Diversity of electron acceptors for respiration

Most microorganisms oxidize carbohydrates as their main source of cellular energy.Microorganisms use two general processes: Cellular respiration and fermentation. Microorganisms also use anaerobic pathway to oxidize glucose. In case of aerobic respiration, the ultimate e-acceptor is O₂ and the reduced form is H₂O. There are f our stages of aerobic respiration:

Oxygen extracts chemical energy from glucose, the glucose molecule must be split up into two molecules of pyruvate. This process also generates two molecules of adenosine triphosphate as an immediate energy yield and two molecules of NADH.

• • The citric acid cycle begins with the transfer of a two-carbon acetyl group from acetyl-CoA to the four-carbon acceptor compound (oxaloacetate) to form a six-carbon compound (citrate).

• The citrate then goes through a series of chemical transformations, losing two carboxyl groups as CO₂. The carbons lost as CO₂ originate from what was oxaloacetate, not directly from acetyl-CoA. The carbons donated by acetyl-CoA become part of the oxaloacetate carbon backbone after the first turn of the citric acid cycle. Loss of the acetyl-CoA-donated carbons as CO₂ requires several turns of the citric acid cycle. However, because of the role of the citric acid cycle in anabolism, they may not be lost, since many TCA cycle intermediates are also used as precursors for the biosynthesis of other molecules.

• Most of the energy made available by the oxidative steps of the cycle is transferred as energy-rich electrons to NAD⁺, forming NADH. For each acetyl group that enters the citric acid cycle, three molecules of NADH are produced.

• Electrons are also transferred to the electron acceptor Q, forming QH₂.

• At the end of each cycle, the four-carbon oxaloacetate has been regenerated, and the cycle continues.

Anaerobic respiration - Microbes are capable of using all sorts of other terminal electron accepters besides oxygen. A few examples of anaerobic respiration;

•  Final electron acceptor is an inorganic substance other than O₂.

•  Some bacteria such as Pseudomonas and Bacillus can use a nitrate ion (NO-₃), in the presence of an enzyme called nitrate reductase, as a final electron acceptor, the nitrate ion is reduced to nitrite ion (NO₂-).

•  Nitrite ion can be converted to nitrous oxide (N₂O), or nitrogen gas (N₂) (denitrification process) which helps in recycling of nitrogen.

•  Other bacteria like Desulfovibrio use sulfate (SO₄^2-) as the final electron acceptor and forms hydrogen sulfide (H₂S).

•  Still other bacteria use carbonate (CO₃^2-) to form methane (CH₄).

•  Anaerobic respiration by bacteria using nitrate and sulfate as final electron acceptors is essential for the nitrogen and sulfur cycles that occur in nature.

•  Amount of ATP generated varies with the organisms and the pathway. Because only a part of the Krebs cycle operates and since not all the carriers in the electron transport chain participate, ATP yield is less and accordingly anaerobes tend to grow more slowly than aerobes.