Biology: Prokaryotes vs Eukaryotes

Prokaryotes

• Pro means before and karyon means nucleus
• DNA is not enclosed within a membrane
• Chromosome is single and circular
• Chromosome exists within a specific region of the cell called the nucleoid
• They do not have membrane-bound organelles such as mitochondria
• Single-celled organisms only (bacteria, archaea)

Eukaryotes

• Eu means true and karyon means nucleus
• DNA is contained within a membrane-bound nucleus
• DNA is arranged as multiple chromosomes
• Organelles are present and membrane-bound
• Can be either single-celled or multicellular
• Most eukaryotes are multicellular
• Includes: protists, fungi, plants, animals

Bacterial Morphology

• Morphology means shape
• Coccus (pleural: cocci): Spherical (Ex: Streptococcus pyogenes)
• Bacillus (pleural: bacilli): Rods (Ex: Escherichia coli)
• Spirillum (pleural: Spirilla): Spiral (Ex: Treponema pallidum)

External Structures

Glycocalyx

• Sugar coat: sticky polymer
• Can be composed of polysaccharide, protein, or both
• When only sugar, it's called an extracellular polysaccharide
• Secreted from the prokaryote onto the outside of the cell wall
• Capsule: substance firmly attached to the cell wall, organized
• Slime Layer: substance is disorganized, loosely attached to the cell wall

Capsules

• Contribute to organism virulence (ability to cause disease)
• Protects the organism from phagocytosis
• Allows the organism to adhere to and colonize host cells
• Protects the bacterial cell against dehydration and holds nutrients
• Allows the bacterium to survive by attaching to different surfaces within the microbe's environment
• Example: Streptococcus mutans attaches to teeth and causes cavities
• Some organisms use the capsule as an energy source

Flagella

• Found on some prokaryotic cells
• Long, filamentous, used for motility
• Composed of three primary parts:
  1. Filament: composed of flagellin protein, forms a helix around a hollow core
  2. Hook: made of different protein than the flagella
  3. Basal body: anchors the flagellum to the plasma membrane and cell wall

Flagellar Arrangements

• Peritrichous: flagella distributed over the entire cell surface
• Monotrichous: single polar flagella
• Lophotrichous: two or more flagella at one or both ends of the cell
• Amphitrichous: a tuft of flagella at each cell end

Bacterial Motility

• Flagellar proteins are used to differentiate strains of different bacteria (Ex: E.coli 0157:H7)
• Flagella can rotate either clockwise or counterclockwise
• This movement depends on energy production
• Run: movement in one direction
• Tumble: abrupt changes in direction
• Motility allows bacteria to move away from dangerous environments towards favorable ones (taxis)
• Chemotaxis: movement toward a chemical stimulus
• Phototaxis: movement toward a light stimulus
• Bacteria move toward attractants and away from repellants

Pili and Fimbriae

• Found in many Gram-negative bacteria
• Hair-like appendages, shorter, thinner, straighter than flagella, not used for motility
• Made of pilin protein
• Fimbriae:
  • Bacterial cell can contain a few or hundreds
  • Can be all over the cell surface or only at the poles
  • Enables the bacterial cell to adhere to surfaces and other bacterial cells
  • Allow for adherence and colonization (disease occurrence)
  • Example: Neisseria gonorrhoeae
• Pili (pilus-singular):
  • Longer than fimbriae
  • Only one or two per cell
  • Join two bacterial cells to transfer DNA in a process called conjugation

Bacterial Cell Wall

• Semi-rigid, complex, and semi-permeable
• Provides the cell with its characteristic shape
• Protects the cell from environmental changes
• Prevents cell rupture
• Important means of classifying bacteria
• Composed of the polysaccharide peptidoglycan

Peptidoglycan

• Polysaccharide composed of repeating disaccharides
• Polysaccharide chains are layered on top of one another
• Polysaccharides chains are linked together by short polypeptides
• Creates a strong cell wall that is resistant to osmotic changes
• Disaccharide unit composed of N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM).

