Comparison of Eukaryotic and Prokaryotic Cells and Biological Diversity

Comparison of Eukaryotic and Prokaryotic Cells

  • General Comparison
    • Prokaryotes (Bacteria and Archaea)
      • Nucleus: Absent. DNA is located in a specialized region called the nucleoid.
      • DNA structure: Circular DNA molecules.
      • Organelles: No membrane-bound organelles are present.
      • Size: Usually smaller, ranging from 0.1 μm0.1\, \mu m to 5.0 μm5.0\, \mu m.
      • Reproduction: Primarily asexual via binary fission.
      • Complexity: Generally less complex than eukaryotes.
    • Eukaryotes (Plants, Animals, Fungi, Protists)
      • Nucleus: Present. The DNA is enclosed inside a nuclear envelope.
      • Organelles: Contains various membrane-bound organelles.
      • Size: Usually larger, ranging from 10 μm10\, \mu m to 100 μm100\, \mu m.
      • Reproduction: Can be sexual or asexual.
      • Complexity: Significantly more complex than prokaryotes.

Structure of the Animal Cell

  • Definition: The animal cell is the microscopic unit of life that constitutes animals. It consists of various organelles working in unison to maintain cell health and vitality.
  • Key Characteristics:
    • Usually round or irregular in shape.
    • Lack a cell wall and chloroplasts.
  • Major Parts and Organelles:
    • Cell Membrane: A thin, flexible outer layer that regulates the entry and exit of substances.
    • Cytoplasm: A jelly-like substance that fills the cell, holds organelles, and serves as the site for most cellular activities.
    • Nucleus: The central control center containing DNA; it governs all cellular functions.
    • Nucleolus: A small, round body situated inside the nucleus focused on ribosome production.
    • Mitochondria: Known as the "powerhouse" of the cell, it is the primary site for energy (ATP) production.
    • Endoplasmic Reticulum (ER): A network of tubular structures for materials transport.
      • Smooth ER (Blue): Lacks ribosomes.
      • Rough ER (Pink): Studded with ribosomes.
    • Ribosomes: Tiny dots responsible for protein synthesis.
    • Golgi Apparatus: A stack of flattened sacs that modify, package, and distribute proteins throughout the cell.
    • Lysosomes: Small round sacs containing enzymes to break down waste or unwanted materials.
    • Vacuoles: Small sacs used for the storage of water, food, and other essential materials.

Structure of the Plant Cell

  • Definition: Plant cells are the fundamental building blocks of plant organisms.
  • Distinctive Features (Not found in Animal Cells):
    • Cell Wall: A thick, rigid outer layer providing structural shape, support, and protection.
    • Chloroplasts: Organelles containing chlorophyll that perform photosynthesis to manufacture food.
    • Central Vacuole: A large, fluid-filled sac used for storing water and nutrients while maintaining turgidity (turgor pressure).
  • Shared Organelles with Animal Cells:
    • Cell Membrane: Controls movement in and out of the cell.
    • Nucleus & Nucleolus: Control center and ribosome production site.
    • Cytoplasm: The site of metabolic reactions.
    • Mitochondria: The site of cellular respiration and energy (ATP) production.
    • Ribosomes: Locations for protein synthesis.
    • ER & Golgi Apparatus: Involved in transport and packaging of molecules.

Taxonomy and Biological Classification

  • Definition: Taxonomy is the scientific discipline dedicated to naming, defining, and classifying organisms into hierarchical groups based on shared physical and genetic characteristics.
  • Hierarchy of Classification (The Taxonomic Ranks):
    1. Domain: The highest level (e.g., Eukarya - organisms with a nucleus and organelles).
    2. Kingdom: (e.g., Animalia - all animals).
    3. Phylum: (e.g., Chordata - animals with a backbone).
    4. Class: (e.g., Mammalia - animals that produce milk).
    5. Order: (e.g., Carnivora - flesh-eaters; Primates - humans and relatives).
    6. Family: (e.g., Canidae - dogs and dog-like animals; Hominidae - humans).
    7. Genus: (e.g., Canis - wolves, jackals, domestic dogs; Homo - humans).
    8. Species: The most specific level (e.g., Canis lupus - Domestic dog; Homo sapiens - Modern Human).

Diversity of Microorganisms: Domain Bacteria and Domain Archaea

  • Unique Characteristics of Prokaryotes:
    • General: They are unicellular organisms.
    • Genetic Material: Usually a single circular DNA molecule located in the nucleoid region (no nuclear envelope).
    • Histones: In Bacteria, DNA is not associated with histone proteins. In some Archaea, bacterial DNA does associate with histone proteins.
    • Plasmids: Small, extra-chromosomal circular DNA found in both Bacteria and Archaea.
    • Organelles: Few organelles exist; they lack membranes (e.g., ribosomes).
    • Reproduction:
      • Asexual: Binary fission.
      • Sexual: Conjugation, which involves the transfer of DNA through a sex pilus (a flexible tube of protein subunits).
    • Survival Mechanisms: Some bacteria form dormant endospores in unfavorable conditions. Archaea do not form endospores but produce unique protective enzymes.
  • Metabolic Diversity (Modes of Nutrition):
    • Photoautotroph: Uses light as an energy source and CO2CO_2 as a carbon source (e.g., Cyanobacteria).
    • Chemoautotroph: Uses the oxidation of inorganic materials (like H2SH_2S or NH3NH_3) for energy and CO2CO_2 for carbon (e.g., Nitrosomonas sp., Nitrococcus sp.).
    • Photoheterotroph: Uses light for energy but requires organic compounds for carbon (e.g., Rhodobacter sp., Rhodospirillum sp.).
    • Chemoheterotroph: Uses organic compounds for both energy and carbon. These can be saprotrophs or pathogens (e.g., Staphylococcus sp.).

