MicroBiology with disearses by taxonomy 5e by bauman 1

» Invest in your future: Microbiology Matters » Engage with compelling healthcare applications NEW! Micro Matters Video Cases animate and connect concepts across chapters and emphasize the clinical importance of foundational material Micro Matters videos are accessible via QR codes in select chapters and are also assignable in MasteringMicrobiology CHAPTER 13 M I C R o M A t t erS Make the connection between chapters 11, 12, and 13 Scan this Qr code to see how Micro Matters » Characterizing and Classifying Viruses, Viroids, and Prions 401 visit the masteringmicrobiology study area to challenge your understanding with practice tests, animation quizzes, and clinical case studies! chapter SummARy Viruses, viroids, and prions are acellular disease-causing agents that lack cell structure and cannot metabolize, grow, self-reproduce, or respond to their environment Characteristics of Viruses (pp 378–383) A virus is a tiny infectious agent with nucleic acid surrounded by proteinaceous capsomeres that form a coat called a capsid A virus exists in an extracellular state and an intracellular state A virion is a complete viral particle, including a nucleic acid and a capsid, outside a cell The genomes of viruses include either DNA or RNA Viral genomes may be dsDNA, ssDNA, dsRNA, or ssRNA They may exist as linear or circular and singular or multiple molecules of nucleic acid, depending on the type of virus Viruses are specific for their hosts’ cells because viral attachment molecules are complementary in shape to specific receptor molecules on the host’s cells All types of organisms can be infected by viruses A bacteriophage (or phage) is a virus that infects a bacterial cell Virions can have a membranous envelope or be naked—that is, have no envelope Classification of Viruses (pp 383–385) Viruses are classified based on type of nucleic acid, presence of an envelope, shape, and size The International Committee on Taxonomy of Viruses (ICTV) has recognized viral family and genus names With the exception of three orders, higher taxa are not established Replication NEW!Viral Interactive Microbiology (pp 385–393) Viruses depend on random contact with a specific host cell type for replication a virus in a cell proceeds with a lytic is a dynamic suite Typically, of interactive replication cycle with five stages: attachment, entry, synthesis, assembly, and release tutorials and animations that teach ANIMATIONS: Viral Replication: Overview key concepts in microbiology Once attachment has been made between virion and host cell, the nucleic acid enters the cell Withand phages, only the nucleic acid including Operons; Biofilms enters the host cell With animal viruses, the entire virion often Quorum Sensing; Complement; enters the cell, where the capsid is then removed in a process called uncoating Antibiotic ANIMATIONS: Resistance, Mechanisms Viral Replication: Animal Viruses and Selection; Aerobic Respiration in Within the host cell, the viral nucleic acid directs synthesis of more viruses using metabolic enzymes and ribosomes of the host cell Prokaryotes, and more Each tutorial presents the concept within a real healthcare scenario and allows you to learn from manipulating variables, predicting outcomes, and answering assessment questions M13_BAUM9192_05_SE_C13.indd 401 Assembly of synthesized virions occurs in the host cell, typically as capsomeres surround replicated or transcribed nucleic acids to form new virions Virions are released from the host cell either by lysis of the host cell (seen with phages and animal viruses) or by the extrusion of enveloped virions through the host’s cytoplasmic membrane (called budding), a process seen only with certain animal viruses If budding continues over time, the infection is persistent An envelope is derived from a cell membrane ViDEO TUToR ANIMATIONS: Viral Replication: Virulent Bacteriophages Temperate phages (lysogenic phages) enter a bacterial cell and remain inactive in a process called lysogeny or a lysogenic replication cycle Such inactive phages are called prophages and are inserted into the chromosome of the cell and passed to its daughter cells Lysogenic conversion results when phages carry genes that alter the phenotype of a bacterium At some point in the generations that follow, a prophage may be excised from the chromosome in a process known as induction At that point the prophage again becomes a lytic virus ANIMATIONS: Viral Replication: Temperate Bacteriophages With the exception of hepatitis B virus, dsDNA viruses use their DNA like cellular DNA in transcription and replication Some ssRNA viruses have positive-sense single-stranded RNA (+ssRNA), which can be directly translated by ribosomes to synthesize protein From the positive-strand RNA (+ssRNA), complementary negative-sense single-stranded RNA (−ssRNA) is transcribed to serve as a template for more +ssRNA Retroviruses, such as HIV, are +ssRNA viruses that carry reverse transcriptase, which transcribes DNA from RNA This reverse process (DNA transcribed from RNA) is reflected in the name retrovirus 10 −ssRNA viruses carry an RNA-dependent RNA transcriptase for transcribing mRNA from the −ssRNA genome, and the mRNA is translated to protein Transcription of RNA from RNA is not found in cells 11 In dsRNA viruses, the positive strand of RNA functions as mRNA, and each strand functions as a template for an RNA complement 12 In latency, a process similar to lysogeny, an animal virus remains inactive in a cell, possibly for years, as part of a chromosome or in the cytosol A latent virus is also known as a provirus A provirus that has become incorporated into a host’s chromosome remains there 10/8/15 3:40 PM NEW! Disease in Depth One- or two-page spreads feature important and representative diseases » These highly visual spreads contain illustrations, micrographs, and infographics, providing in-depth overviews of selected diseases for comprehensive study and review DISEASE IN DEPTH PATHOGEN SIGNS AND SYMPTOMS About week after infection, patients experience fever, headache, chills, muscle pain, nausea, and vomiting In most cases (90%), a spotted, non-itchy rash develops on the trunk and appendages, including palms and soles, sites not involved in rashes caused by the chickenpox or measles viruses In about 50% of patients, the rash develops into subcutaneous hemorrhages called petechiae In severe cases, the respiratory, central nervous, gastrointestinal, and renal systems fail Even with treatment, almost 5% of patients die ROCKY MOUNTAIN SPOTTED FEVER Rickettsia Rickettsias R PATHOGENESIS Rickettsias require a vector for transmission between hosts For R rickettsii, this vector is a hard tick of the genus Dermacentor Male ticks infect female ticks during mating Female ticks transmit bacteria to eggs forming in their ovaries—a process called transovarian transmission Dermacentor can survive without feeding for for more than four years, making tick elimination in the wild problematic Rickettsias cannot use glucose as a nutrient; instead, they oxidize amino acids and Krebs cycle intermediates, such as glutamic acid and succinic acid For this reason, rickettsias are obliged to live inside other cells, where these nutrients are provided LM ickettsia rickettsii causes Rocky Mountain spotted fever (RMSF), the most severe and most reported spotted fever rickettsiosis Hard ticks in the genus Dermacentor transmit R rickettsii among humans and rodents R rickettsii is typically dormant in the salivary glands of its tick vectors; only when the arachnids feed for several hours is the bacterium infective VECTOR Rickettsia rickettsii is a small (0.3–1 µm), nonmotile, aerobic, Gram-negative, intracellular parasite that has a cell wall of peptidoglycan and an outer membrane of lipopolysaccharide surrounded by an organized slime layer Rickettsias not Gram stain well, so scientists use Gimenez-stained yolk sac smear (shown here) 20 μm INSIDE BLOOD VESSEL INSIDE BLOOD VESSEL R rickettsii secretes no toxins, and disease is not the product of immune response Apparently, damage to the endothelial cells leads to leakage of blood into the tissues, which results in low blood pressure and insufficient nutrient and oxygen delivery to the body’s organs R rickettsi escaping into cytosol Endothelial cells lining small blood vessel Blood leaks into tissue Endosome Engorged Dermacentor R rickettsi triggers endocytosis by cells lining blood vessels (endothelium); then