The Gram Positive Cell Wall

• Contains a thick layer of peptidoglycan outside the plasma membrane
• Also contains teichoic acids (only found in gram-positive organisms):
  • Wall teichoic acids extend out from the peptidoglycan
  • Lipoteichoic acids connect the plasma membrane to the peptidoglycan
• Gram-positive bacteria have only one membrane (plasma membrane)

The Gram Negative Cell Wall

• Thin peptidoglycan layer
• Gram-negative bacteria contain a plasma membrane and an outer membrane.
• The outer membrane contains lipids (phospholipids), proteins, and lipopolysaccharides (LPS).
• Lipid portion is toxic (endotoxin).
• Polysaccharide portion composed of O sugars (used to distinguish gram-negative organisms)

The Gram Stain

• Gram-positive cells retain crystal violet due to thick peptidoglycan.
• Gram-negative cells do not retain crystal violet because outer membrane is disrupted by alcohol.

The Importance of Peptidoglycan

• Unique to bacteria; eukaryotes lack a similar compound
• Common target for both host defenses and chemotherapies
• Lysozyme is a host enzyme that degrades peptidoglycan
• Many antibiotics are active against peptidoglycan, including penicillin

The Plasma Membrane

• Composed of a classic phospholipid bilayer
• Provides a barrier between the intracellular and extracellular environments
• Semi-permeable barrier: selectively allows the inflow and outflow of materials
• Exists in a semi-fluid state: fluid enough for membrane proteins to move and function, solid enough to maintain cell shape
• Alcohol disrupts the plasma membrane

Cytoplasm

• Material contained within the plasma membrane
• Composed of ~80% water
• Contains many materials needed for life (amino acids, carbohydrates, nucleotides, enzymes, inorganic ions)
• Aqueous, thick, semi-transparent
• Contains major cellular structures (nucleoid containing the cell's genetic material, ribosomes, inclusion bodies)
• Some bacteria may also have endospores

Nucleoid

• Nuclear area of the bacterium
• Contains the bacterial chromosome (all genetic information for cell structure and function)
• Not surrounded by a nuclear membrane
• Bacteria may also have plasmids (small, circular, double-stranded DNA molecules, help survive conditions including high antibiotic concentrations (e.g., antibiotic resistance genes))

Ribosomes

• Site of protein synthesis
• Made of protein and ribosomal RNA (rRNA)
• Consist of two subunits, a large subunit and a small subunit (50S and 30S, respectively)
• These two subunits come together to form a 70S ribosome

Ribosomes (Prokaryotic vs. Eukaryotic)

• Prokaryotic ribosomes are different than those of eukaryotes (both function in protein synthesis)
• Eukaryotic ribosomes are larger and heavier (80S).
• Several antibiotics target bacterial ribosomes to execute selective toxicity, won't harm host cells

Inclusion Bodies

• Deposits of nutrient granules stored for later use
• Different bacterial species contain various inclusion bodies (basis for identification)
• Types include: sulfur granules, polysaccharide granules, lipid inclusions, enzymes

Endospores

• Only gram-positive bacteria form endospores
• Specialized structures allowing bacteria to resist heat, desiccation, chemicals, and radiation
• Bacterium remains dormant for long periods and leaves dormancy only when good growth conditions occur
• Spores are very resilient (can survive in boiling water for hours)
• Examples include Bacillus anthracis and Clostridium botulinum

Sporulation

• The process of endospore formation
• 5 steps involved in sporulation

Eukaryotic Cells

• Includes both unicellular and multicellular organisms
• Larger and more complex than prokaryotes
• Simple eukaryotes (e.g., protozoa, algae)
• Higher eukaryotes (e.g., fungi, plants, animals)

Eukaryotic Flagella and Cilia

• Long, flexible structures also containing cytoplasm and proteins
• Move in a whip-like motion (cork screw motion in prokaryotes).
• Used for motility
• Both cilia and flagella are used for motility

Eukaryotic Cell Wall

• Not present in animal cells.
• Structurally simpler than peptidoglycan (bacteria).
• Composed of a single polysaccharide:
  • Cellulose (algae, plants)
  • Chitin (fungi)

Eukaryotic Plasma Membrane

• Same basic structure as prokaryotic cells
• Contains phospholipids, proteins, and sterols
• Eukaryotic plasma membrane is more rigid than that of bacteria

Eukaryotic Cytoplasm

• Similar to prokaryotic cytoplasm but with key differences
• Located within the plasma membrane boundary and outside of the nuclear membrane
• Has a complex internal structure called the cytoskeleton which:
  • Provides support and shape
  • Transports substances
  • Composed of protein filaments inside the plasma membrane

Membrane-Bound Organelles

• Absent in bacteria
• Structures with specialized functions
• Examples include:
  • Nucleus: contains the genetic material
  • Mitochondria: powerhouse of the cell
  • Chloroplasts: site of photosynthesis