Bio number 2 Prokaryotes: Bacteria and Archaea

Domains of Life and Prokaryotic Classification

  • The Three Domains of Life:

    • Domain Bacteria: Unicellular prokaryotic organisms; representative scale depicted at 2 μm2\,\mu m.

    • Domain Archaea: Unicellular prokaryotic organisms; representative scale depicted at 2 μm2\,\mu m.

    • Domain Eukarya: Organisms with eukaryotic cellular organization; representative scale depicted at 100 μm100\,\mu m. Comprises four primary groups:

      • Kingdom Plantae

      • Kingdom Fungi

      • Protists

      • Kingdom Animalia

  • Defining Characteristics of Prokaryotes:

    • Prokaryotes are divided into Domain Bacteria and Domain Archaea.

    • Both domains lack a membrane-bound nucleus and lack the membrane-bound cytoplasmic organelles characteristic of eukaryotic cells.

    • Separation between Bacteria and Archaea is based on distinct molecular and cellular differences.

    • Table 27.2 provides a systematic baseline comparison among all three domains of life.

Mechanisms of Reproduction

  • Asexual Reproduction:

    • A single individual acts as the sole parent.

    • Passes complete copies of all its genes directly to its offspring.

    • Offspring produced via asexual reproduction are exact genetic copies or clones of the parent.

  • Sexual Reproduction:

    • Two parents contribute genetic material to give rise to offspring.

    • Offspring possess unique combinations of genes inherited from both parents.

    • Offspring are genetically variable and distinct from both parents and siblings.

Nutritional Strategies and Metabolic Diversity

  • Autotrophs:

    • Organisms capable of synthesizing their own organic compounds from simple inorganic substances present in their surroundings.

    • Can sustain themselves without consuming anything derived from other living organisms.

    • Photoautotrophs: Capture and utilize light energy from sunlight.

    • Chemoautotrophs: Capture energy released from inorganic chemical molecules.

  • Heterotrophs:

    • Organisms requiring an external source of organic compounds for energy and cellular building blocks.

    • Saprotrophs (or Saprophytes): A specialized category of heterotrophs that perform extracellular digestion on dead or decaying organic matter.

Ecological Roles and Species Interactions

  • Roles in the Ecosystem:

    • Autotrophs: Function as the primary producers of the biosphere.

    • Heterotrophs: Function as consumers within the biosphere.

    • Saprotrophs: Function as decomposers, breaking down organic waste to return chemicals to their inorganic forms.

  • Trophic Energy and Nutrient Flow:

    • Energy and nutrients pass sequentially through food webs along the following pathway:         Primary Producers (Autotrophs)→Primary Consumers (Herbivores)→Secondary Consumers (Carnivores)→Tertiary Consumers (Carnivores feeding on carnivores)→Decomposers\text{Primary Producers (Autotrophs)} \rightarrow \text{Primary Consumers (Herbivores)} \rightarrow \text{Secondary Consumers (Carnivores)} \rightarrow \text{Tertiary Consumers (Carnivores feeding on carnivores)} \rightarrow \text{Decomposers}

  • Ecological Interactions (Symbiosis):

    • Symbiosis: An ecological relationship in which two distinct species live in close physical contact with one another. The larger organism is designated as the host, and the smaller organism is designated as the symbiont.

    • Mutualism: A symbiotic interaction in which both organisms derive mutual benefit.

    • Commensalism: An interaction in which one organism benefits while the other is neither significantly harmed nor helped.

    • Parasitism: An interaction in which a parasite harms the host organism without immediately killing it.

    • Pathogens: Parasites specifically capable of causing disease.

  • Prokaryotic Abundance and Environmental Adaptations:

    • Prokaryotes thrive virtually everywhere, including extreme environments that are too acidic, salty, cold, or hot for most eukaryotic life.

    • Although microscopic in size, their environmental impact is driven by immense numerical abundance.

    • A single handful of fertile soil contains more individual prokaryotes than the total number of human beings who have ever lived.

Prokaryotic Morphology, Size, and Microscopic Examples

  • Cellular Size Ranges:

    • Prokaryotic cells generally range between 0.5 μm0.5\,\mu m and 5 μm5\,\mu m in size.

    • They are significantly smaller than eukaryotic cells, which typically range from 10 μm10\,\mu m to 100 μm100\,\mu m in size.

  • Basic Morphological Shapes:

    • Bacillus (plural: Bacilli): Rod-shaped bacteria.

      • Example: Bacillus anthracis (observed via Scanning Electron Microscopy at 35,000×35{,}000\times magnification).

    • Coccus (plural: Cocci): Spherical bacteria.

      • Example: Streptococcus thermophilus (observed via Scanning Electron Microscopy at 3,520×3{,}520\times magnification).

    • Spirillum (plural: Spirillia): Spiral, helical, or curved bacteria.

      • Example: Spirillum volutans (observed via Scanning Electron Microscopy at 6,250×6{,}250\times magnification).

  • Cellular Arrangements:

    • Prokaryotes can exist as solitary single cells or form characteristic multicellular groupings and arrangements.

Prokaryotic Cell Envelope and Internal Anatomy

  • Cell Envelope Components:

    • Plasma Membrane:

      • Present in all prokaryotes as a membrane sheath enclosing the cytoplasm.

      • Regulates the entrance and exit of molecules.

      • Chemical structure differs fundamentally between Domain Bacteria and Domain Archaea.

    • Cell Wall:

      • External layer providing mechanical support, maintaining cell shape, and offering protection.

      • Most bacterial cell walls contain peptidoglycan.

      • The thickness of the peptidoglycan wall forms the structural basis for the Gram stain reaction.

      • Gram-negative bacteria usually possess a secondary outer membrane consisting of a lipid bilayer external to their thin peptidoglycan layer.

      • Archaeal cell walls contain proteins and polysaccharides, but completely lack peptidoglycan.

    • Capsule:

      • A gel-like outer coating located exterior to the cell wall in certain species.

      • Serves as a protective layer against environmental stress and host immune system responses.

  • Appendages and Internal Structures:

    • Fimbriae: Hairlike protein bristles on the surface that mediate adhesion to environmental surfaces or host tissues.

    • Flagellum: A rotating protein filament present in certain bacteria that pushes the cell forward to provide motility.

    • Ribosomes: Cytoplasmic complexes that serve as the site of protein synthesis.

Genetic Organization and Nucleic Information

  • Nucleoid:

    • The non-membrane-enclosed region of the cytoplasm where the primary bacterial DNA is concentrated.

    • Contains a single circular chromosome holding essential survival and operational genes.

  • Plasmids:

    • Small, accessory rings of extrachromosomal DNA found in many bacteria.

    • Replicate independently of the primary chromosome.

    • Carry supplementary genetic information, such as:

      • Antibiotic resistance genes

      • Virulence factors

    • Structural spatial arrangement of the chromosome and plasmids is shown relative to a 1 μm1\,\mu m scale bar in Figure 27.8.