Respiration-II, Biology tutorial

Electron Transport and Oxidative Phosphorylation:

When compared with glycolysis and Krebs cycle, oxidative phosphorylation is the bonanza ATP synthesis. Energy for oxidative phosphorylation comes from NADH and ubiquinol that are produced by j first two stages of respiration. High energy electrons of NADH and ubiquinol are not utilized directly for ATP synthesis; instead, they begin series of oxidation reduction reaction which move electrons by several carriers. Sequence of electron carriers is called as electron transport chain. Energy from electron flow by the carriers maintains proton gradient across inner mitochondrial membrane that drives synthesis of ATP. Oxidative phosphorylation refers to combination of oxidative - reduction reactions of election transport which allow the cell to use energy in NADH and ubiquinol to phosphorylate.

Electron Transport:

Electron transport chain is like the series of small, successively stronger magnets, i.e., each has higher potential than its predecessor. This signifies that every component of chain pulls electrons away from weaker neighbor and gives them up to stronger one. Therefore strongest acceptor in chain is terminal acceptor, oxygen that has potentials of 0.82 volts.

There are at least 9 electron carriers in series, most of which are proteins. Some of the carriers also accept proteins and release them in inter-membrane space. Though scientists are not rather sure how the components work together for electron and proton transport illustrates extensively accepted model for sequence of steps in chain. According to model, in first step of electron transport, Flavin mononucleotide (FMN) takes electrons from NADH in mitochondrial matrix. FMN also takes two protons (H4), one from NADH and one from aqueous matrix. In second step iron and sulphur-containing protein is reduces as FMNH2 is oxidized back to FMN (i.e. iron-sulphur protein pulls electrons from FMNH2. In the meantime protons from FMNH2 are released in inter-membrane space. Net result of the first two steps is that energy from oxidation reduction reactions drives transport of protons from one side of inner mitochondrial membrane to other. Carriers liable for the steps are known as NADH dehydrogenase complex named after enzyme which removes hydrogen from NADH.

In third step of electron transport, electrons are pulled from NADH dehydrogenase complex by ubiquinone (also known as coenzyme Q), that shuttles electrons to complex of two cytochromes and another iron sulphur protein. This complex is known as cytochrome bc1 complex. Protons are again transported from matrix side of inner membrane to inter-membrane space employing energy of electron flow from ubiquinone by cytochrome b-c1 complex. This ubiquinone works as second proton pump in electron transport system and cytochrome b-c1 complex acts as third proton pump. Ubiquinone can also accept electrons directly from ubiquinone in matrix.

Lastly, electrons from cytochrome b-c1 complex are pulled away by mobile cytochrome, cytochrome c1 and shuttled to terminal carriers complex. Terminal carrier complex comprises of two more cytochromes, cytochrome a1 and chtochrome at which form cytochrome oxidase complex. Cytochrome a3 donates the electrons to oxygen that is terminal electron acceptor in electron transport chain. Reduced oxygen then combines with protons in mitochondria matrix producing water. One of the main characteristics of electron transport chain is that it involves iron. This element happens not only in iron sulphur proteins but also in cytochromes. Therefore oxidation and reduction of the electron carrier involves interchanges between reduced (Fe2+) and oxidised (Fe3) forms of iron. In cytochromes, this change happens in complex ring group, known as heme which is attached to protein. Structural masks of cytochrome C1 showing iron-containing heme group. Heme group is blue and iron is red. In electron transport, every iron accepts one electron and is reduced from Fee "to Se". Note that transport of protons from one side of inner mitochondrial membrane to other requires energy that is given by movement of electrons by electron transport chain. Electron movement gives energy as every reduced election carrier has Jess free energy than does donor immediately before it in series. Lowest level of free energy in chain occurs in water which forms when oxygen is reduced in last step between NADH at start of electron transport and oxygen at the end, total change in free energy is -53Kcalmol-l. Energy change between ubiquinol and oxygen is approx one-third less as ubiquinol enters the chain at lower energy level.

