The second metabolic pathway of biological oxidation in bacteria is termed respiration.
In this pathway, an inorganic substance serves as the final acceptor of electrons or hydrogen atoms.
When oxygen plays a role of the final electron acceptor, it is known as aerobic respiration. In this case the end products of respiration pathway are H2O and CO2.
In anaerobic bacteria inorganic substances or ions other than O2 serve as terminal oxidants in respiration process. This ability, essential for many microbial groups, is termed as anaerobic respiration. Suitable electron acceptors for anaerobic respiration include nitrate, sulfate, carbon dioxide, and some others.
Respiratory metabolism dependent on carbon dioxide as the final electron acceptor is an intrinsic property of members of distinct prokaryotic domain of archaebacteria. In particular, archaebacteria are capable of reducing carbon dioxide to acetate, thus generating energy for their cells.
Basic chemical mechanism for generation of ATP associated with respiration includes the addition of inorganic phosphate to ADP molecule. In ground state this reaction is energetically unfavorable; therefore, it needs energy support. Energy for ATP synthesis is acquired from proton gradient, established on opposite sides of bacterial cytoplasmic membranes and their specialized structures (mesosomes).
Therefore, respiration is a membrane-located pathway. In the process of biological oxidation electrons pass from a chemical reductant to a chemical oxidant by a specific set of electron carriers associated with cytoplasmic membrane. This leads to the creation of electrochemical proton gradient across the membrane, resulting in generation of proton motive force. Backward flow of protons through the membrane is coupled with the synthesis of ATP by ATP synthase enzyme.
In aerobic respiration the biologic reductant is commonly NADH, and the oxidant is oxygen.
In the process of fermentation bacterium extracts only a very small fraction of the total energy available in glucose molecule. The energetic yield of fermentation is 2 molecules of reduced NADH and final 2 molecules of ATP – 4 are newly synthesized, whereas 2 are consumed within the pathway.
In respiration large amounts of energy are gained as the result of biological oxidation, following the transfer of electrons from a high-energy to a low-energy level through the set of membrane-associated enzymes of electron transport chain, also termed as the respiratory chain. Proton-motive force, generated by this mechanism, causes phosphorylation of ADP into high-energy ATP molecules. This reaction is named as oxidative phosphorylation.
The high yield of energy of oxidative phosphorylation ensues from further oxidation of pyruvic acid into compounds with a less sum of bond energy. This process is accomplished by the enzymes of tricarboxylic acid (TCA) cycle, or the Krebs cycle.
The TCA cycle generates a lot of energy as the acetic acid (in the form of acetyl CoA) is oxidized to the final low-energy products CO2 and water. Total ATP yield from oxidative phosphorylation here is in the range of 30-36 ATP molecules.
In addition, several compounds formed in the TCA serve as the important precursors for the next cellular metabolic reactions.
In case of anaerobic respiration, total energetic yield is expectedly lower than in aerobic one.
Overall, the processes of respiration in bacteria are very complex and include a long chain of oxidation-reduction reactions with the participation of many enzymatic systems transporting the electrons. The detailed biochemical mechanisms of respiration are described elsewhere within biochemistry course.