Anatomy of Microorganisms

Anatomy of Microorganisms

Recommended Reading

  • Chapter 2 and 3

  • Steven Bowden, Dept. Food Science and Nutrition

Anatomy of Cells and Tools for Studying Microorganisms

  • Objectives:

    • (i) Understand the difference in the structures of prokaryotic and eukaryotic cells.

    • (ii) Importance of the cell membrane for the movement of materials between the cytoplasm and the environment.

    • (iii) Describe the structure and function of the glycocalyx.

    • (iv) Differentiate flagella, cilia, fimbriae, and pili.

Prokaryotic and Eukaryotic Cells

  • Prokaryote: Comes from the Greek words for 'prenucleus'.

  • Eukaryote: Comes from the Greek words for 'true nucleus'.

  • All cells contain:

    • Cytoplasm: A gel-like substance composed of water and dissolved chemicals needed for growth.

    • Plasma Membrane: Also called the cell membrane or cytoplasmic membrane.

    • Chromosomes: One or more chromosomes containing the genetic blueprints of the cell.

    • Ribosomes: Organelles used for the production of proteins.

Differences Between Prokaryotic and Eukaryotic Cells

  • Prokaryotic Cells:

    • One circular chromosome not enclosed in a membrane.

    • No histones.

    • No organelles.

    • Bacteria: Have peptidoglycan cell walls (most cells).

    • Archaea: Have pseudomurein cell walls.

    • Reproduce by binary fission.

  • Eukaryotic Cells:

    • Paired chromosomes enclosed in a nuclear membrane.

    • Histones present.

    • Have organelles.

    • Some have polysaccharide cell walls (few cells).

    • Utilize mitotic spindle for cell division.

Prokaryotic Cells: Shapes and Sizes

  • Average Size: 0.2–1.0 µm x 2–8 µm

  • Most bacteria: Monomorphic (having one shape).

  • Some bacteria: Pleomorphic (having multiple shapes).

  • Shapes of Prokaryotic Cells:

    • Bacillus: Rod-shaped.

    • Coccus: Spherical.

    • Spiral:

    • Spirillum: Spiral.

    • Vibrio: Comma-shaped.

    • Spirochete: Flexible spiral.

  • Individual cells of a prokaryotic organism typically have similar shapes or cell morphology.

Arrangements of Prokaryotic Cells

  • Pairs:

    • Diplococci: Pairs of spherical bacteria.

    • Diplobacilli: Pairs of rod-shaped bacteria.

  • Clusters:

    • Staphylococci: Clusters of spherical bacteria.

  • Chains:

    • Streptococci: Chains of spherical bacteria.

    • Streptobacilli: Chains of rod-shaped bacteria.

Figures

  • Figure 4.1: Different arrangements of cocci (spherical bacteria).

  • Figure 4.2: Displays various arrangements of bacilli (rod bacteria).

  • Figure 4.4: Shows spiral bacteria forms (Vibrio, Spirillum, and Spirochete).

  • Figure of Rectangular Bacteria: Displays sheet-like arrangements of certain bacteria.

Osmotic Pressure and Cell Wall

  • Osmotic pressure: Occurs due to differences in solute concentrations across a semipermeable membrane.

  • Cell Situations:

    • Isotonic Solution: No net movement of water; cell membrane attached to the cell wall.

    • Hypertonic Solution: Water moves out of the cell; cell membrane shrinks—this is called plasmolysis.

    • Hypotonic Solution: Water enters the cell; cell wall prevents lysis (bursting).

Cell Structure

  • Structures found in prokaryotes include:

    • Capsule

    • Cell wall

    • Plasma membrane

    • Fimbriae

    • Cytoplasm

    • Pilus

    • Ribosomes (70S)

    • Nucleoid with DNA

    • Plasmids (circular DNA)

    • Flagella

  • Not all bacteria display all structures; red labeled structures are universal.

Cell Wall and Its Functions

  • Primary function: Protect the cell from harsh external conditions.

