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Unformatted text preview: Bio B ,om 1 3.10 Organelles of the Eukaryotic Cell The cell is in a dynamic flux. In the light microscope, a live cell exhibits myriad movements ranging from the translocation of chromosomes and vesicles to the changes in shape associated with cell crawling and swimming. Investigation of intracellular structures begins with micrographs of fixed, sectioned cells in which all cell movements are frozen. Such static pictures of the cell reveal the organization of the cytoplasm into compartments and the stereotypic location of each type of organelle within the cell. In this section, we d escribe the basic structures and functions of the major organelles in animal and plant cells (Figure 5-19). Plant and fungal cells contain most of the organelles found in an animal cell but lack lysosomes. Instead, they contain a large central vacuole that subserves many of the functions of a lysosome. A plant cell also contains chloroplasts, and its membrane is strengthened by a rigid cell wall . Unique proteins in the interior and membranes of each type of organelle largely determine its specific functional characteristics, which are examined in more detail in later chapters. Those organelles bounded by a single membrane are covered first, followed by the i three types that have a double membrane the nucleus, mitochondrion , and chloroplast. Endosomes Take Up Soluble Macromolecules from the Cell Exterior Although transport proteins in the plasma membrane mediate the movement of ions and small molecules across the lipid bilayer, proteins and some other soluble macromolecules in the extracellular milieu are internalized by endocytosis . In this process, a- segment of the plasma membrane invaginates into a coated pit, whose cytosolic face is lined by a specific set of proteins including clathrin. - The pit pinches from the membrane into a small membrane-bounded vesicle that contains extracellular material and - is delivered to an early endosome , a sorting station of membrane-limited tubules and vesicles (Figure 5-20a, b). o From this compartment, some membrane proteins are recycled back to the plasma membrane; 1 o other membrane proteins are transported to a late endosome where further sorting takes place. - The endocytic pathway ends when a late endosome delivers its membrane and internal contents to lysosomes for degradation. The entire endocytic pathway is described in some detail in Chapter 17. ----------------------------------- 1. Plasma membrane controls movement of molecules in and out of the cell and functions in cell-cell signaling and cell adhesion. 2. Mitochondria, which are surrounded by a double membrane, generate ATP by oxidation of glucose and fatty acids....
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- Spring '10