Cell structure

Cell Structure

Prokaryotic vs Eukaryotic Cells

  • Prokaryotic Cells (Bacteria, Archaea)

    • Small, high surface area to volume ratio

    • Facilitates rapid uptake of nutrients, excretion of wastes

    • Allows rapid growth

    • Vulnerable to predators, parasites, and competitors

  • Eukaryotic Cells (Eukarya)

    • Larger and more complex than prokaryotic cells

    • Contain membrane-bound compartments (organelles) for cellular processes

    • Defined by the presence of a nucleus

Morphology of Prokaryotic Cells: Shapes

  • Bacterial Shapes

    • Spheres (Cocci)

      • Diplococci: Pairs of cocci (e.g., Streptococcus pneumoniae)

      • Streptococci: Chains of cocci (e.g., Streptococcus pyogenes)

      • Staphylococci: Clusters of cocci (e.g., Staphylococcus aureus)

      • Tetrad: Groups of four cocci

      • Sarcina: Cubic arrangement of cocci (e.g., Sarcina ventriculi)

    • Rods (Bacilli)

      • Chains of bacilli (e.g., Bacillus anthracis)

      • Flagellate rods (e.g., Salmonella typhi)

    • Spirals

      • Vibrios: Comma-shaped bacteria (e.g., Vibrio cholerae)

      • Spirilla: Rigid spiral-shaped bacteria (e.g., Helicobacter pylori)

      • Spirochaetes: Flexible, spiral-shaped (e.g., Treponema pallidum)

Groupings of Prokaryotic Cells

  • Binary Fission: Division of bacterial cells often leads to cells sticking together, forming characteristic groupings useful for identification.

    • Example: Staphylococcus resembles a bunch of grapes.

Prokaryotic Cells: Medical Significance

  • Unique bacterial cell components are potential targets for antibacterial medications to treat infectious diseases.

The Cytoplasmic (Plasma) Membrane

  • Structure: Phospholipid bilayer embedded with proteins.

    • Hydrophilic heads face outward while hydrophobic tails face inward.

    • Semipermeable membrane that allows some substances to pass freely (e.g., O2, CO2, and small hydrophobic molecules).

    • Proteins serve various functions such as selective gates and environmental sensors.

Differences in Cytoplasmic Membranes Between Prokaryotes

  • Bacteria and Archaea have similar membrane structures but differ in phospholipid compositions.

    • Archaeal lipid tails are made of isoprenoid chains and are connected via ether bonds.

Permeability of Lipid Bilayer

  • Selectively Permeable: Only certain molecules can cross the membrane freely; others require transport mechanisms.

  • Molecules like O2 and H2O can pass freely, while others may need transport systems or aquaporins.

Transport Mechanisms Across Cytoplasmic Membrane

  1. Simple Diffusion: Movement from high to low concentration; speed depends on concentration gradient.

  2. Osmosis: Specifically refers to water movement across a selectively permeable membrane due to solute concentration differences. It can be in three conditions:

    • Hypotonic

    • Isotonic

    • Hypertonic

Energy Transformation in Cytoplasmic Membrane

  • Electron Transport Chain: Located in the membrane, critical in converting energy into ATP. It creates an electrochemical gradient by pumping protons out as electrons move down the chain.

Transport of Small Molecules

  • Transport Systems:

    • Proteins act as selective gates to facilitate moving substances across the cytoplasmic membrane.

    • Efflux Pumps: Remove waste products and antimicrobial medications, allowing bacteria to survive amidst hostile environments.

Transport Types

  • Facilitated Diffusion: Passive movement down a concentration gradient with no energy required. Less effective in low-nutrient environments.

  • Active Transport: Energy required to move substances against concentration gradients; may involve proton motive force or ATP-driven systems.

  • Group Translocation: A process specific to bacteria that involves chemically modifying compounds during transport, commonly involves phosphorylation.

Protein Secretion

  • Refers to active movement of proteins out of the cell, such as exoenzymes that perform functions outside the cell.

