CHAPTER 7 A TOUR OF THE CELL Section F: The...

CHAPTER 7A TOUR OF THE CELL

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Section F: The Cytoskeleton1. Providing structural support to the cell, the cytoskeleton also functions in

cell motility and regulation

• The cytoskeleton is a network of fibers extendingthroughout the cytoplasm.

• The cytoskeletonorganizes thestructures andactivities ofthe cell.

Introduction

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Fig. 7.20

• The cytoskeleton provides mechanical support andmaintains shape of the cell.

• The fibers act like a geodesic dome to stabilize abalance between opposing forces.

• The cytoskeleton provides anchorage for manyorganelles and cytosolic enzymes.

• The cytoskeleton is dynamic, dismantling in one partand reassembling in another to change cell shape.

1. Providing structural support to the cell,the cytoskeleton also functions in cellmotility and regulation

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• The cytoskeleton also plays a major role in cellmotility.• This involves both changes in cell location and limited

movements of parts of the cell.

• The cytoskeleton interacts with motor proteins.• In cilia and flagella motor proteins pull components

of the cytoskeleton past each other.

• This is also truein muscle cells.

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Fig. 7.21a

• Motor molecules also carry vesicles or organellesto various destinations along “monorails’ providedby the cytoskeleton.

• Interactions of motor proteins and the cytoskeletoncirculates materials within a cell via streaming.

• Recently, evidence is accumulating that thecytoskeleton maytransmit mechanicalsignals that rearrangethe nucleoli andother structures.

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Fig. 7.21b

• There are three main types of fibers in thecytoskeleton: microtubules, microfilaments, andintermediate filaments.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Microtubules, the thickest fibers, are hollow rodsabout 25 microns in diameter.• Microtubule fibers are constructed of the globular

protein, tubulin, and they grow or shrink as moretubulin molecules are added or removed.

• They move chromosomes during cell division.

• Another function isas tracks that guidemotor proteinscarrying organellesto their destination.

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Fig. 7.21b

• In many cells, microtubules grow out from acentrosome near the nucleus.• These microtubules resist compression to the cell.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 7.22

• In animal cells, the centrosome has a pair ofcentrioles, each with nine triplets of microtubulesarranged in a ring.

• During cell division thecentrioles replicate.

• Microtubules are the central structural supports incilia and flagella.• Both can move unicellular and small multicellular

organisms by propelling water past the organism.

• If these structures are anchored in a large structure, theymove fluid over a surface.

• For example, cilia sweep mucus carrying trappeddebris from the lungs.

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Fig. 7.2

• Cilia usually occur in large numbers on the cellsurface.• They are about 0.25 microns in diameter and 2-20

microns long.

• There are usually just one or a few flagella per cell.• Flagella are the same width as cilia, but 10-200 microns

long.

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• A flagellum has an undulatory movement.• Force is generated parallel to the flagellum’s axis.

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Fig. 7.23a

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 7.23b

• Cilia move more like oars with alternating powerand recovery strokes.• They generate force perpendicular to the cilia’s axis.

• In spite of their differences, both cilia and flagellahave the same ultrastructure.• Both have a core of microtubules sheathed by the

plasma membrane.

• Nine doublets of microtubules arranged around a pair atthe center, the “9 + 2” pattern.

• Flexible “wheels” of proteins connect outer doublets toeach other and to the core.

• The outer doublets are also connected by motorproteins.

• The cilium or flagellum is anchored in the cell by abasal body, whose structure is identical to a centriole.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 7.24

• The bending of cilia and flagella is driven by thearms of a motor protein, dynein.• Addition to dynein of a phosphate group from ATP and

its removal causes conformation changes in the protein.

• Dynein arms alternatelygrab, move, and releasethe outer microtubules.

• Protein cross-links limitsliding and the force isexpressed as bending.

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Fig. 7.25

• Microfilaments, the thinnest class of thecytoskeletal fibers, are solid rods of the globularprotein actin.• An actin microfilament consists of a twisted double

chain of actin subunits.

• Microfilaments are designed to resist tension.

• With other proteins, they form a three-dimensionalnetwork just inside the plasma membrane.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 7.26 The shape of themicrovilli in this intestinal cellare supported by microfilaments,anchored to a network ofintermediate filaments.

• In muscle cells, thousands of actin filaments arearranged parallel to one another.

• Thicker filaments, composed of a motor protein,myosin, interdigitate with the thinner actin fibers.• Myosin molecules walk along the actin filament, pulling

stacks of actin fibers together and shorteningthe cell.

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Fig. 7.21a

• In other cells, these actin-myosin aggregates are lessorganized but still cause localized contraction.• A contracting belt of microfilaments divides the

cytoplasm of animals cells during cell division.

• Localized contraction also drives amoeboid movement.

• Pseudopodia, cellular extensions, extend and contractthrough the reversible assembly and contraction ofactin subunits into microfilaments.

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Fig. 7.21b

• In plant cells (and others), actin-myosin interactionsand sol-gel transformations drive cytoplasmicstreaming.• This creates a circular flow of cytoplasm in the cell.

• This speeds the distribution of materials within the cell.

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Fig. 7.21c

• Intermediate filaments,intermediate in size at 8 - 12nanometers, are specializedfor bearing tension.• Intermediate filaments are

built from a diverse class ofsubunits from a family ofproteins called keratins.

• Intermediate filaments aremore permanent fixtures ofthe cytoskeleton than arethe other two classes.

• They reinforce cell shapeand fix organelle location.

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Fig. 7.26