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Lesson 13 - Taking a Deeper Look at Cellular Structure
(An optional lesson about the cellular cytoskeleton)

Student Performance Objectives

1. List and describe the locations of the 3 major types of cytoskeletal elements found in cells.
2. Describe the functions of the microfilaments, intermediate filaments and the microtubules
    found in the cell's cytoplasm.

Lesson Outline
A. A cell is not a watery soup with organelles floating around at random.
B. Cells have a cytoskeleton composed of:
    1. Microfilaments, of which actin is the specific protein type. About 6/1000 of a micron
        a. Microfilaments form a network at the junction of the outermost border of the
            cytoplasm and the inner border of the cell membrane phospholipids. The
            association of microfilaments and phospholipids holds the inner membrane
            in place thus acting as a membrane stabilizing force. Appropriately, this band
            of microfilaments underlying the cell membrane is called the membrane
        b. Microfilaments extend upward from the cytoplasm into microvilli, those
            cellular extension seen best in intestinal epithelial cells and kidney tubular
            epithelial cells where they increase absorptive surface area. The filaments can
            then pull down on the microvillus, physically propelling absorbed materials
            deeper into the cell for processing.
    2. Intermediate filaments, of which keratin is a major type, are seen in epidermal cells.
        Slightly thicker than microfilaments - about 8-10/1000 of a micron thick         

 intermediate filaments

 cytoskeletal system

        a. Extend throughout the cytoplasm like a spider's web.
        b. Help to maintain cell shape by resisting mechanical stresses placed on the cell.
    3. Microtubules, which are more complex structures composed of the protein, tubulin. Significantly
        larger than the other cytoskeletal elements: 25/1000 of a micron.

        a. Each microtubule is composed of the globular protein, tubulin, arranged into 13 tiny filaments that
           form a cylinder.
        b. The cylinders radiate from the cell's centrosomal region throughout the cell like the intermediate
            filaments, only they are larger.
        c. Functions:
            (1) Anchor organelles in place in specific cytoplasmic regions.
            (2) Allow organelles, like chromosomes during mitosis, to glide to
                 different locations as required. The mitotic spindle and centrioles are composed of
                 (3) Work with the intermediate filaments to help maintain cell shape.
                 (4) Make up the inner structure of cilia and flagella and are responsible
                      for their movements.


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