Protein Structures Within the Cytoplasm

Eukaryotic cells have unique protein structures that distinguish them from prokaryotic cells. These include the cytoskeleton, a dynamic filamentous network that provides structure and shape to the cell, and characteristic flagella and cilia, which allow the cell to move.

Another structure that characterizes the eukaryotic cell is the size and density of its ribosomes. The 80S eukaryotic ribo-some is made up of a 60S and a 40S subunit.


The threadlike proteins that make up the cytoskeleton continually reconstruct to adapt to the cell's constantly changing needs. The network is composed of three elements: microtubules, actin filaments, and intermediate fibers (figure 3.51).

Microtubules, the thickest of the cytoskeleton structures, are long hollow cylinders composed of protein subunits called tubulin. Microtubules form the mitotic spindles, the machinery that partitions chromosomes between two cells in the process of cell division. Without mitotic spindles, cells could not reproduce. Microtubules also are the main structures that make up the cilia and flagella, the mechanisms of locomotion in certain eukaryotic cells. In addition, microtubules also function as the framework along which organelles and vesicles move within a cell. Organelles called centrioles are involved in the assembly of microtubules. The antifungal drug griseofulvin is thought to

Fungus Microtubules Images

interfere with the structural integrity of the microtubules of some fungi. ■ mitosis, p. 303

Actin filaments enable the cell cytoplasm to move. They are composed of a polymer of actin, which can rapidly assemble and subsequently disassemble, causing motion. For example, pseudopod formation relies on actin polymerization in one part of the cell and depolymerization in another. Some intracellular pathogens exploit this process and trigger a rapid polymerization of actin, creating an "actin tail," which moves them within that cell. Sometimes, this mechanism propels them with enough force to be ejected into an adjacent cell.

Intermediate fibers function like ropes, strengthening the cell mechanically. They enable cells to resist physical stresses.

Flagella and Cilia

Flagella and cilia are flexible structures that appear to project out of a cell yet are covered by an extension of the plasma membrane (figure 3.52). Both are composed of long microtubules grouped in what is called a 9 + 2 arrangement: nine pairs of microtubules surrounding two individual ones. They originate from a basal body within the cell; the basal body has a slightly different arrangement of microtubules.

Eukaryotic flagella function in motility, but are otherwise completely different from their prokaryotic counterparts. Using ATP as a source of energy, they either propel the cell with a whiplike motion or thrash back and forth to pull the cell forward. Recall that flagella in bacteria use proton motive force as a source of energy and rotate like a propeller to push the organism.

Cilia are shorter than flagella, often covering a cell and moving in synchrony (see figure 1.8). This motion can move a cell forward in an aqueous solution, or propel surrounding material along a stationary cell. For example, epithelial cells that line the respiratory tract have cilia that beat together in a directed fashion. This propels the mucus film that covers those cells, directing it upward toward the mouth, where it can be swallowed. This action removes microorganisms that have been inhaled before they can enter the lungs.

Outer microtubule pair

Central microtubule pair

Outer microtubule pair

Cilia Flagella

Central microtubule pair


Figure 3.52 Flagella Flexible structures involved in movement.


Figure 3.52 Flagella Flexible structures involved in movement.

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  • pinja
    What protein makes up microtubules?
    3 years ago

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