Question

Describe how a bacterial cell could harvest and convert the unstable high energy electrons contained in the redox cofactors to ATPs.

Answer 1

Unstable high-energy electrons are present in molecules like glucose. When they are oxidized to form carbon dioxide and water, they get stabilize. Along with the generation of these two product, ATP which is the energy currency of the cell is also form. This overall process of conversion of glucose to carbon dioxide and water for the generation of ATP is called as cellular respiration. Cellular respiration in eukaryotes occurs in mitochondria but in prokaryotes it occurs in cytoplasm and plasma membrane.

The process begins with the conversion of one molecule of glucose into two molecules of pyruvic acid. This is called as glycolysis and occurs in cytoplasm. This process results in the generation of two molecules of ATP by substrate level phosphorylation and two molecules of NADH.

Nextstep is the conversion of pyruvic acid to acetyl coenzyme a along with the release of carbon dioxide with the help of enzyme pyruvate dehydrogenase. This process is called as pyruvate oxidation or pyruvate decarboxylation and occurs in cytoplasm of bacteria. This process occurs twice for one molecule of glucose. So this process results in the formation of two molecules of NADH.

Third step is the entry of acetyl coenzyme a in to citric acid cycle which also takes place in cytoplasm. This process result in complete oxidation of acetyl coenzyme a into carbon dioxide and water. This process also occurs twice for one molecule of glucose. Therefore it generates two molecules of GTP by substrate level phosphorylation, 3 molecules of NADH and two molecules of FADH2.

All the high energy electron carriers (NADH and FADH2​​​) which are also called as redox cofactors enter electron transport chain which take place in plasma membrane in bacteria. Electrons are transferred from one electron carrier to the other which are arranged in increasing order of their electrode potential. As the electrode potential increases, the affinity for electron also increases. Ultimately the electrons are accepted by oxygen to form water.

When electrons are transferred from one electron carrier to the other, protons are also pumped from cytoplasm to intermembrane space. These protons are diffuse back into the cytoplasm by ATP synthase enzyme. The electrochemical energy stored in proton gradient is used by which enzyme for the synthesis of ATP by phosphorylation of ADP. This is called as oxidative phosphorylation.

Therefore the electrons present in glucose were passed on to high energy electron carrier and ultimately accepted by oxygen to form water. This also resulted in the generation of of ATP by oxidative phosphorylation.

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