it lyses the endosome’s membrane, escaping into the cytosol Rickettsias divide every 8–12 hours in the host cell's cytosol Daughter rickettsias escape from long cytoplasmic extensions of the host cell and infect other endothelial cells R rickettsii escaping an endothelial cell INVESTIGATE IT! How rickettsias avoid being phagocytize by macrophages and neutrophils? EPIDEMIOLOGY Though the earliest documented cases of Rocky Mountain spotted fever were in the Rocky Mountains, the disease is actually more prevalent in the Appalachian Mountains μm Subcutaneous hemorrhages (petechiae) of RMSF patient Infected tick introduces R rickettsii in its saliva This occurs only after the tick has fed for at least six hours Active bacteria are released into the mammalian host’s circulatory system Scan this QR code to watch Dr Bauman's Video Tutor explore Rocky Mountain Spotted Fever Then go to MasteringMicrobiology to investigate further and record your research findings on the following question: TEM 1–400 401–800 801–1200 1201–1600 1601–2000 >2000 TREATMENT AND PREVENTION DIAGNOSIS Cases of Rocky Mountain spotted fever in the United States, 2002–2014 Negative Positive Latex agglutination test Serological tests such as latex agglutination and fluorescent antibody stains are used to confirm an initial diagnosis based on sudden fever and headache following exposure to hard ticks, plus a rash on the soles or palms Nucleic acid probes of specimens from rash lesions provide specific and accurate diagnosis Early diagnosis is crucial because prompt treatment often makes the difference between recovery and death Physicians treat RMSF by removing the tick and prescribing doxycycline for most adults or chloramphenicol for children and pregnant women An effective vaccine is not available Wearing tight-fitting clothing, using tick repellents, promptly removing ticks, and avoiding tick-infested areas, especially in spring and summer when ticks are most voracious, help prevent infection » Disease in Depth Video Tutors walk through the presented disease, concluding with an “Investigate It!” question for independent research, furthering your understanding of microbiology’s relevancy and importance Dr Bauman also includes video tutors to coach students through key process art figures in the book NEW! Disease in Depth Coaching Activities feature personalized hints and feedback and provide guidance through each disease, prompting students to explore further with independent research NEW! Connecting Concepts Coaching Activities reinforce a “big picture” understanding of microbiology by showing how concepts in a particular chapter connect across other chapters in the text » Master Microbiology at your own pace, wherever you go! » NEW! MicroBoosters offer a mobilefriendly way for you to review (or learn for the first time) foundational concepts that are important in order to understand Microbiology, including Study Skills, Basic General and Organic Chemistry, Cell Biology, and more MicroBoosters can be assigned through MasteringMicrobiology and are available for self-study as Dynamic Study Modules » NEW! Mobile-friendly Dynamic Study Modules help students acquire, retain, and recall information faster and more effectively than ever before These flashcardstyle modules are available as a self-study tool or can be assigned by instructors NEW! Adaptive Follow-Up Assignments in MasteringMicrobiology are based on each student’s performance on the original homework assignment and, when assigned, provide additional coaching and practice EXPANDED! Dr Bauman’s Video Tutors, developed and narrated by the author, carefully teach key concepts using textbook art, bringing the illustrations to life and helping you visualize and understand complex topics and important processes The Fifth Edition includes new video tutors on key concepts as well as the Disease in Depth overviews You can quickly access the video tutors by scanning QR codes with a mobile device for on-the-go tutoring; instructors may also assign them as coaching activities in MasteringMicrobiology » Learn how today’s microbiologists think UPDATED! Every chapter has been revised to reflect the current State of the Science, including the latest research and technology Highlights of content updates include extensive discussions on the impact of genomics in understanding disease diagnosis and treatment options » Develop higher-level thinking skills and conceptual understanding NEW! Learning Catalytics is a “bring your own device” (laptop, smartphone, or tablet) classroom system for student engagement and assessment With Learning Catalytics, instructors can assess students in real time using open-ended tasks to probe student understanding NEW! ASM Curriculum Guidelines pre-test and post-test assessments are assignable in MasteringMicrobiology to facilitate efficient and customizable assessment of the six underlying concepts and 22 related topics of lasting importance in undergraduate microbiology courses as determined by the American Society of Microbiology » » Connect Lecture and Lab MicroLab Tutors help instructors and students get the most out of lab time and make the connection between microbiology concepts, lab techniques, and real-world applications » These tutorials combine live-action video and molecular animation with assessment and answer-specific feedback to coach students in how to interpret and analyze different lab results MicroLab Tutor Coaching Activities include the following topics: » Use and Application of the Acid-Fast Stain » Multitest Systems—API 20E » Aseptic Transfer of Bacteria » ELISA » Gram Stain » Use and Application of Microscopy » Polymerase Chain Reaction (PCR) » Safety in the Microbiology Laboratory » Quantifying Bacteria with Serial Dilutions and Pour Plates » Smear Preparation and Fixation » Streak Plate Technique » Survey of Protozoa » Identification of Unknown Bacteria » » Lab Technique Videos give students an opportunity to see techniques performed correctly and quiz themselves on lab procedures both before and after lab time Lab Technique videos can be assigned as pre-lab quizzes in MasteringMicrobiology and include coaching and feedback on the following techniques: » NEW!  The Scientific Method » NEW!  How to Write a Lab Report » Acid-Fast Staining » Amylase Production » Carbohydrate Catabolism » Compound Microscope » Differential and Selective Media » Disk-Diffusion Assay » ELISA » Gram Stain » Hydrogen Sulfide Production » Litmus Milk Reactions » Negative Staining » Respiration » Serial Dilutions » Simple Staining » Smear Preparation » Structural Stains » Safety in the Microbiology Laboratory Also Available for the Microbiology Lab Course NEW EDITION! Microbiology: A Laboratory Manual, Global Edition, Eleventh Edition by James G Cappuccino and Chad Welsh â2017 978-1-292-17578-2 ã 1-292-17578-8 The Eleventh Edition of this popular laboratory manual has been thoroughly revised with easy-to-adapt Lab Reports and the latest protocols from governing agencies including the EPA, ASM, and AOAC, along with two new lab exercises: one on Food Safety, and another covering the BSL Biological Safety Level Techniques in Microbiology: A Student Handbook by John M Lammert 978-0-13-224011-6 • 0-13-224011-4 This concise, visually-appealing handbook provides step-by-step instructions for the most frequentlyused microbiology lab techniques Laboratory Experiments in Microbiology, Eleventh Edition by Ted R Johnson and Christine L Case 978-0-32-199493-6 • 0-32-199493-0 » The very best instructor & student support MasteringMicrobiology with Pearson eText MasteringMicrobiology is the most effective and widely used online homework, tutorial, and assessment system for the sciences, delivering self-paced tutorials that focus on your course objectives, provide individualized coaching, and respond to your students’ progress Instructor’s Resource Material The Instructor’s Resource Material offers a wealth of instructor media resources, including presentation art, lecture outlines, test items, and answer keys—all in one convenient location These resources help instructors prepare for class—and create dynamic lectures—in half the time! The Instructor’s Resource Materials include: » All figures from the text with and without labels in both JPEG and PowerPoint® formats » All figures from the book with the Label Edit feature and selected “process” figures from the text with the Step Edit feature in PowerPoint format » All tables from the text » PowerPoint lecture outlines, including figures and tables from the book and links to the animations and videos All items provided on the IRC can also be downloaded from the “Instructor Resources” area of MasteringMicrobiology, which also includes: Video Tutors, MicroFlix™ Animations, Microbiology Animations, Microbiology Videos, Lab Technique Videos, and more NEW! Learning Catalytics is a “bring your own device” (laptop, smartphone, or tablet) classroom system for student engagement and assessment With Learning Catalytics, instructors can assess students in real time using open-ended tasks to probe student understanding Test Bank (Download Only) by Robert W Bauman, Nichol Dolby The Fifth Edition Test Bank includes hundreds of multiple choice, true/false, and short answer/essay questions that are correlated to the book’s Learning Outcomes and Bloom’s Taxonomy rankings Available electronically in the “Instructor Resources” area of MasteringMicrobiology, in both Microsoft Word® and in TestGen formats Instructor’s Manual (Download Only) by Robert W Bauman, Nichol Dolby This guide can be downloaded from the “Instructor Resources” area of MasteringMicrobiology and includes a detailed chapter outline and summary for each chapter as well as answers to in-text Clinical Case Studies, “Tell Me Why” questions, Critical Thinking questions, and endof-chapter Questions for Review FIFTH EDITION GLOBAL EDITION MICROBIOLOGY WITH DISEASES BY TAXONOMY ROBERT W BAUMAN, PH.D Amarillo College CO N T R I B U T I O N S BY: Todd P Primm, Ph.D Sam Houston State University Elizabeth Machunis-Masuoka, Ph.D University of Virginia C L I N I C A L C O N S U LTA N T S : Cecily D Cosby, Ph.D., FNP-C, PA-C Jean E Montgomery, MSN, RN Harlow, England • London • New York • Boston • San Francisco • Toronto • Sydney • Dubai • Singapore • Hong Kong Tokyo • Seoul • Taipei • New Delhi • Cape Town • Sao Paulo • Mexico City • Madrid • Amsterdam • Munich • Paris • Milan Cell Structure and Function Magnetospirillum magnetotacticum, a spiral-shaped magnetobacterium, contains a clearly visible line of inclusions of magnetite, which together make a magnetosome Can a microbe be a magnet? The answer is yes, if it is a magnetobacterium Magnetobacteria are microorganisms with an unusual feature: cellular structures called magnetosomes Magnetosomes are stored deposits (also called inclusions) of the mineral magnetite These deposits align magnetobacteria with the lines of the Earth’s magnetic field, much like a compass In the Southern Hemisphere, magnetobacteria exist as south-seeking varieties; in the Northern Hemisphere, they exist as north-seeking varieties How these bacteria benefit from magnetosomes? Magnetobacteria prefer environments with little or no oxygen, such as those that exist below the surfaces of land and sea The magnetosomes point toward the underground magnetic poles, helping magnetobacteria move toward regions with little oxygen 86 CHAPTER 3  Cell Structure and Function All living things—including our bodies and the bacterial, protozoan, and fungal pathogens that attack us—are composed of living cells If we want to understand disease and its treatment, therefore, we must first understand the life of cells How pathogens attack our cells, how our bodies defend themselves, how current medical treatments assist our bodies in recovering—all of these activities have their basis in the biology of our, and our pathogens’, cells In this chapter, we will examine cells and structures within cells We will discuss similarities and differences among the three major kinds of cells—bacterial, archaeal, and eukaryotic The differences are particularly important because they allow researchers to develop treatments that inhibit or kill pathogens without adversely affecting a patient’s own cells We will also learn about cellular structures that allow pathogens to evade the body’s defenses and cause disease Processes of Life Learni n g Out co m e 3.1 Describe four major processes of living cells Microbiology is the study of particularly small living things That raises a question: What does living mean; how we define life? Scientists once thought that living things were composed of special organic chemicals, such as glucose and amino acids, that carried a “life force” found only in living organisms These organic chemicals were thought to be formed only by living things and to be very different from the inorganic chemicals of nonliving things The idea that organic chemicals could come only from living organisms had to be abandoned in 1828, when Friedrich Wöhler (1800–1882) synthesized an organic molecule, urea, using only inorganic reactants in his laboratory Today, we know that all living things contain both organic and inorganic chemicals and that many organic chemicals can be made from inorganic chemicals by laboratory processes If organic chemicals can be made even in the absence of life, what is the difference between a living thing and a nonliving thing? What is life? At first, this may seem a simple question After all, you can usually tell when something is alive However, defining “life” itself can prove troublesome, so biologists generally avoid setting a definition, preferring instead to describe characteristics common to all living things Biologists agree that all living things share at least four processes of life: growth, reproduction, responsiveness, and metabolism • Growth Living things can grow; that is, they can increase in size • Reproduction Organisms normally have the ability to reproduce themselves Reproduction means that they increase in number, producing more organisms organized like themselves Reproduction may be accomplished asexually (alone) or sexually with gametes (sex cells) Note that reproduction is an increase in number, whereas growth is an increase in size Growth and reproduction often occur simultaneously (We consider several methods of reproduction when we examine microorganisms in detail in Chapters 11–13.) • Responsiveness All living things respond to their environment They have the ability to change themselves in reaction to changing conditions around or within them Many organisms also have the ability to move toward or away from environmental stimuli—a response called taxis • Metabolism Metabolism can be defined as the ability of organisms to take in nutrients from outside themselves and use the nutrients in a series of controlled chemical reactions to provide the energy and structures needed to grow, reproduce, and be responsive Metabolism is a unique process of living things; nonliving things cannot metabolize Cells store metabolic energy in the chemical bonds of adenosine triphosphate (ă-den´ōsēn trī-fos´fāt), or ATP (Major processes of microbial metabolism, including the generation of ATP, are discussed in Chapters 5–7.) TABLE 3.1 shows how these characteristics, along with cell structure, relate to various kinds of microbes Organisms may not exhibit these four processes at all times For instance, in some organisms, reproduction may be postponed or curtailed by age or disease or, in humans at least, by choice Likewise, the rate of metabolism may be reduced, as occurs in a seed, a hibernating animal, or a bacterial Table 3.1  Characteristics of Life and Their Distribution in Microbes Characteristic Bacteria, Archaea, Eukaryotes Viruses Growth: increase in size Occurs in all Growth does not occur Reproduction: increase in number Occurs in all Host cell replicates the virus Responsiveness: ability to react to environmental stimuli Occurs in all Reaction to host cells seen in some viruses Metabolism: controlled chemical reactions of organisms Occurs in all Viruses use host cell’s metabolism Cellular structure: membrane-bound structure capable of all of the above functions Present in all Viruses lack cytoplasmic membrane or cellular structure 87 CHAPTER 3  Cell Structure and Function endospore,1 and growth often stops when an animal reaches a certain size However, microorganisms typically grow, reproduce, respond, and metabolize as long as conditions are suitable (Chapter discusses the proper conditions for the metabolism and growth of various types of microorganisms.) TEM Inclusions Tell Me Why The smallest free-living microbe—the bacterium Mycoplasma—is nonmotile Why is it alive, even though it cannot move? Ribosome Prokaryotic and Eukaryotic Cells: An Overview Cytoplasm 0.5 μm L e arning O u t c o m e In the 1800s, two German biologists, Theodor Schwann (1810– 1882) and Matthias