The Cytoskeleton

In addition to its many membrane-enclosed organelles, the eukaryotic cytoplasm contains a set of long, thin fibers called the cytoskeleton. The cytoskeleton fills at least three important roles:

► It maintains cell shape and support.

► It provides for various types of cellular movement.

► Some of its fibers act as tracks or supports for motor proteins, which help move things within the cell.

In the discussion that follows, we'll look at three components of the cytoskeleton: microfilaments, intermediate filaments, and microtubules (Figure 4.21).

Microfilaments function in support and movement

Microfilaments can exist as single filaments, in bundles, or in networks. They are about 7 nm in diameter and several micrometers long. They are assembled from actin, a protein that exists in several forms and has many functions among members of the animal phyla. The actin found in microfilaments (which are also known as actin filaments) is extensively folded and has distinct "head" and "tail" sites. These sites interact with other actin molecules to form long, double helical chains (see Figure 4.21). The polymerization of actin into microfilaments is reversible, and they can disappear from cells, breaking down into units of free actin. Microfilaments have two major roles:

► They help the entire cell or parts of the cell to move.

► They stabilize cell shape.

In muscle cells, actin fibers are associated with another protein, myosin, and the interactions of these two proteins account for the contraction of muscles. In non-muscle cells, actin fibers are associated with localized changes of shape in the cell.

4.21 The Cytoskeleton Three highly visible and important structural components of the cytoskeleton are shown here in detail.These structures maintain and reinforce cell shape and contribute to cell movement.

Ribosomes

Ribosomes

4.21 The Cytoskeleton Three highly visible and important structural components of the cytoskeleton are shown here in detail.These structures maintain and reinforce cell shape and contribute to cell movement.

Components Cytoskeleton

Mitochondrion

25 nm

Tubulin ß-Tubuiin a-Tubuiin dimer monomer monomer

Mitochondrion

25 nm

Tubulin ß-Tubuiin a-Tubuiin dimer monomer monomer

Microfilaments are made up of strands of the protein actin and often interact with strands of other proteins. They change cell shape and drive cellular motion, including contraction, cytoplasmic streaming, and the "pinched" shape changes that occur during cell division. Microfilaments and myosin strands together drive muscle action.

Intermediate Filaments

Intermediate filaments are made up of fibrous proteins organized into tough, ropelike assemblages that stabilize a cell's structure and help maintain its shape. Some intermediate filaments help to hold neighboring cells together. Others make up the nuclear lamina.

The Nuclear Lamina

Microtubules are long, hollow cylinders made up of many molecules of the protein tubulin. Tubulin consists of two subunits, a-tubulin and P-tubulin. Microtubules lengthen or shorten by adding or subtracting tubulin dimers. Microtubule shortening moves chromosomes. Interactions between microtubules drive the movement of cells. Microtubules serve as "tracks" for the movement of vesicles.

Microfilaments are made up of strands of the protein actin and often interact with strands of other proteins. They change cell shape and drive cellular motion, including contraction, cytoplasmic streaming, and the "pinched" shape changes that occur during cell division. Microfilaments and myosin strands together drive muscle action.

Intermediate filaments are made up of fibrous proteins organized into tough, ropelike assemblages that stabilize a cell's structure and help maintain its shape. Some intermediate filaments help to hold neighboring cells together. Others make up the nuclear lamina.

Microtubules are long, hollow cylinders made up of many molecules of the protein tubulin. Tubulin consists of two subunits, a-tubulin and P-tubulin. Microtubules lengthen or shorten by adding or subtracting tubulin dimers. Microtubule shortening moves chromosomes. Interactions between microtubules drive the movement of cells. Microtubules serve as "tracks" for the movement of vesicles.

For example, microfilaments are involved in a flowing movement of the cytoplasm called cytoplasmic streaming and in the "pinching" contractions that divide an animal cell into two daughter cells. Microfilaments are also involved in the formation of cellular extensions called pseudopodia (pseudo-, "false;" podia, "feet") that enable some cells to move.

In some cell types, microfilaments form a meshwork just inside the plasma membrane. Actin-binding proteins then cross-link the microtubules to form a rigid structure that supports the cell. For example, microfilaments support the tiny microvilli that line the intestine, giving it a larger surface area through which to absorb nutrients (Figure 4.22).

