Cell Membranes

Structure of the Plasma Membrane

The plasma membrane is a fundamental structure in all cells, serving as a selective barrier and interface with the environment.

• Phospholipid Bilayer: Composed of two layers of phospholipids, each with a hydrophilic (water-attracting) head and hydrophobic (water-repelling) tails. This arrangement makes the membrane amphipathic.

• Proteins: Embedded within the bilayer, these proteins regulate the movement of substances in and out of the cell.

Example: The plasma membrane of Escherichia coli contains transport proteins that allow glucose to enter the cell.

Movement of Molecules Across Membranes

Transport across the plasma membrane can occur via passive or active mechanisms.

• Passive Mechanisms: Do not require energy.

  • Simple Diffusion: Movement of molecules from high to low concentration.

  • Facilitated Diffusion: Movement via membrane proteins (channels or carriers).

  • Osmosis: Diffusion of water across a selectively permeable membrane from a high to low concentration.

    • Isotonic: Equal solute concentration inside and outside the cell.

    • Hypotonic: Lower solute concentration outside; water enters the cell.

    • Hypertonic: Higher solute concentration outside; water leaves the cell.

• Active Mechanisms: Require energy (usually ATP).

  • Active Transport: Movement of molecules against their concentration gradient using energy.

Example: Sodium-potassium pumps in eukaryotic cells use ATP to maintain ion gradients.

Structures Internal to the Plasma Membrane in Prokaryotes

The Nucleoid

The nucleoid is the region in prokaryotic cells where the genetic material (DNA) is located, not surrounded by a membrane.

• Plasmids: Small, circular DNA molecules that replicate independently of the chromosomal DNA.

Ribosomes

Ribosomes are the sites of protein synthesis in all cells.

• Prokaryotic Ribosomes: 70S type, composed of a 50S large subunit and a 30S small subunit.

• Importance of 16S rRNA: The 16S ribosomal RNA is crucial for the function and identification of prokaryotes.

Inclusions

Inclusions are reserve deposits found in prokaryotic cells, storing nutrients or other substances.

Endospores

Endospores are highly resistant, dormant structures formed by some bacteria for survival in harsh conditions. Most prokaryotes do not make endospores.

The Archaea

Characteristics

Archaea are a distinct domain of life, often found in extreme environments.

• Often live in habitats similar to extremes (e.g., high temperature, salinity, acidity).

• More closely related to Domain Eukarya than to Bacteria, based on genetic and biochemical traits.

Example: Halobacterium species thrive in highly saline environments.

Bacterial Taxonomy and Classification

Methods of Classification

Bacteria are classified based on observable traits and genetic information.

• Observable Traits: Morphology, cultural characteristics, and biochemical reactions.

• Clinical Microbiology: Uses these traits to identify bacteria in laboratory settings, focusing on diagnostic themes.

Bergey's Manual of Systematic Bacteriology

This manual is a key reference for bacterial classification, emphasizing genetic relationships and phylogeny.

• Based on relatedness and sequencing information.

• Details phylogenetic relationships (taxonomic scheme).

Species, Strains, and Subspecies

A species is a collection of bacterial cells sharing similar traits, distinct from other groups. Strains or subspecies are variations within a species.

• Species: Group of bacteria with similar characteristics.

• Strain/Subspecies: Subdivisions within a species, often with minor genetic or phenotypic differences.

Additional info: The notes have been expanded to provide context and examples for each topic, ensuring completeness and clarity for microbiology students.