The overall purpose of this Learning Object is to learn the chemical makeup and the functions associated with the cytoskeleton in eukaryotic cells.
LEARNING OBJECTIVES FOR THIS SECTION
The Cytoskeleton (def)
The cytoskeleton is a network of microfilaments, intermediate filaments, and microtubules. The cytoskeleton functions to:
1) give shape to cells lacking a cell wall;
2) allow for cell movement, eg, the crawling movement of white blood cells and amoebas or the contraction of muscle cells;
3) movement of organelles within the cell and endocytosis;
4) cell division, ie, the movement of chromosomes during mitosis and meiosis and the constriction of animal cells during cytokinesis.
We will now take a closer look at microtubules, microfilaments, intermediate filaments, centrioles, flagella, and cilia.
Microtubules are hollow tubes made of subunits of the protein tubulin. They provide structural support for the cell and play a role in cell division, cell movement, and movement of organelles within the cell. Microtubules are components of centrioles, cilia, and flagella (see below).
Microfilaments are solid, rodlike structures composed of actin. They provide structural support, and play a roll in phagocytosis, cell and organelle movement, and cell division.
c. Intermediate filaments
Intermediate filaments are tough fibers made of polypeptides. They help to strengthen the cytoskeleton and stabilize cell shape.
Centrioles are located near the nucleus and appear as cylindrical structures consisting of a ring of nine evenly spaced bundles of three microtubules. Centrioles play a role in the formation of cilia and flagella. During animal cell division, the mitotic spindle forms between centrioles.
e. Flagella and Cilia
Flagella are long and few in number whereas cilia are short and numerous. Both consist of 9 fused pairs of protein microtubules with side arms of the motor molecule dynein that originate from a centriole. These form a ring around an inner central pair of microtubules that arise from a plate near the cell surface (see Fig. 34). The arrangement of microtubules is known as a 2X9+2 arrangement. This complex of microtubules is surrounded by a sheath continuous with the cytoplasmic membrane. In the presence of ATP, the dynein side arms of the microtubules in the outer ring elongate and attempt to move along the neighboring pair, causing the flagellum or the cilium to bend. Flagella and cilia function in locomotion. Cilia also function to move various material that may surround a cell.
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