Intermediate filaments are tough supporting elements

Intermediate filaments (see Figure 4.21) are found only in multicellular organisms. In contrast to the other components of the cytoskeleton, there are at least 50 different kinds of intermediate filaments, often specific to a few cell types. They generally fall into six molecular classes, based on amino acid sequence, and share the same general structure, being composed of fibrous proteins of the keratin family, similar to the protein that makes up hair and fingernails. In cells, these proteins are organized into tough, ropelike assemblages 8 to 12 nm in diameter.

Retinula Microvilli
4.22 Microfilaments for Support Microfilaments form the backbone of the microvilli that increase the surface area of some cells, such as intestinal cells that absorb nutrients.

Intermediate filaments have two major structural functions:

► They stabilize cell structure.

► They resist tension.

In some cells, intermediate filaments radiate from the nuclear envelope and may maintain the positions of the nucleus and other organelles in the cell. The lamins of the nuclear lamina are intermediate filaments. Other kinds of intermediate filaments help hold a complex apparatus of microfilaments in place in muscle cells. Still other kinds stabilize and help maintain rigidity in surface tissues by connecting "spot welds" called desmosomes between adjacent cells (see Figure 5.6).

Microtubules are long and hollow

Microtubules are long, hollow, unbranched cylinders about 25 nm in diameter and up to several micrometers long. Mi-crotubules have two roles in the cell:

► They form a rigid internal skeleton for some cells.

► They act as a framework along which motor proteins can move structures in the cell.

Microtubules are assembled from molecules of the protein tubulin. Tubulin is a dimer—a molecule made up of two monomers. The polypeptide monomers that make up this protein are known as a-tubulin and b-tubulin. Thirteen chains of tubulin dimers surround the central cavity of the micro-tubule (see Figure 4.21). The two ends of a microtubule are different. One end is designated the plus (+) end, the other the minus (-) end.

Tubulin dimers can be added or subtracted, mainly at the plus end, lengthening or shortening the microtubule. This ca pacity to change length rapidly makes mi-crotubules dynamic structures. This dynamic property of microtubules is seen in animal cells, where they are often found in parts of the cell that are changing shape.

Many microtubules radiate from a region of the cell called the microtubule organizing center. Tubule polymerization results in rigidity, and tubule depolymerization leads to a collapse of this rigid structure.

In plants, microtubules help control the arrangement of the cellulose fibers of the cell wall. Electron micrographs of plants frequently show microtubules lying just inside the plasma membrane of cells that are forming or extending their cell walls. Experimental alteration of the orientation of these microtubules leads to a similar change in the cell wall and a new shape for the cell.

In many cells, microtubules serve as tracks for motor proteins, specialized molecules that use energy to change their shape and move. Motor proteins bond to and move along the microtubules, carrying materials from one part of the cell to another. Microtubules are also essential in distributing chromosomes to daughter cells during cell division. And they are intimately associated with movable cell appendages: the flagella and cilia.

Microtubules power cilia and flagella

Many eukaryotic cells possess flagella* or cilia, or both. These whiplike organelles may push or pull the cell through its aqueous environment, or they may move surrounding liquid over the surface of the cell (Figure 4.23a). Cilia and eukary-otic (but not prokaryotic) flagella are both assembled from specialized microtubules and have identical internal structures, but they differ in their relative lengths and their patterns of beating:

► Flagella are longer than cilia and are usually found singly or in pairs. Waves of bending propagate from one end of a flagellum to the other in snakelike undulation.

► Cilia are shorter than flagella and are usually present in great numbers. They beat stiffly in one direction and recover flexibly in the other direction (like a swimmer's arm), so that the recovery stroke does not undo the work of the power stroke.

*Some prokaryotes have flagella, as we saw earlier, but prokaryotic flagella lack microtubules and dynein. The flagella of prokaryotes are neither structurally nor evolutionarily related to those of eukaryotes. The prokaryotic flagellum is assembled from a protein called flagellin, and it has a much simpler structure and a smaller diameter than a single eukaryotic microtubule. And whereas eukaryotic flagella beat in a wavelike motion, prokaryotic flagella rotate (see Figure 4.6).

