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C HAPTER  9T HE  C YTOSKELETON  A ND  C ELL  M OTILITY Overview of the Major Functions of the Cytoskeleton I. Cytoskeleton - composed of 3 well-defined filamentous structures that form an elaborate interactive network; A. Each of the 3 cytoskeletal filaments is a polymer of protein subunits connected to one another by weak, noncovalent bonds 1. Such construction lends itself to rapid assembly & disassembly, which is dependent upon complex cellular regulation 2. Each cytoskeletal element has distinct mechanical properties B. The 3 types of cytoskeletal elements 1. Microtubules (MTs) - hollow, rigid cylindrical tubes with walls composed of tubulin subunits 2. Microfilaments (MFs) - solid, thinner structures composed of actin 3. Intermediate filaments (IFs) - tough, ropelike fibers composed of a variety of related proteins C. Until recently, it was widely held that the cytoskeleton was a strictly eukaryotic invention that was absent from prokaryotic cells, however…… 1. Numerous prokaryotes contain tubulin- & actin-like proteins that polymerize into cytoplasmic filaments that carry out cytoskeletal-like activities 2. Proteins distantly related to those of intermediate filaments have also been discovered in certain prokaryotes 3. Thus, it appears that all 3 types of cytoskeletal elements had their evolutionary roots in prokaryotic structures II. Function in number of interrelated activities – they are highly dynamic; can do rapid & dramatic reorganization; often require accessory proteins that are not part of filaments A. Dynamic scaffold; provides structural support that helps determine cell shape (ex.: RBC spectrin-actin) 1. The flat, rounded shape of many cultured cells depends on a radial array of MTs in cell cytoplasm B. Internal framework that is responsible for positioning organelles in cell interior (ex.: polarized epithelial cells - organelles arranged in defined pattern along an axis from the apical to the basal end of the cell) C. 1. Delivery of mRNA molecules to specific parts of cell 2. Movement of membranous carriers from ER to Golgi complex 3. Transport of vesicles containing neurotransmitters down the length of nerve cell - carry vesicles from synthesis site to axon terminal 4. Peroxisome transport over tracks of microtubules; the two are closely associated as shown by labeling D. Force-generating apparatus - move cells from place to place (cilia, flagella, pseudopodia) 1. Single-celled organisms - crawl over surface of solid substratum or propel themselves through aqueous environment with aid of specialized locomotor organelles (cilia from cell surface) 2. Multicellular animals have variety of cells capable of independent locomotion - sperm, white blood
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