Lecture Prokary Cell Struct Funct

Learning Objectives

• Cell Structure & Function

  • Incorporate Reading 5.5 and sections 2.1, 2.2, 2.3, 2.4.

  • Define LUCA (Last Universal Common Ancestor).

  • Compare and contrast structures of prokaryotic and eukaryotic cells, and viruses.

  • Review all organelles from prior Biology course (Bio 1).

  • Identify and categorize microbes based on morphological features of the cell.

  • Discuss the benefits of smallness in cell structure.

  • List, sketch, and label the components of the plasma membrane and how they contribute to membrane fluidity.

  • Compare and contrast Bacteria, Archaea, and Eukarya.

  • Describe plasma membrane functions.

  • List and describe the three main classes of transport systems.

  • Describe the features and steps of the phosphotransferase system in E. coli.

  • Describe the structure and importance of peptidoglycan.

  • Compare and contrast gram-positive and gram-negative cell wall structure in detail, including sketches.

  • Sketch and label components of the outer membrane of gram-negative bacteria.

  • Sketch and label components of the outer peptidoglycan of gram-positive bacteria.

  • Describe features of the cell wall of Archaea.

  • Define and identify structures such as pili, fimbriae, endospores, flagella, taxis, capsule, slime layer.

  • Define endospores and describe their adaptive significance, life cycle, and structure.

  • Describe behavioral responses in bacteria (taxis).

  • Connect morphological features to environmental adaptations for survival in various habitats.

Elements of Cell Structure

• All cells share the following:

  • Cytoplasmic membrane

  • Cytoplasm

  • Ribosomes

  • DNA chromosome (Not universally found in all cells)

Phylogenetic Tree of Life

• Based on genetic sequence analysis of small subunit ribosomal RNA gene:

  • Placement inference for the last universal common ancestor (LUCA).

Eukaryotic vs. Prokaryotic Cells

Eukaryotes

• DNA is enclosed in a membrane-bound nucleus.

• Generally larger and more complex.

• Contains a variety of organelles.

Prokaryotes

• Lack membrane-enclosed organelles.

• No nucleus; DNA is in the nucleoid region.

• Generally smaller than eukaryotic cells.

Internal Structure of Prokaryotic Cell

• Cytoplasm

• Nucleoid

• Ribosomes

• Plasmid

• Cell wall (also includes Cytoplasmic membrane)

Internal Structure of Eukaryotic Cell

• Cytoplasmic membrane

• Ribosomes

• Nucleus with nucleolus and nuclear membrane

• Cytoplasm

• Endoplasmic reticulum, Golgi apparatus, Mitochondrion, and Chloroplast (in plant cells).

Elements of Viral Structure

• Viruses: Not considered cells.

  • Composed of a protein coat and nucleic acid genome.

  • Lack metabolic abilities; rely on host cells for replication.

  • Infect a wide variety of cell types.

  • Smallest size recorded is 10 nm in diameter.

Arrangement of DNA in Microbial Cells

• Genome: Total genetic content of a cell.

  • Prokaryotic chromosomes are typically single, circular DNA molecules.

  • DNA forms the nucleoid (unbound by membrane).

  • Prokaryotes may also contain plasmids that confer special traits (e.g., antibiotic resistance).

Cell Morphology

• Morphology refers to cell shape.

• Major cell shapes include:

  • Coccus (spherical or ovoid)

  • Rod (cylindrical, also termed bacillus)

  • Spirilla (spiral-shaped)

Representative Cell Morphologies of Prokaryotes

• Morphologies reflect shared environments and selective pressures, rather than purely evolutionary pathways.

Cell Size and the Significance of Smallness

Size Range for Prokaryotes

• Ranging from 0.2 µm to > 700 µm in diameter.

• Most cultured rod-shaped bacteria measure between 0.5 and 4.0 µm wide, unchanged lengths of < 15 µm.

Notable Examples of Large Prokaryotes

• Epulopiscium fishelsoni (gut inhabitant of sturgeon).

• Thiomargarita namibiensis (sulfur-oxidizing chemolithotroph).

Size Range for Eukaryotic Cells

• Varied from 10 µm to > 200 µm in diameter.

Advantages of Smaller Cell Size

• Smaller cells possess a higher surface area-to-volume ratio, facilitating:

  • Enhanced nutrient exchange.

  • Faster growth rates compared to larger cells.

