Chapter 4: Introduction to Eukaryotic Cells PDF

4 Introduction to Eukaryotic Cells What Will We Explore? Eukaryotic cells tant in medicine. Various pathogenic fungi and protists as well make up plants, animals, fungi, and as parasitic worms (helminths) afflict humans. As you read, protists. This chapter reviews the you will note variation among...

4 Introduction to Eukaryotic Cells What Will We Explore? Eukaryotic cells tant in medicine. Various pathogenic fungi and protists as well make up plants, animals, fungi, and as parasitic worms (helminths) afflict humans. As you read, protists. This chapter reviews the you will note variation among eukaryotes as well as differcellular features and physiology ences between prokaryotes and eukaryotes. These differences of eukaryotic cells. It is projected are central to medicine, because they tend to be drug therapy that over eight million different targets. Fungal and parasitic infections can be especially chaleukarylenging to treat, often requirotic speing long courses of treatment cies call with a fairly limited drug arseThe Case of the Killer Fungus NCLEX Earth home. nal. This is, in part, because Did you know there is a deadly HESI Amazingly, it is there are fewer differences to FPO fungus spreading across the TEAS estimated that we have characterized exploit between eukaryotic northwestern United States? only about 16 percent of land-dwellpathogens and our own eukarScan this code or visit the ing eukaryotes and about 9 percent yotic cells. As such, the conMastering Microbiology Study Area CLINICAL of water-dwelling eukaryotes.1 tinuous exploration of basic to watch the case and find out how CASE eukaryotes can explain this medical cellular differences between mystery. “them” and “us” is key to Why Is It Important? A variety developing new therapies. of eukaryotic organisms are impor- 1 Mora, C., Tittensor, D. P., Adl, S., Simpson, A. G. B., & Worm, B. (2011). How many species are there on Earth and in the ocean? PLoS Biology, 9 (8), e1001127. 93 M04_NORM8290_01_SE_C04.indd 93 05/06/17 5:22 PM OVERVIEW OF EUKARYOTES After reading this section, you should be able to: 1 Describe the endosymbiotic theory as it relates to the evolution of eukaryotes. 2 Provide a basic description of a eukaryotic cell and state how eukaryotes differ from prokaryotes. 3 Compare and contrast mitosis and meiosis. 4 Describe and compare the main transport mechanisms used by eukaryotic cells. EVOLUTIONARY TIME Ancient nonphotosynthetic prokaryote (possibly a Rickettsia species) Evolving nucleus Protoeukaryote Mitochondrion Nucleus Photosynthetic bacterium (possibly cyanobacteria) Primitive nonphotosynthetic eukaryote Chloroplast The endosymbiotic theory proposes how eukaryotes evolved. Life on Earth started about 3.5 billion years ago with the evolution of prokaryotes. Eukaryotic cells developed about 2.5 billion years ago. The endosymbiotic theory describes the evolution of eukaryotes as a series of sequential, cellmerging events between an ancient eukaryotic ancestor and certain prokaryotes.2 The prefix endo means inside, while symbiotic refers to collaboration between organisms. Therefore, the word endosymbiotic reflects the idea that these cell-merging events came about through a mutually beneficial relationship among the participants. There are several variations of the endosymbiotic theory. The most widely accepted version states that nonphotosynthetic prokaryotes, possibly an ancient Rickettsia-like species, merged with an ancestral cell to produce a protoeukaryote.3 It is still debated whether the protoeukaryote was a fullfledged, nucleus-containing eukaryote or not. However, it’s generally agreed that the engulfed bacteria eventually lost the ability to live outside of host cells, becoming what we now know as mitochondria. Later, some of these protoeukaryotes engaged in a second merging event with a photosynthetic prokaryote—possibly a cyanobacterium. These engulfed cyanobacteria also lost their ability to live freely outside of their hosts, becoming what we now know as chloroplasts. The sequence of merging events also explains why photosynthetic eukaryotes have both mitochondria and chloroplasts (FIG. 4.1). Abundant evidence supports the endosymbiotic theory. Mitochondria and chloroplasts both have their own circular DNA and 70S ribosomes that are similar to those found in bacteria. These organelles also have a double-membrane structure, are similar in size to bacteria, and replicate by a process similar to binary fission. Furthermore, many mitochondrial genes resemble select proteobacteria genes, while the genes in chloroplasts resemble those found in certain photosynthetic bacteria. These clues all point to mitochondria and chloroplasts having once been independent, prokaryotic life forms. Even if the exact logistics are still debated, scientists widely accept the basic premise that eukaryotes evolved from prokaryotes. Eukaryotic cell structures, as well as processes for cell division and transport, differ from prokaryotic cells. Plants, animals, protists, and fungi are all modern-day eukaryotes. Eukaryotic cells are usually larger in size and structurally more complex than prokaryotic cells. They also tend to have larger genomes that are spread across multiple linear chromosomes, while most prokaryotes have a single, circular chromosome. TABLE 4.1 compares and contrasts features of eukaryotic and prokaryotic cells. Membrane-Bound Organelles Primitive photosynthetic eukaryote Animals, fungi, and nonplantlike protists Plants and plantlike protists FIGURE 4.1 Endosymbiotic theory Eukaryotes evolved by cell-merging events between a protoeukaryote and prokaryotes. 94 Eukaryotic cells have a defined nucleus, as well as a variety of other membranebound organelles, such as mitochondria and chloroplasts. Eukaryotic cells are 2 Margulis, L. (2004). Serial endosymbiotic theory (SET) and composite individuality. Microbiology Today, 31 (4), 172–175. 3 Rickettsia bacteria invade eukaryotes. A number of these bacteria are human pathogens that cause diseases like Rocky Mountain spotted fever (R. rickettsii) and typhus (R. prowazekii). Emelyanov, V. V. (2001). Evolutionary relationship of Rickettsiae and mitochondria. FEBS Letters, 501 (1), 11–18. CHAPTER 4 • Introduction to Eukaryotic Cells M04_NORM8290_01_SE_C04.indd 94 22/06/17 3:41 PM TABLE 4.1 Eukaryotic versus Prokaryotic Cells Characteristic Eukaryotes Prokaryotes Organisms Unicellular: Protists and yeast (a type of fungi) Unicellular archaea and bacteria Multicellular: Animals, plants, and most fungi Size Usually much larger than prokaryotes Usually much smaller than eukaryotes Cell division Asexual (mitosis) and sexual (meiosis) Asexual (binary fission) Plasma membrane Often contain sterols Rarely contain sterols Cell wall Only in plants, fungi, and certain protists Most (except Mycoplasma and L-forms) Nucleus Yes No Ribosomes 80S: Cytoplasm, rough endoplasmic reticulum 70S only 70S: Mitochondria and chloroplasts Genetic material DNA DNA Chromosomes Multiple linear chromosomes Usually a single circular chromosome Membrane-bound organelles Yes No (but may have membranous inclusions) amazingly diverse in both structure and physiological capabilities. A sample eukaryotic cell is shown in FIG. 4.2. Later in this chapter we’ll review the organelles of eukaryotic cells and, where applicable, point out similarities and differences between prokaryotes and