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Lecture 9

BIOL 112 Lecture 9: Lecture 9

14 Pages
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Department
Biology (Sci)
Course Code
BIOL 112
Professor
Frieder Schoeck

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Lecture 9 Mitochondria and chloroplast are NOT part of the endomembrane system. The mitochondria and chloroplasts are Plastids. The mitochondria are in all cells in both animals and plants. It is responsible for ATP (energy making). Themitochondriahave two membranes thatsurroundthem (aninnerandanoutermembrane).They areevendifferentintheirphospholipidcomposition.Theinnermembraneisfoldedintothesecristae. The point of these indentions is to generate a large surface area, because they are full of membrane proteinimportantforrespiration.Thecytosolofthesemitochondriais calledmatrix. A lotofchemical reactions happen in the matrix, we found DNA, RNA and ribosomes in the matrix. (in the endomembrane system we would never find DNA outside of the nucleus). The chloroplast is only found in plants. Plants also have mitochondria. They have two membranes ( inner and outer). They also have internal membranes that are completely separated and they are called thylakoids. Photosynthesis happens in the thylakoids. The stroma is the inner space and its where carbon fixation takes place (transform carbon dioxide gas into sugar, amino acids, fatty acids). We also find DNA, RNA, and ribosomes in the stroma. What really sets them apart from endomembrane system? It is that they are endosymbiotic organelles. Originally, the mitochondria and chloroplast were derived from independently living bacteria. There was a prokaryotic cell that lived independently from the eukaryotic cell and then the eukaryotic cell took it up by phagocytosis. The prokaryotic (mitochondria & chloroplast) cell was integrated in the host eukaryotic cell. Evidence that the mitochondria and the chloroplast were prokaryotic bacteria in the beginning “endosymbiont theory”: the mitochondria still lives independently from cell and there is a double membrane. The phospholipids in the inner membrane looks more like bacterial phospholipids and the phospholipids in the outer membrane look more like typical phospholipids. Also, to move lipids from the ER to the mitochondria or the chloroplast there is a unique system apart from the normal one. Also, they have their own genome which is very similar to bacteria. They also have their own ribosomes which are themselves also more similar to bacteria. *THEORY in biology means that we have a lot of evidence, something that is established* We need mitochondria to provide energy and the mitochondria need to be surrounded by the eukaryotic cell. Cytoskeleton: we find them in bacteria and eukaryotic cells. We will only talk about the cytoskeleton of the eukaryotes. The cytoskeleton is full of proteins. There is not a lot of space in the cytoskeleton. There are 3 types of cytoskeleton filament: actin (the smallest filament), keratin (medium filament), microtubules (largest filament). Actin A monomer: means an entire protein. It is a entire polypeptide but it is still called monomer. The actin monomeric proteins polymerize (but it is a non-covalent protein-protein interaction). We have a lot of these monomers that come together to form a long filament which is polar (there is a minus end and a plus end). Both ends look different. The actin monomer prefers one of those ends. There are a lot of proteins that can bind to these actin filaments or monomers and they regulate the polymerization and depolymerisation. Actin provides structural support to the cell. Example: actin in gut cells. Food passes through the gut. The cytoskeleton underlies the cell membrane. We also call this: cortical. We find the cytoskeleton in the upper lines as well. The cytoskeletonjob (theactin job) is tomakesure thatthe gut cells have therectangular shape that they have. Thereisanimportantmoleculethatinteractswithactin:myosin.Myosinisamotorprotein.Itiscalled a motor protein because it has the ability to walk long the actin filament in ONE direction with the aid of ATP. At each step you have to hydrolyse ATP. When you combine several of these myosin with a lot of actin filaments, it gives rise to actin-myosin contraction. The reason it works is because the myosin motors forms at least a bipolar complex. The headsarestickingoutinbipolardirections.Youcanintegrateupto100or200ofthesemyosinmotors and then they can interact with actin. When the myosin ends move along in their own direction, you contract the actin-myosin. At the end, the myosin filaments overlap entirely with the actin, and this is the contraction (muscle is shorter). This happens in all cells, not only muscle cells. We use slightly smaller myosin in non-muscle cells. A lot of processes can be explained by this actin-myosin contraction: Cytokinesis: separation of cells after DNA and chromosomes were separated. Cytoplasmic streaming: plants don’t move so they need cytoplasmic streaming to move the interior components around and this is mediated by actin-myosin contraction. Microtubules (biggest filament) are made up subunits that polymerize. These are protein. You have a beta and an alpha tubulin monomeric protein. They dimerize and the tubulin dimers then form a long structure called microtubules. These tubulin monomers interact non covalent
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