Cilia and Flagella o Use core of microtubules from basal body generate the

Cilia and flagella o use core of microtubules from

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Cilia and Flagella o Use core of microtubules from basal body- generate the axoneme Cilium beats by performing repetitive rounds of a “power stroke” Actin filaments o Actin filaments are thin and flexible o Many proteins bind to actin and modify its properties o An actin rich cortex underlies the plasma membrane of most eukaryotic cells cell crawling depends on actin o Actin associates with myosin to form contractile structures o IMPORTANT- extracellular signals control arrangement of actin filaments allowing cell to respond to environment o 5% total protein in a typical animal cell is actin o ½ of actin is assembled into filaments, and other in actin monomer in cytosol Actin filaments allow cells to adopt a variety of shapes o Actin fibers like microtubules are unstable o Made stable by interactions with other proteins “actin binding proteins” o Differential interactions determine “stiffness” of structure o Can for stuff microvilli or flexible cellular muscles o Can create temporary protrusions or a contractile ring which pinches off Actin binding proteins control behavior of actin filaments in vertebrate cells- formins and actin related proteins (ARP’s) control actin assembly The cortex- actin rich network under cytoplasm, and gives cell its shape and allows it to move Cell movement o Cell first pushes out protrusions at its front or leading edge o The protrusion adheres to surface over which cell is crawling
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o The rest of the cell drags itself forward Cell crawling relies on actin o Integrins- transmembrane proteins which bind to extra cellular matrix A web of actin filaments pushes the leading edge forward Actin associates with myosin motor proteins to form contractile structures o Carry vesicle along actin filament tracks o Bind plasma membrane and move relative to cortical actin = change shape Extracellular signals control the arrangement of actin filaments- allows the cell to respond to its environments How do cells contract? o Muscle contraction depends on bundles of actin and myosin o During muscle contraction, actin filaments slide against myosin filaments o Muscle contraction is triggered by a sudden rise in Ca2+ o Muscle cells perform highly specialized functions in the body Myosin II molecules can associate to form myosin filaments o Tails associate to form bipolar myosin filament- heads project outward from the middle in opposite directions; globular heads- bind ATP Actin filaments slide along myosin filaments o Slide actin filaments over each other = shortening of actin bundle o The myosin head walks towards the actin plus end of the actin, causing contraction Skeletal muscle is packed with myofibrils o In skeletal muscles, cells fuse to form long fibers, the cytoplasms contain myofibrils o Myofibrils- contractile elements of the muscle cell o Sarcomeres- contractile units of the myofibrils
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Sacromeres are the contractile unit of the muscle o Stacks of actin and myosin filaments o Contraction of a muscle is due to simultaneous shortening of all the sarcomeres o 2 types- thin and thick. Thick filament (myosin)- centrally located; thin filament
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