Schleiden (1804–1881), developed the theory that all living things are composed of cells Cells are living entities, surrounded by a membrane, that are capable of growing, reproducing, responding, and metabolizing The smallest living things are single-celled microorganisms There are many different kinds of cells (FIGURE 3.1) Some cells are free-living, independent organisms; others live together in colonies or form the bodies of multicellular organisms Cells also exist in various sizes, from the smallest bacteria to bird eggs, which are the largest of cells All cells may be described as either prokaryotes (prō-kar´ē-ōts) or eukaryotes (yū-kar´ē-ōts) Scientists categorize organisms based on shared characteristics into groups called taxa “Prokaryotic” is a characteristic of organisms in two taxa—domain Archaea and domain Bacteria—but “prokaryote” is not itself a taxon The distinctive feature of prokaryotes is that they can read their DNA genetic code and simultaneously make proteins—a typical prokaryote does not have a membrane surrounding its genetic material SEM μm (b) LM ▲ Figure 3.1  Examples of types of cells (a) Escherichia coli bacterial cells (b) Paramecium, a single-celled eukaryote Note the differences in magnification 1Endospores Flagellum Cell wall Cytoplasmic membrane 3.2 Compare and contrast prokaryotic and eukaryotic cells (a) Nucleoid Glycocalyx are resting stages, produced by some bacteria, that are tolerant of environmental extremes 40 μm ▲ Figure 3.2  Typical prokaryotic cell Prokaryotes include archaea and bacteria The artist has extended an electron micrograph to show three dimensions Not all prokaryotic cells contain all these features In other words, a typical prokaryote does not have a nucleus (FIGURE 3.2) (Researchers have discovered a few prokaryotes with internal membranes that look like nuclei, but further investigation is needed to determine what these structures are.) The word prokaryote comes from Greek words meaning “before nucleus.” Moreover, electron microscopy has revealed that prokaryotes typically lack various types of internal structures bound with membranes that are present in eukaryotic cells Bacteria and archaea differ fundamentally in such ways as the type of lipids in their cytoplasmic membranes and in the chemistry of their cell walls In many ways, archaea are more like eukaryotes than they are like bacteria (Chapter 11 discusses archaea and bacteria in more detail.) Eukaryotes have a membrane called a nuclear envelope surrounding their DNA, forming a nucleus (FIGURE 3.3), which sets eukaryotes in domain Eukarya Indeed, the term eukaryote comes from Greek words meaning “true nucleus.” Besides the nuclear membrane, eukaryotes have numerous other internal membranes that compartmentalize cellular functions These compartments are membrane-bound organelles—specialized structures that act like tiny organs to carry on the various functions of the cell Organelles and their functions are discussed later in this chapter The cells of algae, protozoa, fungi, animals, and plants are eukaryotic Eukaryotes are usually larger and more complex than prokaryotes, which are often about 1.0 µm in diameter or smaller, as compared to 10–100 µm for eukaryotic cells (FIGURE 3.4) Although there are many kinds of cells, they all share the characteristic processes of life as previously described, as well as certain physical features In this chapter, we will distinguish among bacterial, archaeal, and eukaryotic “versions” of physical features common to cells, including (1) external structures, (2) the cell wall, (3) the cytoplasmic membrane, and (4) the cytoplasm We will also discuss features unique to each type 88 CHAPTER 3  Cell Structure and Function TEM Nuclear envelope Nuclear pore Nucleolus Lysosome Mitochondrion Centriole Secretory vesicle Golgi body Cilium Transport vesicles Ribosomes Rough endoplasmic reticulum Smooth endoplasmic reticulum Cytoplasmic membrane 10 μm Cytoskeleton ▲ Figure 3.3  Typical eukaryotic cell Not all eukaryotic cells have all these features The artist has extended the electron micrograph to show three dimensions Note the difference in magnification between this cell and the prokaryotic cell in the previous figure Besides size, what major difference between prokaryotes and eukaryotes was visible to early microscopists? Figure 3.3  Eukaryotic cells contain nuclei, which are visible with light microscopes, whereas prokaryotes lack nuclei Virus Orthopoxvirus 0.3 μm diameter Bacterium Staphylococcus μm diameter Chicken egg 4.7 cm diameter (47,000 μm)* Parasitic protozoan Giardia 14 μm length *Actually, the inset box on the egg would be too small to be visible (Width of box would be about 0.002 mm.) ◀ Figure 3.4  Approximate size of various types of cells Birds’ eggs are the largest cells Note that Staphylococcus, a bacterium, is smaller than Giardia, a unicellular eukaryote A smallpox virus (Orthopoxvirus) is shown only for comparison; viruses are not cellular CHAPTER 3  Cell Structure and Function (Chapters 11, 12, and 19–23 examine further details of prokaryotic and eukaryotic organisms, their classification, and their ability to cause disease.) Tell Me Why In 1985, an Israeli scientist discovered the single-celled microbe Epulopiscium fishelsoni This organism is visible with the naked eye Why did the scientist initially think Epulopiscium was eukaryotic? What discovery revealed that the microbe is really a giant bacterium? Next, we explore characteristics of bacterial cells, beginning with external features and working into the cell External Structures of  Bacterial Cells Many cells have special external features that enable them to respond to other cells and their environment In bacteria, these features include glycocalyces, flagella, fimbriae, and pili Glycocalyces 89 or both These chemicals are produced inside the cell and are extruded onto the cell’s surface When the glycocalyx of a bacterium is composed of organized repeating units of organic chemicals firmly attached to the cell’s surface, the glycocalyx is called a capsule (FIGURE 3.5a) In contrast, a loose, water-soluble glycocalyx is called a slime layer (FIGURE 3.5b) Glycocalyces protect cells from drying (desiccation) and can also play a role in the ability of pathogens to survive and cause disease For example, slime layers are often sticky and provide one means for bacteria to attach to surfaces as biofilms, which are aggregates of many bacteria living together on a surface Oral bacteria colonize the teeth as a biofilm called dental plaque Bacteria in a dental biofilm can produce acid and cause dental caries (cavities) The chemicals in many bacterial capsules can be similar to chemicals normally found in the body, preventing bacteria from being recognized or devoured by defensive cells of the host For example, the capsules of Streptococcus pneumoniae (streptō-kok´ŭs nū-mō´nē-ī) and Klebsiella pneumoniae (kleb-sē-el´ă nū-mō´nē-ī) enable these prokaryotes to avoid destruction by defensive cells in the respiratory tract and to cause pneumonia Unencapsulated strains of these same bacterial species not cause disease because the body’s defensive cells destroy them Flagella L e arning O u t c o m e s 3.3 Describe the composition, function, and relevance to human health of glycocalyces 3.4 Distinguish capsules from slime layers Some cells have a gelatinous, sticky substance that surrounds the outside of the cell This substance is known as a glycocalyx (plural: glycocalyces), which literally means “sweet cup.” The glycocalyx may be composed of polysaccharides, polypeptides, Glycocalyx (capsule) L ea rn in g Ou tcomes 3.5 Discuss the structure and function of bacterial flagella 3.6 List and describe four bacterial flagellar arrangements A cell’s motility may enable it to flee from a harmful environment or move toward a favorable environment, such as one where food or light is available The most notable structures responsible for such bacterial movement are flagella Bacterial Glycocalyx (slime layer) ◀ Figure 3.5  Glycocalyces (a) Micrograph of a single cell of the bacterium Streptococcus attached to a human tonsil cell (blue), showing a prominent capsule (b) An unidentified skin bacterium has a slime layer surrounding the cell What advantage does a glycocalyx provide a cell? Figure 3.5  A glycocalyx provides protection from drying and from being devoured; it may also help attach cells to one another and to surfaces in the environment (a) TEM 250 nm (b) TEM 250 nm 