eukaryotes. This information is important because it helps explain why some drugs affect bacteria, but do not kill or damage our own eukaryotic cells. For example, the drug penicillin targets peptidoglycan production in the bacterial cell wall, so it won’t impact a eukaryotic cell since eukaryotes don’t make peptidoglycan. This basic information on differences between cell types lays the foundation for the antibiotic therapies discussion found later in the book (see Chapter 15). Plasma membrane Glycocalyx Rough endoplasmic reticulum Nucleus Smooth endoplasmic reticulum Lysosome Centrosome Cytoskeleton Mitochondrion Ribosome Flagellum Peroxisome Golgi apparatus FIGURE 4.2 Eukaryotic cell A typical animal cell is shown. Overview of Eukaryotes M04_NORM8290_01_SE_C04.indd 95 95 05/06/17 5:22 PM Cell Division in Eukaryotes Unlike prokaryotes, which do not conduct sexual reproduction, eukaryotic cells can exhibit sexual and/or asexual reproduction. Before a cell performs any form of cell division it must copy its genetic material (see Chapter 5). Because eukaryotes have a larger genome and membranous organelles to replicate and separate into daughter cells, these processes are much more involved, and take longer than binary fission does in prokaryotes. Mitosis This form of asexual reproduction is the most common way eukaryotic cells divide. Mitosis generates two genetically identical offspring from one parent cell. The offspring cells maintain the same number of chromosomes as the parent cell. All human cells, except for egg and sperm cells, divide using mitosis. Meiosis As you are aware, sexual reproduction (meiosis) in humans requires two contributors: sperm from a male and an egg from a female. These specialized cells, called gametes, combine to make a genetically unique zygote. Fungal sexual spores are another example of gametes. In some cases, two separate partners must contribute gametes in meiosis, but other times the gametes may come from a single parent, so that interaction with another organism of the same species is not necessary. Regardless of the number of parents involved, the gametes are all made by meiosis. Meiosis consists of two cell division stages. Consequently, one parent cell produces four gametes (daughter cells). Due to a genetic recombination event called crossing over, each gamete is genetically unique. The gametes are also haploid, which means they have half the number of chromosomes of their diploid parent, which originally had its chromosomes arranged in pairs before meiosis started. When two complementary gamete cells combine, the resulting zygote has the full number of chromosomes necessary to mature into the next generation of that particular organism. Binary Fission As a reminder, prokaryotic cells divide by binary fission. However, the eukaryotic organelles mitochondria or chloroplasts replicate using a process very similar to binary fission. The fact that these eukaryotic organelles use a binary fission-like method of division distinct from how the eukaryotic cell itself divides is additional evidence that supports the endosymbiotic theory. Binary fission, mitosis, and meiosis are compared in FIG. 4.3. Mitosis Eukaryotes Chromosomes Asexual Diploid reproduction parent Makes 2 genetically cell identical cells Copies DNA before DNA copied division Diploid daughter cells (paired chromosomes) Meiosis Eukaryotes Sexual reproduction Makes 4 genetically unique cells Copies DNA before 1st division Haploid daughter cells (unpaired chromosomes) Cell division Diploid daughter cells Binary Fission Chromosomes Diploid parent cell DNA copied Cell division Prokaryotes (mitochondria Chromosome and chloroplasts also use a process that resembles binary fission) Asexual reproduction Makes 2 genetically DNA copied identical cells Copies DNA before division Same number of Cell division chromosomes as parent Cell division Haploid daughter cells FIGURE 4.3 Comparing types of cell division 96 CHAPTER 4 • Introduction to Eukaryotic Cells M04_NORM8290_01_SE_C04.indd 96 05/06/17 5:22 PM Eukaryotic Cell Transport: Endocytosis and Exocytosis Eukaryotes use many of the same transport processes as prokaryotes. (For more on osmosis, diffusion, and active transport, see Chapter 3.) But they also employ endocytosis to import things into the cell, and exocytosis to remove things from the cell (FIG. 4.4). Endocytosis and exocytosis both require ATP, and are used as generalized (bulk or mass) transport mechanisms, as well as specialized transport regulated by receptors on the cell’s surface. While endocytosis was thought to be exclusive to eukaryotes, recently it was discovered in aquatic prokaryotes belonging to the bacterial phylum Planctomycetes.4 These bacteria blur the lines between prokaryotic and eukaryotic cells in other ways aside from their ability to perform endocytosis. For example, they have some basic membrane compartmentalization and even separate their DNA in a membranous inclusion.5 During endocytosis, substances from the extracellular environment enter the cell in membranous endocytic vesicles that form as the cell’s plasma membrane folds inward (invaginates) and pinches off. Nutrients, macromolecules, dissolved substances, and even viruses or other cells enter eukaryotic cells by endocytosis. The three main mechanisms of endocytosis are pinocytosis, phagocytosis, and receptor-mediated endocytosis. Pinocytosis The term pinocytosis means “cell drinking,” and describes endocytosis of dissolved substances in small vesicles. Most eukaryotic cells are constantly carrying out pinocytosis as a form of nonspecific mass transport that is important for cell survival. ENDOCYTOSIS EXOCYTOSIS FIGURE 4.4 Endocytosis and exocytosis Hydrolytic enzymes Target Lysosome Phagosome Phagocytosis The term phagocytosis means “cell eating,” and describes endocytosis of undissolved substances. Eukaryotic cells often engulf whole cells or viruses. Because the undisPhagolysosome solved cargo imported via phagocytosis is larger than what is imported by pinocytosis, it typically involves larger vesicles. Phagocytosis is often mediated by specific receptors on the cell surface. Features on the agent being engulfed often bind to these receptors. Antibodies and other immune system factors may also coat pathogens to help target them for clearance by phagocytes––specialized immune system cells, such as macrophages, aim to destroy the targets they engulf. Amoebas and a variety of other protists also use phagocytosis to obtain food from their environments. Generally speaking, a phagocytic cell engulfs its target in an endocytic vesicle known as a phagosome. Soon after the phagosome enters the cell, it fuses with a lysosome––a vesicle-like organelle packed with hydrolytic enzymes. The fusion of the phagosome and a lysosome creates a phagolysosome. The hydrolytic enzymes delivered by the lysosome destroy most cells and viruses engulfed by phagocytosis and then waste products are expelled (FIG. 4.5). However, certain pathogens have evolved mechanisms for escaping the phagolysosome and/or neutralizing the enzymes. Once these agents escape destruction, they replicate in the cytoplasm of the phagocytic cell and cause disease. (Escaping phagocytosis is discussed further in Chapter 10.) 1 Nondissolved target (like a cell or virus) is engulfed. 2 Phagosome containing the target enters the cytoplasm. 