Structure of Endomembrane System and Cytoskeleton

Endomembrane System and Cytoskeleton

Endomembrane System

The endomembrane system consists of organelles that collectively produce and degrade biologic molecules (also storing and exporting them) and that degrade substances that may be harmful. This system is made up of the ER, Golgi apparatus, secretory vesicles, ­lysosome, and nuclear membrane. Its components include all membranous organelles or elements that are structurally continuous or that arise due to fusing or forming transport vehicles. Continuities exist between the nuclear envelope, the RER, and the SER. Although not an actual endomembrane, the plasma membrane is also part of the endomembrane system. Throughout this system, many indirect interactions occur. Certain vesicles that begin in the ER are eventually fused with the Golgi apparatus or plasma membrane. Vesicles from the Golgi apparatus can become part of the lyso-somes, plasma membrane, or secretory vesicles.

Cytoskeleton

The cytoskeleton is a network of many rods and hun-dreds of proteins (FIGURE 3-11). The rods run through the cytosol, whereas the proteins link them to other structures of the cell. The cytoskeleton, therefore, functions as the cell’s skeleton, muscles, and ligaments. Various cell movements are also generated by the cyto-skeleton. The three types of rods in the cytoskeleton, all of which lack covering membranes, are microfilaments,intermediate filaments, and microtubules.

Microfilaments are the thinnest type of cytoskel-etal rods, are semiflexible, and are made of actin, a protein. They are the most fragile of the cytoskeletal elements. Because no two cells are identical, each has its own unique microfilaments. However, almost all cells have a cross-linked microfilament network (the terminal web) that is attached to the cytoplasmic side of the plasma membrane. The cell surface is strength-ened by this web, which also acts against compres-sion and transmits force during shape changes and cellular movements.

Microfilaments are usually involved in cell move-ment and shape changes. Actin filaments interact with unconventional myosin, another protein, to generate contractile cellular forces. It is by this mechanism that cells are “pinched” into two cells during cell division. Microfilaments are used for the motion of amoeba and when membranes change during exocytosis and endocytosis. In most cells except for muscle cells, actin filaments breakdown regularly and reform from smaller subunits as needed.

Intermediate filaments resemble ropes and are made of strong, insoluble protein fibers. Twisted tetramer fibrils form intermediate filaments, which arethicker (in diameter) than microfilaments but thinner than microtubules. Intermediate filaments have high tensile strength, and of all cytoskeletal elements are the most permanent and stable. They attach to desmo-somes to resist pulling forces that may be exerted on the cell. Their protein composition differs in various cell types. Therefore, they are named differently based on the cells they exist in. For example, in nerve cells, the intermediate filaments are referred to as neurofilaments.

Microtubules are hollow organelles, made up of spherical tubulins, which are protein subunits. They usually radiate from a small area of cytoplasm near the nucleus (the centrosome or cell center). ­Microtubules are always active, growing from the centrosome, then dis-assembling, and then reassembling in either different­ sites or the same site. They are stiff but bendable and determine the cell’s overall shape as well as how cellu-lar organelles are distributed. From the microtubules, structures appear to “hang.” These include lysosomes, mitochondria, and ­secretory vesicles. Tiny motor pro-teinsconstantly move and reposition organelles­ along the microtubules. These motor proteins include dyneins,kinesins, and others. They function by changing shape,energized by ATP. Some move substances along the microtubules evenly, whereas others grip and release the microtubule, repeating these actions again and again.