90 CHAPTER 3  Cell Structure and Function Structure Bacterial flagella are composed of three parts: a filament, a hook, and a basal body (FIGURE 3.6) A filament is a long hollow shaft, about 20 nm in diameter, which extends out of the cell into its environment No membrane covers a filament Filament k H o o Direction of rotation during run Rod A bacterial flagellum is composed of many identical globular molecules of a protein called flagellin A flagellum lengthens by growing at its tip as the cell secretes molecules of flagellin through the hollow core of the flagellum, to be deposited in a clockwise helix at the tip of the filament Bacterial flagella react to external wetness, inhibiting their own growth in dry habitats At its base, a filament inserts into a curved structure, the hook, which is composed of a different protein The basal body, which is composed of still different proteins, anchors the filament and hook to the cell wall and cytoplasmic ◀ Figure 3.6  Proximal structure of bacterial flagella (a) Detail of flagellar structure of a Gram-positive cell (b) Detail of the flagellum of a Gram-negative bacterium How flagella of Gram-positive bacteria differ from those of Gram-negative bacteria? Figure 3.6  Flagella of Gram-positive cells have a single pair of rings in the basal body that function to attach the flagellum to the cytoplasmic membrane The flagella of Gram-negative cells have two pairs of rings: one pair anchors the flagellum to the cytoplasmic membrane, the other pair to the cell wall flagella (singular: flagellum) are long structures that extend beyond the surface of a cell and its glycocalyx and propel the cell through its environment Not all bacteria have flagella, but for those that do, their flagella are very similar in composition, structure, and development. ANIMATIONS: Motility: Overview Peptidoglycan layer (cell wall) Protein rings Cytoplasmic membrane (a) Cytoplasm Filament H o o k Outer protein rings Outer membrane Rod Gram + Gram – Basal body Peptidoglycan layer Integral protein Inner protein rings Integral protein (b) Cytoplasmic membrane Cytoplasm Cell wall 91 CHAPTER 3  Cell Structure and Function membrane by means of a rod and a series of either two or four rings of proteins Together, the hook, rod, and rings allow the filament to rotate 360° Differences in the proteins associated with bacterial flagella vary enough to allow classification of species into strains called serovars  ANIMATIONS: Flagella: Structure Arrangement Bacteria may have one of several flagellar arrangements (FIGURE 3.7) For example, flagella that cover the surface of the (a) Peritrichous flagella SEM cell are termed peritrichous;2 in contrast, polar flagella are only at the ends Other bacteria have tufts of polar flagella Some spiral-shaped bacteria, called spirochetes (spī´rōkēts),3 have flagella at both ends that spiral tightly around the cell instead of protruding into the surrounding medium These flagella, called endoflagella, form an axial filament that wraps around the cell between its cytoplasmic membrane and an outer membrane (FIGURE 3.8) Rotation of endoflagella evidently causes the axial filament to rotate around the cell, causing the spirochete to “corkscrew” through its medium Treponema 0.5 μm SEM (a) Endoflagella rotate Axial filament μm Axial filament rotates around cell Outer membrane (b) Single polar flagellum SEM 0.5 μm Cytoplasmic membrane Spirochete corkscrews and moves forward Axial filament (b) ▲ Figure 3.8  Axial filament (a) Scanning electron micrograph of two spirochete cells of Borrelia burgdorferi, which causes Lyme disease (b) Diagram of axial filament wrapped around a spirochete Cross section reveals that an axial filament is composed of endoflagella (c) Tuft of polar flagella SEM 0.5 μm ▲ Figure 3.7  Micrographs of basic arrangements of ­bacterial flagella 2From 3From Greek peri, meaning “around,” and trichos, meaning “hair.” Greek speira, meaning “coil,” and chaeta, meaning “hair.” 92 CHAPTER 3  Cell Structure and Function pallidum (trep-ō-nē´mă pal´li-dŭm), the agent of syphilis, and Borrelia burgdorferi (bō-rē´lē-ă burg-dōr´fer-ē), the cause of Lyme disease, are notable spirochetes Some scientists think that the corkscrew motility of these pathogens allows them to invade human tissues. ANIMATIONS: Flagella: Arrangement; Spirochetes Function Although the precise mechanism by which bacterial flagella move is not completely understood, we know that they rotate 360° like boat propellers rather than whipping from side to side The flow of ions (electrically charged atoms) through the cytoplasmic membrane near the basal body powers the rotation, propelling the bacterium through the environment at about 60 cell lengths per second—equivalent to a car traveling at 670 miles per hour! Flagella can rotate at more than 100,000 rpm and can change direction from counterclockwise to clockwise Bacteria move with a series of “runs” interrupted by “tumbles.” Counterclockwise flagellar rotation produces movements of a cell in a single direction for some time; this is called a run If more than one flagellum is present, the flagella align and rotate together as a bundle Runs are interrupted by brief, abrupt, random changes in direction called tumbles Tumbles result from clockwise flagellar rotation where each flagellum rotates independently Receptors for light or chemicals on the surface of the cell send signals to the flagella, which then adjust their speed and direction of rotation If, during its random movements, a cell finds itself approaching a favorable stimulus, it increases the duration of the runs, which tends to move the cell toward an attractant—the bacterium positions itself into a more favorable environment (FIGURE 3.9) Unfavorable stimuli increase Attractant the duration of runs that are in a direction away from a repellent This tends to move the cell toward a more favorable environment Movement in response to a stimulus is termed taxis Stimuli are often either light (phototaxis) or a chemical (chemotaxis) Movement toward a favorable stimulus is positive taxis, whereas movement away from an unfavorable stimulus is negative taxis For example, movement toward a nutrient would be positive chemotaxis  ANIMATIONS: Flagella: Movement Fimbriae and Pili L ea rn in g Ou tcome 3.7 Compare and contrast the structures and functions of fimbriae, pili, and flagella Many bacteria have rodlike proteinaceous extensions called fimbriae (fim´brē-ī; singular: fimbria) These sticky, bristlelike projections adhere to one another and to substances in the environment There may be hundreds of fimbriae per cell, and they are usually shorter than flagella (FIGURE 3.10) An example of a bacterium with fimbriae is Neisseria gonorrhoeae (nī-se´rē-ă go-nor-rē´ī), which causes gonorrhea Pathogens must be able to adhere to their hosts if they are to survive and cause disease This bacterium is able to colonize the mucous membrane of the reproductive tract by attaching with fimbriae Neisseria cells that lack fimbriae are nonpathogenic Some fimbriae carry enzymes that render soluble, toxic metal ions into insoluble, nontoxic forms Bacteria may use fimbriae to move across a surface via a process similar to pulling an object with a rope The bacterium extends a fimbria, which attaches at its tip to the surface; then the bacterium retracts the fimbria, pulling itself toward the attachment point Fimbriae also serve an important function in biofilms, slimy masses of microbes adhering to a substrate and to one Run Tumble Flagellum Fimbria Run Tumble ▲ Figure 3.9  Motion of a peritrichous bacterium In peritrichous bacteria, runs occur when all of the flagella rotate counterclockwise and become bundled Tumbles occur when the flagella rotate clockwise, become unbundled, and the cell spins randomly In positive chemotaxis (shown), runs last longer than tumbles, resulting in motion toward the chemical attractant What triggers a bacterial flagellum to rotate counterclockwise, producing a run? SEM ▲ Figure 3.10  Fimbriae Proteus vulgaris has flagella and fimbriae μm Figure 3.9  Favorable environmental conditions induce runs 93 CHAPTER 3  Cell Structure and Function another by means of fimbriae and glycocalyces It has been estimated that at least 99% of bacteria in nature exist in biofilms Some fimbriae act as electrical wires, conducting electrical signals among cells in a biofilm Researchers are interested in biofilms because of the roles they play in human diseases and in industry (See Highlight: Biofilms: Slime Matters.) A special type of fimbria is a pilus (pī´lus; plural: pili, pī´lī), also called a conjugation pilus Pili are usually longer than other fimbriae but shorter than flagella Typically, only one to a few pili are present per cell in bacteria that have them Cells use pili to transfer DNA from one cell to the other via a process termed conjugation (FIGURE 3.11) (Chapter deals with conjugation in more detail.) Pilus Tell Me Why Why is a pilus a type of fimbria, but a flagellum is not a fimbria? TEM 0.5 μm ▲ Figure 3.11  Pili Two Escherichia coli cells are connected by a pilus How are pili different from bacterial flagella? Bacterial Cell Walls L ea rn in g Ou tcom es Describe common shapes and arrangements of bacterial cells 3.9 Describe the sugar and peptide portions of peptidoglycan 3.10 Compare and contrast the cell walls of Gram-positive and Gram-negative bacteria in terms of structure and Gram staining 3.8 Figure 3.11  Bacterial flagella are flexible structures that rotate to propel the cell; pili are hollow tubes used to transfer DNA from one cell to another Highlight Biofilms: Slime Matters They form plaque on teeth; they are the slime on rocks in rivers and streams; and they can cause disease, clog drains, or help clean up hazardous waste They are biofilms—organized, layered systems of bacteria and other microbes attached to a surface Understanding biofilms holds the key to many important clinical and industrial applications Biofilm bacteria communicate via chemical and electrical signals that help them organize and form threedimensional structures The architecture of a biofilm provides protection that free-floating bacteria lack For example, the lower concentrations of oxygen found in the interior of biofilms thwart the effectiveness of some antibiotics Furthermore, bacteria in biofilms behave in significantly different ways from individual, free-floating bacteria For example, as a free-floating cell, the soil bacterium Pseudomonas putida propels itself through water with its flagella; however, once it becomes part of a biofilm, it turns off the genes for flagellar proteins and starts synthesizing pili instead In addition, the genes for antibiotic resistance in P putida are more active within cells in a biofilm than within a free-floating cell The more we understand biofilms, the more readily we can reduce their harmful effects or put them to good use Biofilms account for about two-thirds of bacterial infections in humans, such as the serious lung infections frequently suffered by individuals with cystic fibrosis Biofilms Biofilm on medical tubing SEM μm are also the culprits in many industrial problems, including corroded pipes and clogged water filters, which cause millions of dollars of damage each year Fortunately, not all biofilms are detrimental; some show potential as aids in preventing and controlling certain kinds of industrial pollution 94 CHAPTER 3  Cell Structure and Function Sugar chain NA NA NA NA (a) (b) ▲ Figure 3.12  Bacterial shapes and arrangements (a) Spherical cocci may be in arrangements such as single, chains (streptococci), clusters (staphylococci), and cuboidal packets (sarcinae) (b) Rod-shaped bacilli may also be single or in arrangements such as chains N-acetylglucosamine NAG Glucose CH2OH H HO H OH H H OH O H H O OH OH H H NH CH3 (b) M NA G NA G NA M NA NA NA M G NA NA G NA G NA NA G NA NA M M M NA NA M M M NA M M M NA M NA G M M M Connecting chain of amino acids ▲ Figure 3.14  Possible structure of peptidoglycan Peptidoglycan is composed of chains of NAG and NAM linked by tetrapeptide crossbridges and, in some cases, as shown here, connecting chains of amino acids to form a tough yet flexible structure The amino acids of the crossbridges differ among bacterial species NAM alternate These chains are the “glycan” portions of peptidoglycan Chains of NAG and NAM are attached to other NAGNAM chains by crossbridges of four amino acids (tetrapeptides) between neighboring NAMs FIGURE 3.14 illustrates one possible configuration Such peptide crossbridges are the “peptido” portion of peptidoglycan Depending on the bacterium, tetrapeptide bridges are either bonded to one another or held together by short connecting chains of other amino acids, as shown in Figure 3.14 Peptidoglycan covers the entire surface of a cell, which must insert millions of new NAG and NAM subunits if it is to grow and divide Scientists describe two basic types of bacterial cell walls as Gram-positive cell walls or Gram-negative cell walls They distinguish Gram-positive and Gram-negative cells by the use of the Gram staining procedure (described in Chapter 4), which was invented long before the structure and chemical nature of bacterial cell walls were known CH2OH CH2OH O C (a) N-acetylmuramic acid NAM NA NA NA NA Tetrapeptide (amino acid) crossbridge The cells of most prokaryotes are surrounded by a cell wall that provides structure and shape to the cell and protects it from osmotic forces (described shortly) In addition, a cell wall assists some cells in attaching to other cells or in resisting antimicrobial drugs Note that animal cells not have cell walls, a difference that plays a key role in treatment of many bacterial diseases with certain types of antibiotics For example, penicillin attacks the cell wall of bacteria but is harmless to human cells, because the latter lack walls Cell walls give bacterial cells characteristic shapes Spherical cells, called cocci (kok´sī), may appear in various arrangements, including singly or in chains (streptococci), clusters (staphylococci), or cuboidal packets (sarcinae, sar´si-nī) (FIGURE 3.12), depending on the planes of cell division Rod-shaped cells, called bacilli (bă-sil´ī), typically appear singly or in chains Bacterial cell walls are composed of peptidoglycan (pep´ti-dō-glī´kan), a meshlike complex polysaccharide Peptidoglycan in turn is composed of two types of regularly alternating sugar molecules, called N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), which are structurally similar to glucose (FIGURE 3.13) Millions of NAG and NAM molecules are covalently linked in chains in which NAG and G M M O H O H H O O HC C H CH3 O O H NH C O CH3 OH ▲ Figure 3.13  Comparison of the structures of glucose, NAG, and NAM (a) Glucose (b) N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) molecules linked as in peptidoglycan Blue shading indicates the differences between glucose and the other two sugars Orange boxes highlight the difference between NAG and NAM Gram-Positive Bacterial Cell Walls L ea rn in g Ou tcome 3.11 Compare and contrast the cell walls of acid-fast bacteria with typical Gram-positive cell walls Gram-positive bacterial cell walls have a relatively thick layer of peptidoglycan that also contains unique chemicals called teichoic4 acids (tī-kō´ik) Some teichoic acids are covalently linked to lipids, forming lipoteichoic acids that anchor the peptidoglycan 4From Greek teichos, meaning “wall.” CHAPTER 3  Cell Structure and Function 95 Peptidoglycan layer (cell wall) Cytoplasmic membrane (a) Gram-positive cell wall Lipoteichoic acid Teichoic acid Integral protein Porin Outer membrane of cell wall Porin (sectioned) Peptidoglycan layer of cell wall (b) Gram-negative cell wall Periplasmic space Cytoplasmic membrane Lipopolysaccharide (LPS) layer, containing lipid A Phospholipid layers Integral proteins ▲ Figure 3.15  Comparison of cell walls of Gram-positive and Gram-negative bacteria (a) The Gram-positive cell wall has a thick layer of peptidoglycan and lipoteichoic acids that anchor the wall to the cytoplasmic membrane (b) The Gram-negative cell wall has a thin layer of peptidoglycan and an outer membrane composed of lipopolysaccharide (LPS), phospholipids, and proteins What effects can lipid A have on human physiology? Figure 3.15  Lipid A can cause shock, blood clotting, and fever in humans to the cytoplasmic membrane of the cell (FIGURE 3.15a) Teichoic acids have negative electrical charges, which help give the surface of a Gram-positive bacterium a negative charge and may play a role in the passage of ions through the wall The thick cell wall of a Gram-positive bacterium retains the crystal violet dye used in the Gram staining procedure, so the stained cells appear purple under magnification Video Tutor Some additional chemicals are associated with the walls of some Gram-positive bacteria For example, species of Mycobacterium (mī´kō-bak-tēr´ē-ŭm), which include the