Prokaryote Overview and Bacteria Classification

Prokaryote Overview and Bacteria Classification

• Prokaryotes are ancient and simple life forms, single-celled organisms lacking a nucleus and membrane-bound organelles.
• Two domains of prokaryotes: Bacteria and Archaea.
• Key features of prokaryote structure:
  • No nucleus: Genetic material (DNA) located in the nucleoid region, directly in contact with the cytoplasm.
  • Plasma membrane: Controls substance entry and exit.
  • Cell wall: Provides structural support and protection, mostly composed of peptidoglycan in bacteria.
  • Cytoplasm: Jelly-like substance where cellular activities occur.
  • Ribosomes: Sites of protein synthesis.
  • Flagella: Long, whip-like structures for movement.
  • Pili: Short, hair-like structures for attachment.

Bacteria

• Most well-known prokaryotes, found in various habitats.
• Essential roles in ecosystems: decomposition, nutrient cycling, symbiotic relationships.
• Gram-positive and Gram-negative bacteria: Classified based on cell wall structure and staining technique.
  • Gram-positive: Thick layer of peptidoglycan, retaining purple dye in Gram stain.
  • Gram-negative: Thinner layer of peptidoglycan, outer membrane present, staining pink or red.
• Beneficial bacteria: Essential for human digestion, vitamin production, and protection against harmful microbes.
• Pathogenic bacteria: Can cause diseases in humans, animals, and plants.

Archaea

• Separate domain of life, but share similarities with bacteria (e.g., single-celled, lack a nucleus).
• Key differences from bacteria:
  • Cell wall composition: No peptidoglycan, composed of different materials.
  • Membrane lipids: Chemically distinct, more stable in extreme environments.
• Extremophiles: Thrive in harsh conditions where other life forms cannot survive (e.g., Thermophiles, Halophiles).
• Methanogenesis: Unique metabolic process where methane is a byproduct.

Importance of Prokaryotes

• Decomposers and nutrient cycling: Break down dead organic matter, recycle nutrients.
• Symbiotic relationships: Benefit both prokaryotes and their host organisms.
• Essential for human health: Gut microbiome plays vital role in digestion, immune function, and mental health.
• Carbon and nitrogen cycling: Prokaryotes crucial for converting atmospheric nitrogen into usable forms for plants.

Bacterial Structure and Nutrition

Bacteria: Structure and Nutrition

• Single-celled organisms, belonging to the domain Bacteria.
• Characteristics:
  • Prokaryotic cells: Lack a membrane-bound nucleus and organelles.
  • Size: Microscopic, ranging from 0.2 to 10 micrometers.
  • Shapes: Cocci (spherical), Bacilli (rod-shaped), Spirilla (spiral), Vibrios (comma-shaped), Spirochaetes (flexible spiral).
• Importance: Decomposition, nitrogen fixation, symbiotic relationships.

Bacterial Cell Structure

• Cell Wall: Provides shape, protection, and prevents bursting in hypotonic environments.
  • Peptidoglycan: Main component, a polymer of sugars and amino acids, forming a mesh-like layer.
• Types of Bacterial Cell Walls:
  • Gram-positive: Thick peptidoglycan layer, retains crystal violet dye (appears purple).
  • Gram-negative: Thin peptidoglycan layer, outer membrane present, does not retain crystal violet (appears pink/red).
• Plasma Membrane: Selectively permeable barrier controlling substance movement. Composed of a phospholipid bilayer with embedded proteins.
• Cytoplasm: Jelly-like substance within the plasma membrane, containing:
  • Ribosomes: Sites of protein synthesis.
  • Nucleoid: Region containing bacterial chromosome.
  • Plasmids: Small, circular DNA molecules carrying extra genes (e.g., antibiotic resistance).
• External Structures:
  • Flagella: Long, whip-like structures for movement.
  • Pili (Fimbriae): Hair-like structures for attachment and conjugation.
  • Capsule: Gelatinous layer protecting against desiccation, phagocytosis, and immune system.

Bacterial Nutrition

• Autotrophic Bacteria: Produce their own organic compounds from inorganic sources.
  • Photoautotrophs: Obtain energy from light through photosynthesis (e.g., cyanobacteria).
  • Chemoautotrophs: Obtain energy by oxidizing inorganic substances (e.g., in extreme environments).
• Heterotrophic Bacteria: Obtain organic carbon from other organisms or matter.
  • Saprotrophs (Saprophytes): Feed on dead/decaying organic matter (important decomposers).
  • Parasites: Obtain nutrients from a living host, often causing disease.
  • Symbionts: Mutually beneficial relationships with other organisms (e.g., Rhizobium bacteria in legume roots).
• Nutrient Uptake Mechanisms:
  • Passive Transport: Movement down concentration gradient, no energy required (simple diffusion, facilitated diffusion).
  • Active Transport: Requires energy (ATP) to move molecules against concentration gradient.
  • Group Translocation: Substance chemically modified during transport (e.g., phosphotransferase system for glucose).

Bacterial Metabolism

• Sum of chemical reactions within a bacterial cell.
• Catabolism: Breakdown of complex molecules into simpler ones, releasing energy (ATP).
  • Glycolysis: Breakdown of glucose into pyruvate, producing a small amount of ATP and NADH.
  • Fermentation: Conversion of pyruvate into organic compounds in oxygen-free environments (e.g., lactic acid, ethanol).
  • Respiration: Aerobic respiration (using oxygen) produces much more ATP via the Krebs cycle and electron transport chain. Anaerobic respiration uses other electron acceptors.
• Anabolism: Synthesis of complex molecules from simpler ones, requiring energy input.
  • Used to build macromolecules needed for cell structure and function (e.g., proteins, nucleic acids, lipids).