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Chapter 3

BISC300 Chapter 3: Chapter 3 Study Guide

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Biological Sciences
Cooper Carlton

3 Bacteria CHAPTER OVERVIEW This chapter provides a description of the prokaryotic cell, including bacterial and archaeal cells. The discussion begins with the general features of size, shape, and arrangement of prokaryotic cells. Then the general features of biological membranes and the specific features of prokaryotic membranes are given. The cell wall is discussed with special reference to the differences between the cell walls of gram-positive bacteria and gram- negative bacteria, differential staining reactions, and the nature of S-layers. Important internal structures of prokaryotes, such as the cytoplasmic matrix, ribosomes, inclusion bodies, and the nucleoid are described. Structures external to the cell, such as pili, fimbriae, and flagella lead into a discussion of cell motility and chemotaxis. The chapter concludes with a description of the structure and functions of bacterial endospores. CHAPTER OBJECTIVES After reading this chapter and listing to the lecture, you should be able to: • identify major prokaryotic cell structures in a drawing or photomicrograph and be able to draw it, gram(-) and gram (+). You must be able to draw in full detail a eukaryotic cell, both animal and plant, knowing the function of organelles and other structures • describe the various sizes, shapes, and cellular arrangements exhibited by prokaryotes • describe the bacterial plasma membrane and the limited internal membrane structures found in prokaryotes • compare the structure of gram-positive and gram-negative bacterial cell walls and explain how the differences between the two contribute to their Gram reaction • describe the appearance, composition, and function of the various internal structures found in prokaryotic organisms (such as inclusions, ribosomes, and the nucleoid) • describe external structures such as capsules and flagella • describe how bacteria use their locomotive ability to swim toward chemical attractants and away from chemical repellents • Briefly describe the production of the bacterial endospore and how it enables endospore-forming bacteria to survive harsh environmental conditions Is DNA replication required for sporulation? What is germinations? CHAPTER OUTLINE I. Prokaryotes A. Bacteria are grouped together as prokaryotes because they lack a nucleus and generally do not have membrane-bound organelles or an endomembrane system II. of Bacterial Cell Structure A. Shape, arrangement, and size 1. Most prokaryotes are spheres (cocci) or rods (bacilli) a. During the reproductive process, some cocci remain attached to each other to form pairs (diplococci), chains, clusters, square planar configurations (tetrads), or cubic configurations (sarcinae) b. Some rods are so short and wide that they appear to be ovals (coccobacilli); most rods occur singly, but some form pairs or chains 2. Other shapes include: curved rods (vibrios); rigid helices (spirilla), and flexible helices (spirochetes); a few are flat; filaments, which can produce a network, are called a mycelium; bacteria that exhibit more than one form are called pleomorphic 3. Prokaryotic cells vary in size (generally 1 to 5 µm) although they are typically smaller than most eukaryotic cells; B. Prokaryotic cell structure 1. A variety of structures (cell wall, periplasmic space, plasma membrane, nucleoid, ribosomes, inclusion bodies, flagella, capsules, and slime layers) are observed in prokaryotic cells 2. Not all structures are found in every genus; prokaryotic cells are morphologically simpler than eukaryotic cells C. Cell organization 1. Bacterial cells often share a common organization 2. Bacterial cells are surrounded by a cell envelope with complex cell walls; they lack many internal features common in eukaryotic cells III. Bacterial Cell Envelopes A. The cell envelope includes the plasma membrane, cell wall, and other external layers of the cell B. The plasma membrane serves several functions: 1. It retains the cytoplasm and separates the cell from its environment 2. It serves as a selectively permeable barrier 3. It contains transport systems used for nutrient uptake, waste excretion, and protein secretion 4. It is the location of a variety of crucial metabolic processes including respiration, photosynthesis, lipid synthesis, and cell wall synthesis 5. It contains special receptor molecules that enable detection of and response to chemicals in the surroundings C. The