Biology: Flagella Structure and Function

Flagella Structure and Function

• Flagella are locomotory organelles that move cells through a propeller-like action.
• Embedded in the cell membrane, they project as strands and are composed of flagellin (protein) subunits.
• Some bacteria are motile, responding to stimuli such as chemicals (chemotaxis), light (phototaxis), osmotic pressure (osmotaxis), oxygen (aerotaxis), and temperature (thermotaxis).

Ultrastructure of Flagella

• A flagellum has three basic parts: the filament, hook, and basal body.
• The filament is the long outermost region, containing globular flagellin protein arranged in helical chains around a hollow core.
• The hook is a short, curved tubular structure that connects the basal body to the filament, composed of numerous hook protein (FlgD) subunits.
• The basal body anchors the flagellum to the cell wall and plasma membrane, comprising a small central rod inserted into a series of rings (two pairs in Gram-negative bacteria).

Patterns of Flagella Distribution

• Flagella distribution patterns are unique to each species and include:
  • Monotrichous (single polar flagellum)
  • Lophotrichous (tuft of flagella at one end of the cell)
  • Amphitrichous (flagella at each end of the cell)
  • Peritrichous (flagella distributed over the entire cell)

Axial Filaments (Endoflagella)

• Axial filaments are bundles of fibrils that arise at the ends of the cell beneath an outer sheath and spiral around the cell.
• They are anchored at one end of the spirochete and have a structure similar to that of flagella.

Pili and Fimbriae

• Pili and fimbriae are thin, protein tubes originating from the cytoplasmic membrane of many bacteria.
• They stick bacteria to surfaces, with pili being longer and fewer in number than fimbriae.
• Fimbriae are short, fine appendages surrounding the cell, found in most Gram-negative bacteria, but not in many Gram-positive bacteria.
• They help bacteria adhere to solid surfaces and are a major virulence factor.

Bacterial Morphology

• Bacterial morphology includes:
  • Cocci (spherical shape)
  • Rods or bacilli (cylindrical shape)
  • Spirilla (twisted, spiral shape)

Bacterial Growth

• Binary fission is the process of bacterial growth, involving:
  • DNA replication and attachment to the cell membrane
  • Cell membrane growth between the two DNA molecules
  • Cell membrane pinching inward, followed by cell wall formation, dividing the original cell into two identical daughter cells

Cyanobacteria

• Cyanobacteria (Blue-greens) have both blue and green photosynthetic pigments.
• They contain chlorophyll a, phycocyanin (blue), and phycoerythrin (red).
• They exist as colonies, filaments, and single cells, are prokaryotic, and predominantly aquatic.
• Some species are found in hot water springs and have beneficial vitamins, minerals, proteins, and amino acids.
• Cell structure consists of a definite cell wall (peptidoglycan) surrounded by a mucilagenous sheath.