Plus Minus Ends Microtubules Cilia

4.23 Cilia are Made up of Microtubules (a) A ciliated protist. (b) Three cilia on a protist cell. (c) Cross section of a single cilium.

Basal body

4.23 Cilia are Made up of Microtubules (a) A ciliated protist. (b) Three cilia on a protist cell. (c) Cross section of a single cilium.

Basal body

In cross section, a typical cilium or eukaryotic flagellum is surrounded by the plasma membrane and contains a "9 + 2" array of microtubules. As Figure 4.23c shows, nine fused pairs of microtubules—called doublets—form an outer cylinder, and one pair of unfused microtubules runs up the center. A spoke radiates from one microtubule of each doublet and connects the doublet to the center of the structure.

In the cytoplasm at the base of every eukaryotic flagellum or cilium is an organelle called a basal body. The nine mi-crotubule doublets extend into the basal body. In the basal body, each doublet is accompanied by another microtubule, making nine sets of three microtubules. The central, unfused microtubules do not extend into the basal body.

Centrioles are almost identical to the basal bodies of cilia and flagella. Centrioles are found in all eukaryotes except the flowering plants, pine trees and their relatives, and some pro-tists. Under the light microscope, a centriole looks like a small, featureless particle, but the electron microscope reveals that it is made up of a precise bundle of microtubules arranged as nine sets of three fused microtubules each. Cen-trioles lie in the microtubule organizing center in cells that are about to divide. As you will see in Chapter 9, they are involved in the formation of the mitotic spindle, to which the chromosomes attach (see Figure 9.8).

Motor proteins move along microtubules

The nine microtubule doublets of cilia and flagella are linked by proteins. The motion of cilia and flagella results from the sliding of the microtubules past each other. This sliding is driven by a motor protein called dynein, which can undergo changes in its shape.

All motor proteins work by undergoing reversible shape changes powered by energy from ATP. Dynein molecules attached to one microtubule doublet bind to a neighboring doublet. As the dynein molecules change shape, they move the microtubule past its neighbor (Figure 4.24a).

Dynein and another motor protein, kinesin, are responsible for carrying protein-laden vesicles from one part of the cell to another. These motor proteins bind to a vesicle or other organelle, then "walk" it along a microtubule by changing their shape. Recall that microtubules have a plus end and a minus end. Dynein moves attached organelles toward the minus end, while kinesin moves them toward the plus end (Figure 4.24b).

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Responses

  • Robert
    What are thin protein strands that change cell shape?
    8 years ago
  • jouni salpa
    What is the make up of a plasma membrane?
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  • iida
    Do microfilaments make up the nuclear lamina?
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  • melampus longhole
    What rope like cytoskeleton fibers make up the nuclear lamina?
    8 years ago
  • May Bolger-Baggins
    What are the components of the cellular cytoskeleton?
    8 years ago
  • rian simpson
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    7 years ago
  • primrose diggle
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  • Ali
    Which molecule makes up the bulk of a cell's membrane?
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  • asmara yusef
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  • franco
    What is a dynamic framework of protein filaments that support, organize and move eukary otic cell?
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  • troy
    What are the components of a cykoskeloton?
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  • Graham Gray
    What connects plasma membrane to the cytoskeleton?
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  • John
    WHERE IS THE CILIUM IN A PLASMA MEMBRANE?
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  • ISUMBRAS ROPER
    Do microfilaments have a membrane?
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  • Cindy
    Are microfilaments structural components of plasma membrane?
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  • ville
    Is the microtubules of the cytoskeleton made of thin fibrous?
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  • MORGAN
    Is the plus end of microtubule in the plasma membrane?
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  • halfred
    What are thin strands of protein from part of a cytoskeleton called?
    6 years ago
  • Hope
    Do microfilaments have membranes?
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  • pontus
    Is the nuclear membrane part of the cytoskeleton?
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    Does the plasma membrane surround cilia?
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    How microtubules interact with membrane?
    5 years ago
  • Maria Reilly
    Do microfilaments support the plasma membrane?
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  • Walter
    What is the function of the filamentsof the cytoskeleton in the plasma membrane?
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  • markus
    Which cytoskeleton protein helps a cell maintain its shape?
    3 years ago
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