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Chapter 27 Prokaryotes PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Overview: They’re (Almost) Everywhere! • Most prokaryotes are microscopic, but what they lack in size they more than make up for in numbers • The number of prokaryotes in a single handful of fertile soil is greater than the number of people who have ever lived • Biologists are discovering that these organisms have an astonishing genetic diversity Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Prokaryotes thrive almost everywhere – Including places too acidic, too salty, too cold, or too hot for most other organisms Figure 27.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 27.1: Structural, functional, and genetic adaptations contribute to prokaryotic success • Most prokaryotes are unicellular – Although some species form colonies Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Prokaryotic cells have a variety of shapes – The three most common of which are spheres (cocci), rods (bacilli), and spirals 1 m Figure 27.2a–c (a) Spherical (cocci) 2 m (b) Rod-shaped (bacilli) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 5 m (c) Spiral Cell-Surface Structures • One of the most important features of nearly all prokaryotic cells is their cell wall, which maintains cell shape, provides physical protection, and prevents the cell from bursting in a hypotonic environment • Using a technique called the Gram stain scientists can classify many bacterial species into two groups based on cell wall composition, Gram-positive and Gramnegative Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lipopolysaccharide Cell wall Outer membrane Peptidoglycan layer Cell wall Peptidoglycan layer Plasma membrane Plasma membrane Protein Protein Grampositive bacteria Gramnegative bacteria 20 m (a) Gram-positive. Gram-positive bacteria have a cell wall with a large amount of peptidoglycan that traps the violet dye in the cytoplasm. The alcohol rinse does not remove the violet dye, which masks the added red dye. Figure 27.3a, b Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (b) Gram-negative. Gram-negative bacteria have less peptidoglycan, and it is located in a layer between the plasma membrane and an outer membrane. The violet dye is easily rinsed from the cytoplasm, and the cell appears pink or red after the red dye is added. • The cell wall of many prokaryotes is covered by a capsule, a sticky layer of polysaccharide or protein 200 nm Capsule Figure 27.4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Some prokaryotes have fimbriae and pili – allows them to stick to their substrate or other individuals in a colony Fimbriae 200 nm Figure 27.5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Motility • Most motile bacteria propel themselves by flagella – structurally and functionally different from eukaryotic flagella • In a heterogeneous environment, many bacteria exhibit taxis – The ability to move toward or away from certain stimuli Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Flagellum Filament 50 nm Cell wall Hook Basal apparatus Plasma membrane Figure 27.6 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Internal and Genomic Organization • Prokaryotic cells usually lack complex compartmentalization • Some prokaryotes do have specialized membranes that perform metabolic functions 0.2 m 1 m Respiratory membrane Thylakoid membranes Figure 27.7a, b (a) Aerobic prokaryote Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (b) Photosynthetic prokaryote • The typical prokaryotic genome is a ring of DNA that is not surrounded by a membrane and that is located in a nucleoid region • Some species of bacteria also have smaller rings of DNA called plasmids Chromosome Figure 27.8 1 m Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Reproduction and Adaptation • Prokaryotes reproduce quickly by binary fission and can divide every 1–3 hours • Rapid reproduction and horizontal gene transfer (via conjugation) facilitate the evolution of prokaryotes to changing environments Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Many prokaryotes form endospores – Which can remain viable in harsh conditions for centuries Endospore 0.3 m Figure 27.9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings