10.1 TISSUES AND ORGANS ARE COMMUNITIES OF CELLS THAT PERFORM A SPECIFIC FUNCTION.
10.2 THE CYTOSKELETON IS COMPOSED OF MICROTUBULES, MICROFILAMENTS, AND INTERMEDIATE FILAMENTS THAT HELP TO MAINTAIN CELL SHAPE.
10.3 THE CYTOSKELETON INTERACTS WITH MOTOR PROTEINS TO PERMIT THE MOVEMENT OF CELLS AND SUBSTANCES WITHIN CELLS.
10.4 CELLS ADHERE TO OTHER CELLS AND THE EXTRACELLULAR MATRIX BY MEANS OF CELL ADHESION MOLECULES AND JUNCTIONAL COMPLEXES.
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10.5 THE EXTRACELLULAR MATRIX PROVIDES STRUCTURAL SUPPORT AND INFORMATIONAL CUES.
Name three types of filaments that make up the cytoskeleton, including the subunits of each, and state which cytoskeletal element is not present in plant cells.
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Three types of filaments that make up the cytoskeleton are microtubules, made up of alpha- and beta-tubulin dimers, microfilaments, made of actin monomers, and intermediate filaments, made of intermediate filament protein subunits. Intermediate filament subunits vary by cell type. For example, the intermediate filament subunit is keratin in epithelial cells, and vimentin in fibroblasts. Animal cells have all three filaments types whereas plant cells only have microtubules and microfilaments.
Explain how microtubules and microfilaments are dynamic structures.
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Microtubules and microfilaments are dynamic in structure because they can become longer or shorter with the addition or subtraction of their subunits. This addition or deletion is influenced by many factors including the concentration of free subunits and the activity of regulatory proteins.
Describe the functions of the three major motor proteins that are required for cellular movements involving the cytoskeleton.
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Three major motor proteins that are required for cellular movement involving the cytoskeleton are myosin, kinesin and dynein. Myosin binds to microfilaments in the cell and can cause these filaments to move relative to each other, as in muscle cell contraction. Myosin can also attach to various types of cellular cargo and move along the microfilament, transporting these materials from one part of the cell to another. Kinesin transports cargo towards the plus end of microtubules, while dynein transports cargo in the opposite way, towards the minus end of the microtubule.
Compare the cytoskeletal dynamics in cells that move by beating cilia or flagella with cells that move by crawling.
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Movement of cells through cilia and flagella is made possible by the microtubules extending along the length of the flagella or cilia. Dynein undergoes a conformational change, powered through ATP hydrolysis, which causes the pairs of microtubules to slide past each other giving the cilia or flagella its characteristic movement. In contrast, when a cell crawls it is due to the polymerization of actin in the microfilaments. The microfilaments assemble and disassemble in a directional manner and cause the cell to move in a particular direction. The interaction of myosin and actin also help this movement by contracting the microfilaments of the cell and squeezing the cytoplasm and its contents forward.
Identify the cytoskeletal component linked to adherens junctions, desmosomes, and hemidesmosomes.
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Cadherins are transmembrane proteins that are clustered together in sites called adherens junctions. The extracellular domains of the cadherins associate with cadherins from another cell, attaching one cell to another. The cytosolic domains of these cadherins associate with microfilaments. This establishes a physical connection among the actin cytoskeletons of all cells present in the tissue. Desmosomes are cell junctions that also hold adjacent cells together. They also utilize cadherins but unlike adherens junctions, the cytosolic domains of these cadherins bind to intermediate filaments of the cytoskeleton. Hemidesmosomes anchor epithelial cells to the basal lamina. They are composed of transmembrane proteins called integrins. The integrin extracellular domain binds to extracellular matrix proteins in the basal lamina, and the integrin cytosolic domain binds to intermediate filaments. This results in a firmly anchored cell.
Compare and contrast the structural features and functional roles of cadherins and integrins.
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Cadherins are transmembrane proteins that interact with the cytoskeleton on their cytoplasmic domain and with other like cadherin proteins on their extracellular domain. This results in the ability of cells to connect to one another. Integrin proteins are also transmembrane proteins that connect specific extracellular proteins, through interactions with their extracellular domain, to the cytoskeleton of the cell. This results in the cells being stably attached to proteins of the extracellular matrix.
Name examples of the extracellular matrix in plants and animals.
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In plants, the extracellular matrix makes up the cell wall, which is composed of proteins like cellulose and lignin. The interconnected cell walls of the plant support the entire organism. In animals, one example of the extracellular matrix is connective tissue. Here, proteins like collagen, elastin and laminin provide support and protection to the tissues surrounding them.
Explain the functional relationships among the basal lamina, integrins, and intermediate filaments in epithelial tissues.
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The basal lamina is a specialized layer of extracellular matrix that is present beneath all epithelial tissues where it provides structural foundation. The epithelial tissue is connected to the basal lamina through hemidesmosomes, which are clusters of integrin transmembrane proteins. The extracellular domains of the integrins interact with proteins in the extracellular matrix. The cytosolic domains of integrin interact with the intermediate filaments of the cytoskeleton.
Predict the effect of inadequate or inappropriate adhesion of a cell to the extracellular matrix on gene expression in the cell.
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Malignant tumors are an example of a cell type that has a heightened ability to attach to extracellular matrix proteins. This allows the cancerous cells to go through basal laminas of blood vessels, travel to different locations and adhere. One mechanism thought to play a role in this heightened ability to adhere is increased gene expression and production of adherence proteins. In contrast, if a white blood cell, for example, was unable to attach to the basal lamina of the blood vessels due to lowered adherence gene expression, it would be unable to travel to and stay at sites of infection and would therefore not be able to perform its function.