Advanced Cell Bio Chapter 14 Part 1 Notes!
Advanced Cell Bio Chapter 14 Part 1 Notes! BCMB 311
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This 5 page Class Notes was uploaded by Izabella Nill Gomez on Wednesday March 2, 2016. The Class Notes belongs to BCMB 311 at University of Tennessee - Knoxville taught by Dr. Barry Bruce, Dr. J. Park in Spring 2016. Since its upload, it has received 18 views.
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Date Created: 03/02/16
Advanced Cell Bio Chapter 14 Notes Part 1 It is thought that the earliest cells may have produced ATP by breaking down organic molecules generated by geochemical processes--fermentation occurs in present day cells, where they use energy from paerial oxidation of energy rich foods for ATP. Main chemical currency of energy is ATP< small amounts generated during glycolysis in the cytosol of all cells--but for most cells, ATP is produced by oxidative phosphorylation (O.P.). Generation of ATP by O.P. differs in the way ATP is produced during glycolysis in that it requires a membrane--in eukarya, takes place in mitochondria and depends on electron transport that moves H+ across the inner mitochondrial membrane. Consists of 2 stages, one set up E.G. proton gradient, other uses the gradient to generate ATP. Both are carried out by special protein complexes in the membrane. 1. In stage 1, high energy electrons derived from oxidation of food molecules, from sunlight or other sources, are transferred along a series of electron carriers--electron transport chain (E.T.C.)--embedded in the membrane. Electron transfers release energy used to pump protons, derived from H2O ubiquitous in cells, across membrane and thus generated an electron proton gradient. Ion gradient across a membrane is a form of stored energy to do work when ions are allowed to flow back down E.G. 2. Protons flow down E.G. through protein complex ATP synthase, which catalyzes the energy a synthesis of ATP from ADP and inorganic Phosphate (Pi). Ions like to bind, allowing proton gradient to drive ATP production. Together, these two stages are called chemiosmotic coupling Mechanism for making ATP arose very early in life’s history. Some type of ATP generating processes occur in the plasma membrane of modern bacteria and archaea. Process in eukaryotic mitochondria and chloroplasts evolved from engulfed bacterial cells more than 1 billion ya. Also harbor bacteria-like biosynthesis machinery to make RNA and protein and retain their own genomes. Many chloroplasts like cyanobacteria--photosynthetic bacteria from which chloroplasts are thought to have originated from. Although mitochondria/chloroplasts still have DNA, the bacteria that gave rise gave up many genes for independent living. The genes moved to the cell nucleus, however, and continue to carry out products of proteins that they import to carry out special functions, including the generation of ATP. Mitochondria present in nearly all eukaryotic cells, where they produce the bulk of the cell’s ATP. Without mitochondria, eukarya would rely on inefficient glycolysis. When glucose is converted to pyruvate by glycolysis in the cytosol, only 2 ATP/glucose procures, 10%< of free energy available. 30 molecules of ATP/glucose through mitochondria are produced. Patients with MERRF are deficient in multiple proteins required for electron transport, results in muscle weakness, heart problems, dementia. Muscle and nerve cells are especially sensitive to mitochondrial defects because of a need for ATP. Isolated mitochondria are generally similar in size and shape to bacterial ancestors. Although no longer capable of living independently, mitochondria are remarkably adaptable and can adjust to location, shape and number to suit the needs of the cell. Usually placed in an area of high energy consumption (ex: around sperm tail or contractile fiber). In other cells, fuse to form elongated, dynamic tubular networks, diffusely distributed though the cytosol--networks are dynamic, constantly breaking and fusion. Mitochondria are present in large numbers but vary in cell type and energy needs. Individual mitochondrion is bound by 2 highly specialized membranes--one surrounding the other. Outer and inner membranes create large internal space-- matrix--and narrower intermembrane space. When purified mitochondria are gently fractured into separate components and contents analyzed, each space has a unique collection of proteins. The outer membrane contains many molecules of transport protein porin which forms wide aqueous channels through the lipid bilayer. Center membrane like a sieve permeable to all molecules of 5000 daltons or less, including