  • Lower metabolic rates inversely correlate with cell size.

Lower Limits of Cell Size

• Cellular organisms below 0.15 µm are rare.

• Open ocean habitats generally host cells between 0.2–0.4 µm to maximize nutrient exchange.

The Cytoplasmic Membrane in Bacteria and Archaea

General Structure

• A thin structure (6–8 nm thick) that encompasses the cell.

• Functions as a selective barrier allowing for concentration and waste expulsion.

• Phospholipid bilayer structure, featuring:

  • Hydrophobic (fatty acids) and hydrophilic components (exposed regions).

Membrane Functions

1. Permeability barrier: Prevents leakage; critical for nutrient transport.

2. Protein anchor: Hosts proteins essential for transport and bioenergetics.

3. Energy conservation: Site for generation of proton motive force.

Lipid Composition

• Phospholipids are amphipathic (hydrophilic head, nonpolar fatty acid tails).

• Fatty acids are common in bacteria; isoprenes are prevalent in archaea.

Membrane Proteins in Bacteria

• Integral membrane proteins: Firmly embedded and function as channels or transporters.

• Peripheral membrane proteins: Anchored to the membrane, involved in various functions.

Membrane-Strengthening Agents

• Sterols: Rigid lipids found primarily in eukaryotes.

• Hopanoids: Similar to sterols; prevalent in bacterial membranes and enhance stability.

Archaeal Membranes

• Utilize ether linkages, differing from the ester linkages in bacterial and eukaryotic membranes.

• Major components include glycerol diethers and diglycerol tetraethers.

Transport Systems in Prokaryotes

• Nutrient uptake across a permeability barrier:

  • For polar/charged molecules, specialized transport systems are required, e.g., active transport.

  • Three major classes of transport systems:

    1. Simple transport

    2. Group translocation

    3. ABC system

The Phosphotransferase System in E. coli

• Functions in sugar uptake (glucose, fructose, and mannose).

• Operates via group translocation and energy comes from phosphoenolpyruvate (PEP).

ABC Systems

• Over 200 systems identified; important in organic compound uptake, featuring high substrate specificity.

Bacterial Cell Walls

• Provide rigidity and prevent cell lysis due to internal pressure.

• Differences between gram-positive and gram-negative cell walls:

  • Gram-positive: Thick peptidoglycan layer, often with teichoic acids.

  • Gram-negative: Thin peptidoglycan, outer membrane predominantly composed of lipopolysaccharides (LPS).

Peptidoglycan Structure

• Rigid mesh of:

  • N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

• Comprises amino acids, forming a network crucial for maintaining cell shape.

Gram-Positive vs. Gram-Negative Cell Walls

Gram-Positive

• Contains mainly peptidoglycan (up to 90%), teichoic acids present; provides structural integrity.

Gram-Negative

• Composed of minimal peptidoglycan (~10%) plus an outer membrane, which is crucial for its structural configurations.

  • Characterized by a periplasmic space.

Cell Walls of Archaea

• Generally lack peptidoglycan.

• May possess pseudomurein or S-layers, differing in composition from bacterial cell walls.

Bacterial Surface Layers

• Capsules and slime layers: Polysaccharide structures aiding in attachment and immune evasion.

• Fimbriae and pili: Protein-based structures facilitating adhesion and motility; pili play roles in genetic exchange and twitching motility.

Endospores

• Endospores are resilient cells aiding in survival under extreme conditions.

  • Dormant lifecycle stage; important for dispersal.

  • Found in some gram-positive bacteria, particularly in Clostridium and Bacillus species.

Flagella and Mobility

• Flagella facilitate swimming; can be characterized by arrangement (polar, lophotrichous, peritrichous).

• Movement influenced by the proton motive force.

  • Tumble and run patterns observed in response to stimuli (taxis).

Types of Flagellar Arrangement

1. Polar/Monotrichous: single flagellum at one end.

2. Lophotrichous: multiple flagella at one end.

3. Amphitrichous: flagella on both ends.

4. Peritrichous: flagella distributed across the entire cell body.

Microbial Taxis

• Taxis: directional movement in response to stimuli.

  • Chemotaxis, phototaxis, aerotaxis, osmotaxis, and hydrotaxis are primary forms of taxis in microorganisms.

Chemotaxis in E. coli

• Best studied example of chemotaxis; uses a run and tumble mechanism to navigate nutrient gradients.