《C CHAPTER 4 Eucaryotic Cell Structure and Function nd con Outline Concepts Ine cy 43 70 ed of m ratus 80 4.6 mes 82 d Cell proc 86 ngs 88
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)
er What is an organelle? ormally cyto tures that partici -the mitotic spindle,cilia,and 4.3 The Endoplasmic Reticulum ubules and intermediate filaments are maior com nents of a vast role in both cell shape and movement.Pro es lack a true or tein for purposes such as se retion.a large part of the ER is stud- ganized cytoskeleton and may not possess actinlike proteins ng large quantities of lipid have er tha This teins,lipids,and probably other materials through thesized on RER proteins nd lipids.RER then loses its conected rib 4.7 The En and smooth
Prescott−Harley−Klein: Microbiology, Fifth Edition I. Introduction to Microbiology 4. Eucaryotic Cell Structure and Function © The McGraw−Hill Companies, 2002 1. What is an organelle? 2. Define cytoplasmic matrix, bulk or free water, bound water, microfilament, microtubule, and tubulin. Discuss the roles of microfilaments, intermediate filaments, and microtubules. 3. Describe the cytoskeleton. What are its functions? 4.3 The Endoplasmic Reticulum Besides the cytoskeleton, the cytoplasmic matrix is permeated with an irregular network of branching and fusing membranous tubules, around 40 to 70 nm in diameter, and many flattened sacs called cisternae (s., cisterna). This network of tubules and cisternae is the endoplasmic reticulum (ER) (figure 4.2a and figure 4.8). The nature of the ER varies with the functional and physiological status of the cell. In cells synthesizing a great deal of protein for purposes such as secretion, a large part of the ER is studded on its outer surface with ribosomes and is called rough or granular endoplasmic reticulum (RER or GER). Other cells, such as those producing large quantities of lipids, have ER that lacks ribosomes. This is smooth or agranular ER (SER or AER). The endoplasmic reticulum has many important functions. It transports proteins, lipids, and probably other materials through the cell. Lipids and proteins are synthesized by ER-associated enzymes and ribosomes. Polypeptide chains synthesized on RERbound ribosomes may be inserted either into the ER membrane or into its lumen for transport elsewhere. The ER is also a major site of cell membrane synthesis. New endoplasmic reticulum is produced through expansion of the old. Many biologists think the RER synthesizes new ER proteins and lipids. “Older” RER then loses its connected ribosomes and is modified to become SER. Not everyone agrees with this interpretation, and other mechanisms of growth of ER are possible. 4.3 The Endoplasmic Reticulum 79 Figure 4.7 The Eucaryotic Cytoskeleton. (a) Antibody-stained microfilament system in a mammal cell (400). (b) Antibody-stained microtubule system in a mammal cell (1,000). (a) (b) Figure 4.8 The Endoplasmic Reticulum. A transmission electron micrograph of the corpus luteum in a human ovary showing structural variations in eucaryotic endoplasmic reticulum. Note the presence of both rough endoplasmic reticulum lined with ribosomes and smooth endoplasmic reticulum without ribosomes (26,500). heart cells are exposed to colchicine, they simultaneously lose their microtubules and their characteristic shapes. The shapeless cells seem to wander aimlessly as if incapable of directed movement without their normal form. Their microfilaments are still intact, but due to the disruption of their microtubules by colchicine, they no longer behave normally. Microtubules also are present in structures that participate in cell or organelle movements—the mitotic spindle, cilia, and flagella. For example, the mitotic spindle is constructed of microtubules; when a dividing cell is treated with colchicine, the spindle is disrupted and chromosome separation blocked. Microtubules also are essential to the movement of eucaryotic cilia and flagella. Other kinds of filamentous components also are present in the matrix, the most important of which are the intermediate filaments (about 8 to 10 nm in diameter). The microfilaments, microtubules, and intermediate filaments are major components of a vast, intricate network of interconnected filaments called the cytoskeleton (figure 4.7). As mentioned previously, the cytoskeleton plays a role in both cell shape and movement. Procaryotes lack a true, organized cytoskeleton and may not possess actinlike proteins
