However, anaerobic respirers use altered ETS 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. 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.
Figure 1. 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 1. The tendency for movement in this way is much like water accumulated on one side of a dam, moving through the dam when opened. 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 Table 1.
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.
Table 1 summarizes the theoretical maximum yields of ATP from various processes during the complete aerobic respiration of one glucose molecule.
Skip to main content. Microbial Metabolism. Search for:. Cellular 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.
Think about It Do both aerobic respiration and anaerobic respiration use an electron transport chain? Think about It What are the functions of the proton motive force? Key Concepts and Summary Most ATP generated during the cellular respiration of glucose is made by oxidative phosphorylation. Nitrate, like oxygen, has a relatively positive reduction potential.
This process is widespread, and used by many members of the Proteobacteria. Denitrification involves the stepwise reduction of nitrate to nitrite NO 2 — , nitric oxide NO , nitrous oxide N 2 O , and, eventually, to dinitrogen N 2. Some organisms e. Others e.
Paracoccus denitrificans or Pseudomonas stutzeri reduce nitrate completely. Complete denitrification is an environmentally significant process because nitrous oxide is a powerful greenhouse gas, while nitric oxide reacts with sunlight and ozone to produce nitric acid, a component of acid rain.
Denitrification is also important in biological wastewater treatment, where it can be used to reduce the amount of nitrogen released into the environment, thereby reducing eutrophication. Denitrification takes place under special conditions in both terrestrial and marine ecosystems, where oxygen consumption exceeds the oxygen supply and sufficient quantities of nitrate are present.
These environments may include certain soils and groundwater, wetlands, oil reservoirs, and in sea floor sediments. Sulfate reduction uses sulphate SO 4 2- as the electron acceptor, producing hydrogen sulphide H 2 S as a metabolic end product. Sulfate reduction is a relatively energetically poor process, though it is a vital mechanism for bacteria and archaea living in oxygen-depleted, sulphate-rich environments.
Many sulphate-reducing organisms are organotrophic, using carbon compounds, such as lactate and pyruvate among many others as electron donors. Other sulphate reducers are lithotrophic, using hydrogen gas H 2 as an electron donor.
Some species of sulphate-reducing bacteria SRBs; e. Sulfate-reducing bacteria are common in anaerobic environments such as seawater, sediment, and water rich in decaying organic material where they aid in the degradation of organic materials.
In these anaerobic environments, fermenting bacteria extract energy from large organic molecules; the resulting smaller compounds such as organic acids and alcohols are further oxidized by acetogens, methanogens, and the competing SRBs. Acetogens and methanogens are further discussion in Methanogens and Syntrophy. Many bacteria reduce small amounts of sulphates in order to synthesize sulphur-containing cell components; this is known as assimilatory sulphate reduction.
Most SRBs can also reduce other oxidized inorganic sulphur compounds, such as sulphite, thiosulphate, or elemental sulphur S 0. Toxic hydrogen sulphide is one waste product of sulphate-reducing bacteria; its rotten egg odour is often a marker for the presence of SRBs. Common examples are salt marshes and mud flats. Much of the hydrogen sulphide will react with metal ions in the water to produce metal sulphides.
These metal sulphides, such as ferrous sulphide FeS , are insoluble and often black or brown. Thus, the black colour of sludge on a pond, or in the Winogradsky sulphuretum Figure 8. Electron flow in these organisms is similar to those in electron transport, ending in oxygen or nitrate, except that in ferric iron-reducing organisms the final enzyme in this system is a ferric iron reductase. Since some ferric iron-reducing bacteria e.
Geobacter metallireducens can use toxic hydrocarbons e. Organic compounds may also be used as electron acceptors in anaerobic respiration. Methanogenesis is another form of anaerobic respiration involving the reduction of CO 2.
It is discussed in Methanogens and Syntrophy. 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.
However, anaerobic respirers use altered ETS 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. 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.
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 8.
The tendency for movement in this way is much like water accumulated on one side of a dam, moving through the dam when opened. 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 8.
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 8. As an Amazon Associate we earn from qualifying purchases. Want to cite, share, or modify this book? This book is Creative Commons Attribution License 4.
0コメント