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MitochondriaMitochondria are responsible for energy production. They are also theresponsible location for which respiration takes place. Mitochondria containenzymes that help convert food material into adenosine triphosphate (ATP), whichcan be used directly by the cell as an energy source. Mitochondria tend to beconcentrated near cellular structures that require large inputs of energy, suchas the flagellum. The role of the mitochondria is very important in respiration. In the presence of oxygen, pyruvate or fatty acids, can be furtheroxidized in the mitochondria. Each mitochondrion is enclosed by two membranesseparated by an intermembrane space.
The intermembrane space extends into thefolds of the inner membrane called cristae which dramatically increase thesurface area of the inner membrane. Cristae extend into a dense material calledthe matrix, an area which contains RNA, DNA, proteins, ribosomes and range ofsolutes. This is similar to the contents of the chloroplast stroma and like thechloroplast, the mitochondrion is a semi-autonomous organelles containing themachinery for the production of some of its own proteins. The main function ofthe mitochondrion is the oxidation of the pyruvate derived from glycolysis andrelated processes to produce the ATP required to perform cellular work.(Campbell182-9) Pyruvate, or fatty acids from the breakdown of triglycerides orphospholipids, pass easily through pores in the outer mitochondrial membranemade up of a channel protein called porin. The inner membrane is a moresignificant barrier and specific transport proteins exist to carry pyruvate andfatty acids into the matrix. Once inside the matrix, pyruvate and fatty acidsare converted to the two carbon compound acetyl coenzyme A (acetyl CoA). Forpyruvate this involves a decarboxylation step which removes one of the threecarbons of pyruvate as carbon dioxide. The energy released by the oxidation ofpyruvate at this stage is used to reduce NAD to NADH. (185) The C2 acetyl CoA is then taken into a sequence of reactions known asKrebs cycle which completes the oxidation of carbon and regenerates an acceptorto keep the cycle going.
The oxidation of the carbon is accompanied by thereduction of electron acceptors and the production of some ATP by substratephosphorylation. The C2 acetyl CoA is coupled to oxaloacetate, a C4 acceptor inthe cycle. The product is citrate a C6 compound. This first product, citrate,is the reason the cycle is sometimes called the citric acid or ticarboxylic acidcycle, referring it after the scientist whose lab most advanced ourunderstanding of it, Sir Hans Krebs. (Comptons 160) Two of the early reactions of the cycle are decarboxylations whichshorten citrate to succinate a C4 compound. The CO2 lost does not actuallyderive from acetyl CoA, during that cycle, but two carbons are lost which arethe equivalent of the two introduced by acetyl CoA. The decarboxylation stepsare again accompanied by the reduction of NAD to NADH.
The formation ofsuccinate also sees the formation of an ATP molecule by substratephosphorylation. (Brit 1041) The last part of the cycle converts C4 succinate back to C4 oxaloacetate.In the process another reaction generates NADH while another reduces theelectron acceptor FAD (Flavin Adenine Dinucleotide) to FADH. The final stage of respiration in the mitochondria involves the transferof energy from the reduced compounds NADH and FADH to the potential energy storerepresented by ATP. The process is oxidative phosphorylation and it is drivenby a chemiosmotic system analogous to that seen in chloroplasts. (Moore 88-9) The inner membrane contains an electron transport chain that can receiveelectrons from reduced electron carriers. The energy lost as electrons flowbetween the components of the electron transport chain is coupled to the pumpingof protons from the matrix to the intermembrane space.
The matrix isalkalinized while the intermembrane space is acidified. The electrons areultimately combined with molecular oxygen and protons to produce water.Respiration is aerobic when oxygen is the terminal electron acceptor. (Brit1042) The energy that was contained in the pyruvate molecule has at this pointbeen converted to ATP by substrate phosphorylation in glycolysis and Krebs cycleand to a free energy gradient of protons across the inner membrane known as theproton motive force (PMF). The gradient of protons will tend to diffuse toequilibrium but charged substances like protons do not easily cross membranes.Proton complexes in the inner membrane provide a channel for the protons toreturn to the matrix. Those protein complexes function as an ATPase, an enzymethat synthesizes ATP, because the energy liberated as the protons work todiffuse back to the matrix is used to push the equilibrium between ADP+Pi andATP strongly toward ATP. (Campbell 182) The electron transport chain has three sites along it that pump protonsfrom the matrix. NADH donates its electrons to the chain at a point where theenergy input is sufficient to drive all three proton pumping sites.
FADH is lessenergetic than NADH and its electrons are donated at a point that drives twoproton pumping sites. It is also possible for the NADH produced in glycolysis toenter the mitochondrial matrix and donate electrons to the electron transportchain. Depending on the system, NADH from glycolysis may be able to drive two orthree proton pumping sites. For eukaryotes, only two pumping sites are driven;for prokaryotes, three. (184-5) The importance of mitochondria is unremarkably, a key element in theprocess of respiration. Between the three distinct sections of respiration,glycolysis, Krebs Cycle, and Electron Transport, the mitochondrion is the siteof which most of it takes place, either inside of the mitochondrion or outsideit..
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