(165,000)in (and diagrammatically n(b) 4.4 The Golgi Apparatus different fates by adding specific and then sends the eins on their way to thep nave phosphates added to their mannose sugars). smooth ER.lack bound rib The may be many more.Each sac is 15 to 20 nm thick and s 4.5 Lysosomes and Endocytosis Avery important function of the Golgi apparatus and endoplasmic the cistemae.The stack of cisternae has a definite polarity be some algae and n a single membrane:they average ab out 500nm in diameter.but size.They are involved in i etimes it consis sts of a single stack of cistemae:however tain an acidic environment by pumping protons s into their interior ER The surtace with the s through endoc ossIn this process a cell takes up solute les of some rlagell ac and radic y enclosing them in vacuo esicles pinc and then ce in ities that contain fuid,and mat al.Lan participate in th ler cavitie 5m a the er to the 410 Most often ves cles bud off the ER.travel to the th its s ytotic ves from ed by e
Prescott−Harley−Klein: Microbiology, Fifth Edition I. Introduction to Microbiology 4. Eucaryotic Cell Structure and Function © The McGraw−Hill Companies, 2002 4.4 The Golgi Apparatus The Golgi apparatusis a membranous organelle composed of flattened, saclike cisternae stacked on each other (figure 4.9). These membranes, like the smooth ER, lack bound ribosomes. There are usually around 4 to 8 cisternae or sacs in a stack, although there may be many more. Each sac is 15 to 20 nm thick and separated from other cisternae by 20 to 30 nm. A complex network of tubules and vesicles (20 to 100 nm in diameter) is located at the edges of the cisternae. The stack of cisternae has a definite polarity because there are two ends or faces that are quite different from one another. The sacs on the cis or forming face often are associated with the ER and differ from the sacs on the trans or maturing face in thickness, enzyme content, and degree of vesicle formation. It appears that material is transported from cis to trans cisternae by vesicles that bud off the cisternal edges and move to the next sac. The Golgi apparatus is present in most eucaryotic cells, but many fungi and ciliate protozoa may lack a well-formed structure. Sometimes it consists of a single stack of cisternae; however, many cells may contain up to 20, and sometimes more, separate stacks. These stacks of cisternae, often called dictyosomes, can be clustered in one region or scattered about the cell. The Golgi apparatus packages materials and prepares them for secretion, the exact nature of its role varying with the organism. The surface scales of some flagellated algae and radiolarian protozoa appear to be constructed within the Golgi apparatus and then transported to the surface in vesicles. It often participates in the development of cell membranes and in the packaging of cell products. The growth of some fungal hyphae occurs when Golgi vesicles contribute their contents to the wall at the hyphal tip. In all these processes, materials move from the ER to the Golgi apparatus. Most often vesicles bud off the ER, travel to the Golgi apparatus, and fuse with the cis cisternae. Thus the Golgi apparatus is closely related to the ER in both a structural and a functional sense. Most proteins entering the Golgi apparatus from the ER are glycoproteins containing short carbohydrate chains. The Golgi apparatus frequently modifies proteins destined for different fates by adding specific groups and then sends the proteins on their way to the proper location (e.g., lysosomal proteins have phosphates added to their mannose sugars). 4.5 Lysosomes and Endocytosis A very important function of the Golgi apparatus and endoplasmic reticulum is the synthesis of another organelle, the lysosome. This organelle (or a structure very much like it) is found in a variety of microorganisms—protozoa, some algae, and fungi—as well as in plants and animals. Lysosomes are roughly spherical and enclosed in a single membrane; they average about 500 nm in diameter, but range from 50 nm to several m in size. They are involved in intracellular digestion and contain the enzymes needed to digest all types of macromolecules. These enzymes, called hydrolases, catalyze the hydrolysis of molecules and function best under slightly acid conditions (usually around pH 3.5 to 5.0). Lysosomes maintain an acidic environment by pumping protons into their interior. Digestive enzymes are manufactured by the RER and packaged to form lysosomes by the Golgi apparatus. A segment of smooth ER near the Golgi apparatus also may bud off lysosomes. Lysosomes are particularly important in those cells that obtain nutrients through endocytosis. In this process a