causative agents of tuberculosis and leprosy, have walls with up to 60% mycolic acid, a waxy lipid Mycolic acid helps these cells survive desiccation (drying out) and makes them difficult to stain with regular water-based dyes Researchers have 96 CHAPTER 3  Cell Structure and Function developed a special staining procedure called the acid-fast stain to stain these Gram-positive cells that contain large amounts of waxy lipids Such cells are called acid-fast bacteria (see Chapter 4) Gram-Negative Bacterial Cell Walls Learni n g Out co m e 3.12 Describe the clinical implications of the structure of the Gram-negative cell wall Gram-negative cell walls have only a thin layer of peptidoglycan outside the cytoplasmic membrane (FIGURE 3.15b), but outside the peptidoglycan there is another, outer membrane This outer membrane is a bilayer; that is, it is composed of two different layers or leaflets The inner leaflet of the outer membrane is composed of phospholipids and proteins, but the outer leaflet is made of lipopolysaccharide (LPS) Integral proteins called porins form channels through both leaflets of the outer membrane, allowing midsize molecules such as glucose to move freely across the outer membrane LPS is a union of lipid with sugar The lipid portion of LPS is known as lipid A The erroneous idea that lipid A is inside Gram-negative cells led to the use of the term endotoxin5 for this chemical Dead Gram-negative cells release lipid A when the outer membrane disintegrates This is medically important because lipid A may trigger fever, vasodilation, inflammation, shock, and blood clotting in humans Killing large numbers of Gram-negative bacteria with antimicrobial drugs within a short time period releases large amounts of lipid A, which might threaten the patient more than the live bacteria; thus, any internal infection by Gram-negative bacteria is cause for concern The outer membrane of a Gram-negative cell can also be an impediment to the treatment of disease For example, the outer membrane may prevent the movement of penicillin to the underlying peptidoglycan, thus rendering this drug ineffectual against many Gram-negative pathogens Between the cytoplasmic membrane and the outer membrane of Gram-negative bacteria is a periplasmic space (see Figure 3.15b) The periplasmic space contains the peptidoglycan and periplasm, the name given to the gel between the membranes Periplasm contains water, nutrients, and substances secreted by the cell, such as digestive enzymes and proteins involved in specific transport The enzymes function to catabolize large nutrient molecules into smaller molecules that can be absorbed or transported into the cell Because the cell walls of Gram-positive and Gram-negative bacteria differ, the Gram stain is an important diagnostic tool After the Gram staining procedure, Gram-negative cells appear pink, and Gram-positive cells appear purple Bacteria Without Cell Walls A few bacteria, such as Mycoplasma pneumoniae (mī´kō-plaz-mă nū-mō´nē-ī), lack cell walls entirely In the past, these bacteria were often mistaken for viruses because of their small size and lack of walls However, they have other features of prokaryotic cells, such as prokaryotic ribosomes (discussed later in the chapter) Tell Me Why Why is the microbe illustrated in Figure 3.2 more likely a Gram-positive bacterium than a Gram-negative one? Bacterial Cytoplasmic Membranes Beneath the glycocalyx and the cell wall is a cytoplasmic membrane The cytoplasmic membrane may also be referred to as the cell membrane or a plasma membrane Structure L ea rn in g Ou tcomes 3.13 Diagram a phospholipid bilayer and explain its significance in reference to a cytoplasmic membrane 3.14 Explain the fluid mosaic model of membrane structure Cytoplasmic membranes are about nm thick and composed of phospholipids (see Figure 2.16) and associated proteins Some bacterial membranes also contain sterol-like molecules, called hopanoids, that help stabilize the membrane The structure of a cytoplasmic membrane is referred to as a phospholipid bilayer (FIGURE 3.16) A phospholipid molecule is bipolar; that is, the two ends of the molecule are different The phosphate-containing heads of each phospholipid molecule are hydrophilic,6 meaning that they are attracted to water at the two surfaces of the membrane The hydrocarbon tails of each phospholipid molecule are hydrophobic7 and huddle together with other tails in the interior of the membrane, away from water Phospholipids placed in a watery environment naturally form a bilayer because of their bipolar nature About half of a bacterial cytoplasmic membrane is composed of integral proteins inserted amidst the phospholipids Some integral proteins penetrate the entire bilayer; others are found in only half the bilayer In contrast, peripheral proteins are loosely attached to the membrane on one side or the other Proteins of cell membranes may act as recognition proteins, enzymes, receptors, carriers, or channels The fluid mosaic model describes our current understanding of membrane structure The term mosaic indicates that the membrane proteins are arranged in a way that resembles the tiles in a mosaic, and fluid indicates that the proteins and lipids are free to flow laterally within a membrane  ANIMATIONS: Membrane Structure 6From 5From Greek endo, meaning “inside,” and toxikon, meaning “poison.” 7From Greek hydro, meaning “water,” and philos, meaning “love.” Greek hydro, meaning “water,” and phobos, meaning “fear.” CHAPTER 3  Cell Structure and Function Head, which contains phosphate (hydrophilic) 97 Phospholipid Tail (hydrophobic) Integral proteins Cytoplasm Integral protein Phospholipid bilayer Peripheral protein Integral protein ▲ Figure 3.16  Structure of a prokaryotic cytoplasmic membrane: a phospholipid bilayer Function L e arning O u t c o m e s 3.15 Describe the functions of a cytoplasmic membrane as they relate to permeability 3.16 Compare and contrast the passive and active processes by which materials cross a cytoplasmic membrane 3.17 Define osmosis, and distinguish among isotonic, hypertonic, and hypotonic solutions A cytoplasmic membrane does more than separate the contents of a cell from its outside environment The cytoplasmic membrane also controls the passage of substances into and out of the cell Nutrients are brought into the cell, and wastes are removed The membrane also functions for harvesting light energy in photosynthetic bacteria producing molecules of ATP (Chapter discusses photosynthesis and ATP synthesis.) In its function of controlling the contents of a cell, the cytoplasmic membrane is selectively permeable, meaning that it allows some substances to cross while preventing the crossing of others How does a membrane exert control over substances that move across it and the contents of the cell?  ANIMATIONS: Membrane Permeability A phospholipid bilayer is naturally impermeable to most substances Large molecules cannot cross between the packed phospholipids; ions and molecules with an electrical charge are repelled by it; and hydrophilic substances cannot easily cross its hydrophobic interior However, cytoplasmic membranes contain proteins, and some of these proteins function as pores, channels, or carriers that allow substances to cross the membrane Movement across a cytoplasmic membrane occurs by either passive or active processes Passive processes not require the expenditure of a cell’s metabolic energy store (usually ATP), whereas active processes require the expenditure of cellular energy, either directly or indirectly Active and passive processes will be discussed shortly, but first you must understand another feature of selectively permeable cytoplasmic membranes: their ability to maintain a concentration gradient Membranes enable a cell to concentrate chemicals on one side of the membrane or the other The difference in concentration of a chemical on the two sides of a membrane is the chemical's concentration gradient (also known as a chemical gradient) Because many of the substances that have concentration gradients across cell membranes are electrically charged chemicals, a corresponding electrical gradient, or voltage, also exists across the membrane (FIGURE 3.17) For example, a greater concentration of negatively charged proteins exists inside the membrane, and positively charged sodium ions are more concentrated outside the membrane One result of the segregation of electrical charges by a membrane is that the interior of a cell