Prokaryotic and Eukaryotic Cells: Structure, Function, and Medical Significance

Posted on Mar 26, 2025 in Biology

Prokaryotic and Eukaryotic Cells

Endospore Formation

• Spores formed many millions of years ago (mya) are still viable.
• Looking for these is what that phase contrast microscope was for.

Endospore Formation

• Stimuli for sporulation
  • Depletion of nutrients
  • Chemical signals
    • Are there a lot of me around?
    • Only a subset sporulate → rest wait it out (or try)
• Vegetative cell → sporangium
• Sporangium → endospore
• Hardiest of all life forms
  • Withstand extremes in:
    • heat, drying, freezing, radiation, and chemicals
  • Regulated dehydration → heat and random drying resistance
  • Some viable endospores have been found tens to hundreds of million years old

Endospore Germination

• Breaking of dormancy
• Requires:
  • Water
  • Specific germination agent
    • Germinant
    • Varies depending on species
    • Small molecule representative of conditions favorable to growth
    • Stimulates the formation of hydrolytic enzymes
    • Enzymes digest the cortex
    • Allow water to re-enter core/rehydrate cell
    • Growth resumes
• Relatively rapid (1 ½ hours)
  • vs. 8-24 hours for sporulation

Medical Significance of Bacterial Spores

• Most are harmless
• Resist ordinary cleaning methods
  • Boiling, bleach, etc. will not lyse spores
  • Main import → special conditions required to sterilize
  • Geobacillus stearothermophilus is reason autoclave conditions are what they are
• Several bacterial pathogens
  • Bacillus anthracis
  • Bacillus cereus
  • Clostridium perfringens
  • Clostridium tetani
  • Clostridium botulinum
  • Clostridium difficile
    • Important to cleaning medical treatment areas

Special Stains

• Emphasize cell parts not revealed by conventional staining
• Examples:
  • Capsule staining
  • Endospore staining
  • Flagellar staining

Bacterial Shapes, Arrangements, and Sizes

• Several general shapes
  • Coccus – roughly spherical
  • Bacillus – rod-shaped
    • Coccobacillus – short and plump
    • Vibrio – gently curved
  • Spiral – curviform or spiral-shaped
    • Spirillum – external flagella
    • Spirochete – periplasmic flagella (axial filaments)
  • Pleomorphism – cells of species vary in shape and size
    • Mycoplasma (no cell wall) extreme example of this

Arrangement or Grouping

• Cocci – greatest variety in arrangement
  • Single
  • Pairs (diplococci)
  • Tetrads
  • Irregular clusters (staphylococci and micrococci)
  • Chains (streptococci)
  • Cubical packet (sarcina)
• Bacilli – less varied
  • Single
  • Pairs (diplobacilli)
  • Chain (streptobacilli)
  • Row of cells oriented side by side (palisades)

• Spirilla
  • Occasionally found in short chains

Archaea

• Prokaryotic cells
• Many are found in extreme environments
  • Psychrophiles
    • Love cold
  • Thermophiles and Hyperthermophiles
    • Love extreme heat
  • Halophiles
    • Like high salt
    • Give red color to Red Sea and solar salt evaporating ponds
  • Acidophiles
    • Low pH
  • Methanogens
    • Can’t survive in presence of oxygen
    • Make methane
• Not all are extremophiles
  • In addition to methanogens, many live under normal conditions, just rarely cause disease, so we only think about cool ones
• Archaea play important roles but are usually non-pathogens
  • Usually – may be associated with periodontal disease
  • Also associated with other human tissues
  • 1/3 carry methanogens

• Wide variety of morphologies
• Different from Bacteria in:
  • Cell structure
  • Metabolism
  • Genetics

Classifying the Prokaryotes

• One of the original classification systems – shape, variations in arrangement, growth characteristics, and habitat
• Definitive published source for bacterial classification
  • Bergey’s Manual (est. 1923)
    • Early classification – the phenotypic traits of bacteria
      • Aerobic vs anaerobic
      • Cell wall type
      • Metabolism type
    • Species → collection of bacterial cells, all of which share an overall similar pattern of traits
    • 2 problems:
    • Individual members of a bacterial species can show variations
      • Subspecies, strain, or type
        • bacteria of the same species that have differing characteristics
      • Serotype
        • representatives of a species that stimulate a distinct pattern of antibody responses in their hosts
    • Need to be able to grow representatives to determine characteristics
• Now compare sequence of nitrogen bases in rRNA
  • 95-97% similar is same species
  • Even better and more recent → whole genomes where available