3 Phagosome fuses with lysosome, forming a phagolysosome. enzymes 4 Hydrolytic usually destroy contents of the phagolysosome. products are 5 Waste released. SEM A human neutrophil engulfiing two Staphylococcus aureus bacteria. FIGURE 4.5 Phagocytosis This multistep process 4 Lonhienne, T. G., et al. (2010). Endocytosis-like protein uptake in the bacterium Gemmata obscuriglobus. Proceedings of the National Academy of Sciences of the United States of America, 107 (29), 12883–12888. 5 Lindsay, M. R., et al. (2001). Cell compartmentalisation in planctomycetes: Novel types of structural organisation for the bacterial cell. Archives of Microbiology, 175 (6), 413–429. starts with engulfing the target and ends with expelling waste products. Critical Thinking How would this diagram look for a pathogen that manages to escape into the cytoplasm before the phagolysosome stage? Overview of Eukaryotes M04_NORM8290_01_SE_C04.indd 97 97 05/06/17 5:22 PM FIGURE 4.6 Receptor-mediated endocytosis Clathrin-mediated endocytosis, shown here, is the most common form of receptor-mediated endocytosis. The imported ligand and receptor can have diverse final destinations. This schematic shows the receptor being recycled and the ligand being broken down. Critical Thinking How would this image look different if the ligand and the receptor were both broken down? Hydrolytic enzymes use water to break chemical bonds. They are important for breaking down organic molecules in cells. CHEM • NOTE Polymerization is a process in which building block subunits are linked to make a larger molecule, or polymer. Membranebound cargo Secreted cargo 1 Vesicles form, often by budding from organelles like the Golgi apparatus. 2 Vesicle Vesicles move to the plasma membrane where they fuse. 3 Vesicle contents may stay in the cell membrane (certain proteins and lipids) or leave the cell (waste, toxins, neurotransmitters). FIGURE 4.7 Exocytosis Vesicles transport substances to the cell surface. This removes waste, replenishes cell membrane lost to endocytosis, and allows secretion of substances like cell-signaling factors. Critical Thinking If there was more endocytosis than exocytosis, what do you think would generally happen to the cell’s plasma membrane? 98 Receptor Clathrin 1 Clathrincoated vesicle Ligand binds to receptor on the cell surface, and clathrin coats the corresponding intracellular surface of the plasma membrane. 2 Clathrin forms a pit that leads to the membrane pinching off and creating a vesicle. 3 The vesicle sheds its clathrin coat and fuses with an acidic endosome. The pH change separates ligand and receptor. Acidic endosome the ligand is broken down 4 Here by lysosomal enzymes and the Lysosome CHEM • NOTE Golgi apparatus Ligand Ligand broken down Receptor recycled receptor is recycled to the cell surface. (The contents of the endocytotic vesicle can have diverse final destinations and may not always be broken down as shown here.) Receptor-mediated endocytosis A highly specific importation tool is receptormediated endocytosis. Here ligands, which are target substances such as hormones, nutrients, or pathogens, bind to specific cell-surface receptors. The hormone insulin enters cells by this process, as do a variety of viruses, including the one that causes polio. There are several forms of receptor-mediated endocytosis, but the most common form is clathrin-mediated endocytosis (FIG. 4.6). When a ligand binds to a receptor on the cell surface, the inner surface of the plasma membrane where the receptor–ligand complex is located becomes coated with a protein called clathrin. As the clathrin polymerizes, it causes a pit to form. Eventually this clathrin-coated pit, which contains the receptor–ligand complex, pinches off from the inner plasma membrane surface, forming a clathrin-coated vesicle. Soon after the vesicle enters the cell, it sheds its clathrin coat and fuses with an endosome, a small vesicle with an acidic interior. Fusing with the endosome alters pH so that the ligand and receptor separate from one another. Finally, the ligand and receptor are sorted and delivered to their respective final destinations (either somewhere within the cell or back to the cell surface). Alternatively, the sorting process could lead the endosome to fuse with a lysosome—forming an endolysosome—that breaks down the contents. This mirrors the process in phagocytes when a phagosome fuses with a lysosome to form a phagolysosome. Exocytosis The cellular exportation process called exocytosis involves vesicles delivering their contents to the plasma membrane. These exocytic vesicles are formed inside the cell––often by budding from cellular organelles like the Golgi apparatus (discussed more later). The cell transports the vesicles to the plasma membrane, with which they then fuse in order to expel their contents from the cell (FIG. 4.7). Exocytosis can rid the cell of unwanted waste products, as occurs in the last step of phagocytosis. The process can also be used to secrete specific substances, like signaling factors, into the cell’s environment. For example, our own neurons secrete neurotransmitters via exocytosis. Also, as exocytic vesicles fuse with the plasma membrane, they replace membrane that was removed by endocytosis. Highly phagocytic cells like macrophages are estimated to endocytose the equivalent of their entire plasma membrane in just half an hour. Such cells must constantly repair and restore their plasma membrane by exocytosis to avoid self-consumption of their plasma membrane. CHAPTER 4 • Introduction to Eukaryotic Cells M04_NORM8290_01_SE_C04.indd 98 05/06/17 5:22 PM TRAINING TOMORROW’S HEALTH TEAM Endocytosis and HIV Many viruses rely on endocytosis to invade host cells, which means blocking endocytosis is one possible approach to treating a variety of viral infections, including human immunodeficiency virus (HIV). Drugs that use this mode of Fuzeon blocks action are called fusion inhibitors. endocytosis to limit Normally, HIV invades cells by binding to specific HIV entry into host proteins on the plasma membrane of T cells, a type of cells. human immune system cell. When HIV binds to the T cell’s surface proteins, it triggers endocytosis, allowing the virus to enter the cell. An HIV fusion inhibitor drug called Enfuvirtide limits HIV endocytosis by blocking HIV’s binding to T cells. Since it doesn’t actually eradicate infection, it is not a cure for HIV. However, it lowers the amount of HIV present in the blood, which is key for preserving better health for the patient. QU E ST I ON 4 .1 While specifically blocking endocytosis is useful as an anti-HIV therapy, an overall generalized block on endocytosis would be dangerous for a cell. Based on what you read in previous sections of this chapter, why would a general block on all cellular endocytosis be hazardous to eukaryotic cells? BUILD YOUR FOUNDATION 1. 2. 3. 