Bacterial and Archaeal Cell Structure and Function

Cell Structure and Function

Introduction to Prokaryotic Cell Structure

Prokaryotes, including Bacteria and Archaea, are single-celled organisms with diverse structures and functions. Understanding their cellular architecture is fundamental to microbiology, as it underpins their physiology, survival, and ecological roles.

Structural Differences and Similarities between Bacteria and Archaea

Bacteria and Archaea share several structural features but also exhibit key differences, especially in their cell walls and membrane composition.

Structural Characteristic	Bacteria	Archaea
Cell type	Prokaryotic	Prokaryotic
Cell morphology	Variable	Variable
Cell wall	Contains peptidoglycan	Does not contain peptidoglycan
Cell membrane type	Lipid bilayer	Lipid bilayer or monolayer
Plasma membrane lipids	Fatty acids-glycerol ester	Phytanyl-glycerol ethers
Chromosome	Typically circular	Typically circular
Replication origins	Single	Multiple
RNA polymerase	Single	Multiple
Initiator tRNA	Formyl-methionine	Methionine
Streptomycin inhibition	Sensitive	Resistant
Calvin cycle	Yes	No

Biochemical Composition of Prokaryotes

All cells share common chemical components, but their proportions and types can vary with species, growth phase, and environmental conditions.

• Water

• Essential ions

• Small organic molecules

• Macromolecules

Example: Escherichia coli cell composition (approximate values):

Component	% of total weight	Approximate number of molecules/cell	Number of different kinds
Water	70	2,000,000,000	1
Proteins	15	2,400,000	2,000
RNA	6	180,000	4,000
Lipids	2	25,000,000	50
Polysaccharides	2	1,000,000	8
DNA	1	1	1
Metabolites and biosynthetic precursors	0.5	500,000	1,000
Ions	1	500,000,000	20
Polyamines	0.1	1,000,000	2

Model of a Bacterial Cell

Major Structural Components

• Cell membrane: Encloses the cytoplasm and is composed of a phospholipid bilayer with embedded proteins. It acts as a selective barrier.

• Cell wall: Covers the cell membrane, providing structural support and shape. In bacteria, it is primarily composed of peptidoglycan.

• Cytoplasm: Gel-like network containing water, proteins, RNA, ribosomes, DNA, and small molecules.

• Nucleoid: Non-membrane-bound region containing the chromosome in looped coils.

• Flagellum: External helical filament with a rotary motor for motility.

Cell Membrane Structure and Function

• Phospholipid bilayer: Composed of hydrophilic heads and hydrophobic tails.

• Membrane proteins: Facilitate transport, energy generation, and signal transduction.

• Membrane lipid diversity: Fatty acid composition varies (e.g., palmitic acid, oleic acid, cyclopropane fatty acid).

• Planar molecules (e.g., hopanoids): Fill gaps between hydrocarbon chains, stabilizing the membrane.

Archaeal membranes have unique features such as ether-linked lipids and can form monolayers or bilayers with cyclopentane rings for stability in extreme environments.

Transport Across the Cell Membrane

• Semipermeable barrier: Allows selective movement of substances.

• Passive transport: Molecules move along their concentration gradient (no energy required).

• Active transport: Molecules move against their concentration gradient (requires energy, often from ATP hydrolysis).

• Membrane-permeant weak acids and bases: $\text{Weak acid: } HA \rightleftharpoons H^+ + A^-$ $\text{Weak base: } B + H_2O \rightleftharpoons BH^+ + OH^-$

The Cell Wall and Envelope

Peptidoglycan Structure

• Peptidoglycan: Unique to bacteria, composed of repeating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) cross-linked by peptide bridges.

• Provides rigidity and protection against osmotic pressure.

• Enzymes for peptidoglycan biosynthesis are targets for antibiotics (e.g., penicillin).

Gram-Positive vs. Gram-Negative Bacteria

• Gram-positive bacteria: Thick peptidoglycan layer, often threaded by teichoic acids. Example: phylum Firmicutes.

• Gram-negative bacteria: Thin peptidoglycan layer, outer membrane containing lipopolysaccharide (LPS). Example: phylum Proteobacteria.

Gram stain procedure distinguishes between these groups based on cell wall structure.

Archaeal Cell Walls

• Do not contain peptidoglycan.

• May have S-layers (protein or glycoprotein), pseudomurein, or other polymers.

• Envelope structure varies widely among archaeal species.

Bacterial Cytoskeleton

Bacteria possess cytoskeletal proteins that help maintain cell shape, polarity, and division. Mutations in these proteins can drastically alter cell morphology.

Key Questions for Understanding Prokaryotic Cells

• What are the similarities and differences between Archaea and Bacteria?

• How do cells maintain selective permeability?

• What provides structural integrity and protection from osmotic pressure?

• How do hydrophilic and hydrophobic molecules interact with the membrane?

• Where is genetic information stored and how is it expressed?

• Where does protein synthesis occur?

• What is the nature of the cytoplasm and outer membrane?

Example Application: Antibiotics such as penicillin target bacterial cell wall synthesis, exploiting the unique presence of peptidoglycan in bacteria.

Additional info: Archaeal cell envelopes are highly variable and adapted to extreme environments, often lacking peptidoglycan and containing unique surface layers.