cell takes up solutes or particles by enclosing them in vacuoles and vesicles pinched off from its plasma membrane. Vacuoles and vesicles are membranedelimited cavities that contain fluid, and often solid material. Larger cavities will be called vacuoles, and smaller cavities, vesicles. There are two major forms of endocytosis: phagocytosis and pinocytosis. During phagocytosislarge particles and even other microorganisms are enclosed in a phagocytic vacuole or phagosome and engulfed (figure 4.10a). In pinocytosissmall amounts of the surrounding liquid with its solute molecules are pinched off as tiny pinocytotic vesicles (also called pinocytic vesicles) or pinosomes. Often phagosomes and pinosomes are collectively called endosomes because they are formed by endocytosis. The type of pinocytosis, receptormediated endocytosis, that produces coated vesicles (see p. 403) is important in the entry of animal viruses into host cells. 80 Chapter 4 Eucaryotic Cell Structure and Function Dictyosome (a stack of flattened cisternae or lamelliae) Trans or maturing face Peripheral tubules Secretory vesicle Cis or forming face Figure 4.9 Golgi Apparatus Structure. Golgi apparatus of Euglena gracilis. Cisternal stacks are shown in the electron micrograph (165,000) in (a) and diagrammatically in (b). (a) (b)
tic ov nd functio diges ed by ×141371 sec 410 s,lysc catastrophe th would r secondary lys s and other macromolecules while allowing small di. n products to leave com who materials( Invading ba ure 4.11).Chr istian e()has of its functional uniry.The ER manufactures secretor (white blood cells)of vertebrates. fuse with the plasma mem rane and de.It also 0a)It is cytoplasm(figure 4.11.or Membrane me ovement in the me ly a ves ing betwe led and Golgi atus and plasma membrane rather than being de ysosomes aid in digestion and removal of cell debris. into the cytoplasmic matrix
Prescott−Harley−Klein: Microbiology, Fifth Edition I. Introduction to Microbiology 4. Eucaryotic Cell Structure and Function © The McGraw−Hill Companies, 2002 4.5 Lysosomes and Endocytosis 81 Figure 4.10 Lysosome Structure, Formation, and Function. (a) A diagrammatic overview of lysosome formation and function. (b) Lysosomes in macrophages from the lung. Secondary lysosomes contain partially digested material and are formed by fusion of primary lysosomes and phagocytic vacuoles (14,137). Endoplasmic reticulum Primary lysosome Secondary lysosome Residual body Golgi apparatus Phagocytic vacuole Pinocytotic vesicle Autophagic vacuole Plasma membrane Mitochondrion Golgi apparatus Lobe of nucleus Primary lysosomes (a) (b) Material in endosomes is digested with the aid of lysosomes. Newly formed lysosomes, or primary lysosomes, fuse with phagocytic vacuoles to yield secondary lysosomes, lysosomes with material being digested (figure 4.10). These phagocytic vacuoles or secondary lysosomes often are called food vacuoles. Digested nutrients then leave the secondary lysosome and enter the cytoplasm. When the lysosome has accumulated large quantities of indigestible material, it is known as a residual body. Lysosomes join with phagosomes for defensive purposes as well as to acquire nutrients. Invading bacteria, ingested by a phagocytic cell, usually are destroyed when lysosomes fuse with the phagosome. This is commonly seen in leukocytes (white blood cells) of vertebrates. Phagocytosis and resistance to pathogens (pp. 718–20) Cells can selectively digest portions of their own cytoplasm in a type of secondary lysosome called an autophagic vacuole (figure 4.10a). It is thought that these arise by lysosomal engulfment of a piece of cytoplasm (figure 4.11), or when the ER pinches off cytoplasm to form a vesicle that subsequently fuses with lysosomes. Autophagy probably plays a role in the normal turnover or recycling of cell constituents. A cell also can survive a period of starvation by selectively digesting portions of itself to remain alive. Following cell death, lysosomes aid in digestion and removal of cell debris. A most remarkable thing about lysosomes is that they accomplish all these tasks without releasing their digestive enzymes into the cytoplasmic matrix, a catastrophe that would destroy the cell. The lysosomal membrane retains digestive enzymes and other macromolecules while allowing small digestion products to leave. The intricate complex of membranous organelles composed of the Golgi apparatus, lysosomes, endosomes, and associated structures seems to operate as a coordinated whole whose main function is the import and export of materials (figure 4.11). Christian