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Prescott−Harley−Klein: Microbiology, Fifth Edition I. Introduction to Microbiology 4. Eucaryotic Cell Structure and Function © The McGraw−Hill Companies, 2002 CHAPTER 4 Eucaryotic Cell Structure and Function Often we exclusively emphasize procaryotes and viruses, but eucaryotic microorganisms also have major impacts on human welfare. For example, the protozoan parasite Trypanosoma brucei gambiense is a cause of African sleeping sickness. The organism invades the nervous system and the victim frequently dies after suffering several years from symptoms such as weakness, headache, apathy, emaciation, sleepiness, and coma. Outline 4.1 An Overview of Eucaryotic Cell Structure 76 4.2 The Cytoplasmic Matrix, Microfilaments, Intermediate Filaments, and Microtubules 76 4.3 The Endoplasmic Reticulum 79 4.4 The Golgi Apparatus 80 4.5 Lysosomes and Endocytosis 80 4.6 Eucaryotic Ribosomes 82 4.7 Mitochondria 83 4.8 Chloroplasts 85 4.9 The Nucleus and Cell Division 86 Nuclear Structure 86 The Nucleolus 87 Mitosis and Meiosis 87 4.10 External Cell Coverings 88 4.11 Cilia and Flagella 89 4.12 Comparison of Procaryotic and Eucaryotic Cells 91 Concepts 1. Eucaryotic cells differ most obviously from procaryotic cells in having a variety of complex membranous organelles in the cytoplasmic matrix and the majority of their genetic material within membrane-delimited nuclei. Each organelle has a distinctive structure directly related to specific functions. 2. A cytoskeleton composed of microtubules, microfilaments, and intermediate filaments helps give eucaryotic cells shape; microtubules and microfilaments are also involved in cell movements and intracellular transport. 3. In eucaryotes, genetic material is distributed between cells by the highly organized, complex processes called mitosis and meiosis. 4. Despite great differences between eucaryotes and procaryotes with respect to such things as morphology, they are similar on the biochemical level
OG emphasizes Chapter 4 fo any valuable traordinarily complex.interesting in their own right and promi ent members of the ecosystem (figure 4.1).In addition,fung .and (a
Prescott−Harley−Klein: Microbiology, Fifth Edition I. Introduction to Microbiology 4. Eucaryotic Cell Structure and Function © The McGraw−Hill Companies, 2002 (and to some extent, algae) are exceptionally useful in industrial microbiology. Many fungi and protozoa are also major human pathogens; one only need think of either malaria or African sleeping sickness (see chapter opener) to appreciate the significance of eucaryotes in pathogenic microbiology. So, although this text emphasizes bacteria, eucaryotic microorganisms are discussed at many points. Chapter 4 focuses on eucaryotic cell structure and its relationship to cell function. Because many valuable studies on eucaryotic cell ultrastructure have used organisms other than microorganisms, some work on nonmicrobial cells is presented. At the end of the chapter, procaryotic and eucaryotic cells are compared in some depth. 4.1 An Overview of Eucaryotic Cell Structure 75 The key to every biological problem must finally be sought in the cell. —E. B.Wilson I n chapter 3 considerable attention is devoted to procaryotic cell structure and function because bacteria are immensely important in microbiology and have occupied a large portion of microbiologists’ attention in the past. Nevertheless, eucaryotic algae, fungi, and protozoa also are microorganisms and have been extensively studied. These organisms often are extraordinarily complex, interesting in their own right, and prominent members of the ecosystem (figure 4.1). In addition, fungi Figure 4.1 Representative Examples of Eucaryotic Microorganisms. (a) Paramecium as seen with interference-contrast microscopy (115). (b) Mixed diatom frustules (100). (c) Penicillium colonies, and (d) a microscopic view of the mold’s hyphae and conidia (220). (e) Stentor. The ciliated protozoa are extended and actively feeding, dark-field microscopy (100). (f) Amanita muscaria, a large poisonous mushroom (5). (a) (b) (c) (d) (e) (f )
incanaee e and Functio d cell wall (w 4.1 An Overview of Eucaryotic Cell Structure n different cell locations.Thus abundant membrane discussed.Table 4.1 briefly summarizes the functions of the ma between the relationship of organelles to a celland that of organs side the membrane are discussed. mem and b 4.2 gure 31p how es for several When a eucaryotic cell is examined at low power with the clec so that they aer
Prescott−Harley−Klein: Microbiology, Fifth Edition I. Introduction to Microbiology 4. Eucaryotic Cell Structure and Function © The McGraw−Hill Companies, 2002 4.1 An Overview of Eucaryotic Cell Structure The most obvious difference between eucaryotic and procaryotic cells is in their use of membranes. Eucaryotic cells have membranedelimited nuclei, and membranes also play a prominent part in the structure of many other organelles (figures 4.2 and 4.3). Organelles are intracellular structures that perform specific functions in cells analogous to the functions of organs in the body. The name organelle (little organ) was coined because biologists saw a parallel between the relationship of organelles to a cell and that of organs to the whole body. It is not satisfactory to define organelles as membrane-bound structures because this would exclude such components as ribosomes and bacterial flagella. A comparison of figures 4.2 and 4.3 with figure 3.11 (p. 51) shows how much more structurally complex the eucaryotic cell is. This complexity is due chiefly to the use of internal membranes for several purposes. The partitioning of the eucaryotic cell interior by membranes makes possible the placement of different biochemical and physiological functions in separate compartments so that they can more easily take place simultaneously under independent control and proper coordination. Large membrane surfaces make possible greater respiratory and photosynthetic activity because these processes are located exclusively in membranes. The intracytoplasmic membrane complex also serves as a transport system to move materials between different cell locations. Thus abundant membrane systems probably are necessary in eucaryotic cells because of their large volume and the need for adequate regulation, metabolic activity, and transport. Figures 4.2, 4.3, and 4.26b provide generalized views of eucaryotic cell structure and illustrate most of the organelles to be discussed. Table 4.1 briefly summarizes the functions of the major eucaryotic organelles. Those organelles lying inside the plasma membrane are first described, and then components outside the membrane are discussed. 4.2 The Cytoplasmic Matrix, Microfilaments, Intermediate Filaments, and Microtubules When a eucaryotic cell is examined at low power with the electron microscope, its larger organelles are seen to lie in an apparently featureless, homogeneous substance called the cytoplasmic matrix. The matrix, although superficially uninteresting, is actually one of the most important and complex parts of the cell. It is the “environment” of the organelles and the location of many important biochemical processes. Several physical changes seen in cells—viscosity changes, cytoplasmic streaming, and others— also are due to matrix activity. 76 Chapter 4 Eucaryotic Cell Structure and Function Figure 4.2 Eucaryotic Cell Ultrastructure. (a) A lymphoblast in the rat lymph node (17,500). (b) The yeast Saccharomyces (7,200). Note the nucleus (n), mitochondrion (m), vacuole (v), endoplasmic reticulum (er), and cell wall (w). W N (a) (b)
lipid drop RE Table 4.1 Functions of Eucaryotic Organelles 61. and o Microfilaments form the that bound terials.protein and lipid pelyFor exampe,protozoan digestive vacuoles may reach and shape changes.Some examples of cellular movements ass Nucleus Repo netic information,contro slime molds (see chapler25) onst and ive shape to the cell app Tem and o oplasm in plant ells and slime molds ments using the drug cytochalasin B have provided additional
Prescott−Harley−Klein: Microbiology, Fifth Edition I. Introduction to Microbiology 4. Eucaryotic Cell Structure and Function © The McGraw−Hill Companies, 2002 Water constitutes about 70 to 85% by weight of a eucaryotic cell. Thus a large part of the cytoplasmic matrix is water. Cellular water can exist in two different forms. Some of it is bulk or free water; this is normal, osmotically active water. Osmosis, water activity, and growth (pp. 61, 121–23) Water also can exist as bound water or water of hydration. This water is bound to the surface of proteins and other macromolecules and is osmotically inactive and more ordered than bulk water. There is some evidence that bound water is the site of many metabolic processes. The protein content of cells is so high that the cytoplasmic matrix often may be semicrystalline. Usually matrix pH is around neutrality, about pH 6.8 to 7.1, but can vary widely. For example, protozoan digestive vacuoles may reach pHs as low as 3 to 4. Probably all eucaryotic cells have microfilaments, minute protein filaments, 4 to 7 nm in diameter, which may be either scattered within the cytoplasmic matrix or organized into networks and parallel arrays. Microfilaments are involved in cell motion and shape changes. Some examples of cellular movements associated with microfilament activity are the motion of pigment granules, amoeboid movement, and protoplasmic streaming in slime molds (see chapter 25). The participation of microfilaments in cell movement is suggested by electron microscopic studies showing that they frequently are found at locations appropriate for such a role. For example, they are concentrated at the interface between stationary and flowing cytoplasm in plant cells and slime molds. Experiments using the drug cytochalasin B have provided additional 4.2 The Cytoplasmic Matrix, Microfilaments, Intermediate Filaments, and Microtubules 77 CI PV F DV SV GA PL AV RB C GE CH MT N M P CR NU P RER R M G SER PM LD Figure 4.3 Eucaryotic Cell Ultrastructure. This is a schematic, three-dimensional diagram of a cell with the most important organelles identified in the illustration. AV, autophagic vacuole; C, centriole; CH, chloroplast; CI, cilium; CR, chromatin; DV, digestion vacuole; F, microfilaments; G, glycogen; GA, Golgi apparatus; GE, GERL; LD, lipid droplet; M, mitochondrion; MT, microtubules; N, nucleus; NU, nucleolus; P, peroxisome; PL, primary lysosome; PM, plasma membrane; PV, pinocytotic vesicle; R, ribosomes and polysomes; RB, residual body; RER, rough endoplasmic reticulum; SER, smooth endoplasmic reticulum; SV, secretion vacuole. Table 4.1 Functions of Eucaryotic Organelles Plasma membrane Mechanical cell boundary, selectively permeable barrier with transport systems, mediates cell-cell interactions and adhesion to surfaces, secretion Cytoplasmic matrix Environment for other organelles, location of many metabolic processes Microfilaments, Cell structure and movements, form the intermediate filaments, cytoskeleton and microtubules Endoplasmic reticulum Transport of materials, protein and lipid synthesis Ribosomes Protein synthesis Golgi apparatus Packaging and secretion of materials for various purposes, lysosome formation Lysosomes Intracellular digestion Mitochondria Energy production through use of the tricarboxylic acid cycle, electron transport, oxidative phosphorylation, and other pathways Chloroplasts Photosynthesis—trapping light energy and formation of carbohydrate from CO2 and water Nucleus Repository for genetic information, control center for cell Nucleolus Ribosomal RNA synthesis, ribosome construction Cell wall and pellicle Strengthen and give shape to the cell Cilia and flagella Cell movement Vacuole Temporary storage and transport, digestion (food vacuoles), water balance (contractile vacuole)
78 Chapter 4 Eucaryonc Cell Structure and Functio B-Tubulin and B-tubulin evidence.Cytochalasin B disrupts microfilament structure and ents is sometimes difficult ament prot ted and ana zed chem 广evc for make us onure he diamct slightly different spherical p subunits named tubulins.ac average of 13 subunits (figure 4.5) maintain cell shap movements. and (3)participate transpor b colchic n.Long.thin cell st ctures requiring support such as th b) e the parallel array of
Prescott−Harley−Klein: Microbiology, Fifth Edition I. Introduction to Microbiology 4. Eucaryotic Cell Structure and Function © The McGraw−Hill Companies, 2002 evidence. Cytochalasin B disrupts microfilament structure and often simultaneously inhibits cell movements. However, because the drug has additional effects in cells, a direct cause-and-effect interpretation of these experiments is sometimes difficult. Microfilament protein has been isolated and analyzed chemically. It is an actin, very similar to the actin contractile protein of muscle tissue. This is further indirect evidence for microfilament involvement in cell movement. Some pathogens such as Listeria monocytogenes make use of eucaryotic actin to move rapidly through the host cell. The ActA protein released by Listeria causes the polymerization of actin filaments at the end of the bacterium. A tail of actin is formed and trapped in the host cytoskeleton. Its continued elongation pushes the bacterium along at rates up to 11 m/minute. The bacterium can even be propelled through the cell surface and into neighboring cells (figure 4.4). A second type of small filamentous organelle in the cytoplasmic matrix is shaped like a thin cylinder about 25 nm in diameter. Because of its tubular nature this organelle is called a microtubule. Microtubules are complex structures constructed of two slightly different spherical protein subunits named tubulins, each of which is approximately 4 to 5 nm in diameter. These subunits are assembled in a helical arrangement to form a cylinder with an average of 13 subunits in one turn or circumference (figure 4.5). Microtubules serve at least three purposes: (1) they help maintain cell shape, (2) are involved with microfilaments in cell movements, and (3) participate in intracellular transport processes. Evidence for a structural role comes from their intracellular distribution and studies on the effects of the drug colchicine. Long, thin cell structures requiring support such as the axopodia (long, slender, rigid pseudopodia) of protozoa contain microtubules (figure 4.6). When migrating embryonic nerve and 78 Chapter 4 Eucaryotic Cell Structure and Function Listeria Actin tail Figure 4.4 Listeria Motility and Actin Filaments. A Listeria cell is propelled through the cell surface by a bundle of actin filaments. Microtubule β-Tubulin α-Tubulin Figure 4.5 Microtubule Structure. The hollow cylinder, about 25 nm in diameter, is made of two kinds of protein subunits, -tubulin and -tubulin. Figure 4.6 Cytoplasmic Microtubules. Electron micrographs of pseudopodia with microtubules. (a) Microtubules in a pseudopodium from the protozoan Reticulomyxa (65,000). (b) A transverse section of a heliozoan axopodium (48,000). Note the parallel array of microtubules organized in a spiral pattern. (a) (b)