4. How did eukaryotic mitochondria and chloroplasts likely develop? Name at least three ways that eukaryotes differ from prokaryotes. List the ways that mitosis and meiosis differ and some ways they are similar. What are the two main classes of endocytosis and how are they different from one another? What is exocytosis and why is it important? QUICK QUIZ Build your foundation by answering the Quick Quiz: scan this code or visit the Mastering Microbiology Study Area to quiz yourself. CLASSIFICATION OF EUKARYOTES Eukaryotic organisms fall into four different kingdoms. Eukaryotic cells make up single-celled and multicellular organisms that fall into four different kingdoms (TABLE 4.2). We will review the general features of these different kingdoms. All of these kingdoms (except for plants) include potential pathogens or parasites. Animals Animals are multicellular organisms that do not carry out photosynthesis, so they must obtain their organic carbon from nutrients. It is estimated that there are over 7.5 million animal species on our planet, making this the largest of the eukaryotic kingdoms. In their mature form, animals are not typically microscopic, yet we cover them in microbiology because certain parasitic worms, or helminths, are usually spread in a microscopic form. The term helminth refers to a broad collection of organisms that spans roundworms and flatworms. In general, such organisms act as parasites, meaning they live in or on a host. They tend to have complex life cycles that can involve different host species. Most helminthic parasites in humans spend at least some part of their life cycle in the gastrointestinal tract. (See more discussion of these pathogens in Chapter 19.) After reading this section, you should be able to: 5 Name and describe the four kingdoms of eukaryotes. 6 Name and describe the two main groups of parasitic helminths. 7 Explain how hyphae relate to fungal growth. 8 Name and describe the five main classes of fungal spores. 9 Define the term mycosis and give examples of human mycoses. 10 Explain why Protista is sometimes described as a catchall kingdom. 11 Define the term protozoan, list the four main groupings of protozoans, and state how they are classified. Classification of Eukaryotes M04_NORM8290_01_SE_C04.indd 99 99 05/06/17 5:22 PM TABLE 4.2 Summary of Eukaryotic Organisms Kingdom Animalia Plantae Fungi Protista Examples Birds, helminths, reptiles, mammals, fish, amphibians, sponges, arthropods Plants Yeasts, molds, mushrooms Euglena, diatoms, amoebas, paramecia, algae, slime molds Organization Multicellular Multicellular Some unicellular (yeasts) but most are multicellular (nonyeast fungi) • Animal-like protists (protozoans): Unicellular • Plant-like protists: Unicellular or multicellular • Fungus-like protists: Unicellular or multicellular Reproduction Sexual and asexual Sexual and asexual Sexual and asexual Sexual and asexual Cell wall No Yes Yes Some (Examples: Slime molds at certain life stages, and algae) Chloroplasts (or plastids) No Yes No Some (Example: Algae) Mitochondria Yes Yes Yes Most (some have only remnants of mitochondria called mitosomes) Medical examples Parasitic worms; many arthropods like ticks and mosquitoes transmit infectious diseases No pathogens, but some may produce toxins dangerous to animals Candida albicans causes yeast infections; Pneumocystis jirovecii causes infections in the immune compromised Various Plasmodium species cause malaria; Entamoeba histolytica causes amoebic dysentery; certain algae make toxins (such as the ones that cause red tide blooms) Helminths represent a significant medical burden around the world. Improved water and food sanitation practices in developed countries reduce helminthic infections, but they are by no means eliminated. The World Health Organization (WHO) estimates that at least half of the world’s population is infected with some sort of helminth at any given time. The general characteristics of the main groups of medically important helminths are presented in TABLE 4.3. Parasitic worms are not the only members of the animal kingdom explored in microbiology. A number of arthropod species, especially ticks and mosquitos, are medically important due to their capacity to serve as disease vectors. (See Chapter 9 for more on vectors.) TABLE 4.3 Overview of Parasitic Helminths Phylum Roundworms Flatworms Subtypes Nematodes Tapeworms (cestodes) Flukes (trematodes) Hookworm Tapeworm Liver fluke Structure Non-segmented; elongated, cylindrical Segmented, flat, ribbon-like Non-segmented; flattened leaf shaped Size range Microscopic- 1 meter 1 millimeter-10 meters 1 millimeter-7 centimeters Reproduction Sexual reproduction; two sexes Sexual reproduction; hermaphroditic (male and female reproductive organs in same individual) Sexual reproduction; except for blood flukes, all are hermaphroditic Examples in humans Hookworm, pinworm, Ascaris, filarial worms, Trichinella, Strongyloides, whip worm Six tapeworms infect humans (Examples: Diphyllobothrium latum from fish, Taenia saginata from beef) Blood flukes: Schistosoma species; Lung fluke: Paragonimus westermani, Liver fluke: Fasciola hepatica; Intestinal flukes: Fasciolopsis buski and Heterophyes heterophyes Transmission mechanisms Fecal/oral (eat eggs in contaminated food, water, or soil) or eat undercooked meat; some species’ larvae burrow into skin, migrate to lungs, get coughed up and swallowed to arrive at the targeted intestines Fecal/oral through contaminated food or water; eating undercooked meat or fish from an infected animal Embryonated eggs from host feces enter water and hatch; released larvae mature in snails and then are either ingested in contaminated food/water or burrow into human host 100 CHAPTER 4 • Introduction to Eukaryotic Cells M04_NORM8290_01_SE_C04.indd 100 05/06/17 5:22 PM Plants Septate hyphae Plants are multicellular organisms that carry out photosynthesis to make their own organic carbon using light energy. There are over 290,000 different plant species, none of which cause infectious disease—although vegetation can serve as a vehicle for infectious pathogens, such as improperly cleaned fruits and vegetables that retain microbes on their surfaces. Plant cells contain chloroplasts, which are organelles that a variety of photosynthetic eukaryotic organisms rely on to perform photosynthesis. Chloroplasts will be explored later in this chapter. (For more on photosynthesis, also refer to the online appendix for Chapter 8.) Nuclei Nuclei Cell wall Cell wall Septum Fungi Septum The Kingdom Fungi is believed to include over 600,000 different species, although most have not yet been characterized. Most fungi (singular: fungus) are multicellular or colonial; only yeasts are unicellular. Fungi do not carry out photosynthesis and instead rely on extracting carbon from the nutrients they absorb from their environment. Aside from pathogenic fungi, most fungi are saprobes, meaning they absorb nutrients from dead plants and animals in the environment. Cell wall FIGURE 4.8 Fungal hyphae Fungal hyphae can be described as septate or aseptate. Hyphae Most fungi (aside from yeasts) grow as a collection of tubular structures called hyphae (singular: hypha). Fungal hyphae can be described as septate or aseptate (or coenocytic). Septate hyphae include divisions between each cell in the filament, and apConidiospores pear as a string of individual cells. Aseptate hyphae do not have divisions and appear as a long continuous chain with many nuclei (FIG. 4.8). Some fungi cycle between having hyphae and living as a yeast-like form and are called dimorphic fungi (having two forms). Many pathogenic fungi are dimorphic, exhibiting a yeast-like growth in humans and a hyphae growth form in the environment. (See Chapter 16 for more on fungal respiratory infections and dimorphism.) Fungal Spores Fungi have a variety of reproductive strategies, but the most prevalent approach to reproduction involves the production of spores. Fungal spores are not like spores made by bacteria. They are