Krebs Cycle In Mitochondria? 9 Facts You Should Know

by

Krebs cycle is one of the common pathway for aerobic respiration. Let us explore more facts about krebs cycle and mitochondria.

Citric acid cycle and tricarboxylic acid cycle are other names for the Krebs cycle. This is a typical metabolic pathway for the oxidation of carbohydrates, lipids, and amino acid.

Aerobic respiration is a multi-step enzyme mediated energy releasing catabolic process. It involves complete oxidation of food substrate with the help of oxygen to form carbon dioxide and water along with the generation of high amount of ATP.

Does Krebs cycle occur in mitochondria?

Mitochondria is the main site for ATP synthesis. Let us discuss whether Krebs cycle occur in mitochondria or in the cytosol. 

Krebs cycle occurs in mitochondria. It is mainly operated in the mitochondrial matrix. In eukaryotes, the site for krebs cycle is mitochondria but in prokaryotes it occurs in cytosol region.

Respiratory enzymes are present in both mitochondrial matrix and inner membrane. Krebs cycle and the electron transport chain are interlinked. The process of substrate level phosphorylation produces few ATP molecules. Oxygen gets reacted with hydrogen to produce metabolic water during terminal oxidation.

Why does Krebs cycle occur in mitochondria?

Mitochondria has its own DNA and can function without the cell nucleus. Let us find out the reason why krebs cycle occurs in mitochondria instead of cytoplasm. 

Krebs cycle takes place in mitochondria because it contain the enzymes necessary for the aerobic oxidation of pyruvate. Mitochondria has prehistoric origin of producing energy.  

The endosymbiotic theory states that mitochondria are engulfed by the primitive cells for the energy exchange. Therefore, complete cell depends on the mitochondria for the production of ATP.

How does Krebs cycle occur in mitochondria?

Krebs cycle take place in mitochondria of eukaryotes to generate energy. Let us discover how it occurs in mitochondria.

The pyruvate from glycolysis cannot directly enter the krebs cycle in mitochondria. Listed below are the steps which helps us understand how krebs cycle occurs in mitochondria.

  • The pyruvate from glycolysis is first converted into acetyl CoA.
  • The latter enters the krebs cycle for further degradation.
  • Oxaloacetate is the first acceptor molecule which combines with acetyl CoA to form first product named tricarboxylic acid.
  • Organic molecules are split into pyruvate during glycolysis in the cytoplasm.
  • Then, oxidative decarboxylation of pyruvate occurs to form acetyl CoA.
  • With the help of pyruvate dehydrogenase complex, pyruvate undergoes oxidation and decarboxylation, produces a two carbon acetate group or acetyl CoA along with NADH and CO2.

Where does Krebs cycle occur in mitochondria?

Krebs cycle does not take place in the cytoplasm of the cell, in contrast to glycolysis. Let us determine the precise location where it happens.

Krebs cycle mainly occurs in the mitochondrial matrix. Unlike glycolysis, Krebs cycle is an eight step closed pathway. Oxaloacetate is created in the final stage of the citric acid cycle and is then ready to be combined with acetyl CoA to begin a new cycle.

Krebs cycle in mitochondria diagram

One molecule of glucose initiates the commencement of cellular respiration. Lets us explore the different steps of krebs cycle with the help of the diagram.

In the mitochondrial matrix, two pyruvate molecules produced during glycolysis will enter the Krebs cycle. In the preparatory step, two pyruvate molecules converted into 2 acetyl CoA with help of enzyme pyruvate dehydrogenase complex.

krebs cycle in mitochondria
Krebs Cycle in Mitochondria

The krebs cycle occurs in eight step in a cyclic pathway, producing energy in the form of ATP. All the enzymes required for the citric acid cycle are present inside the matrix except succinate dehydrogenase which is present int the inner mitochondrial membrane. The complete cyclic pathway can be seen in the above diagram.

Functions of Krebs cycle in mitochondria

Mitochondria is the power house of the cell. It supplies energy to the entire cell in the form of ATP. Let us find out the functions of krebs cycle in the production of energy.

List of functions of krebs cycle in mitochondria are as follows:

  • The primary function of Krebs cycle is to generate energy, which is then transferred and stored as ATP or GTP.
  • Krebs cycle act as amphibolic, means it has both anabolic and catabolic roles.
  • The common pathway for the oxidation of amino acids, carbohydrates and fats is Krebs cycle.
  • The highest number of energy molecules ATP are generated from Krebs cycle.
  • The synthesis and degradation of fatty acids, steroids, carotenoids, terpenes, and aromatic chemicals are both aided by acetyl CoA. 
  • Krebs cycle is a cyclic pathway consisting of eight enzymes out of which one is present in the inner mitochondrial membrane i.e., succinate dehydrogenase.
  • Krebs cycle do not require any oxygen molecules. However, oxygen is required during final step of aerobic respiration.
  • The majority of the energy utilized by aerobic cells is produced by the Krebs cycle when it enters the terminal oxidation.
  • Krebs cycle plays a crucial role in cell development and growth.
  • Energy produced from the krebs cycle helps in the growth of endothelial cells.

How many ATP are produced in Krebs cycle?

One NADH molecule will generate 3 ATP and One FADH produces 2 ATP in the metabolic pathway. Let us explore how many ATP molecules are produced in Krebs cycle.

Only two ATP is produced during Krebs cycle from two pyruvate molecules at substrate level phosphorylation. There is also a production of six NADH and two FADH2 molecule, but they will generate  ATP in the Electron transport system.

In glycolysis, there is a production of 8 ATP ( 2ATP are direct and 6 ATP are formed from 2 NADH). During pyruvate oxidation or gateway step, it forms 6 ATP from 2 NADH. In Krebs cycle, maximum number of ATP are produced, it yields 24 ATP molecules. Therefore, a single molecule of glucose yields around 36 ATP molecules in many eukaryotes.

What is electron transport chain?

The reduced coenzymes formed in the krebs cycle will enter into electron transport chain. Let us discuss about the electron transport chain and how it works.

Electron transport chain is a grouping of coenzymes and cytochromes which helps in transporting the electrons downhill along the decreasing redox potential with loss of energy at each step.There are five complex present in electron transport chain and different electron carriers.

These electron carriers are flavins, FeS complex, quinones and cytochromes. These carriers carry electrons and push protons to the outer mitochondrial chamber. These protons creates a gradient across inner mitochondrial membrane, which is then converted into ATP with the help of  F0-F1 complex.

Where does electron transport chain occur?

Krebs cycle is linked to electron transport chain. Let us find out the location where it occurs.

ETC or electron transport chain occurs in the inner membrane of mitochondria. It contains electron carriers and different complexes in it. All the NADH produced in the mitochondrial matrix will give electrons to the electron carriers and get oxidized from NADH to NAD+.

When electrons moves with the electron carrier system, they are moving from higher energy state to lower energy state. So, this makes the collection of electrons in the inner membrane space. These electrons will then move towards the F0-F1 complex  and will generate ATP.

Conclusion

To wrap up this post, we conclude that Krebs cycle occurs in mitochondria. It only occurs in mitochondria because special respiratory enzymes are only present in mitochondrial matrix which are required for oxidation of pyruvate. Krebs cycle is linked to electron transport chain which takes in the inner membrane of mitochondria.

Also Read: