One of the most heard sentences while studying biology is that "the mitochondrion is the powerhouse of the cell." But how many of us questioned how is it so? Some of us know the answer to this question while many don't. So, get ready to find the answer to how mitochondrion is the powerhouse of the cell.
Once the process of glycolysis reaches its final step in the cytoplasm, it forms two molecules of pyruvic acid. It undergoes another step named Link Reaction that leads to the formation of Acetyl-CoA. This Acetyl-CoA then enters the ten-step long Krebs Cycle.
The Ten Steps of the Krebs Cycle
So, as it happens, the first step of the Krebs Cycle is dehydration. Oxaloacetate, Acetyl-CoA, and Water react to form Citric Acid and CoA-SH. Citrate synthase, as the name suggests, catalyzes the formation of citric acid.
Over the next two steps, Citric acid undergoes dehydration and hydration, leading to the synthesis of cis-Aconitate and Isocitrate respectively. Both these reactions are catalyzed by the enzyme Aconitase.
Finally, we come to the good stuff i.e. synthesis of NADH + H+ that will eventually lead to the synthesis of ATP in the ETC reaction. Isocitrate and NAD+ form Oxalosuccinate and NADH + H+.
The next step leads to the first release of a carbon dioxide molecule of the Krebs cycle (though second in the aerobic respiration, first being in the Link reaction). α-Ketoglutarate and a molecule of CO2 are formed from Oxalosuccinate.
α-Ketoglutarate, NAD+, and CoA-SH reacts, catalyzed by a bunch of catalysts and co-factors, to form Succinyl-CoA, NADH + H+, and CO2.
Succinyl-CoA, GDP (Guanosine diphosphate), and Pi (phosphate ion) undergo substrate-level phosphorylation to give Succinate, CoA-SH, and GTP (equivalent to ATP). Succinate synthetase catalyzes this step.
The final three steps of the Krebs cycle are oxidation, hydration, and oxidation that leads to the formation of Oxaloacetate in the end.
Succinate reacts with ubiquinone to give Fumaric acid. Fumaric acid binds with a molecule of water to give L-Malate. The last step is the formation of oxaloacetate so that the Krebs cycle would continue without any hindrance, given pyruvate is available too. L-Malate oxidizes to Oxaloacetate and forms NADH + H+ as a by-product.
Oxaloacetate then once again reacts with Acetyl-CoA to form Citric acid and the cycle continues. In prokaryotes, the mitochondrion is absent, so the Krebs Cycle takes place in the cytoplasm itself.
The Production of bio-fuel
You must be thinking that only a single molecule of ATP, that too in the form of GTP, forms in this 10-step reaction, that we call Krebs Cycle. And, you know that we need a lot of energy to do our daily activities and just one molecule of ATP per molecule of pyruvate is not enough to power it,
The answer to this mystery lies in the Electron Transport Chain Reaction that synthesizes the quantity of ATP suitable for our bulky bodies, NADH + H+ plays a big role in the ETC Reactions and the role of oxygen comes into play at this stage only. And, that is why this process of ATP synthesis is known as aerobic respiration.
We will look into the ETC reactions and the formation of ATP in the next article.
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