Key Takeaways 1. Prokaryotes as a group are capable of a large set of chemical reactions to gain energy. Numerous fermentations, anaerobic respiration, anoxygenic photosynthesis, oxygenic photosynthesis, as well as aerobic respiration are found in prokaryotes. 2. Energy in the form of ATP is generated in three general ways by organisms: substrate level phosphorylation, electron transport level phosphorylation, and photophosphorylation. 3. Anaerobic fermentation involves the transfer of electrons between organic molecules and does not involve molecular oxygen.
4. Energy yields from fermentation are low, so a large amount of substrate is needed to produce appreciable growth. 5. Many microbes (and most larger organisms) use the Embden-Meyerhof pathway to catabolize sugars, with the pathway ending with pyruvate. 6. Alcoholic fermentation reduces pyruvate to ethanol in two steps. 7. Lactic fermentation reduces pyruvate to lactate. 8. The heterolactic pathway results in the formation of ethanol, CO 2 , and lactic acid. It shares some enzymes with the EMP pathway, but has several unique enzymes. Only 1 ATP is generated per glucose fermented. 9. The Entner-Doudoroff pathway oxidizes glucose before cleavage and results in only 1 ATP generated per glucose converted into pyruvate. Chapter 8-5 Respiration involves donation of electrons to an inorganic terminal electron acceptor An overall picture of respiration can stated as follows. Respiration is the oxidation of a source of energy by removal of electrons and donation to an inorganic terminal electron acceptor. The path the electrons follow from source to acceptor involves a membrane-bound system that creates a proton gradient between the cytoplasm and the outside environment. This proton gradient can then be used to create ATP. If you can understand this paragraph, you understand the basics of respiration. What follows fills in the details and gives specific examples, but if you get this right now, you are well on your way to understanding this aspect of metabolism . The diversity of respiring microbes is vast, but they do have many things in common. ATP is generated by electron transport level phosphorylation (ETLP) involving a membrane system where the redox potential of the electrons on NADH are used to create a proton gradient across a membrane. This then drives ATP synthesis. This will be covered in more detail later on. The energy yield from respiration is much higher than fermentation because much more of the potential energy stored in the reduced substrates is extracted with strong oxidants. Moreover, the reducing equivalents produced are more efficiently converted to ATP. Because of this energy advantage, respiring organisms, including ourselves, have come to dominate the earth.
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- Spring '08