protein tubulin. These tubulin complexes are the organizing centers for growth of the mitotic spindle, which plays a critical role in cell division, and for microtubule formation in nondividing cells. Tubulin - Surrounding the centrioles is pericentriolar material, which contains hundreds of ring-shaped complexes composed of the protein tubulin. These tubulin complexes are the organizing centers for growth of the mitotic spindle, which plays a critical role in cell division, and for microtubule formation in nondividing cells.Cilium (plural is cilia) - are numerous, short, hair-like projections that extend from the surface of the cell. Each cilium contains a core of 20 microtubules surrounded by plasma membrane.Flagellum (plural is flagella) - are similar in structure to cilia but are typically much longer. Flagella usually move an entire cell. A flagellum generates forward motion along its axis by rapidly wiggling in a wavelike patternRibosome - are the sites of protein synthesis. The name of these tiny structures reflects their high content of one type of ribonucleic acid, but each one also includes more than 50 proteins.Endoplasmic reticulum (ER) - is a network of membranes in the form of flattened sacs or tubules. The ER extends from the nuclear envelope (membrane around the nucleus), to which it is connected and projects throughout the cytoplasm. The ER is so extensive that it constitutes more than half of the membranous surfaces within the cytoplasm of most cells.Rough ER - is continuous with the nuclear membrane and usually is folded into a series of flattened sacs. The outer sur- face of rough ER is studded with ribosomes, the sites of protein synthesis. Proteins synthesized by ribosomes attached to rough ER enter spaces within the ER for processing and sorting. In some cases, enzymes attach the proteins to carbohydrates to form glycoproteins. In other cases, enzymes attach the proteins to phospholipids, also synthesized by rough ER.Smooth ER - extends from the rough ER to form a network of membrane tubules. Unlike rough ER, smooth ER does not have ribosomes on the outer surfaces of its membrane. However, smooth ER contains unique enzymes that make it functionally more diverse than rough ER. Because it lacks ribosomes, smooth ER does not synthesize proteins, but it doessynthesize fatty acids and steroids, such as estrogens and testosterone.Golgi complex - first step in the transport pathway, consists of 3 to 20 cisternae.
Cisternae - small, flattened membranous sacs with bulging edges that resemble a stack of pita bread. The cisternae are often curved, giving the Golgi complex a cuplike shape.Secretory vesicles - stores some of the processed proteins that leave the exit face. These vesicles deliver the proteins to the plasma membrane, where they are discharged by exocytosis into the extracellular fluid.Membrane vesicles - deliver their contents to the plasma membrane for incorporation into the membrane.Lysosome - are membrane-enclosed vesicles that form from the Golgi complex. They can contain as many as 60 kinds of