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5.2: Cytoskeleton

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  • Page ID
    161207
    • Ying Liu, Serena Chang, Grace Murphy, Esther Ajayi-Akinsulire, Isobel Ardren, Izabella Guy, Kai Johnston, Saskia Lee, and Lauren Russell
    • City College of San Francisco

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    Learning Objectives
    • Explain the distinguishing characteristics of eukaryotic cells
    • Describe internal and external structures of prokaryotic cells in terms of their physical structure, chemical structure, and function
    • Identify and describe structures and organelles unique to eukaryotic cells
    • Compare and contrast similar structures found in prokaryotic and eukaryotic cells

    Cytoskeleton

    Eukaryotic cells have an internal cytoskeleton made of microfilaments, intermediate filaments, and microtubules. This matrix of fibers and tubes provides structural support as well as a network over which materials can be transported within the cell and on which organelles can be anchored (Figure \(\PageIndex{10}\)). For example, the process of exocytosis involves the movement of a vesicle via the cytoskeletal network to the plasma membrane, where it can release its contents.

    A micrograph shows many lines emminating from the nucleus and extending throughout the cell. These are shown in diagram form as small spheres forming the outside of a long tube. Each pair of spheres is a tubulin dimer and columns of these dimers can be seen on the outside of the large tube they form. The diameter of the tube is 25 µm. The same micrograph shows lines throughout the cell; these are drawn as spheres forming a braided structures (a double helix). The diameter of the helix is 7 nm. The spheres are labeled actin subunit. Another micrograph shows many lines forming a webbing in the cell. These are drawn as a rope; each strand of the rope is labeled fibrous subunit (keratins coiled together). The diameter of the rope is 8 – 12 nm.
    Figure \(\PageIndex{10}\): The cytoskeleton is a network of microfilaments, intermediate filaments, and microtubules found throughout the cytoplasm of a eukaryotic cell. In these fluorescently labeled animal cells, the microtubules are green, the actin microfilaments are red, the nucleus is blue, and keratin (a type of intermediate filament) is yellow.

    Microfilaments

    Microfilaments are composed of two intertwined strands of actin, each composed of actin monomers forming filamentous cables 6 nm in diameter2 (Figure \(\PageIndex{11}\)). The actin filaments work together with motor proteins, like myosin, to effect muscle contraction in animals or the amoeboid movement of some eukaryotic microbes. In ameboid organisms, actin can be found in two forms: a stiffer, polymerized, gel form and a more fluid, unpolymerized soluble form. Actin in the gel form creates stability in the ectoplasm, the gel-like area of cytoplasm just inside the plasma membrane of ameboid protozoans.

    Temporary extensions of the cytoplasmic membrane called pseudopodia (meaning “false feet”) are produced through the forward flow of soluble actin filaments into the pseudopodia, followed by the gel-sol cycling of the actin filaments, resulting in cell motility. Once the cytoplasm extends outward, forming a pseudopodium, the remaining cytoplasm flows up to join the leading edge, thereby creating forward locomotion. Beyond amoeboid movement, microfilaments are also involved in a variety of other processes in eukaryotic cells, including cytoplasmic streaming (the movement or circulation of cytoplasm within the cell), cleavage furrow formation during cell division, and muscle movement in animals (Figure \(\PageIndex{11}\)). These functions are the result of the dynamic nature of microfilaments, which can polymerize and depolymerize relatively easily in response to cellular signals, and their interactions with molecular motors in different types of eukaryotic cells.

    a) A diagram of the plasma membrane shows the filaments of the cytoskeleton as thin lines on the cytoplasmic side of the membrane. B) A closeup of the filaments shows spheres labeled actin subunits forming into a long chain labeled actin filaments. C)examples of how actin is used in various cells. Some cells use actin for amoeboid movement. This is done when actin polymerizes and depolymerizes to allow a portion of the cell to project out, attach to a surface and pull the rest of the cell behind it. Cytoplasmic streaming is the movement of cytoplasm due to the actions of actin. Contractile ring formation during cytokinesis is when actin pinches a dividing cell off into two separate cells. Muscle contraction in animals is when actin strands are pulled together by myosin; this shortens the length of the muscle cell and contracts the muscle.
    Figure \(\PageIndex{11}\): (a) A microfilament is composed of a pair of actin filaments. (b) Each actin filament is a string of polymerized actin monomers. (c) The dynamic nature of actin, due to its polymerization and depolymerization and its association with myosin, allows microfilaments to be involved in a variety of cellular processes, including ameboid movement, cytoplasmic streaming, contractile ring formation during cell division, and muscle contraction in animals.

