Topic 3. The Prokaryotes Introduction, Structure & Function, Classification, Examples September 21, 2005 Biology 1001

3.1 Introduction to the Prokaryotes First life to evolve about 3.5 BYA; alone on Earth for 2 BYA  Two groups diverged early in life’s history – Archaea & Bacteria  Genetically diverse lineages due to almost 4 billion years of evolution

3.1 Introduction to the Prokaryotes Prokaryotes are microscopic, unicellular, and simple in form, but they dominate the biosphere  Biomass 10X greater than all eukaryotes  More bacteria in a handful of soil than people who have ever lived  More than 4000 species described; perhaps as many as 4 million exist

3.1 Introduction to the Prokaryotes Inhabit diverse environments – they’re almost everywhere –Salty, acidic, hot, cold, anaerobic…places where nothing else lives –A wealth of metabolic diversity and other evolutionary adaptations Serve vital ecological roles  Chemical recycling  Mutually beneficial symbiotic relationships  Most are not pathogenic!

3.2 Structure and Function of Prokaryotes Small (usually 1-5 µM) and structurally simple  Evolution and diversity at the chemical or metabolic level Three common shapes – spheres, rods, and spirals Most are unicellular  Some stick together and form clumps or chains Thiomargarita namibiensis (750 µm)

Prokaryotic Vs. Eukaryotic Cells All cells have the following components Plasma membrane – membrane enclosing the cytoplasm Cytoplasm – space between plasma membrane and nucleus, interior of cell in prokaryotes Cytosol – semi-fluid substance in the cytoplasm Ribosomes – “organelles” that synthesize proteins Chromosomes – contain DNA and associated proteins Eukaryotic (eu = true, karyon=kernel) cells also have a membrane-bound nucleus that contains the chromosomes, are larger ( µm), and contain other membranous organelles and structures

Features of the Prokaryotic Cell In prokaryotic (pro=before, karyon=kernel) cells the single chromosome is concentrated in a non-membrane-bound region called the nucleoid  In addition, prokaryotes may have smaller rings of DNA called plasmids that contain only a few genes (usually for antibiotic resistance or metabolism of rare nutrients) and replicate independently of the main chromosome

Features of the Prokaryotic Cell Hairlike appendages called fimbriae (Sl. fimbria) or pili (Sl. pilus) allow prokaryotes to stick to their substrate or each other An external capsule (layer of polysaccharide or protein) also enables adherence, and provides protection for pathogens Nearly all prokaryotes have a cell wall, a rigid structure found outside the plasma membrane, that protects the cell & helps maintains cell shape

The Structure and Function of the Cell Wall (Section 5.2 of Course Outline) Most Bacteria cell walls contain peptidoglycan – a modified sugar polymer cross-linked by short polypeptides Archaea cell walls contain a variety of polysaccharides and proteins A technique called Gram stain is often used to classify Bacterial species on the basis of differences in cell wall composition  Gram-positive bacteria have simpler walls with a large amount of peptidoglycan  Gram-negative bacteria less peptidoglycan and are structurally more complex, with an outer membrane containing lipopolysaccharides

Gram Staining Figure 27.3

Motility  About half of all prokaryotes are capable of directed movement or taxis, at speeds up to or exceeding 50 µm/sec  The most common structures enabling prokaryotes to move are the flagella (Sl. flagellum)

Internal Organization of Prokaryotes Lack complex organization but some do have specialized membranes that perform metabolic functions. These are usually infoldings of the plasma membrane.

The Prokaryotic Cell Figure 6.6