Chapter 27 Prokaryotes

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!

Prokaryotes thrive almost everywhere Including places too acidic, too salty, too cold, or too hot for most other organisms Figure 27.1

Biologists are discovering That these organisms have an astonishing genetic diversity

Most prokaryotes are unicellular Concept 27.1: Structural, functional, and genetic adaptations contribute to prokaryotic success Most prokaryotes are unicellular Although some species form colonies

Prokaryotic cells have a variety of shapes The three most common of which are spheres (cocci), rods (bacilli), and spirals 1 m 2 m 5 m (a) Spherical (cocci) (b) Rod-shaped (bacilli) (c) Spiral Figure 27.2a–c

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 Gram-negative (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. (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. Figure 27.3a, b Peptidoglycan layer Cell wall Plasma membrane Protein Gram- positive bacteria 20 m Outer membrane Lipopolysaccharide negative

The cell wall of many prokaryotes Is covered by a capsule, a sticky layer of polysaccharide or protein 200 nm Capsule Figure 27.4

Some prokaryotes have fimbriae and pili Which allow them to stick to their substrate or other individuals in a colony 200 nm Fimbriae Figure 27.5

Motility Most motile bacteria propel themselves by flagella Which are structurally and functionally different from eukaryotic flagella Flagellum Filament Hook Cell wall Plasma membrane Basal apparatus 50 nm Figure 27.6

In a heterogeneous environment, many bacteria exhibit taxis The ability to move toward or away from certain stimuli

Internal and Genomic Organization Prokaryotic cells usually lack complex compartmentalization. Some prokaryotes do have specialized membranes that perform metabolic functions. (a) Aerobic prokaryote (b) Photosynthetic prokaryote 0.2 m 1 m Respiratory membrane Thylakoid membranes Figure 27.7a, b

The typical prokaryotic genome Is a ring of DNA that is not surrounded by a membrane and that is located in a nucleoid region Figure 27.8 1 m Chromosome

Some species of bacteria Also have smaller rings of DNA called plasmids

Reproduction and Adaptation Prokaryotes reproduce quickly by binary fission And can divide every 1–3 hours

Many prokaryotes form endospores Which can remain viable in harsh conditions for centuries Endospore 0.3 m Figure 27.9

What creates variation in prokaryotes? Mutations are the main source of variation in prokaryotic populations Rapid reproduction and horizontal gene transfer (ex. transduction, conjugation) facilitate the evolution of prokaryotes to changing environments as well. Transduction – when a bacteriophage carries bacterial genes from one host cell to another Conjugation – genetic material is transferred between two bacterial cells that are temporarily joined