Download Chapter 4: Cell Structure and Function in the Bacteria and Archaea

Chapter 4 Outline
Cell Structure and Function in Bacteria and Archaea
Introduction
4.1 Diversity among the Bacteria and Archaea
 The Domain Bacteria Contains Some of the Best Studied Prokaryotes
o The majority of the 18 phyla of Bacteria play a positive role in nature
o Proteobacteria contain the largest and most diverse group of species,
including E. coli and many human pathogens
o Firmucutes consist of many gram-positive species, such as Bacillus,
Clostridium, Staphylococcus, and Streptococcus
o Actinobacteria include Streptomyces, the genus that produces the
antibiotic streptomycin
o Cyanobacteria carry out photosynthesis using chlorophyll, responsible
for the appearance of oxygen in Earth’s early atmosphere
o Chlamydiae are extremely small, mostly pathogenic bacteria
 The Domain Archaea Contains Many Extremophiles
o Euryarchaeota contain:
 methanogens (live in anoxic environments)
 extreme halophiles (salt-lovers)
 thermoacidophiles (grow at high temperatures with low pH)
o Crenarchaeota tend to grow in hot, acidic environments such as hot
springs and volcanic vents
4.2 Cell Shapes and Arrangements
 Prokaryotes Vary in Cell Shape and Cell Arrangement
o A bacillus is a prokaryotic cell with a rod shape
o A spherical bacterial cell is a coccus
 Many cocci and some bacilli appear in groups or chains
o Spiral-shaped bacteria can appear as vibrios (comma-shaped), spirilla
(helical), or spirochete
4.3 An Overview to Prokaryotic Cell Structure
 Structure and organization are based on specific processes cells need to carry
out:
o sensing/responding to stimuli
o compartmentation of metabolism
o growth reproduction
4.4 External Prokaryotic Cell Structures
 Pili Are Protein Fibers Extending from the Surface of Many Prokaryotes
o Pili help:
 attach cells to surfaces
 to form biofilms and microcolonies
o Some bacterial species have conjugation pili, used to transfer genetic
material between cells
 Prokaryotic Flagella Are Long Appendages Extending from the Cell Surface
o Flagella can be used for cell motility
o Prokaryotic flagella contain a helical filament, a hook, and a basal
body
 Prokaryotic Flagella Are Long Appendages Extending from the Cell Surface
(Cont.)
o Bacteria exhibit chemotaxis, moving up the concentration gradient of a
chemical attractant
o Some prokaryotes move without flagella by gliding across a solid
surface
o Spirochetes contain endoflagella, which move the cell through torsion
exerted on the cell by endoflagellar rotation
 The Glycocalyx Is an Outer Layer External to the Cell Wall
• The glycocalyx is an adhering layer of polysaccharides (and
sometimes small proteins)
• A thick glycocalyx bound to the cell is a capsule
• A capsule is a thick, diffuse layer of polysaccharides is a slime
layer
• It protects cells from the environment and allows them to attach to
surfaces
4.5 The Cell Envelope
 The Prokaryotic Cell Wall Is a Tough and Protective External Shell
• The cell wall protects the cell from injury and maintains cell shape and
water balance
• Cell walls in prokaryotes contain peptidoglycan
• Gram-positive bacteria have thick peptidoglycan cell walls containing
teichoic acid
• Gram-negative bacteria have a two-dimensional peptidoglycan layer and
no teichoic acid
• The gram-negative cell wall has an outer membrane, separated from the
cell membrane by the periplasmic space
• The outer membrane contains proteins called porins that selectively allow
small molecules into the periplasmic space
 The Archaeal Cell Wall Also Provides Mechanical Strength
• No archaea have peptidoglycan in the cell wall, but some have
pseudopeptidoglycan
• Others have polysaccharides, proteins, or both
• The S-layer is the most common archaeal cell wall, consisting of protein
or glycoprotein in a crystal lattice
 The Cell Membrane Represents the Interface between the Cell Environment
and the Cell Cytoplasm
• The cell membrane is a fluid layer of phospholipid and protein (the fluid
mosaic model)
• The phospholipid molecules are arranged in a bilayer
• Hydrophobic fatty acid chains in the phospholipids form a permeability
barrier
 The Cell Membrane Represents the Interface between the Cell Environment
and the Cell Cytoplasm (cont.)
• Antimicrobial substances may disrupt or dissolve the bilayer
• Membrane proteins perform or aid in many functions, such as:
• cell wall synthesis
• energy metabolism
• DNA replication
• sensation of stimuli
• molecule transport
• Transport of molecules can be passive (facilitated diffusion) or active
(active transport)
 The Archaeal Cell Membrane Differs from Bacterial and Eukaryal
Membranes
• Hydrophobic lipid tails are attached to glycerol differently in archaea
• Fatty acids are usually absent
• Adjacent lipid tails are bound together forming a lipid monolayer, instead
of a bilayer
4.6 The Cell Cytoplasm and Internal Structures
 The Nucleoid Represents a Subcompartment Containing the Chromosome
• The nucleoid is a central subcompartment in the cytoplasm where
DNA aggregates
• The chromosome is usually a closed loop of DNA and protein
• The DNA contains the genes (hereditary information)
• The complete set of genes is called the genome
• Most cells have only one copy of each gene (are haploid), so cannot
undergo mitosis like eukaryotes
 Plasmids Are Found in Many Prokaryotic Cells
• Plasmids are molecules of DNA smaller than the chromosome
• Each plasmid is a closed loop, containing 5–10 genes
• Plasmids can be transferred between cells and can be used as vectors in
genetic engineering
• R plasmids carry genes for resistance to antibiotics
 Other Subcompartments Exist in the Prokaryotic Cytoplasm
• There are hundreds of thousands of ribosomes, used for protein synthesis
• Inclusion bodies store nutrients or building blocks for cellular structures
• Some aquatic bacteria use gas vesicles to float on the water’s surface
• Magnetosomes contain crystals of magnetite or greigite, allowing cells to
respond to magnetic fields
 Prokaryotic Cells Have a “Cytoskeleton”
• Prokaryotes contain a homolog of eukaryotic tubulin
• It forms filaments similar to those found in microtubules
• Proteins homologous to eukaryotic actin can help determine cell shape
• Crescentin, a homolog to eukaryotic intermediate filaments, also assists in
cell shape
4.7 The Prokaryotic/Eukaryotic Cell—Revisited
 Prokaryotes can carry out the “complex” metabolic and biochemical processes
typically associated with eukaryotic cells

Spatial separation of transcription and translation occurs in prokaryotes as
well as eukaryotes