    Intermediate Filaments 

    Intermediate filaments (Figure \(\PageIndex{12}\)) are a diverse group of cytoskeletal filaments that act as cables within the cell. They are termed “intermediate” because their 10-nm diameter is thicker than that of actin but thinner than that of microtubules.3 They are composed of several strands of polymerized subunits that, in turn, are made up of a wide variety of monomers. Intermediate filaments tend to be more permanent in the cell and maintain the position of the nucleus. They also form the nuclear lamina (lining or layer) just inside the nuclear envelope. Additionally, intermediate filaments play a role in anchoring cells together in animal tissues. The intermediate filament protein desmin is found in desmosomes, the protein structures that join muscle cells together and help them resist external physical forces. The intermediate filament protein keratin is a structural protein found in hair, skin, and nails.

    a) Intermediate filaments are shown as a rope-like structure. B) These are found in the nuclear lamina (lamina intermediate filaments) which are just under the nuclear envelope. C) Intermediate filaments are also found in desmosomes. Desmosomes are connections between two cells (shown here as two small regions of plasma membranes next to each other. The intermediate filaments connect these two membranes together across the extracellular space. A micrograph shows these as dark lines running across the membranes between two cells.
    Figure \(\PageIndex{12}\): (a) Intermediate filaments are composed of multiple strands of polymerized subunits. They are more permanent than other cytoskeletal structures and serve a variety of functions. (b) Intermediate filaments form much of the nuclear lamina. (c) Intermediate filaments form the desmosomes between cells in some animal tissues. (credit c “illustration”: modification of work by Mariana Ruiz Villareal)

    Microtubules 

    Microtubules (Figure \(\PageIndex{13}\)) are a third type of cytoskeletal fiber composed of tubulin dimers (α tubulin and β tubulin). These form hollow tubes 23 nm in diameter that are used as girders within the cytoskeleton.4 Like microfilaments, microtubules are dynamic and have the ability to rapidly assemble and disassemble. Microtubules also work with motor proteins (such as dynein and kinesin) to move organelles and vesicles around within the cytoplasm. Additionally, microtubules are the main components of eukaryotic flagella and cilia, composing both the filament and the basal body components (Figure \(\PageIndex{20}\)).

    A) a computer simulation shows micrtububles as spheres forming a rube structure. This can be drawn as spheres forming a ring; stacks of these rings form the tube. Each ring is 13 polymerized dimers of alpha-tubulin and beta-tubulin. C) The long tubes that are formed create a structure similar to a railroad track; motor proteins move along the microtubule track to carry vesicles throughout the cell.
    Figure \(\PageIndex{13}\): (a) Microtubules are hollow structures composed of polymerized tubulin dimers. (b) They are involved in several cellular processes, including the movement of organelles throughout the cytoplasm. Motor proteins carry organelles along microtubule tracks that crisscross the entire cell. (credit b: modification of work by National Institute on Aging)

    In addition, microtubules are involved in cell division, forming the mitotic spindle that serves to separate chromosomes during mitosis and meiosis. The mitotic spindle is produced by two centrosomes, which are essentially microtubule-organizing centers, at opposite ends of the cell. Each centrosome is composed of a pair of centrioles positioned at right angles to each other, and each centriole is an array of nine parallel microtubules arranged in triplets (Figure \(\PageIndex{14}\)).

    a) Centrosomes are shown as short tubes. The outside of these tubes is made of 9 sets of microtubule triplets. These sets are held together by lines labeled centrioles. B) Centrosomes are shown on the two poles of a cell. Lines connect the centrosomes to chromosomes in the center of the cell.
    Figure \(\PageIndex{14}\): (a) A centrosome is composed of two centrioles positioned at right angles to each other. Each centriole is composed of nine triplets of microtubules held together by accessory proteins. (b) In animal cells, the centrosomes (arrows) serve as microtubule-organizing centers of the mitotic spindle during mitosis.

