5 C H A P T E R Cellular Movement Cellular Movement and Muscles and Muscles
Cytoskeleton and Motor Proteins • All physiological processes depend on movement • Intracellular transport • Changes in cell shape • Cell motility • Animal locomotion • All movement is due to the same cellular “machinery” • Cytoskeleton • Protein-based intracellular network • Motor proteins • Enzymes that use energy from ATP to move
Figure 5.1 Use of Cytoskeleton for Movement Cytoskeleton elements Microtubules Microfilaments Three ways to use the cytoskeleton for movement a. Cytoskeleton “road” and motor protein carriers b. To reorganize the cytoskeletal network c. Motor proteins pull on the cytoskeletal “rope”
Microtubules • Are tube-like polymers of tubulin • Similar protein in diverse animal groups • Multiple isoforms • Form spontaneously • Are anchored at both ends (polarized) • MTOC (–) near the nucleus • Integral proteins (+) in the plasma membrane
Microtubule Assembly 1. Monomer activation 2. Dimer formation (tubulin) 3. Protofilament formation 4. Protofilaments sheet microtubule 5. Dimers can be added or removed from ends of tubule • Asymmetrical growth • Growth is faster at + end 6. Cell regulates rates of growth and shrinkage
Microtubule Growth and Shrinkage • Factors affecting growth/shrinkage are • High [tubulin] promotes growth • Low temp. promotes shrinkage • Microtubule-associated proteins (MAPs) • Dynamic instability • GTP hydrolysis on -tubulin causes disassembly • Chemicals that disrupt the dynamics • For example, plant poisons such as taxol and colchicine Treadmilling
Movement Along Microtubules • Motor proteins move along microtubules • Direction is determined by polarity and the type of motor protein • Kinesin move in (+) direction • Dynein moves in (–) direction • Movement is fueled by ATP hydrolysis • Rate determined by the ATPase domain of motor protein and regulatory proteins • Dynein is larger than kinesin and moves five times faster
Cilia and Flagella • Cilia • Numerous, wavelike motion • Flagella • Single or in pairs, whiplike movement • Composed of microtubules arranged into axoneme • Bundle of parallel microtubules • “Nine-plus-two” • Asymmetric activation of dynein causes movement
Microfilaments • Polymers composed of the protein actin • Found in all eukaryotic cells • Often use the motor protein myosin • Movement arises from • Actin polymerization ( -actin) • Sliding filaments using myosin ( -actin ) • More common than movement by polymerization
Microfilament Structure and Growth • G-actin monomers polymerize to form a polymer called F-actin • Energy not required • Spontaneous growth • 6–10 times faster at + end • Treadmilling • Assembly and disassembly occur simultaneously and overall length is constant • Capping proteins • Increase length by stabilizing “–” end and slowing disassembly
Cellular Microfilaments • Tangled networks • Microfilaments linked by filamin protein • Bundles • Cross-linked by fascin protein • Networks and bundles of microfilaments are attached to cell membrane by dystrophin protein • Maintain cell shape and can be used for movement
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