is usually electrically negative compared to the exterior This tends to repel negatively charged chemicals and attract positively charged substances into cells  ANIMATIONS: Passive Transport: Principles of Diffusion Passive Processes In passive processes, electrochemical gradients provide energy to move substances across the membrane; the cell does not expend its ATP Passive processes include diffusion, facilitated diffusion, and osmosis 98 CHAPTER 3  Cell Structure and Function Clinical Case Study The Big Game College sophomore Nadia is a star point guard for her school’s basketball team She is excited about the divisional finals Friday night—she’s even heard rumors that a professional scout will be in the stands On Thursday morning, she wakes up with a sore throat Her forehead doesn’t feel warm, so she forces herself to attend her Thursday classes; but when she wakes up on Friday morning, her throat is noticeably worse Still, she forces herself to attend Friday morning class but feels tired and much worse by noon It is downright painful to swallow, and she skips lunch Nearly crying, she heads back to the dorm and checks her temperature—101°F Desperate, she walks to the student health center, where a nurse practitioner notices white spots on the back of Nadia’s throat and on her tonsils The divisional basketball game starts in six hours, but it only takes a few minutes for the nurse practitioner to perform a rapid streptococcal antigen test and determine that Nadia has streptococcal pharyngitis—strep throat She will miss the big game Strep throat is caused by an encapsulated, Gram-positive bacterium, Streptococcus pyogenes The only good news is that by taking the prescribed penicillin, Nadia should be ready for her next big game—hopefully, the quarterfinals How does the capsule of Streptococcus contribute to the bacterium’s ability to cause disease? What bacterial structures, besides the capsule, may be allowing Streptococcus to infect Nadia’s throat? Penicillin works by interrupting the formation of peptidoglycan What bacterial structure contains peptidoglycan? In a Gram-positive organism like Streptococcus, is this structure typically thicker or thinner than it would be in a Gram-negative bacterium? Diffusion  Diffusion is the net movement of a chemical down its concentration gradient; that is, from an area of higher concentration to an area of lower concentration It requires no energy output by the cell, a common feature of all passive processes In fact, diffusion occurs even in the absence of cells or their membranes In the case of diffusion into or out of cells, only chemicals that are small or lipid soluble can diffuse through the lipid portion of the membrane (FIGURE 3.18a) For example, oxygen, carbon dioxide, alcohol, and fatty acids can freely diffuse + Na+ – Cl– Cell exterior (extracellular fluid) + + + + + + + + + + mV + + + – –30 –70 + + + + + + – + + Cytoplasmic membrane – Integral protein + – _ – – + – – + _ – _ – _ – – + – – – – – – Protein – DNA – Protein – – Cell interior (cytoplasm) ▲ Figure 3.17  Electrical potential of a cytoplasmic membrane The electrical potential, in this case −70 mV, exists across a membrane because there are more negative charges inside the cell than outside it through the cytoplasmic membrane, but molecules such as glucose and proteins cannot Facilitated Diffusion  The phospholipid bilayer blocks the movement of large or electrically charged molecules, so they not cross the membrane unless there is a pathway for diffusion As we have seen, cytoplasmic membranes contain integral proteins Some of these proteins act as channels or carriers to allow certain molecules to diffuse down their concentration gradients into or out of the cell This process is called facilitated diffusion because the proteins facilitate the process by providing a pathway for diffusion The cell expends no energy in facilitated diffusion; electrochemical gradients provide all of the energy necessary Some channel proteins allow the passage of a range of chemicals that have the right size or electrical charge (FIGURE 3.18b) Other channel proteins, known as permeases, are more specific, carrying only certain substrates (FIGURE 3.18c) A permease has a binding site that is selective for one substance Osmosis  When discussing simple and facilitated diffusion, we considered a solution in terms of the solutes (dissolved chemicals) it contains because it is those solutes that move into and out of the cell In contrast, with osmosis it is useful to consider the concentration of the solvent, which in organisms is always water Osmosis CHAPTER 3  Cell Structure and Function Extracellular fluid 99 ◀ Figure 3.18  Passive processes of movement across a cytoplasmic membrane Passive processes always involve movement down an electrochemical gradient Cytoplasm (b) Facilitated diffusion of several types of chemicals through a nonspecific channel (c) Facilitated diffusion through a permease of a specific chemical (d) Osmosis (diffusion of water through a specific channel or through the membrane) is the special name given to the diffusion of water across a semipermeable membrane—that is, across a membrane that is permeable to water molecules but not to most solutes that are present, such as proteins, amino acids, salts, or glucose (FIGURE 3.18d) Because these solutes cannot freely penetrate the membrane, they cannot diffuse across the membrane, no matter how unequal their concentrations on either side may be Instead, the water diffuses Water molecules cross from the side of the membrane that contains a higher concentration of water (lower concentration of solute) to the side that contains a lower concentration of water (higher concentration of solute) In osmosis, water moves across the membrane until equilibrium is reached, or until the pressure of water is equal to the force of osmosis (FIGURE 3.19) We commonly compare solutions according to their concentrations of solutes When solutions on either side of a selectively permeable membrane have the same concentration of solutes, the two solutions are said to be isotonic.8 In an isotonic situation, neither side of a selectively permeable membrane will experience a net loss or gain of water (FIGURE 3.20a) When the concentrations of solutions are unequal, the solution with the higher concentration of solutes is said to be hypertonic9 to the other The solution with a lower concentration of solutes is hypotonic10 in comparison Note that the terms hypertonic and hypotonic refer to the concentration of solute, even though osmosis refers to the movement of water The terms isotonic, hypertonic, and hypotonic are relative For example, a glass 8From Greek isos, meaning “equal,” and tonos, meaning “tone.” Greek hyper, meaning “more” or “over.” 10From Greek hypo, meaning “less” or “under.” 9From of tap water is isotonic to another glass of the same water, but it is hypertonic compared to distilled water, and hypotonic when compared to seawater In biology, the three terms are traditionally used relative to the interior of cells Most cells are hypertonic to their environments (a) Solutes (chemicals dissolved in water) (b) Semipermeable membrane allows movement of water, but not of solutes ▲ Figure 3.19  Osmosis, the diffusion of water across a semipermeable membrane (a) A membrane separates two solutions of different concentrations in a U-shaped tube The membrane is permeable to water, but not to the solute (b) After time has passed, water has moved down its concentration gradient until water pressure prevented the osmosis of any additional water Which side of the tube more closely represents a living cell? Figure 3.19  The right-hand side represents the cell, because cells are typically hypertonic to their environment (a) Diffusion of small or lipid-soluble chemicals through the membrane ... (Figs 11 .1, 11 .2a, 11 .5, 11 .7, 11 .11 a, 11 .16 , 11 .17 , 11 .19 , 11 . 21, 11 .22, 11 .23, 11 .24b, 11 .27b) • Ten revised figures for better pedagogy (Figs 11 .1, 11 .3, 11 .4, 11 .6, 11 .10 , 11 .14 , 11 .17 , 11 . 21, ... incidence data • Modified/updated nine figures (Figs 21. 1, 21. 2, 21. 3, 21. 5, 21. 8, 21. 12, 21. 13, 21. 17, 21. 20) • Two new photos (Figs 21. 11, 21. 19) • Updated treatment regimen for rickettsial spotted... pedagogy and currency (Figs 13 .5, 13 .8, 13 .12 , 13 .13 , 13 .14 , 13 .16 , 13 .18 , 13 .22) • One new figure showing prion templating (Fig 13 .23) • Two new Learning Outcomes concerning (1) structures of viruses