    Query \(\PageIndex{1}\)

     

    Key Concepts and Summary

    • Eukaryotic cells are defined by the presence of a nucleus containing the DNA genome and bound by a nuclear membrane (or nuclear envelope) composed of two lipid bilayers that regulate transport of materials into and out of the nucleus through nuclear pores.
    • Eukaryotic cell morphologies vary greatly and may be maintained by various structures, including the cytoskeleton, the cell membrane, and/or the cell wall.
    • The nucleolus, located in the nucleus of eukaryotic cells, is the site of ribosomal synthesis and the first stages of ribosome assembly.
    • Eukaryotic cells contain 80S ribosomes in the rough endoplasmic reticulum (membrane bound-ribosomes) and cytoplasm (free ribosomes). They contain 70s ribosomes in mitochondria and chloroplasts.
    • Eukaryotic cells have evolved an endomembrane system, containing membrane-bound organelles involved in transport. These include vesicles, the endoplasmic reticulum, and the Golgi apparatus.
    • The smooth endoplasmic reticulum plays a role in lipid biosynthesis, carbohydrate metabolism, and detoxification of toxic compounds. The rough endoplasmic reticulum contains membrane-bound 80S ribosomes that synthesize proteins destined for the cell membrane
    • The Golgi apparatus processes proteins and lipids, typically through the addition of sugar molecules, producing glycoproteins or glycolipids, components of the plasma membrane that are used in cell-to-cell communication.
    • Lysosomes contain digestive enzymes that break down small particles ingested by endocytosis, large particles or cells ingested by phagocytosis, and damaged intracellular components.
    • The cytoskeleton, composed of microfilaments, intermediate filaments, and microtubules, provides structural support in eukaryotic cells and serves as a network for transport of intracellular materials.
    • Centrosomes are microtubule-organizing centers important in the formation of the mitotic spindle in mitosis.
    • Mitochondria are the site of cellular respiration. They have two membranes: an outer membrane and an inner membrane with cristae. The mitochondrial matrix, within the inner membrane, contains the mitochondrial DNA, 70S ribosomes, and metabolic enzymes.
    • The plasma membrane of eukaryotic cells is structurally similar to that found in prokaryotic cells, and membrane components move according to the fluid mosaic model. However, eukaryotic membranes contain sterols, which alter membrane fluidity, as well as glycoproteins and glycolipids, which help the cell recognize other cells and infectious particles.
    • In addition to active transport and passive transport, eukaryotic cell membranes can take material into the cell via endocytosis, or expel matter from the cell via exocytosis.
    • Cells of fungi, algae, plants, and some protists have a cell wall, whereas cells of animals and some protozoans have a sticky extracellular matrix that provides structural support and mediates cellular signaling.
    • Eukaryotic flagella are structurally distinct from prokaryotic flagella but serve a similar purpose (locomotion). Ciliaare structurally similar to eukaryotic flagella, but shorter; they may be used for locomotion, feeding, or movement of extracellular particles.

    Footnotes

    1. 1 A.E. Barnhill, M.T. Brewer, S.A. Carlson. “Adverse Effects of Antimicrobials via Predictable or Idiosyncratic Inhibition of Host Mitochondrial Components.” Antimicrobial Agents and Chemotherapy 56 no. 8 (2012):4046–4051.
    2. 2 Fuchs E, Cleveland DW. “A Structural Scaffolding of Intermediate Filaments in Health and Disease.” Science 279 no. 5350 (1998):514–519.
    3. 3 E. Fuchs, D.W. Cleveland. “A Structural Scaffolding of Intermediate Filaments in Health and Disease.” Science 279 no. 5350 (1998):514–519.
    4. 4 E. Fuchs, D.W. Cleveland. “A Structural Scaffolding of Intermediate Filaments in Health and Disease.” Science 279 no. 5350 (1998):514–519.
    5. 5 N. Yarlett, J.H.P. Hackstein. “Hydrogenosomes: One Organelle, Multiple Origins.” BioScience 55 no. 8 (2005):657–658.
    6. 6 M. Dudzick. “Protists.” OpenStax CNX. November 27, 2013. http://cnx.org/contents/f7048bb6-e46...ef291cf7049c@1

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