used for reproduction, whereas bacterial spores are used to survive harsh conditions. A variety of features such as morphology, coloration, genetics, physiological features, and the type of reproductive spores made are all used to classify fungi. Fungal spores are classified as either asexual or sexual. Fungal spores are summarized in TABLE 4.4 and examples are shown in FIG. 4.9. Asexual spores arise from mitosis, and do not result in genetic variation. Two classes of asexual fungal spores are conidiospores and sporangiospores. By contrast, sexual spores arise from the union of complementary mating strains of fungi generated by meiosis and do result in genetic variation. Three types of sexual fungal spores exist: zygospores, ascospores, and basidiospores. The type of sexual spore made is used to taxonomically group fungi into a given phylum. Fungal Diseases Diseases caused by fungi are called mycoses. Of the fungi described to date, the vast majority are not pathogens. Most mycoses are seen in individuals with a weakened immune system, such as Pneumocystis pneumonia that occurs in AIDS patients. In other cases, people who develop fungal Aseptate hyphae Conidiospores from Penecillium Sporangiospores Sporangiospores from Absidia Ascus Zygospore Ascospores Zygospores from Rhizopus Ascospores from cup fungus Basidiospores Basidium Basidiospores from mushrooms FIGURE 4.9 Examples of fungal spores Classification of Eukaryotes M04_NORM8290_01_SE_C04.indd 101 101 05/06/17 5:22 PM TABLE 4.4 Fungal Spores Type test up to here Asexual Fungal Spores Sexual Fungal Spores Name Conidiospores Sporangiospores Zygospores Ascospores Basidiospores Form Chains of spores; not enclosed in a sac Spores formed within a sac called a sporangium Haploid gametes found at the tips of hyphae Haploid gametes form within a sac called an ascus Bud off of a pedestal structure called the basidium Examples Penicillium (source of penicillin) and Aspergillus species Absidia species (the cause of mucormycosis in humans) Phylum Zygomycota; includes black bread molds (Rhizopus species) Phylum Ascomycota; includes truffles, morels, many yeasts, and cup fungi Phylum Basidiomycota; includes mushrooms Tinea unguium, or dermatophytic onychomycosis, causes nails to become brittle and discolored. Some risk factors for this fungal infection include an infection with athlete’s foot (tinea pedis), nail damage (as can occur when applying acrylic nails), and immunosuppression. Tinea pedis, or athlete’s foot, is a common infection that often starts between the toes and causes itching and burning as well as scaling skin. This colored SEM shows fungal spores (yellow) growing on skin of a human foot (pink). FIGURE 4.10 Tinea unguium and tinea pedis infections first experienced a disruption of their normal microbiota through interventions like antibiotic therapies; this is the case for many vaginal yeast infections caused by Candida species. However, some fungi are true pathogens that infect even a typically healthy host without a disruption in the normal microbiota. Such infections include histoplasmosis and coccidioidomycosis (cock-SID-ee-oh-doh-my-co-sis), also called valley fever. These infections are discussed later, in the respiratory diseases chapter. Fungi called dermatophytes are also true pathogens; they infect the skin, hair, and nails and break down the protein keratin in these structures (FIG. 4.10). Common dermatophytic infections are tinea, or ringworm infections. Although the term ringworm suggests that a worm is responsible, these infections are actually caused by fungi. Tinea infections tend to be named for the body region they affect. For example, tinea pedis, or athlete’s foot, develops when fungi such as Trichophyton species infect the feet. Even if a fungus doesn’t cause an infection, it could cause other clinical issues by stimulating allergies or by producing potentially deadly toxins called mycotoxins. The fungus Claviceps purpurea produces ergot toxin, a potent neurotoxin that can lead to seizures, psychosis, nausea, vomiting, and even death. While we will not review agriculturally important fungi in this text, you should know that they represent a major challenge in farming, as they can readily infect and destroy crops. Lastly, a number of fungi are medically important not for causing human suffering, but for curing it. Thanks to their production of diverse antibiotics, a number of fungi save lives. Perhaps as more fungal species are discovered we will naturally expand our armory of antimicrobial compounds. The roles of fungi in making antibiotics are discussed more in Chapter 15. Protists Kelp are considered protists. These photosynthetic organisms can grow enormously tall and form kelp forests that are central to marine ecosystems. This harmless “dog vomit slime mold,” named for its texture and coloration, can often be spotted growing on mulch in gardens. It typically starts off yellow and eventually becomes a brownish-red before it hardens to a dry mass filled with spores. FIGURE 4.11 Algae and slime molds 102 The miscellaneous nature of protists makes it difficult for scientists to agree on standard features that should be used to group them.6 While most protists are unicellular, some, like algae are multicellular, while others such as slime molds form multinucleated cell masses when stressed (FIG. 4.11). The earliest eukaryotes probably resembled some of our modern-day protists, making these organisms an interesting evolutionary link to plants, fungi, and animals. As evidence of this evolutionary link, some protists such as algae have plant-like features, including the ability to conduct photosynthesis and the presence of a cell wall. Some, like slime molds, are fungi-like saprobes that have cell walls and don’t carry out photosynthesis. Others are called protozoans, which means “first animal.” The term protozoan is not an official taxonomic rank; rather it is a term of convenience to describe animal-like protists that are unicellular, lack a true 6 In the past, protists were all placed in Kingdom Protista, but genetic analysis has since shown that this kingdom is actually a collection of organisms that are not all related. However, rather than review the complicated modern protist taxonomy here, in this text we opt to continue grouping these organisms together in the traditional manner. It’s hoped that this approach will help you to appreciate the amazing diversity of these organisms without feeling overwhelmed by the taxonomic details that evolutionary biologists debate. CHAPTER 4 • Introduction to Eukaryotic Cells M04_NORM8290_01_SE_C04.indd 102 05/06/17 5:22 PM Amoeboid Flagellated Ciliated Spore forming Cilia Pseudopods Entamoeba histolytica Flagella Giardia lamblia Tetrahymena thermophila Plasmodium sp. (sporozoite stage) FIGURE 4.12 Protozoan groups These animal-like protists are grouped by their means of motility. cell wall, exhibit asexual and sexual reproduction, and typically live by heterotrophic means (which means most don’t conduct photosynthesis, although some like Euglena do). Protozoans that cause disease are always nonphotosynthetic. People in developed countries often think of protozoan pathogens as only infecting those who live in or visit developing countries. This is a dangerous misconception that leads to misdiagnosis and underdiagnosis of protozoan infections in the developed world. In reality, the U.S. Centers for Disease