  • Gram-positive cells:

    • Composed of thick peptidoglycan layers (30–100 nm).

  • Gram-negative cells:

    • Have a much thinner peptidoglycan layer (≈4 nm).

  • Teichoic acids are present in gram-positive bacteria.

  • Outer membrane: Presence of LPS (lipopolysaccharides) in gram-negative bacteria, causing endotoxins.

Damage to the Cell Wall

  • Lysozyme: Digests disaccharides in peptidoglycan, leading to cell lysis.

  • Penicillin: Inhibits peptide bridges in peptidoglycan, compromising cell wall integrity.

Glycocalyx

  • Located outside the cell wall; usually sticky in nature.

  • Capsule: Neatly organized structure, provides protection from phagocytosis.

  • Slime layer: Unorganized and loose; helps in adherence.

Flagella

  • Flagella are apparatus allowing cells to move in aqueous environments.

  • Located outside the cell wall and attached to a protein hook.

  • Anchored to the wall and membrane by a basal body.

  • Basal body configurations:

    • Gram-positive: 2-ring basal body.

    • Gram-negative: 4-ring basal body.

  • Flagella movements are directed by environmental signals:

    • Light (phototaxis)

    • Magnetic fields (magnetotaxis)

    • Chemical gradients (chemotaxis).

Pili and Fimbriae

  • Pili: Longer and less numerous, important for DNA transfer.

  • Fimbriae: Allows attachment to surfaces and other cells, aiding in colonization.

Plasma Membrane

  • Structures enclosing the cytoplasm collectively known as the cell envelope.

  • Exhibits selective permeability to control the transport of molecules.

  • Composed of a phospholipid bilayer with embedded proteins.

  • Membrane proteins facilitate cell-cell communication and environmental sensing.

Movement of Materials Across Membranes

  • Simple diffusion (passive transport): Movement from high to low concentration.

  • Facilitated diffusion: Requires carriers for charged or large molecules.

  • Active transport: Moves molecules against concentration gradients; requires ATP.

Cytoplasm Components

  • Nucleoid Region: Contains DNA concentrated with associated proteins.

  • Plasmids: Small, circular, extrachromosomal DNA; can confer antibiotic resistance.

  • Ribosomes: Prokaryotic ribosomes (70S) differ from eukaryotic ribosomes (80S).

  • Ribosomes are where protein synthesis occurs, vital for cell function.

Endospores in Bacteria

  • Formed under unfavorable conditions; resistant to desiccation, heat, and chemicals.

Eukaryotic Cells

  • Structure Characteristics:

    • Defined by a nucleus surrounded by a complex nuclear membrane.

    • Contain membrane-bound organelles including mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and peroxisomes.

  • Nucleus: Houses organized and condensed DNA in rod-shaped chromosomes wrapped around histones.

  • Nucleolus: Site of ribosomal RNA biosynthesis within the nucleus.

Eukaryotic Ribosomes

  • Eukaryotic ribosomes are slightly larger than prokaryotic ones.

  • Free ribosomes synthesize soluble proteins; membrane-bound ribosomes synthesize proteins for export.

Endomembrane System

  • Comprising organelles like the endoplasmic reticulum (smooth and rough ER), Golgi apparatus, and lysosomes.

Eukaryotic Cells Functions

  • Mitochondria: Site of aerobic cellular respiration.

  • Chloroplasts: Site of photosynthesis; have multiple membrane systems.

  • Cytoskeleton: Composed of microfilaments and microtubules; provides structural support and assists in intracellular transport.

Eukaryotic Cell Cycle

  • G1 Phase: Accumulation of building blocks for DNA replication.

  • S Phase: DNA replication occurs.

  • G2 Phase: Prepares for mitosis; synthesizes proteins.

  • Mitosis: Includes distinct phases (prophase, metaphase, anaphase, telophase) leading to separation of chromatids.

    • Cells experience cytokinesis post-mitosis, leading to the final separation into two daughter cells.

Recommended External Reading