Chemiosmosis and ATP Synthesis:

Chemiosmosis theory of ATP synthesis was estimated by British scientist peter Mitchel based on series of experiments he performed towards understanding of ATP synthesis in electron transport chain. His theory states that:

  • Carriers of electron transport chain are embedded in inner mitochondrial membrane. Flow of electrons by this series of carriers is utilized to pump protons across membrane. As membrane is not very permeable to protons, comparatively few protons leak back across membrane.
  • Continued electron transport pumps more and more protons in space between inner and outer membranes of mitochondria. This develops gradient of protons across membrane.
  • Proton gradient serves as the battery to work, comprising making ATP.

According to Mitchel's model, interaction between electron transport and ATP synthesis is indirect electron transport generates the proton gradient, and that gradient drives synthesis of ATP. Inner membrane of mitochondria is folded several times, so that much surface area is available for membrane dependent metabolism. Thousands of copiers of ATP synthase complex are embedded all through membrane, making inner mitochondrial membrane power house for making ATP.

Other kinds of Respiration:

Aerobic respiration is main kind of respiration prevalent in several organisms. It is known as aerobic respiration as it needs oxygen as terminal electron acceptor. When oxygen is not available, other electron acceptors can be utilized in process known as anaerobic respiration.

Anaerobic Respiration:

Though oxygen is needed only at the end of electron transport in aerobic respiration, electron transport and Krebs cycle are both inhibited when oxygen is not available. Glycolysis works usually in oxygen free environment, but energy stored in electrons of NADH from glycolysis can become problem if NADH can't be used fast adequate elsewhere in cell, or if supply of NAD+ is depleted. In both situations, there would be no NAD+ available to oxidize glyceraldehydes-3 phosphate, as a result, glycolysis would stop. In animals, definite fungi and bacteria, pyruvic acid is reduced to lactic acid by excess NADH. In other fungi and plants, plenty of NADH is relieved when pyruvic acid is converted to acetaldehyde that is then reduced by NADH to ethanol. Together, these anaerobic reactions are known as fermentation. Unless they have adaptations (like aerenchyma) to ease diffusion of oxygen to the roots several plants ferment when they raise up in mud or in oxygen-poor water, due to this most house plants die when they are over watered. Anaerobic respiration is incompetent as it produces small or no ATP.

Anaerobic respiration is economically significant. For instance, fermentation by bacteria and fungi is significant for flavoring cheese and yoghurt. Likewise, fermentation by yeast is significant for making alcoholic beverages and bread, wine, beer and bread are prepared by different strains of brewer's years (Saccharomyces cerevisiae). This carbon dioxide made during fermentation makes bread rise, but alcohol from fermentation evaporates when bread is baked. Wine is generally prepared by yeast which grows on grapes, though fermented honey, applies (cider) and other carbohydrates rich substrates are also utilized to make wine or wine such as beverages. Similarly, though beer is generally prepared from barley, rice beer is common in China, and com beer is prepared in Mexico.

Respiration of Lipids:

Lipids which comprise fatty acids are significant storage compounds in oil containing seeds. When such seeds germinate the fatty acids are metabolized in glyoxysomes to recover energy stored in them. Fatty acids are first removed from glycerol in storage triacylglycerides, and then snipped in two carbon pieces which are released as acetyl-CoA. Respiration of lipids. Triacylglycerides are hydrolysed in glycerol and carry ackiss. Fatty acids are degraded by beta-oxidation into acertyl groups which are joined to coenzyme. A can be utilized in respiration or for carbohydrate synthesis. FAD and NAD are also reduced during into oxidation. Such reaction, known as beta-oxidation is repeated for every part of carbons until all fatty acid is converted in acetyl-CoA molecules. Therefore, molecule of stearic acid, that has eighteen carbons, yields nine molecules of acetyl-CoA. Beta-oxidation happens either in cytosol or in glyoxysomes. Release of acetyl-CoA drives reduction of NAD+ and FAD to NADH and FADH2 respectively. Acetyl-CoA from beta-oxidation utilized in Krebs cycle for further recovery of energy stored in fatty acids, or it can be routed to other metabolic pathways NADH and FADH2 may be transported to mitochondria where the electrons utilized in electron transport chain.

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