Control and Prevention (CDC) estimates that as of 2013, at least 60 million people are chronically infected with the protozoan Toxoplasma gondii and about 3.7 million are infected with the sexually transmitted protozoan Trichomonas vaginalis—and that’s just in the United States alone. Likewise, the CDC estimates that every year roughly 2 million giardiasis cases occur in the United States. Protozoans include a number of pathogens, making them noteworthy in human and veterinary medicine. In general, they fall into four key groups based on their means of motility in their mature form. These groups are amoeboid, flagellated, ciliated, and spore forming (FIG. 4.12). Amoeboid Protozoans (Sarcodina) Amoeboid (uh-MEE-boid) protozoans use “false feet”—extensions of their cytoplasm called pseudopods (SUE-doh-pods)— for movement. Many amoeboid protozoans are free living and don’t need a host—for example, Amoeba proteus. However, others are parasitic. A number of amoebas such as Naegleria fowleri, Acanthamoeba, Hartmannella, and Balamuthia are human pathogens that cause rare forms of encephalitis, a potentially fatal inflammation of the brain. Acanthamoeba also causes corneal keratitis, a painful inflammation of the transparent front portion of the eye called the cornea, which can lead to blindness. Entamoeba histolytica (EN-tuh-mee-buh HIS-toh-lit-ik-ah) is the most common amoeboid infection in humans. It infects people when they ingest food or water contaminated with the cyst form of the parasite, a form in which the parasite is encased in a tough protective layer. The cysts are excreted in feces of infected humans and other primates. About 90 percent of infections are asymptomatic; the other 10 percent of infections lead to amoebic dysentery (amoebiasis)––a form of severe diarrhea that includes the passing of mucus and blood. The World Health Organization estimates that worldwide, there are about 50 million cases of amoebiasis every year, with about 70,000 deaths due to this parasite. E. histolytica infections can be treated, but it is challenging to destroy the cyst forms of the amoeba. Complications of amoebiasis can include liver abscess, intestinal perforation (tearing), and colitis. Amoebiasis is most common in Africa, Latin America, southeast Asia, and India. Classification of Eukaryotes M04_NORM8290_01_SE_C04.indd 103 103 05/06/17 5:22 PM TRAINING TOMORROW’S HEALTH TEAM Amoeboid Infection and Neti Pots Normally free-living protists, Naegleria fowleri exist in moist soil and warm waters like hot springs, lakes, or rivers. In humans, N. fowleri causes primary amoebic meningoencephalitis, an infection with a 97 percent mortality rate. This protozoan generated a large public scare in 2012, when it was associated with neti pots, a treatment many allergy and cold sufferers use to alleviate nasal congestion. You cannot get primary amoebic meningoencephalitis by drinking contaminated water, because stomach acid kills the microbe. Disease only develops if the amoeboid enters through the nose and then migrates to the brain—hence its nickname, the “brain-eating amoeba.” Headache, fever, nausea, or vomiting begin within a week of infection and closely resemble bacterial meningitis. The patient may then develop neck stiffness, compromised balance, hallucinations, cognitive impairment, and seizures. Death usually occurs within 12 days of initial symptoms. 1 Q UE STIO N 4. 2 Based on your readings, if you were to look at N. fowleri under the microscope, what key features would you expect to see? Flagellated Protozoans (Mastigophora) Flagellated protozoans have one or more flagella for motility (more on eukaryotic flagella later). These protozoans include free-living organisms like Euglena and parasitic organisms such as Trichomonas vaginalis, Trypanosoma species, and Giardia lamblia––which are all human pathogens. Trichomonas vaginalis is a sexually transmitted protozoan commonly responsible for infections in industrialized and developing nations alike. African sleeping sickness, which is caused by Trypanosoma species, is a dangerous protozoan disease transmitted by tsetse flies. Giardia lamblia (also referred to as Giardia intestinalis or Giardia duodenalis), has five flagella that propel it through water and help it adhere to the intestinal epithelium of animals. The cyst stage of Giardia has a protective outer coat that can resist chemicals like chlorine, which are commonly used to treat recreational and drinking water. Outbreaks often occur when water sanitation processes are comMerogony Asexual cell divisions promised, as is common following hurricanes, earthquakes, and other produce multiple natural disasters. (See Chapter 18 for more on African sleeping sickness, merozoites. Chapter 19 for giardiasis, and Chapter 20 for trichomoniasis.) Sporozoites infect host cell Ciliated Protozoans (Ciliophora) Ciliated (SIL-ee-ay-tid) protozoans use hair-like appendages called cilia for motility (more on cilia later). Ciliates are common in aquatic environments and include diverse species from free-living Paramecium species to parasites like Balantidium coli––the only ciliated protozoan known to cause human disease. Balantidiasis is a rare form of dysentery that can develop when the host ingests Merozoites the cyst form of Balantidium coli in contaminated food or water. The protozoan can then grow in the colon and cause persistent diarrhea and abdominal pain, and in severe cases can lead to Gamogony perforation (tearing) of the colon. Host cell 3 Sporogony Repeated cell divisions of the zygote make sporozoites. Zygote Gametes fuse 2 FIGURE 4.13 Generalized life cycle of protozoans In general, apicomplexans go through three life stages: merogony, gamogony, and sporogony. Critical Thinking At what stage would you expect the organism to invade a new host? 104 From 2004 through 2013, there were only 34 infections reported in the United States, and most were traced to swimming. Just three cases resulted from nasal irrigation with contaminated tap water in neti pots. But given the high mortality rate, health officials urge Improper use of neti pots has been neti pot users to take precautions. Standard associated with fatal tap water should never be placed in a neti N. fowleri infections. pot; sterile or distilled solutions purchased at pharmacies are recommended. Homemade saline solution is also safe if the tap water is boiled for three minutes and cooled before use. Meiosis produces gametes from merozoites. Spore-Forming Protozoans (Apicomplexa) The phylum Apicomplexa (formerly called Sporozoa) is one of the largest phyla of protozoans. In their mature form, they have no flagella, cilia, or pseudopodia, and instead move by gliding. Most apicomplexans are obligate intracellular parasites with complex life cycles that include sexual and asexual stages. In general, the Apicomplexans go through three phases: merogony, gamogony, and sporogony (FIG. 4.13). Merogony is an asexual stage of CHAPTER 4 • Introduction to Eukaryotic Cells M04_NORM8290_01_SE_C04.indd 104 05/06/17 5:22 PM reproduction, in which daughter cells called merozoites are made by repeated asexual cell division. This occurs by a modified form of mitosis in which the nucleus divides multiple times and is later followed by division of the remaining cell components into discrete cells. Gamogony is the sexual phase of reproduction in which the merozoites from the prior stage produce male and female haploid gametes by meiosis. In sporogony, the zygote made by gamete fusion divides to make sporozoites. The produced sporozoites are typically the stage that invades a new host to repeat the reproductive cycle. Apicomplexans are notorious human pathogens, causing diseases such as toxoplasmosis (often spread by cats), malaria, and gastrointestinal illnesses such as cryptosporidiosis and microsporidiosis. Some of the members of this phylum require arthropod vectors like mosquitoes or ticks for one part of their life cycle. For example, Plasmodium species, which cause malaria, have their sexual phase of reproduction in mosquitoes. In contrast, their asexual phase of reproduction occurs in humans. (Protozoan pathogens are discussed more in the disease chapters of this text––Chapters 16 through 21.) BUILD YOUR FOUNDATION 5. What eukaryotic kingdom(s) include unicellular organisms? Which include multicellular organisms? Which have photosynthetic organisms? 