small proteins. Makes intermembrane space chemically equivalent to the cytosol with respect to small uncharged and inorganic ions it contains. Inner membrane is impermeable to the passage of ions and most small molecules, except where a path is provided by a specific membrane transport protein. Mitochondrial matrix only has molecules selectively transported into the matrix across the intermembrane and contents are highly specialized. The inner mitochondrial membrane is the site of O.P., has proteins of ETC, proton pumps and ATP synthase need for ATP production. Also has a variety of transport proteins that allow entry of selected small molecules--ex: pyruvate and fatty acids that will be oxidized by the mitochondria into the matrix. The inner membrane is highly convoluted forming cristae foldings that project into the matrix space. Greatly increases the surface area of the membrane. Generation of ATP powered by the flow of electrons is derived from the burn of carbs, fats, other fuels during glycolysis and the citric acid cycle. High energy electrons provided by activated carriers generated during the 2 stages of catabolism, with the majority churned out by the citric acid cycle that operates in the mitochondrial matrix. Citric acid cycled gets fuel needed to produce activated carriers from food molecules that make their way into the mitochondria by the cytosol. Both pyruvate from the glycolysis and fatty acids derived from the breakdown of fats can enter into the mitochondrial intermembrane space through porins of the outer membrane. Fuel molecules are then transported to the matrix, where they are converted to acetyl CoA--groups oxidized to CO2 via citric acid cycle. Some of the energy derived is saved in the form of high energy electrons, held by NADPH and FADH2--then donate electrons to the ETC in the inner membrane. Chemiosmotic generation of energy begins when activated NADH and FADH2 carriers donate high energy electrons to ETC in the inner membrane, oxidizing to NAD+ and FAD in the process. Electrons are quickly passed along chemically to molecular O2 to make H2O. Stepwise movement of electrons through the ETC releases energy that can be used to pump protons across the inner membrane. Resulting proton gradient is used to drive synthesis of ATP. Inner membrane serves as a device to convert energy in electrons of NADH/FADH2 into the P bond of ATP molecule--oxidative phosphorylation involves the consumption of O2 and the addition of P group to ADP to make ATP. Source of high energy electron differs between different organisms and processes. In cellular respiration, high electron ultimately derived from sugars or fats. In photosynthesis, high electrons come from chlorophyll which captures light energy. And many single celled organisms use inorganic substances to make ATP. ETC or respiratory chain has more than 40 proteins, grouped into 3 large respiratory enzyme complexes---contain multiple individual proteins, including transmembrane proteins that anchor the complex firmly in the inner mitochondrial membrane. 3 enzyme complexes in the order they receive electrons: NADH dehydrogenase complex, cytochrome c reductase complex, cytochrome c oxidase complex. Each contains metal ions and other chemical groups that acts as stepping stones to facilitate the passage of electrons. Movement accompanied by proton pumping form the mitochondrial matrix to the intermembrane space. Each can be though as a proton pump. First respiratory complex in chain, NADH accepts the electron from NADH< extracted in the form of H-, converted to proton and 2 high energy electrons catalyzed by NADH dehydrogenase. Electrons then passed along the chain to other complexes using mobile electron carries to ferry electrons between complexes. Transfer is energetically favorable. Final reaction is O2-requiring step and consumes almost all the O2 we breathe. Without a mechanism for harnessing energy released form the transfer of electron NADH to O2, energy would be released as heat. Cells recover by the energy that is used for proton pumping. Generates H+ (pH) gradient across the inner membrane. pH in the matrix is 7.9 and intermembrane is 7.2 ( same as cytosol). Proton umping generated by a membrane potential across the inner membrane; as H+ flows out, the matrix side becomes negative and intermembrane side is positive. Steep E.G. from H+ to flow into the matrix because of ion concentration and membrane potential--proton motive force--more membrane potential, more energy stored in the proton gradient. In most cells, H+ gradient is used to drive synthesis from ADP and Pi, Device that makes this possible is ATP synthase--large, multisubunit