3.3 Prokaryotic Cell Structure

Prokaryotic cells, like bacteria, have unique structures that set them apart from eukaryotes. Their genetic material floats freely in the cytoplasm, and they have special features like plasmids and a tough cell wall. These structures help them survive and thrive in various environments.

Bacteria also have cool appendages like pili and flagella. Pili help them stick to surfaces and share DNA, while flagella let them swim around. These features, along with their simple internal organization, make prokaryotes incredibly adaptable and successful organisms.

Genetic Material

Nucleoid Structure and Function

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• Nucleoid consists of a single circular molecule of DNA not enclosed within a nuclear membrane
• Houses the bacterial chromosome
• Lacks histone proteins (eukaryotic chromosomes)
• Condenses the DNA molecule to fit inside the cell
• Controls cellular processes through gene expression and regulation

Plasmids: Extrachromosomal DNA

• Plasmids are small, circular pieces of DNA separate from the main bacterial chromosome
• Contain genes that provide additional functions or traits to the cell (antibiotic resistance)
• Can be transferred between bacteria through horizontal gene transfer processes like conjugation
• Replicate independently of the bacterial chromosome
• Utilized in genetic engineering and biotechnology to introduce foreign genes into bacteria

Ribosomes: Protein Synthesis Machinery

• Ribosomes are small, spherical organelles composed of rRNA and proteins
• Serve as the site of protein synthesis in the cell
• Prokaryotic ribosomes are smaller (70S) than eukaryotic ribosomes (80S)
• Consist of two subunits: the small (30S) and large (50S) subunits
• Translate mRNA into polypeptide chains using tRNA molecules and amino acids
• Can be found free in the cytoplasm or attached to the cell membrane

Cell Envelope Structures

Cell Wall Composition and Function

• Cell wall is a rigid structure surrounding the plasma membrane
• Provides shape, support, and protection against osmotic stress
• Composed of peptidoglycan, a mesh-like polymer of sugar and amino acid chains
  • Gram-positive bacteria have a thick cell wall with many layers of peptidoglycan
  • Gram-negative bacteria have a thin cell wall with few layers of peptidoglycan and an outer membrane
• Maintains cell integrity and prevents cell lysis in hypotonic environments
• Acts as a barrier against harmful substances and enzymes

Capsule: Protective Coating

• Capsule is a thick, slimy layer of polysaccharides or proteins that surrounds the cell wall
• Protects the cell from phagocytosis by host immune cells
• Enhances bacterial adherence to surfaces and other cells
• Contributes to biofilm formation, a community of bacteria encased in an extracellular matrix
• Some capsules are virulence factors in pathogenic bacteria (Streptococcus pneumoniae)

Cell Surface Appendages

Pili: Attachment and DNA Transfer

• Pili are short, hair-like protein structures that extend from the cell surface
• Facilitate bacterial attachment to host cells or surfaces
• Sex pili (F pili) are specialized structures that allow for DNA transfer during conjugation
  • Donor cell extends a sex pilus to establish contact with a recipient cell
  • DNA is transferred from the donor to the recipient through the hollow sex pilus
• Type IV pili are involved in twitching motility, a form of surface movement
• Some pili act as receptors for bacteriophages (viruses that infect bacteria)

Flagella: Bacterial Locomotion

• Flagella are long, whip-like protein structures that protrude from the cell surface
• Provide mobility and allow bacteria to swim towards attractants or away from repellents
• Composed of three main parts: the filament, hook, and basal body
  • Filament is the long, helical portion that extends from the cell surface
  • Hook connects the filament to the basal body and acts as a flexible joint
  • Basal body is embedded in the cell membrane and cell wall, anchoring the flagellum
• Rotate to generate propulsive force, enabling bacterial movement through liquid environments
• Flagellar arrangement varies among bacterial species (monotrichous, lophotrichous, amphitrichous, peritrichous)