (6C) entering the Krebs’s cycle, it will enter as two pyruvic molecules concerted to 2 acetyl co-A, and in the end of one cycle yield  2 ATP molecules, 10 NADH, 2 FADH2, and 6 CO2.


Glycolysis is the cellular process which breaks a glucose molecule in to 2 pyruvic acid molecules. Unlike Krebs’s cycle, this process is common to animals, plants and microorganisms. This takes place in the cytoplasm and consists of multi-steps. Although 4 ATP molecules are produced per glucose, in  intermediate steps 2 ATP molecules are used up. Therefore, the net ATP production is 2. In addition, 2 NADH molecules are produced. If the pyruvic acid molecules do not enter Krebs’s cycle, in plants ethanol is generated and, in animals, lactic acid is generated. This process does not need oxygen; therefore, can be used in anaerobic environments. However, due to that, the efficiency is lowered.

What is the difference between Krebs’s Cycle and Glycolysis?

• Krebs’s cycle is a cyclic process while glycolysis is a linear array of enzymatic reactions.

• The substrate for Krebs’s cycle is acetyl co-A, and for glycolysis it is glucose.

• Krebs’s cycle is a part of aerobic respiration while glycolysis can also occur in the absence of oxygen.

• Krebs’s cycle’s location is mitochondrion while glycolysis occurs in the cytoplasm.

• Krebs’s cycle produces oxaloacetic acid, NADH, FADH2, ATP and CO2 while glycolysis produces pyruvic acid, NADH and ATP.

• Krebs’s cycle is an efficient process while glycolysis is inefficient.

• Krebs’s cycle alone cannot happen, but glycolysis alone can occur in cells and can lead to alcohol fermentation in plants or lactic acid fermentation in animals.

• Krebs’s cycle only occurs in eukaryotes, but glycolysis occurs in eukaryotes as well as prokaryotes.