protein embedded in the inner membrane--same enzyme for all cells. Part of the protein that catalyzes phosphorylation of ADP is shape like a lollipop, projects into the mitochondrial matrix and is attached by a central stalk to H+ carrier and spins rapidly like a motor, rubs against the protons to alter the conformation and prompt to form ATP--3 molecules of ATP/revolution produced. ATP synthase can also operate in reverse-- using ATP hydrolysis to pump protons uphill against E.G., synthase operates like H+ pumps--can make/use ATP depending on the magnitude of E. proton G. across the membrane in which the enzyme is embedded, In bacteria that can grow areo/anaerobically, direction in which ATP synthase works routinely reversed when bacteria loses/runs out of O2. ATP synthase uses ATP from glycolysis to pump protons out to create proton gradient to import essential nutrients. Coupled transport across inner mitochondrial membrane also driven by E.P.G.--ex: pyruvate and Pi transported inward with protons as they move down EG to matrix. Other transporters take advantage of the membrane potential generated by EPG which makes the makes the matrix side of the inner membrane more negative than the intermembrane side. The antiport carrier protein exploits this voltage gradient to export ATP and bring ADP in. EPG drives the formation of ATP (-4) and transport of selected metabolites across the inner mitochondrial membrane. Due to nucleotide exchange, ADP (-3) from hydrolysis in the cytosol is randomly driven to the mitochondria for recharging, concentration of ATP in cytosol is kept about 10x higher than ADP--without activity of mitochondria, ATP levels would fall and cell would eventually die (cyanide blocks electron transportation in the inner mitochondria). Much of the energy carried by NADH/FADH2 ultimately converts into bond energy of ATP. How much ATP each of these activated carriers can produce depends on several factors, including where electrons enter the respiratory chain. NADH molecules produced in the mitochondria during the citric acid cycle pass high electrons to NADH hydrogenase complex--first complex. As electrons pass from one complex to the next, they promote proton pumping across the inner membrane. Each NADH=generated 2.5 ATP. FADH2 pass electrons to carrier ubiquinone-- promotes less pumping of protons, produces only 1.5 ATP. Almost 50% of total energy by fats/sugars stored in bonds of ATP during respiration. Transmembrane proton gradients drive the process of ETC ATP production (embedded in membranes). H2O serves as a ready reservoir for accepting/donating protons--these often accompany electrons transferred during oxidation (loses electron + H+)/reduction (gains electron + H+). ET pass spontaneously from a molecule with low affinity for outer shell electrons to higher. NADH with lower electron affinity, passes electron to NADH dehydrogenase complex. Redox pairs: ↔ NADH NAD+ +H+ + 2 electrons (tendency to donate/accept measured by redox potential) Electrons move spontaneously from redox pair with low redox potential (NADH/NAD+ (-320 mV/low)) or low affinity for electrons, to high, like O2/H2O, If Gibbs free energy is negative, it is spontaneous--transfer of electrons must be done in small steps to not cause extensive force and have all energy released as heat. When passing from one respiratory complex to the next, electrons formed by electron carries that diffuse freely within the lipid bilayer. In the mitochondrial respiratory chain, small ubiquinone picks up electrons from NADH complex and delivers to the cytochrome c complex. Quinone functions similarly during electron transport in photosynthesis. Ubiquinone can accept/donate 1 or 2 electrons, picks up 1 H+ from H2O with each electron it carries. Lies between both complexes in tendency to gain/lose electrons. Also serves as entry point for electrons donated by FADH2 generated during the citric acid cycle and from fatty acid oxidation. Redox potential of different level complexes influence where used along the ETC. Iron Sulfur centers have low affinities for electrons, prominent in electron carriers that operate in the early part of the chain. I.S. center in NADH complex passes electrons to ubiquinone. Later, iron atoms in heme groups to the cytochrome proteins common electron carriers. These heme groups give cytochromes; ex: cytochrome c reductase/oxidase their color. Cytochrome proteins increase the redox potential further down the mitochondrial ETC they are located. Cytochrome c accepts electrons from reductase complex--has redox potential between the cytochromes it interacts with.
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