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Lecture 11

Lecture 11:"Cellular Respiration II"

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Biology 1002B
Tom Haffie

Biology Lecture No. 11: Cellular Respiration II Monday February 13 , 2012 RECALL: -The mitochondrion is an energy-transducing organelle, much like the chloroplast. -The mitochondrial matrix is an aqueous environment, much like the stroma. -Respiratory electron transport occurs within the inner mitochondrial membrane. The Electron Transport Chain I: -Along the inner mitochondrial membrane, major proteins known as supramolecular complexes give rise to the transport of electrons. They consist of NADH dehydrogenase (enzyme complex), cytochrome complex (similar to cytochrome along thylakoid membrane), and cytochrome oxidase. -It is important to note that these mitochondrial components are not specific to eukaryotes as bacteria have been known to contain them as well. -NADH dehydrogenase oxidizing NADH signals the beginning of respiratory electron transport. Then the ubiquinone region shuttles electrons from NADH dehydrogenase to the cytochrome complex. + -Once electrons exit the cytochrome complex they are paired with H ions and oxygen in the mitochondrial matrix, producing water (H O)2 -The production of water greatly distinguishes respiratory form photosynthetic electron transport as water is a reactant in the thylakoid reactions. -The direct product (water) may be essential for life, but there is not much energy associated with it. Movement Of Electrons Through Electron Transport: -The electrons are able to flow as a result of the components of respiratory electron transport being organized in order of increasing redox potential (negative to positive redox potential where each consequent component has a greater pull of electrons). -NADH is very easy to oxidize due to its great reducing power and small redox potential. Oxygen on the other hand, is very easy to reduce as it is a powerful oxidizing agent. -Note that the pulling of electrons is not controlled by the proteins involved, but by the cofactors bound to the proteins (FMN group and FeS group). -These organic molecules not encoded by genes, but are rather the products of biosynthetic pathways. Due to iron’s various oxidation states, it becomes clear why FeS can play a role in redox reactions. -NADH spontaneously donates an electron to FMN group as FMN is more electronegative and has a slightly greater redox potential. This process continues to completion by spontaneous flow based on redox potential. -As oxygen is the final electron acceptor for cellular respiration, there is an enormous requirement for oxygen in the body. Since it is needed in such high quantities, a decrease in oxygen abundance (especially in mitochondria) is where problems can occur. Chemiosmosis: -The term chemiosmosis is defined as linking a proton gradient to do work. This does not imply that this method is solely used for making ATP across chloroplast and mitochondrial membranes. -In any energy-requiring processes within a cell, it is common for a proton gradient to be established across a membrane. -The proton-motive force (PMF) describes the magnitude of the force in a proton gradient. This force value is based on both pH difference and difference in charge, which is why proton gradients are useful as a source of energy to do work. PMF =  - 59(pH) - denotes the difference in charge that is established across a membrane, while pH denotes the change in concentration that exists. In mitochondria, most PMF is due to charge difference. -In mitochondria, the PMF is roughly 220mV although the PMF will not always be the same due to change in contributions by each of the two components. The Electron Transport Chain II: -The energy associated with NADH is conserved by proton pumping as water has no energy associated with it. - When NADH is oxidized, some of the energy is used to pump protons (H ions) into the intermembrane + space. Ubiquinone then cycles, picking up H ions and dumping them also into the intermembrane space. There is also proton pumping at the level of cytochrome oxidase. -As protons accumulate in great numbers in the intermembrane space, they eventually are pumped back into the matrix through ATP synthase and form ATP from ADP and P. For this reason, electron transport is linked to proton pumping. -This is a highly conserved arrangement of protein complexes as it is found in all domains of life and has changed little over evolutionary time. -Electron transport may not measure ATP synthesis, but due to the link between oxidation of NADPH and phosphorylation of ADP, the process is better known as oxidative phosphorylation. -Oxidative phosphorylation is distinctly different from substrate level phosphorylation as that is a method of synthesizing ATP in the absence of oxygen, in glycolysis. -Electron transport and ATP synthesis are coupled processes, coupled by chemiosmosis, the proton gradient. This is not to be confused with energy coupling. -Each NADH yields 3 ATP, while each FADH yiel2s 2 ATP. This is due to the fact that the oxidation of FADH b2gins in the ubiquinone cycle and bypasses the NADH dehydrogenase protein complex, not pumping nearly as much protons as the oxida
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