8.3 Cellular respiration  (Page 11/8)

Learning objectives

• Compare and contrast the electron transport system location and function in a prokaryotic cell and a eukaryotic cell
• Compare and contrast the differences between substrate-level and oxidative phosphorylation
• Explain the relationship between chemiosmosis and proton motive force
• Describe the function and location of ATP synthase in a prokaryotic versus eukaryotic cell
• Compare and contrast aerobic and anaerobic respiration

We have just discussed two pathways in glucose catabolism—glycolysis and the Krebs cycle—that generate ATP by substrate-level phosphorylation. Most ATP, however, is generated during a separate process called oxidative phosphorylation , which occurs during cellular respiration. Cellular respiration begins when electrons are transferred from NADH and FADH ₂ —made in glycolysis, the transition reaction, and the Krebs cycle—through a series of chemical reactions to a final inorganic electron acceptor (either oxygen in aerobic respiration or non-oxygen inorganic molecules in anaerobic respiration). These electron transfers take place on the inner part of the cell membrane of prokaryotic cells or in specialized protein complexes in the inner membrane of the mitochondria of eukaryotic cells. The energy of the electrons is harvested to generate an electrochemical gradient across the membrane, which is used to make ATP by oxidative phosphorylation.

Electron transport system

The electron transport system (ETS) is the last component involved in the process of cellular respiration; it comprises a series of membrane-associated protein complexes and associated mobile accessory electron carriers ( [link] ). Electron transport is a series of chemical reactions that resembles a bucket brigade in that electrons from NADH and FADH ₂ are passed rapidly from one ETS electron carrier to the next. These carriers can pass electrons along in the ETS because of their redox potential . For a protein or chemical to accept electrons, it must have a more positive redox potential than the electron donor. Therefore, electrons move from electron carriers with more negative redox potential to those with more positive redox potential. The four major classes of electron carriers involved in both eukaryotic and prokaryotic electron transport systems are the cytochrome s, flavoprotein s, iron-sulfur protein s, and the quinone s.

In aerobic respiration , the final electron acceptor (i.e., the one having the most positive redox potential) at the end of the ETS is an oxygen molecule (O ₂ ) that becomes reduced to water (H ₂ O) by the final ETS carrier. This electron carrier, cytochrome oxidase , differs between bacterial types and can be used to differentiate closely related bacteria for diagnoses. For example, the gram-negative opportunist Pseudomonas aeruginosa and the gram-negative cholera-causing Vibrio cholerae use cytochrome c oxidase, which can be detected by the oxidase test, whereas other gram-negative Enterobacteriaceae, like E. coli , are negative for this test because they produce different cytochrome oxidase types.