Topic 3. The ProkaryotesIntroduction,

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 rolesChemical recyclingMutually beneficial symbiotic relationshipsMost 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 (10-100 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 StainingFigure 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 CellFigure 6.6