Prokaryotic Physical Structures

  • Cell Wall: Maintains shape and provides protection.
    • Bacteria: Composed of peptidoglycan.
    • Archaea: Contains proteins and polysaccharides (lacks peptidoglycan).
  • Capsule: An additional protective layer that enhances resistance to host defenses; often associated with pathogenic (disease-causing) strains.
  • Mesosomes: Tightly folded regions of the plasma membrane that serve as the site for cellular respiration.
  • Flagella: Composed of flagellin protein; used for motility (helical orientation propulsion).
  • Fimbriae: Help prokaryotes attach to surfaces or each other.
  • Pili (Singular: Pilus): Longer than fimbriae; used to assist in conjugation for DNA exchange.

Differences Between Bacteria and Archaea

  • Cell Wall: Bacteria use peptidoglycan; Archaea use protein/polysaccharides.
  • Membrane Lipids:
    • Bacteria/Eukarya: Straight-chain fatty acids linked to glycerol via ester linkage.
    • Archaea: Branched-chain hydrocarbons (Phytanyl sidechains) linked to glycerol via ether linkage. These can form a phospholipid monolayer or a unique bilayer that offers stability in extreme environments.
  • DNA Association: Bacteria lack histones; some Archaea have DNA associated with histones.
  • Archaea Extremophiles:
    • Thermophiles: Thrive at temperatures of 60−80∘C60 - 80^\circ C.
    • Acidophiles: Thrive at low pH levels (e.g., pH 3pH\, 3).
    • Xerophiles: Grow in extremely dry conditions.
    • Halophiles: Require extremely high salt concentrations.

Diversity of Bacteria by Classification

  • Based on Shape:

    1. Spherical (Coccus/Cocci): Can exist as unicellular, two-celled (Diplococcus sp.), chains (Streptococcus sp.), or clusters/clumps (Staphylococcus sp.).
    2. Rod-shape (Bacillus/Bacilli): Examples include Bacillus thuringiensis and Escherichia coli.
    3. Spiral (Spirillum/Spirilla): Examples include Rhodospirillum sp. (e.g., Rhodospirillum rubrum).
    4. Comma-shape (Vibrio): Examples include Vibrio cholerae.

  • Based on Gram Stain:

    • Gram-Positive (Gram +):
      • Have a simpler cell wall with a thick peptidoglycan layer.
      • Stain blue/purple because the thick layer traps the crystal violet dye.
      • Contain teichoic and lipoteichoic acids.
      • Generally less pathogenic.
      • Examples: Lactobacillus sp., Clostridium sp., Bacillus sp.
    • Gram-Negative (Gram -):
      • Have a complex cell wall with two layers: a thin peptidoglycan layer and an outer membrane.
      • Outer membrane contains lipopolysaccharides (LPS) which are often toxic (comprising O-antigen, Core polysaccharide, and Lipid A/endotoxin).
      • Stain pink because they do not retain the crystal violet and take up the safranin counterstain.
      • Outer membrane protects them from host defenses and entry of antibiotics.
      • More pathogenic; cause diseases like typhoid and gonorrhea.
      • Examples: E. coli, Azotobacter sp., Salmonella sp.

  • Based on Position of Flagella:

    • Atrichous: Absence of flagella.
    • Monotrichous: A single flagellum at one end.
    • Amphitrichous: One flagellum or a tuft (cluster) at both ends.
    • Lophotrichous: A tuft (cluster) of flagella at one end.
    • Peritrichous: Multiple flagella distributed all over the bacterial surface.

Importance of Bacteria

  • Ecological Recycling: Help recycle chemical elements. Nitrogen Fixation (e.g., Rhizobium sp.): Bacteria inside the root nodules of legume plants convert atmospheric N2N_2 into ammonia (NH3NH_3) for plant protein synthesis.
  • Symbiosis: Enterobacteria like E. coli in the human intestine assist in digesting complex molecules such as lactose.
  • Pathogenic Bacteria: Cause diseases.
    • Tetanus: Caused by Clostridium tetani.
    • Cholera: Caused by Vibrio cholerae.
    • Syphilis: Caused by Treponema pallidum.
  • Research and Technology:
    • Food Production: Used to make cheese, yogurt, and vinegar.
    • Biotechnology: E. coli is used in gene cloning.
    • Bioremediation: Utilizing microbes to clean up oil spills.
    • Medical Research: Easily cultured for use in making antibiotics; study of antibiotic resistance where resistant strains survive treatment.

Cyanobacteria (Blue-Green Algae)

  • Structure can be single-celled, colonial, or in filament forms.
  • Specialized Cells:
    • Heterocysts: Thick-walled cells containing the enzyme nitrogenase used to fix nitrogen.
    • Akinetes: Climate-resistant spores that form during unfavorable environmental conditions.
    • Vegetative Cells: Contain genes encoding proteins for photosynthesis.
  • Examples: Nostoc sp. and Anabaena sp.