fluid mosaic model of membrane structure 1. This model, proposed by Singer and Nicholson, states that membranes are lipid bilayers with floating proteins 2. Cell membranes are very thin (5–10 nm thick); the lipids are amphipathic, having hydrophilic (interact with water) head groups and long hydrophobic (insoluble in water) tails; the head groups face out of the membrane while the tails are buried in the membrane to form bilayers 3. Two types of proteins are associated with the lipid bilayer of the membrane: peripheral (loosely associated and easily removed) and integral (embedded within the membrane and not easily removed) D. Bacterial lipids 1. The plasma membrane of bacteria consists of a phospholipid bilayer with hydrophilic surfaces and a hydrophobic interior; bacterial membranes lack sterols, but many contain sterol-like molecules called hopanoids that help stabilize the membrane 2. Bacteria do not have membranous organelles, but can have internal membrane systems with specialized functions such as photosynthesis or respiration IV. Bacterial Cell Walls A. The cell wall is a rigid structure that lies just outside the plasma membrane; it creates characteristic shapes for the bacteria and protects from osmotic lysis and toxins, often while increasing pathogenicity B. Overview of bacterial cell wall structure 1. The cell walls of most bacteria contain peptidoglycan (murein) 2. The cell walls of gram-positive bacteria and gram-negative bacteria differ greatly, but both have the periplasmic space between the cell wall material and the plasma membrane C. Peptidoglycan structure 1. Peptidoglycan is a polysaccharide polymer composed of two sugar derivatives with peptide linkers; the polysaccharide polymer is a linear chain of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid(NAM) moieties 2. Polysaccharide chains of peptidoglycan are cross-linked via a peptide interbridge attached to the sugar backbone via a short peptide chain; these peptides contain some amino acids not found in proteins; cross-linking adds strength to the peptidoglycan mesh 3. Variations in peptidoglycan structure are seen in certain bacterial groups and can be diagnostic D. Gram-positive cell walls 1. They consist of a thick wall composed of many layers of peptidoglycan and large amounts of teichoic acids 2. Techoic acids are polymers with a glycerol and phosphate backbone that span the cell wall and likely enhance its structural stability 3. The periplasmic space of gram-positive cells is usually thin and contains only a few secreted proteins (exoenzymes) 4. Most gram-positive bacteria also have a layer of proteins (S-layer proteins) on the outer surface of the peptidoglycan that have a role in wall synthesis and virulence 5. Acid-fast bacteria include mycolic acids in their cell walls E. Gram-negative cell walls 1. The gram-negative cell wall is more complex than the gram-positive cell wall; and has a thin layer of peptidoglycan surrounded by an outer membrane 2. The periplasmic space is often wide and contains many different proteins; some are involved with energy conservation or nutrient acquisition 3. The outer membrane is composed of lipids, lipoproteins, and lipopolysaccharides (LPS); Braun's lipoprotein attaches the outer membrane to the peptidoglycan 4. LPS are large complex molecules composed of lipid A, core polysaccharides, and O antigen carbohydrate side chains; LPS stabilize the outer membrane, protect against some toxins, and can cause strong host immunological responses, acting as an endotoxin 5. The outer membrane is more permeable than the plasma membrane because of porin proteins that form channels through which molecules smaller than 600 daltons can pass F. Mechanism of Gram staining 1. After staining with crystal violet, constriction of the thick peptidoglycan layer of gram-positive cells during decolorization prevents the loss of the crystal violet stain 2. The thinner, less cross-linked peptidoglycan layer of gram-negative bacteria does not retain the stain, and thus more readily decolorized when treated with alcohol G. Cell walls and osmotic protection 1. The cell wall prevents swelling and lysis of bacteria in hypotonic solutions; in hypertonic habitats, the plasma membrane shrinks away from the cell wall in a process known as plasmolysis 2. Gram -Bacteria without cell walls (by removal with lysozyme or through peptidoglycan synthesis inhibition by penicillin) called spheroplasts while Gram + are called protoplast. Both are osmotically sensitive 3. Mycoplasmas lack a cell wall a
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