de Duve (Nobel Prize, 1974) has suggested that this complex be called the vacuome in recognition of its functional unity. The ER manufactures secretory proteins and membrane, and contributes these to the Golgi apparatus. The Golgi apparatus then forms secretory vesicles that fuse with the plasma membrane and release material to the outside. It also produces lysosomes that fuse with endosomes to digest material acquired through phagocytosis and pinocytosis. Membrane movement in the region of the vacuome lying between the Golgi apparatus and the plasma membrane is two-way. Empty vesicles often are recycled and returned to the Golgi apparatus and plasma membrane rather than being destroyed. These exchanges in the vacuome occur without membrane rupture so that vesicle contents never escape directly into the cytoplasmic matrix
2 Chapter 4 Eucaryonic Cell Structure and Functioe Q ATP-dependent process and the ubiquitins are released.The pep 1.How do th 26S Pr de and with C5andtnansf es of the Go 3.How are lysoso es fomed?De scribe the various forms of digestion.What is pinocytosi飞，and phagocytos More al 4.6 Eucaryotic Ribosomes
Prescott−Harley−Klein: Microbiology, Fifth Edition I. Introduction to Microbiology 4. Eucaryotic Cell Structure and Function © The McGraw−Hill Companies, 2002 More recently a nonlysosomal protein degradation system has been discovered in eucaryotic cells, a few bacteria, and many archaea. The majority of eucaryotic proteins may be degraded by this system. In eucaryotes, proteins are targeted for destruction by the attachment of several small ubiquitin polypeptides (figure 4.12). The marked protein then enters a huge cylindrical complex called a 26S proteasome, where it is degraded to peptides in an ATP-dependent process and the ubiquitins are released. The peptides may be hydrolyzed to amino acids. In this case the system is being used to recycle proteins. The proteasome also is involved in producing peptides for antigen presentation during many immunological responses (see section 32.4). 1. How do the rough and smooth endoplasmic reticulum differ from one another in terms of structure and function? List the processes in which the ER is involved. 2. Describe the structure of a Golgi apparatus in words and with a diagram. How do the cis and trans faces of the Golgi apparatus differ? List the major Golgi apparatus functions discussed in the text. 3. How are lysosomes formed? Describe the various forms of lysosomes and the way in which they participate in intracellular digestion. What is an autophagic vacuole? Define endocytosis, pinocytosis, and phagocytosis. What is a proteasome? 4.6 Eucaryotic Ribosomes The eucaryotic ribosome can either be associated with the endoplasmic reticulum or be free in the cytoplasmic matrix and is larger than the bacterial 70S ribosome. It is a dimer of a 60S and 82 Chapter 4 Eucaryotic Cell Structure and Function Endoplasmic reticulum Lysosome Endosome Lysosome 5 7 2 3 6 4 Endosome 5 5 1 Plasma membrane Golgi apparatus Figure 4.11 Membrane Flow in the Vacuome. The flow of material and membranes between organelles in a eucaryotic cell. (1) Vesicles shuttling between the ER and Golgi apparatus. (2) The Golgi–plasma membrane shuttle for secretion of materials. (3) The Golgi-lysosome shuttle. (4) The movement of material and membranes during endocytosis. (5) Pathways of plasma membrane recovery from endosomes, lysosomes, and through the Golgi apparatus. (6) Movement of vesicles from endosomes to lysosomes. (7) Autophagy by a lysosome. Protein Ubiquitin Peptides ATP 26S Proteasome ATP Figure 4.12 Proteasome Degradation of Proteins. See text for details
Cal Both free and RER-bound ribos mbrane protein or non ory an 270cc tof pro omes usually attach to a single messenger RNA and simultane mes.Ribo omal participation n protein synthesis is dealt (b) with later.Thef mes in protein synthesis (pp.267-72) omes and those bound to the ER differ in s mitochondria with cristae in the shane of 4.7 Mitochondria Found in most cre called ium phosphate ondrial rib es and res e o eon m sition.Mitoch I DNA ope,mito the others ranes,for example,poss s different lipic En of electron ytoplasm (figure 4.13).The mitochondrial membrane separated from an iner mito eres.about 5 pm diameter are attac ed by stalks to its inne are 4.14).Spe ace. es and synthesize ATP Tubular cristae are found in a variety of eucaryotes:however. drial proteins,however,are manufactured under the direction of