The History of Eukaryotes

• First cells were prokaryotic
• Evolved into:
  • Modern Prokaryotes
  • Eukaryotes
    • First appeared ~2 billion years ago
    • Evolved from prokaryotic organisms → intracellular symbiosis
• The Endosymbiont Theory
  • Some organelles originated from intracellular prokaryotes
  • First eukaryotes: probably single-celled, independent
  • Eventually → colonies
  • Cells within colonies → specialized
  • Evolved → multicellular organisms

Eukaryotic vs. Prokaryotic Cells

• Larger in size
  • Generally 10-100x bigger
  • Except for those unusual, large prokaryotes like Thiomargarita namibiensis
• Contain nucleus
  • What’s done there?
• Division of labor
  • Organelles
    • Distinct compartments within cell
    • Bound by membranes
    • Specialized roles
  • Different cells with different roles
    • In multicellular eukaryotes only
      • Members of biofilms may divide labor
        • Take different roles in community
        • Different individual cells or members of different species, not different cells of the same individual organism
    • Not all eukaryotes are multicellular

Limits on Cell Size

• Lower limits:
  • Too small → not enough room for genes/enzymes needed
• Upper limits:
  • Gene copy number

• Metabolic requirements
• Surface area to volume ratio
• Takes longer to adapt to new conditions

Eukaryotic vs. Prokaryotic Cells: Size DOES Matter

• Prokaryotes are a lot smaller than eukaryotes
  • They have a higher surface to volume ratio
    • Prokaryotes: things coming from outside the cell → evenly spread throughout the cell rapidly
    • Eukaryotes: nutrients don’t diffuse as quickly to regions farther from surface
  • Affects transport and metabolism
    • Eukaryotes need a lot of extra structure and transport machinery
      • Things prokaryotes don’t need/have

Form and Function of the Eukaryotic Cell

Locomotion: Eukaryote Flagellum

• Used for locomotion
• Different from prokaryotic flagella
  • Much thicker
    • 10x
  • Multi-protein complex
    • Microtubule core (9×2 + 2)
• Found in:
  • Protozoa
  • Algae
  • Fungi
  • Animal cells

• Does not spin
  • Whips back and forth to push – if just one
    • If 2 → breast stroke
  • Dynein arms use ATP to slide the tubules which results in flexion

Locomotion Appendages: Cilia

• Cilia – similar to flagella but some differences
  • Shorter
  • More numerous
    • 100’s to 1000’s
  • Together, beat like oars on viking ship
  • Can also function in:
    • Feeding
    • Filtering structures
    • Pushing substances past cells
    • Sensing
      • Non-motile Cilia

• Found in a few protozoa and animal cells

The Extracellular Space

• Eukaryote Capsule
  • Present in some pathogens
  • Polymeric sugars
  • Important in avoiding immune clearance
  • Similar to prokaryotes
• The glycocalyx
  • Most eukaryotes have some version of
    • Not quite same thing as prokaryote glycocalyx
    • Not a capsule or a slime layer
  • Carbohydrate projections from the plasma membrane
    • Also other CHOs interacting with those
  • Not essential for life:
    • Kind of like a cell’s fur
      • Can remove it → cell will be unhappy, but live
      • Will grow back over time

• Roles include:
  • Mediates cell-cell and cell-substratum interactions
  • Provides mechanical protection to cells
  • Barrier slowing/preventing some particles from reaching membrane
  • Keeps water close to cells – polar CHOs: lots of H-bonds
• The extracellular matrix (ECM)
  • Organized network beyond the plasma membrane
    • Proteins and carbohydrates
  • One step out past glycocalyx – may interact/attach to it
    • Some people say glycocalyx is part of it
  • Often plays a regulatory role
  • Determines shape and activities of the cell
    • Remove it from cultured cells and they change shape
      • Stop doing whatever they were doing. Add it back and they resume

Form and Function of the Eukaryotic Cell: External Structures

• The layer beneath glycocalyx/ECM varies among eukaryotes
  • Fungi and most algae: thick, rigid cell wall
  • Protozoa and animal cells: only have cytoplasmic membrane

Cell Wall

• Not found in all Eukaryotes
• Rigid
• Provide support and shape
• Different chemically from prokaryotic cell walls
  • No peptidoglycan
• Fungi
  • Thick, inner layer of chitin or cellulose
  • Often outer layer of mixed glycans
• Algae
  • Varied in chemical composition
    • May contain cellulose, pectin, other polysaccharides
    • Also minerals like SiO2 or CaCO4

Cytoplasmic Membrane

• Bilayer of phospholipids with protein molecules embedded
• Also contain sterols
  • Gives stability
  • Especially important in cells without a cell wall
• Selectively permeable