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Examples: photosynthesis and cellular respiration
Anoxic incubation induces significant increases in the activities of the NADP-dependent isocitrate dehydrogenase, 3-hydroxyacyl-CoA dehydrogenase (HCDH) and, at lower temperatures (10°C), lactate dehydrogenase (). HCDH normally operates in the -oxidation of fatty acids in the mitochondria. ) proposed that a reversed -oxidation pathway results in the synthesis of excretory fatty acids and provides a sink for electrons to maintain the redox balance under anoxia. The glyceraldehyde-3-phosphate dehydrogenase isoenzymes and are also involved in the oxygen-deprivation response, although it is unlikely that they merely enhance the flux through glycolysis (). L-lactate, acetate, succinate and propionate are the main waste products of anaerobic metabolism, and they are predominantly excreted. Malate excretion is substantially reduced under anoxic conditions. Branched fatty acids like 2-methylbutyrate or 2-methylvalyrate, which are typically excreted by parasitic nematodes, are not produced in measurable amounts by anoxic (). reported that ethanol is an important end product of anaerobic fermentation, but ) found ethanol levels to be below the limits of detection. The reason for this discrepancy is not known, but may be related to differences in culture conditions, analytical methods, or possibly to bacterial contamination. Interestingly, anoxic worms cultured axenically excrete more succinate but almost no propionate, suggesting that the enzymes for the conversion of succinate to propionate are not active or are lacking under these conditions. Axenic culture conditions may also affect the lactate carrier, since axenic animals retain most of the lactate produced under anoxia in their tissues (). These metabolic adaptations to oxygen deprivation are consistent with an induction of the pyruvate fermentation and malate dismutation pathways (). Enhanced malate dismutation is also observed during dauer diapause. Dauers are resistant to several environmental stresses, including hypoxia (; ).
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Electron Transport Chain: The Movie
Transcriptional studies using SAGE analysis on sterile worms and controls did not yield identical results. ) found a clear reduction in transcripts for general metabolic activities (including DNA/RNA, lipid, protein and energy metabolism) in 6-day-old adults compared to age-matched . This led the authors to conclude that mutants must be hypometabolic at mid-life, although metabolic rate appeared to be restored at advanced age. A more recent study of the same SAGE library confirmed the decline in TCA pathway transcripts, but no concomitant upregulation of fermentation-related genes was detected (). On the contrary, higher transcript levels of enzymes associated with the glyoxylate shunt, possibly compensating for the reduced TCA cycle activity and electron transport machinery were found in . It is still unclear, however, how the glyoxylate cycle, which yields at least four times fewer reducing equivalents relative to the TCA cycle, could compensate for the 4-5-fold reported decrease of the TCA cycle and still reduce similar amounts of oxygen through the electron transport chain.
Another anaerobic process that seems to be important in dauers is malate dismutation. Malate, formed from oxaloacetate by malate dehydrogenase or produced by the glyoxylate pathway, can enter mitochondria for aerobic (TCA cycle) or anaerobic combustion (). Here, malate can be oxidized to pyruvate via malic enzyme and further converted into acetyl-CoA and acetate. Alternatively, malate can be reduced to fumarate and succinate, using the reverse action of fumarase and complex II of the electron transport chain. Complex II can act as succinate dehydrogenase or as fumarate reductase (). In addition to the electron transporter ubiquinone, dauers also contain rhodoquinone, which has a lower reduction potential and favors the reverse action of complex II (; ; ). Oxidation of malate to acetate and reduction to succinate in a ratio of 1:2 would maintain the redox balance in equilibrium in the absence of oxygen (). The malate dismutation pathway and the excretion of succinate, propionate and acetate has been well documented for parasitic helminths, including (). These end products of anaerobic mitochondria are also excreted by under anoxic conditions ().
Electron transport chain - Wikipedia
Take a look at the Photosynthesis Animation of the Light Dependent Reactions from the Virtual Cell Animation Collection. You are not required to know the detail in this video, but rather get a feel for the overall process and the roles of the following molecules:
Both electrons are passed down an electron transport chain. One generates ATP and the other NADPH, an electron carrier. Chemical energy has been generated from sunlight energy.
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electron transport chain, fermentation, glucose and ..
Under aerobic conditions (), will metabolize energy through the standard metabolic pathways. In glycolysis, a series of enzymatic reactions will convert sugars (typically glucose) to pyruvate. During this process, small amounts of ATP are generated and electron carriers are loaded with electrons (NAD+ is reduced to NADH + H+). Consequently, pyruvate is translocated to the mitochondria, decarboxylated and converted into acetyl-CoA. In this process the first completely oxidized carbon is released as CO2. Acetyl-CoA then enters the tricarboxylic acid (TCA) cycle by condensing with oxaloacetate to form citrate. A series of oxidation reactions then ensue with the result that two carbons are expelled as CO2, (G/A)TP is produced and the electron carriers (NAD+ and FAD+) are reduced. Finally, the cycle is completed when oxaloacetate is formed. The electron carriers that were reduced during glycolysis and the TCA cycle deliver their electrons to O2 (with the formation of water) through a series of electron transport chain redox proteins located in the inner mitochondrial membrane. During electron transfer, some of these redox proteins shuttle protons from the mitochondrial matrix to the intramembrane space of the mitochondria, thereby creating an electrochemical gradient. The potential energy in this gradient is finally used by ATP-synthase (another protein complex in the inner mitochondrial membrane) to drive ATP synthesis.
The Mitochondrion and the Electron Transport Chain animation.
Lipids and amino acids can also be used as energy sources, but they enter the main pathways at different points. has a functional methylmalonyl-CoA epimerase (racemase) that is involved in propionyl-CoA metabolism for the degradation of branched amino acids and odd-chain fatty acids (). Fatty acid moieties of lipids are broken down by -oxidation into acetyl-CoA (which in turn can enter the TCA cycle). -oxidation occurs in the mitochondrial matrix and also yields reduced electron carriers. Peroxisomal -oxidation of long-chain fatty acids is not linked directly to energy metabolism because the reduced electron carrier is directly oxidized by molecular oxygen (yielding hydrogen peroxide). Amino acids can be broken down via distinct pathways and their carbon skeletons can be metabolized in the TCA cycle.
However both systems use an electron transport chain and ..
Chloroplast (plant cells only) : Chloroplasts are specialized found in all higher plant cells. These organelles contain the plant cell's chlorophyll responsible for the plant's green color and the ability to absorb energy from sunlight. This energy is used to convert water plus atmospheric carbon dioxide into metabolizable sugars by the biochemical process of photosynthesis. Chloroplasts have a double outer membrane. Within the stroma are other membrane structures - the thylakoids. Thylakoids appear in stacks called "grana" (singular = granum). Estrella Moumtain Community College provides a good source of information on photosynthesis.
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