Cell Wall Composition and Function

  • Structure: Strong, rigid, prevents cell lysis, consists mainly of peptidoglycan in bacteria.

  • Peptidoglycan: Made of repeating units of N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) linked by tetrapeptides and cross-bridges.

Gram-Positive vs. Gram-Negative Cell Walls

Gram-Positive Cell Wall

  • Contains a thick peptidoglycan layer with teichoic acids; has a periplasmic gel-like substance beneath.

Gram-Negative Cell Wall

  • Characterized by a thinner peptidoglycan layer and a unique outer membrane of lipopolysaccharide (LPS).

    • LPS serves as an endotoxin and can trigger immune responses.

Therapeutic Targets

  • Peptidoglycan is a good target for antibiotics; drugs like penicillin interfere with its synthesis, particularly effective against Gram-positive bacteria.

Cell Walls of Archaea

  • Diverse walls, possibly due to extreme environments. Generally lack peptidoglycan but may have pseudopeptidoglycan or S-layers.

Capsules and Slime Layers

  • Gel-like Layers: Protect cells or mediate adhesion to surfaces. Capsules are distinct and gelatinous, while slime layers are diffuse and irregular.

  • Capsules can help evade host immune responses and are involved in biofilm formation, such as in dental plaque.

Flagella

  • Structures for motility; function as propellers to move cells. This can relate to pathogenicity (e.g., Helicobacter pylori).

  • Different arrangements (e.g., peritrichous or polar) assist in identification.

Chemotaxis

  • Ability of bacteria to sense and move toward or away from chemical signals. Involves a pattern of runs and tumbles influenced by flagella movement.

    • Additional responses include aerotaxis, magnetotaxis, thermotaxis, and phototaxis.

Pili

  • Shorter than flagella, used for attachment to surfaces (common pili) or for DNA transfer in conjugation (sex pili).

Internal Components of Prokaryotic Cells

  • Nucleoid: Region where single circular DNA is tightly packed; classified as haploid since it carries one copy.

  • Plasmids: Smaller, non-essential DNA molecules that may confer antibiotic resistance, can be shared among bacteria.

Ribosomes in Prokaryotic Cells

  • Involved in protein synthesis, identified by size: Prokaryotic ribosomes are 70S (30S + 50S subunits). They differ from eukaryotic ribosomes (80S) and are a target for certain antibiotics.

Cytoskeleton and Storage granules

  • Cytoskeleton: Protein framework, similar in function to eukaryotes, likely involved in cell shape and division.

  • Storage Granules: Accumulate excess nutrients; may consist of carbon or energy reserves like glycogen.

Protein-Based Compartments in Prokaryotic Cells

  • Gas Vesicles: Allow buoyancy control in aquatic bacteria; only gases can flow through.

  • Bacterial Microcompartments (BMCs): Enzyme containing microcompartments prevent unwanted reactions.

  • Encapsulin Nanocompartments: Newest type for sequestering specific proteins.

Endospores

  • Unique dormant cells formed in response to unfavorable conditions (e.g., nutrient depletion). Extremely resistant to extreme conditions.

    • Sporulation: Formation and liberation of endospores during cell death, allowing revival once conditions improve.

The Eukaryotic Cell

  • Larger, more complex; includes membrane-bound organelles and a cytoplasmic membrane not directly involved in ATP synthesis.

    • Similarities exist with prokaryotic structures, particularly in ribosomes and cytoskeletal elements.

Organelles in Eukaryotic Cells

  • Nucleus: Contains DNA and regulates gene expression.

  • Mitochondria: Energy-producing organelles with their own DNA and 70S ribosomes; ATP generation.

  • Chloroplasts: Site of photosynthesis, found in plants and algae, also contain 70S ribosomes.

Endosymbiotic Theory

  • Suggests mitochondria and chloroplasts originated from symbiotic bacteria. They became integral to eukaryotic cells and lost some independence.

    • Provides explanations for many similarities seen between these organelles and bacteria, including DNA and ribosome structure.