Chapter 13: Prokaryotes and Viruses

Prokaryotes, encompassing Bacteria and Archaea, lack membrane-bound nuclei and organelles but possess circular chromosomes within nucleoids, plasmids carrying supplementary genes, and ribosomes for protein synthesis. Bacterial cell walls contain peptidoglycan, distinguishing gram-positive and gram-negative species, while archaea typically lack walls entirely. Surface structures including capsules, flagella, and pili facilitate protection, motility, and genetic exchange through conjugation, transformation, and transduction. Binary fission serves as the primary reproductive mechanism, though endospore formation enables survival under extreme conditions for extended periods. Prokaryotic metabolism displays remarkable versatility: photoautotrophs like cyanobacteria fix carbon through oxygenic photosynthesis using thylakoids and phycobilin pigments, while chemosynthetic autotrophs derive energy from inorganic substrates. Heterotrophs range from saprophytic decomposers to parasitic pathogens. Oxygen tolerance varies from obligate aerobes to strict anaerobes, and temperature adaptation spans from psychrophiles to extreme thermophiles. Cyanobacteria contribute significantly to planetary oxygen production and nitrogen cycling through heterocyst differentiation; their ecological importance extends to symbiotic partnerships with plants. Archaea occupy specialized niches: extreme halophiles utilize bacteriorhodopsin for energy production in saline environments, methanogens generate methane in anaerobic habitats, and thermophilic archaea thrive in geothermal zones. Viruses are obligate intracellular parasites consisting of nucleic acid genomes encased in protein capsids, sometimes surrounded by lipid envelopes. Plant viruses spread through insect vectors, mechanical transmission, or plasmodesmatal movement using viral movement proteins, causing diseases including mosaics, ringspots, and yellowing. Viral replication strategies vary: RNA viruses often replicate in cytoplasm while DNA viruses typically replicate in nuclei. Host defense mechanisms include hypersensitive response, systemic acquired resistance, and post-transcriptional gene silencing. Viroids, minute circular RNA molecules lacking protein coats, represent the smallest known infectious agents responsible for economically significant plant diseases.