6. What are the two main groupings of helminths? 7. What are the tubular extensions that make up most fungi called? 8. Name the types of sexual and asexual fungal spores and state how they are made. 9. Define the term mycosis and give an example from this chapter. 10. Why is it challenging to develop specific criteria for describing protists? 11. What are animal-like protists called and how are they primarily classified? QUICK QUIZ Build your foundation by answering the Quick Quiz: scan this code or visit the Mastering Microbiology Study Area to quiz yourself. EXTRACELLULAR STRUCTURES All eukaryotes have a plasma membrane. All cells have a plasma membrane, which serves as a selective barrier and interface through which the cell interacts with its outside environment. Typically, cells have a plasma membrane with a phospholipid bilayer structure that’s crowded with a variety of receptors and channels for normal cell functions, including cellular communication and transport processes. (See Chapter 3 to review phospholipid bilayers.) While it’s rare for prokaryotes to contain sterols in their membranes, eukaryotic membranes contain many sterols, which have central roles in maintaining membrane stability and fluidity. Cholesterol is especially abundant in animal cell membranes, while plant cell membranes contain a wide variety of phytosterols as well as low levels of cholesterol. Fungi and protists have a wide range of sterols in their membranes, with ergosterol being a key example. Certain azole drugs and other antifungals such as amphotericin B and nystatin target the ergosterol in plasma membranes of a wide range of fungal and protozoan pathogens. Since animal cells lack ergosterol, these drugs do not target human cells. Certain eukaryotes have a cell wall. When a cell wall is present, it’s external to the plasma membrane. This structure helps maintain cell shape, and protects the cell against mechanical and osmotic stress. Fungi, plants, and certain protists have cell walls, but animal After reading this section, you should be able to: 12 Discuss the basic structural and functional features of eukaryotic plasma membranes and how they differ among kingdoms. 13 Give example of eukaryotes with cell walls and discuss how cell walls differ among eukaryotic kingdoms. 14 Describe the eukaryotic glycocalyx and list some of its roles. 15 Discuss the basic structure of eukaryotic flagella and compare eukaryotic and prokaryotic flagella. 16 Describe the structure and function of cilia. CHEM • NOTE Phospholipids are amphipathic molecules with a hydrophilic (water-loving) phosphate head group and hydrophobic (water-hating) fatty acid chains. See Chapter 2 for more on phospholipids. Extracellular Structures M04_NORM8290_01_SE_C04.indd 105 105 05/06/17 5:22 PM cells don’t. Despite the chemical diversity seen among eukaryotic cell walls, none of them contain peptidoglycan or pseudopeptidoglycan, making them chemically distinguishable from prokaryotic cell walls. In general, chitin (KITEin) is a core compound in many fungal cell walls, while cellulose is abundant in plant cell walls. Protists have diverse cell walls that can include cellulose, calcium carbonate, xylan, silica, and a variety of other protein- and carbohydratebased compounds. CHEM • NOTE Sterols, or steroid alcohols, are organic ringstructured compounds; the most common example is cholesterol. These compounds are hydrophobic, which means they do not like interacting with water. See Chapter 2 for more on sterols. CHEM • NOTE Osmotic stress may occur when a cell tries to maintain its water balance by counteracting the movement of water down its concentration gradient. Such a situation would occur when the solute concentration in the cell and its environment are unequal. Cells in hypotonic environments will burst as they take on water unless they have an intact cell wall. Hypertonicity and hypotonicity are also reviewed in Chapters 2 and 3. FIGURE 4.14 Eukaryotic glycocalyx The eukaryotic glycocalyx serves roles in cell protection, adhesion, and assisting with cellular communication. Critical Thinking You read that pathogens often rely on components of their glycocalyx to adhere to target cells, but how might a host cell’s glycocalyx impact what pathogens can invade it? Glycocalyx Many eukaryotes have structures for protection, adhesion, and movement. Structures external to the plasma membrane and cell wall can have roles in cellular protection, adhesion, and motility. While these eukaryotic cellular tools are not too different in general function to those in prokaryotes, there are some structural differences to consider. Eukaryotic Glycocalyx Like many prokaryotes, most eukaryotic cells have a sticky extracellular layer called a glycocalyx (which means “sugar husk”) as their outermost layer. Researchers have only discovered the very basic roles of the glycocalyx in cells. The glycocalyx contains a diverse collection of carbohydrates, glycoproteins, and glycolipids. Each of these sugary components can have a different role in protecting the cell from a variety of stresses, promoting or preventing cell adhesion where appropriate, and assisting with cellular communication (FIG. 4.14). In multicellular organisms, the eukaryotic glycocalyx is also central to proper tissue development. In contrast, prokaryotic cells do not form higher order tissues, so while their glycocalyx is important to their physiology and their ability to make biofilms, their glycocalyx has a slightly less sophisticated role. The thickness and composition of the glycocalyx in many cancer cells is different from normal cells. For example, certain breast cancer cells that have spread from the initial tumor site (metastatic cancer) have an abnormally thickened glycocalyx.7 The glycocalyx of pathogenic fungi and protozoans is Outside of cell Plasma membrane TEM Glycolipid Various glycoproteins Carbohydrates (sugars) Cytoplasm Glycocalyx Nucleus Plasma membrane Inside of cell 7 Paszek, M. J., et al. (2014). The cancer glycocalyx mechanically primes integrin-mediated growth and survival. Nature, 511 (7509), 319–325. 106 CHAPTER 4 • Introduction to Eukaryotic Cells M04_NORM8290_01_SE_C04.indd 106 05/06/17 5:22 PM also known to impact infectivity and immune responses, making this structure important to consider in drug and vaccine development. Studies done with the protozoan pathogen Leishmania major, which cause skin lesions, showed that mutations to one of the organism’s glycocalyx components decreased the pathogen’s ability to invade host cells and also made it more susceptible to immune system attack.8 One of nine sets of fused microtubule pairs Central microtubules Flagellum Flagella Flagella are long, tail-like structures used for motility. Eukaryotes, like prokaryotes, may have one (called a flagellum) or multiple flagella. Many gametes used for reproduction and a variety of protists are flagellated. The structure of eukaryotic flagella is different from that seen in prokaryotes (TABLE 4.5). Not only is the eukaryotic flagellum thicker and typically longer than seen in prokaryotes, but also it is wrapped in a membrane derived from the cell’s plasma membrane. Therefore, the inner part of the flagellum isn’t a sealed-off compartment––instead, it is continuous with the rest of the cell’s cytoplasm. Eukaryotic flagella are made of the protein tubulin and have what is called a nine-plus-two arrangement (9 + 2), where nine fused pairs of microtubules form the border of the flagellum and two nonfused microtubules are at the center. The eukaryotic flagellum is anchored to the cell by a basal body consisting of a cylindrical collection of microtubules sprouting from a centriole (more on the centriole is found in the next chapter section on intracellular structures) (FIG. 4.15). As you may recall from Chapter 3, prokaryotic flagella have a rotary, or propeller-like, motion; in contrast, eukaryotic flagella have a wavelike, back-and-forth motion (FIG. 4.16 top). Plasma membrane Microtubules sprouting from a centriole FIGURE 4.15 Eukaryotic flagella Eukaryotic flagella have a nine-plus-two arrangement of microtubules. Wave-like motion of eukaryotic flagella Cilia Cilia, the Latin word for “eyelashes,” are structurally similar to flagella except that they are much shorter and far more numerous on a cell. Only eukaryotes have cilia. The synchronized rowing motion they provide helps Paramecium species and other ciliophora protists move in their aquatic environments (FIG. 4.16 bottom). Cilia are also found on certain animal cells. One example is the epithelial cells of our upper airway, which sweep mucus and pathogens up and away from the lungs. (For more on the mucocilliary escalator, see Chapter 11.) Like flagella, cilia are surrounded by a membrane and sprout from centrioles associated with the plasma membrane. TABLE 4.5 Prokaryotic versus Eukaryotic Flagella Flagella Type Prokaryotic Eukaryotic Built from Flagellin protein Tubulin protein Microtubules No Yes; 9 + 2 arrangement Membrane enclosed No, except for periplasmic flagella Yes Anchor Hook-and-filament structure anchored by rings Microtubules sprout from a centriole Motion Rotary (propeller) Wavelike (whips back and forth) Direction of cell’s movement Oar-like motion provided by cilia Power stroke Recovery stroke Direction of cell’s movement 8 Späth, G. F., Garraway, L. A., Turco, S. J., & Beverley, S. M. (2003). The role(s) of lipophosphoglycan (LPG) in the establishment of Leishmania major infections in mammalian hosts. Proceedings of the National Academy of Science, 100 (16), 9536–9541. FIGURE 4.16 Flagella and cilia motion Flagella and cilia differ in how they propel an organism. Extracellular Structures M04_NORM8290_01_SE_C04.indd 107 107 05/06/17 5:22 PM BUILD YOUR FOUNDATION Build your foundation by answering the Quick Quiz: scan this code or visit the Mastering Microbiology Study Area to quiz yourself. QUICK QUIZ 12. How are fungal plasma membranes distinguishable from other eukaryotic plasma membranes? 13. List some eukaryotes that have cell walls. 14. What is the eukaryotic glycocalyx and what does it do? 15. Name three differences between prokaryotic and eukaryotic flagella. 16. What are cilia and where can they be found? INTRACELLULAR STRUCTURES After reading this section, you should be able to: 17 Describe the structure of eukaryotic ribosomes and state where they are located. 18 Explain the structure and function of the cytoskeleton and name the organelle that builds microtubules. 19 Describe the general structure and function of the nucleus. 20 Discuss the basic structural and functional features of the endoplasmic reticulum. 21 Describe the general structural and functional features of the Golgi apparatus. 22 Outline the types of vesicles and vacuoles that exist in eukaryotic cells and state their general functions. 23 Review the structure and function of mitochondria and state what features make them similar to bacteria. 60S 60S Large subunit + 40S 40S Complete 80S ribosome Small subunit FIGURE 4.17 Eukaryotic ribosomes Eukaryotes have 80S ribosomes in their cytoplasm and attached to the rough endoplasmic reticulum. Critical Thinking Why don’t antibacterial drugs necessarily target eukaryotic ribosomes? 108 Ribosomes can be free or membrane associated. As discussed in Chapter 3, both prokaryotic and eukaryotic cells have ribosomes, essential organelles for making proteins. Eukaryotic ribosomes, like prokaryotic ribosomes, are made up of protein and ribosomal RNA (rRNA). Eukaryotic cells have 80S ribosomes that are made up of a small subunit (the 40S subunit) and a large subunit (the 60S subunit) (FIG. 4.17). (Refer back to Chapter 3 for a review of Svedberg units, which are used to denote the sedimentation rates of the subunits.) Eukaryotic ribosomes can be found free in the cytoplasm, or bound to the membrane of an organelle called the endoplasmic reticulum (ER). The main difference between bound and free ribosomes is not in the ribosomes themselves, but in what type of proteins they make. Ribosomes that are bound to the ER tend to produce proteins that are destined for secretion from the cell. That said, ribosomes that are free in the cytoplasm can become bound to the ER if they are making protein bound for secretion. Similarly, bound ribosomes can become free ribosomes as needed. This means that ribosomal populations are not static, and can change from free to bound, based on the protein production demands of the cell. Lastly, in mitochondria and chloroplasts, eukaryotes have ribosomes that resemble 70S ribosomes found in prokaryotes. These mitochondrial and chloroplastic ribosomes are separate from the membrane-bound and free ribosomes previously discussed. Many toxins target eukaryotic ribosomes. For example, ricin, a deadly toxin made by castor beans, binds to the 60S subunit of eukaryotic ribosomes and blocks protein production. Similarly, toxins made by certain Shigella and E. coli species that cause severe dysentery also inactivate the 60S subunit of the eukaryotic ribosome and disrupt protein synthesis in affected cells. The cytoskeleton shapes cells and coordinates cell cargo movement. The cytoskeleton is a dynamic and responsive intracellular network of protein fibers that helps maintain shape, facilitates movement, protects against external forces that may otherwise deform the cell, and directs transport of vesicles, organelles, and other cellular cargo. It also coordinates cell division by moving chromosomes and organelles to developing daughter cells. A number of pathogens interact with the eukaryotic cytoskeleton as a part of their toolbox for causing disease. For example, the bacterium Salmonella enterica, which causes a severe form of gastroenteritis, releases substances that affect the host cell’s cytoskeleton in a way that helps the pathogen invade gut epithelial cells. Listeria monocytogenes bacteri