They form a three dimensional network just inside the plasma membrane to help

They form a three dimensional network just inside the

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They form a three-dimensional network just inside the plasma membrane to help support the cell’s shape, giving the cell cortex the semisolid consistency of a gel. Microfilaments are important in cell motility, especially as part of the contractile apparatus of muscle cells. In muscle cells, thousands of actin filaments are arranged parallel to one another. Thicker filaments composed of myosin interdigitate with the thinner actin fibers. Myosin molecules act as motor proteins, walking along the actin filaments to shorten the cell. In other cells, actin-myosin aggregates are less organized but still cause localized contraction. A contracting belt of microfilaments divides the cytoplasm of animal cells during cell division. Localized contraction brought about by actin and myosin also drives amoeboid movement. Pseudopodia, cellular extensions, extend and contract through the reversible assembly and contraction of actin subunits into microfilaments. Microfilaments assemble into networks that convert sol to gel.
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According to a widely accepted model, filaments near the cell’s trailing edge interact with myosin, causing contraction. The contraction forces the interior fluid into the pseudopodium, where the actin network has been weakened. The pseudopodium extends until the actin reassembles into a network. In plant cells, actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming. This creates a circular flow of cytoplasm in the cell, speeding the distribution of materials within the cell. Intermediate filaments range in diameter from 8–12 nanometers, larger than microfilaments but smaller than microtubules. Intermediate filaments are a diverse class of cytoskeletal units, built from a family of proteins called keratins. Intermediate filaments are specialized for bearing tension. Intermediate filaments are more permanent fixtures of the cytoskeleton than are the other two classes. They reinforce cell shape and fix organelle location. Concept 6.7 Extracellular components and connections between cells help coordinate cellular activities Plant cells are encased by cell walls. The cell wall, found in prokaryotes, fungi, and some protists, has multiple functions. In plants, the cell wall protects the cell, maintains its shape, and prevents excessive uptake of water. It also supports the plant against the force of gravity. The thickness and chemical composition of cell walls differs from species to species and among cell types within a plant. The basic design consists of microfibrils of cellulose embedded in a matrix of proteins and other polysaccharides. This is the basic design of steel-reinforced concrete or fiberglass. A mature cell wall consists of a primary cell wall, a middle lamella with sticky polysaccharides that holds cells together, and layers of secondary cell wall. Plant cell walls are perforated by channels between adjacent cells called plasmodesmata. The extracellular matrix (ECM) of animal cells functions in support, adhesion, movement, and regulation.
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