Prescott−Harley−Klein: Microbiology, Fifth Edition I. Introduction to Microbiology 4. Eucaryotic Cell Structure and Function © The McGraw−Hill Companies, 2002 a 40S subunit, about 22 nm in diameter, and has a sedimentation coefficient of 80S and a molecular weight of 4 million. When bound to the endoplasmic reticulum to form rough ER, it is attached through its 60S subunit. Both free and RER-bound ribosomes synthesize proteins. As mentioned earlier, proteins made on the ribosomes of the RER either enter its lumen for transport, and often for secretion, or are inserted into the ER membrane as integral membrane proteins. Free ribosomes are the sites of synthesis for nonsecretory and nonmembrane proteins. Some proteins synthesized by free ribosomes are inserted into organelles such as the nucleus, mitochondrion, and chloroplast. As discussed in chapters 3 and 12 (see pp. 52, 272–74), molecular chaperones aid the proper folding of proteins after synthesis. They also assist the transport of proteins into eucaryotic organelles such as mitochondria. Several ribosomes usually attach to a single messenger RNA and simultaneously translate its message into protein. These complexes of messenger RNA and ribosomes are called polyribosomes or polysomes. Ribosomal participation in protein synthesis is dealt with later. The role of ribosomes in protein synthesis (pp. 267–72) 1. Describe the structure of the eucaryotic 80S ribosome and contrast it with the procaryotic ribosome. 2. How do free ribosomes and those bound to the ER differ in function? 4.7 Mitochondria Found in most eucaryotic cells, mitochondria (s., mitochondrion) frequently are called the “powerhouses” of the cell. Tricarboxylic acid cycle activity and the generation of ATP by electron transport and oxidative phosphorylation take place here. In the transmission electron microscope, mitochondria usually are cylindrical structures and measure approximately 0.3 to 1.0 m by 5 to 10 m. (In other words, they are about the same size as bacterial cells.) Although cells can possess as many as 1,000 or more mitochondria, at least a few cells (some yeasts, unicellular algae, and trypanosome protozoa) have a single giant tubular mitochondrion twisted into a continuous network permeating the cytoplasm (figure 4.13). The tricarboxylic acid cycle, electron transport, and oxidative phosphorylation (pp. 183–89) The mitochondrion is bounded by two membranes, an outer mitochondrial membrane separated from an inner mitochondrial membrane by a 6 to 8 nm intermembrane space (figure 4.14). Special infoldings of the inner membrane, called cristae (s., crista), greatly increase its surface area. Their shape differs in mitochondria from various species. Fungi have platelike (laminar) cristae, whereas euglenoid flagellates may have cristae shaped like disks. Tubular cristae are found in a variety of eucaryotes; however, amoebae can possess mitochondria with cristae in the shape of vesicles (figure 4.15). The inner membrane encloses the mitochondrial matrix, a dense matrix containing ribosomes, DNA, and often large calcium phosphate granules. Mitochondrial ribosomes are smaller than cytoplasmic ribosomes and resemble those of bacteria in several ways, including their size and subunit composition. Mitochondrial DNA is a closed circle like bacterial DNA. Each mitochondrial compartment is different from the others in chemical and enzymatic composition. The outer and inner mitochondrial membranes, for example, possess different lipids. Enzymes and electron carriers involved in electron transport and oxidative phosphorylation (the formation of ATP as a consequence of electron transport) are located only in the inner membrane. The enzymes of the tricarboxylic acid cycle and the -oxidation pathway for fatty acids (see chapter 9) are located in the matrix. The inner membrane of the mitochondrion has another distinctive structural feature related to its function. Many small spheres, about 8.5 nm diameter, are attached by stalks to its inner surface. The spheres are called F1 particles and synthesize ATP during cellular respiration (see pp. 187–89). The mitochondrion uses its DNA and ribosomes to synthesize some of its own proteins. In fact, mutations in mitochondrial DNA often lead to serious diseases in humans. Most mitochondrial proteins, however, are manufactured under the direction of 4.7 Mitochondria 83 K Figure 4.13 Trypanosome Mitochondria. The giant mitochondria from trypanosomes. (a) Crithidia fasciculata mitochondrion with kinetoplast, K. The kinetoplast contains DNA that codes for mitochondrial RNA and protein. (b) Trypanosoma cruzi mitochondrion with arrow indicating position of kinetoplast. (a) (b)
