HO - 04 Prokaryotes

Prokaryotic Cells Overview

Prokaryotic cells are single-celled organisms that lack a nuclei and membrane-bound organelles, distinguished primarily into two categories based on their cell envelope structure: Gram-positive and Gram-negative bacteria, as identified by Hans Gram.

Gram Staining

  • Gram-positive Bacteria: These bacteria are characterized by a thick peptidoglycan layer that retains the crystal violet stain, appearing purple under a microscope. They do not have an outer membrane.

  • Gram-negative Bacteria: In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and possess an outer membrane that prevents the violet stain from penetrating, leading to a pink color after applying a counterstain.

Cell Wall Structures

Gram-positive Bacteria

  • Composed of a thick peptidoglycan layer, approximately 80 nm thick, providing structural integrity.

  • Lacks an outer membrane, with the cell wall directly adjacent to the plasma membrane.

Gram-negative Bacteria

  • Characterized by a thin peptidoglycan layer (~20 nm thick) sandwiched between two membranes: an inner plasma membrane and an outer membrane made of lipopolysaccharides (LPS). This configuration contributes to its lower retention of stains due to the limited exposure of the peptidoglycan to staining agents.

Peptidoglycan Structure

  • Peptidoglycan is a carbohydrate polymer consisting of alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by β (1→4) glycosidic bonds.

  • Each polymer chain is cross-linked by tetrapeptide sidechains, contributing to the robustness of bacterial cell walls.

Antibiotic Action

Penicillin

  • Penicillin disrupts bacterial cell wall formation by inhibiting enzymes responsible for cross-linking the peptidoglycan, thereby rendering the bacteria susceptible to lysis. Various types of penicillin are derived from Penicillium fungi.

Lysozyme

  • Alexander Fleming discovered lysozyme, an enzyme that cleaves β (1→4) glycosidic bonds in peptidoglycan. Found in bodily secretions such as tears and saliva, it degrades cell walls, leaving bacteria vulnerable to osmotic lysis.

Structures of E. coli

Lipopolysaccharides (LPS)

  • E. coli, a Gram-negative bacterium, has LPS on its cell surface, consisting of an O antigen for pathogen identification and Lipid A which triggers immune responses, potentially causing septic shock.

Appendages

  • Fimbriae: Short, hair-like structures used by E. coli and other bacteria for adherence to surfaces and host cells.

  • Flagella: Longer than fimbriae, used for locomotion, powered by a proton gradient.

Bacterial Capsules and Biofilms

  • Capsule: A thick polysaccharide layer surrounding some bacteria providing protection and contributing to adherence, distinct from a slime layer which is more flexible.

  • Biofilms: Structures formed by the adhesion of bacteria within a polysaccharide matrix; dental plaque serves as an example of a complex biofilm.

Mycoplasma and Pathogenicity

  • Mycoplasma: A class of bacteria that lack a cell wall, making them resistant to β-lactam antibiotics. Some species are pathogenic in humans.

Archaea Characteristics

  • The membranes of archaea differ from bacteria: they consist of ether-linked phospholipids and often feature S-layers (protein shells) instead of peptidoglycan.

Cell Contents

Cytoplasm and Chromosome

  • The cytoplasm contains everything within the plasma membrane apart from the nucleoid, which houses circular double-stranded DNA (the chromosome).

  • The chromosome carries essential genetic information, replicated prior to cell division.

Plasmids

  • Plasmids are small, circular DNA molecules that replicate independently of the chromosomal DNA. They can carry genes for antibiotic resistance (R-plasmids) or traits for genetic conjugation (F-plasmids).

Ribosomes

  • Prokaryotic ribosomes synthesize proteins from amino acids using mRNA, composed of two subunits (50S and 30S), totaling 70S. They play a crucial role in translating genetic information into proteins necessary for cell function.

Compartmentalization in Prokaryotes

  • Prokaryotic cells incorporate various membrane structures for distinct functions, such as respiration and photosynthesis. Unique structures like magnetosomes allow some bacteria to navigate using magnetic field lines, showcasing adaptability.