Download 18.4 – Bacteria and Archaea Prokaryotes are

18.4 – Bacteria and Archaea
Prokaryotes are widespread on earth
Prokaryotes – single-celled, no membrane-bound nucleus or
organelles, smaller than eukaryotic cells, grouped based on their
need for oxygen
 Obligate anaerobes – prokaryotes that cannot live in the
presence of oxygen, ex: archaea prokaryotes that produce
methane gas (live in marshes, bottom of lakes, and in
digestive tracts of herbivores)
 Obligate aerobes – organisms that need oxygen in their
environment, ex: microorganisms that cause tuberculosis and
leprosy
 Facultative aerobe – prokaryote that can survive whether
oxygen is present in the environment or not
Bacteria & Archaea are structurally similar (single-celled
prokaryotes), but have different molecular characteristics
 Domain Bacteria & Domain Archaea contain ALL known
prokaryotes
 Domain Bacteria is more widespread and diverse
 Domain Archaea can survive in extreme environments
 Structural comparisons:
o Both archaea and bacteria are small, single-celled
organisms that have cell walls and plasma membranes
o Archaea come in many shapes, while most bacteria fall
into 3 shapes:
 Rod-shaped: bacilli
 Spiral-shaped: spirilla or spirochetes
 Spherical: cocci
o DNA is in the form of a circle and is surrounded by
cytoplasm
o May have plasmids – small pieces of genetic material
that can replicate separately from the main chromosome
o Most prokaryotes can move on their own by gliding or
using flagella
 Flagellum – long, whip-like structure outside of a
cell that is used for movement
o May also have pili – thinner, shorter structures that
allow prokaryotes to stick to surfaces or other
prokaryotes
 Molecular comparisons:
o Bacteria and archaea are not closely related
o Archaea membranes contain lipids that are not found
anywhere else on earth
o Bacteria cell walls contain polymers called peptidoglycan
 Amount of peptidoglycan in cell walls is an
important characteristic of bacteria
 Gram stain
 Gram positive – thicker peptidoglycan layer,
stains purple
 Gram negative – thinner peptidoglycan layer,
stains red
Bacteria have various strategies for survival
 Gene exchange:
o Prokaryotes reproduce via asexual reproduction
o Binary fission – chromosome copies itself, cell grows and
splits, taking a full set of genes with it
o Conjugation – prokaryotes exchange parts of their
chromosomes through a hollow pili bridge
 Surviving harsh conditions:
o Some bacteria can produce an endospore – a specialized
cell with a thick, protective wall