6.1: Respiration
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
In Chapter 5 we discussed Central Metabolism—glycolysis and the TCA cycle—which generates ATP by substrate-level phosphorylation when used catabolically. In respiration, however, most ATP is generated during a separate process called oxidative phosphorylation. Respiration begins when electrons are transferred from an electron donor through a series of chemical reactions to a final inorganic electron acceptor obtained from the environment (either oxygen in aerobic respiration or non-oxygen inorganic molecules in anaerobic respiration) (Figure \(\PageIndex{1}\)). In chemoheterotrophs the electron donors are NADH and FADH 2 which carry electrons from glycolysis and the TCA cycle, but in chemoautotrophs the electron donor is another source of chemical energy such as hydrogen sulfide or hydrogen. These electron transfers take place on the plasma membrane in prokaryotic cells or 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 Chain
The electron transport chain (ETC) is the last component involved in the process of respiration; it comprises a series of membrane-associated protein complexes and associated mobile accessory electron carriers. Electron transport is a series of chemical reactions that resembles a bucket brigade in that electrons from the electron donor are passed rapidly from one ETC electron carrier to the next. These carriers can pass electrons along in the ETC 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 cytochromes, flavoproteins, iron-sulfur proteins, and the quinones.
Figure \(\PageIndex{1}\). From glucose to ATP in a respiring cell. Yellow buttons bring up additional information.
In aerobic respiration, the final electron acceptor (i.e., the one having the most positive redox potential) at the end of the ETC is an oxygen molecule (O 2 ) that becomes reduced to water (H 2 O) by the final ETC 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.
Although we are most familiar with aerobic respiration, it is far from the only type of respiration used by prokaryotes. As we saw in Chapter 4 , most organisms were anaerobic prior to the increase in atmospheric oxygen due to the oxygenic phototrophy of cyanobacteria. Some prokaryotes may not have the option to aerobically respire while others can use either oxygen or another final electron acceptor depending on the environmental conditions. Some examples of situations where aerobic respiration is not possible or preferable include:
- The cell lacks genes encoding an appropriate cytochrome oxidase for transferring electrons to oxygen at the end of the electron transport system.
- The cell lacks genes encoding enzymes to minimize the severely damaging effects of dangerous oxygen radicals produced during aerobic respiration, such as hydrogen peroxide (H 2 O 2 ) or superoxide (O2–).
- A cell which can aerobically respire when oxygen is present lacks a sufficient amount of oxygen to carry out aerobic respiration.
An alternative to aerobic respiration is anaerobic respiration , respiring using inorganic molecule other than oxygen as a final electron acceptor at the end of the ETC. In the big picture, the processes of aerobic and anaerobic respiration are essentially identical except for the final electron acceptor used at the end of the ETC . There are many types of anaerobic respiration found in bacteria and archaea. Denitrifiers are important soil bacteria that use nitrate (NO3–) and nitrite (NO2–) as final electron acceptors, producing nitrogen gas (N 2 ). When oxygen levels have been depleted, many aerobically respiring bacteria, such as Pseudomonas aeruginosa , can switch to using nitrate as a final electron acceptor and producing nitrite as a waste product.
Microbes using anaerobic respiration have an intact Krebs cycle, so these organisms can access the energy of the NADH and FADH 2 molecules formed. However, anaerobic respirers use altered ETC carriers encoded by their genomes, including distinct complexes for electron transfer to their final electron acceptors. Smaller electrochemical gradients are generated from these electron transfer systems, so less ATP is formed through anaerobic respiration. Because of the very positive redox potential of oxygen, an organism that can use oxygen as its final electron acceptor will be able to generate more ATP than any other final electron acceptor . For instance, a bacterium catabolizing glucose and using nitrate as the final electron acceptor for respiration will generate about half as much ATP as one using oxygen.
Exercise \(\PageIndex{1}\)
Do both aerobic respiration and anaerobic respiration use an electron transport chain?
Chemiosmosis, Proton Motive Force, and Oxidative Phosphorylation
In each transfer of an electron through the ETS, the electron loses energy, but with some transfers, the energy is stored as potential energy by using it to pump hydrogen ions (H + ) across a membrane. In prokaryotic cells, H + is pumped to the outside of the cytoplasmic membrane (called the periplasmic space in gram-negative and gram-positive bacteria), and in eukaryotic cells, they are pumped from the mitochondrial matrix across the inner mitochondrial membrane into the intermembrane space. There is an uneven distribution of H + across the membrane that establishes an electrochemical gradient because H + ions are positively charged (electrical) and there is a higher concentration (chemical) on one side of the membrane. This electrochemical gradient formed by the accumulation of H + (also known as a proton) on one side of the membrane compared with the other is referred to as the proton motive force (PMF). Because the ions involved are H + , a pH gradient is also established, with the side of the membrane having the higher concentration of H + being more acidic. Beyond the use of the PMF to make ATP, as discussed in this chapter, the PMF can also be used to drive other energetically unfavorable processes, including nutrient transport and flagella rotation for motility.
The potential energy of this electrochemical gradient generated by the ETS causes the H + to diffuse across a membrane (the plasma membrane in prokaryotic cells and the inner membrane in mitochondria in eukaryotic cells). This flow of hydrogen ions across the membrane, called chemiosmosis, must occur through a channel in the membrane via a membrane-bound enzyme complex called ATP synthase (Figure \(\PageIndex{2}\)). The tendency for movement in this way is much like water accumulated on one side of a dam, moving through the dam when opened. ATP synthase (like a combination of the intake and generator of a hydroelectric dam) is a complex protein that acts as a tiny generator, turning by the force of the H + diffusing through the enzyme, down their electrochemical gradient from where there are many mutually repelling H + to where there are fewer H + . In prokaryotic cells, H + flows from the outside of the cytoplasmic membrane into the cytoplasm, whereas in eukaryotic mitochondria, H + flows from the intermembrane space to the mitochondrial matrix. The turning of the parts of this molecular machine regenerates ATP from ADP and inorganic phosphate (P i ) by oxidative phosphorylation, a second mechanism for making ATP that harvests the potential energy stored within an electrochemical gradient.
The number of ATP molecules generated from the catabolism of glucose varies. For example, the number of hydrogen ions that the electron transport system complexes can pump through the membrane varies between different species of organisms. In aerobic respiration in mitochondria, the passage of electrons from one molecule of NADH generates enough proton motive force to make three ATP molecules by oxidative phosphorylation, whereas the passage of electrons from one molecule of FADH 2 generates enough proton motive force to make only two ATP molecules. Thus, the 10 NADH molecules made per glucose during glycolysis, the transition reaction, and the Krebs cycle carry enough energy to make 30 ATP molecules, whereas the two FADH 2 molecules made per glucose during these processes provide enough energy to make four ATP molecules. Overall, the theoretical maximum yield of ATP made during the complete aerobic respiration of glucose is 38 molecules, with four being made by substrate-level phosphorylation and 34 being made by oxidative phosphorylation (Figure \(\PageIndex{3}\)). In reality, the total ATP yield is usually less, ranging from one to 34 ATP molecules, depending on whether the cell is using aerobic respiration or anaerobic respiration; in eukaryotic cells, some energy is expended to transport intermediates from the cytoplasm into the mitochondria, affecting ATP yield.
Figure \(\PageIndex{3}\) summarizes the theoretical maximum yields of ATP from various processes during the complete aerobic respiration of one glucose molecule.
Exercise \(\PageIndex{1}\)
What are the functions of the proton motive force?
Summary
- Most ATP generated during the cellular respiration of glucose is made by oxidative phosphorylation .
- An electron transport system (ETS) is composed of a series of membrane-associated protein complexes and associated mobile accessory electron carriers. The ETS is embedded in the cytoplasmic membrane of prokaryotes and the inner mitochondrial membrane of eukaryotes.
- Each ETS complex has a different redox potential, and electrons move from electron carriers with more negative redox potential to those with more positive redox potential.
- To carry out aerobic respiration , a cell requires oxygen as the final electron acceptor. A cell also needs a complete Krebs cycle, an appropriate cytochrome oxidase, and oxygen detoxification enzymes to prevent the harmful effects of oxygen radicals produced during aerobic respiration.
- Organisms performing anaerobic respiration use alternative electron transport system carriers for the ultimate transfer of electrons to the final non-oxygen electron acceptors.
- Microbes show great variation in the composition of their electron transport systems, which can be used for diagnostic purposes to help identify certain pathogens.
- As electrons are passed from NADH and FADH 2 through an ETS, the electron loses energy. This energy is stored through the pumping of H + across the membrane, generating a proton motive force .
- The energy of this proton motive force can be harnessed by allowing hydrogen ions to diffuse back through the membrane by chemiosmosis using ATP synthase . As hydrogen ions diffuse through down their electrochemical gradient, components of ATP synthase spin, making ATP from ADP and P i by oxidative phosphorylation.
- Aerobic respiration forms more ATP (a maximum of 34 ATP molecules) during oxidative phosphorylation than does anaerobic respiration (between one and 32 ATP molecules).
Contributors and Attributions
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Nina Parker, (Shenandoah University), Mark Schneegurt (Wichita State University), Anh-Hue Thi Tu (Georgia Southwestern State University), Philip Lister (Central New Mexico Community College), and Brian M. Forster (Saint Joseph’s University) with many contributing authors. Original content via Openstax (CC BY 4.0; Access for free at https://openstax.org/books/microbiology/pages/1-introduction )