27.1 Prokaryotes are the smallest and most numerous organisms.
The Prevalence of Prokaryotes
• Archaebacteria and bacteria are the oldest, structurally simplest, and most abundant forms of life on earth. (p. 546)
• About 5000 different kinds of prokaryotes are currently recognized. (p. 546)
• In the 1980s, a new discovery led to the division of prokaryotes into two groups: archaebacteria and bacteria. (p. 546)
• Prokaryotes are ubiquitous on earth and exist in some of the most hostile environments on the planet. (p. 546)
• Bacteria exhibit one of three basic structures: bacillus (straight and rod-shaped), coccus (spherical-shaped), and spirillum (long and helical-shaped). (p. 546)
• Prokaryotes differ from eukaryotes in many important features, including unicellularity, cell size, chromosomes, cell division and genetic recombination, internal compartmentalization, flagella, and metabolic diversity. (pp. 546-547)
27.2 Prokaryotes exhibit considerable diversity in both structure and metabolism.
Prokaryotic Diversity
• Key characteristics used in classifying bacteria are photosynthetic or nonphotosynthetic, motile or nonmotile, unicellular or colony-forming or filamentous, and formation of spores or division by transverse binary fission. (p. 548)
• Molecular approaches to classification include analysis of amino acid sequences of key proteins, analysis of nucleic acid base sequences, nucleic acid hybridization, ribosomal RNA sequencing, and whole-genome sequencing. (p. 548)
• Archaebacteria and bacteria differ in four key areas: plasma membrane, cell wall, gene translation machinery, and gene architecture. (p. 548)
27.3 Prokaryotes are more complex than commonly supposed.
The Prokaryotic Cell Surface
• A prokaryotic cell wall usually consists of peptidoglycan, forming either thick or thin walls; prokaryotes can be classified and identified by Gram staining as either gram-positive or gram-negative bacteria. (p. 550)
• Many prokaryotes have slender, rigid flagella for propulsion, and some prokaryotes have pili to help in attachment and genetic exchange. (p. 550)
The Cell Interior
• Prokaryotic cells lack the extensive functional compartmentalization found in eukaryotic cells, but do possess internal membranes, a nucleoid region, and ribosomes. (p. 551)
Prokaryotic Variation
• Bacteria have a very short generation time, thus mutation and genetic recombination play an important role in producing and maintaining genetic diversity. (p. 552)
Prokaryotic Metabolism
• Autotrophs can be broken down into photoautotrophs and chemoautotrophs; heterotrophs can be broken down into photoheterotrophs and chemoheterotrophs. Chemoheterotrophs appear to be the most common. (p. 553)
• Almost every kind of plant is susceptible to one or more bacterial diseases. (p. 553)
27.4 Prokaryotes are responsible for many diseases, but they also make important contributions to ecosystems.
Human Bacterial Diseases
• Tuberculosis and dental caries are just two of the many human diseases caused by bacteria. A few others are cholera, leprosy, tetanus, bacterial pneumonia, whooping cough, diptheria, and Lyme disease. (p. 554)
• A number of bacteria also cause sexually transmitted diseases such as gonorrhea, syphilis, and chlamydia. (p. 556)
Benefits of Prokaryotes
• Prokaryotes affect everyday life in many ways, including decomposition of dead organisms, chemical fixation, symbiotic relationships (such as that between nitrogen-fixing bacteria and plant roots), genetic engineering improvements in agricultural crops, commercial production of pharmaceuticals, and bioremediation. (pp. 557-558)
• Some disease-causing bacteria can even be used as forms of bioterrorism. (p. 558)