Comprehensive Study Notes: Cell Structure and Prokaryotes

Prokaryotic vs. Eukaryotic

• Prokaryotic vs. Eukaryotic

  • Size given in transcript: 0.2 \,\text{–} \,2 \,\text{mm}

  • Prokaryotes lack a nucleus

  • Prokaryotes lack organelles

  • Examples: Eubacteria, Archaea

  • Eukaryotes have a nucleus and organelles

• Prokaryotic vs. Eukaryotic: alternative sizes listed in transcript

  • 10 \,\text{–} \,100 \,\text{mm}

  • Have a nucleus and organelles

  • Examples: Protozoa, Animals, Fungi, Plants

Prokaryotic Cell Structure

• Typical sizes (as listed):

  • 0.3 \,\text{–} \,2 \,\text{mm wide}

  • 2 \,\text{–} \,10 \,\text{mm long}

• Shapes (morphology):

  • Bacillus, bacilli (rod)

  • Coccus, cocci (round)

  • Spirillum, spirilli (spiral)

Prokaryotic Cell Arrangements & Imaging

• Figure 3:12 and SEM images depict cell arrangements

  • Scale bars shown as 2 \,\mu\text{m}, 1-5 \,\mu\text{m} depending on image

  • (Images labelled a, b, c, d in transcript)

Bacilli & Spiral Shaped Bacteria

• Shapes shown in examples:

  • (a) Single bacillus

  • (b) Diplobacilli

  • (c) Spirochete

  • (a) Vibrio

  • (b) Spirillum

  • (c) Spirillum (additional) / Spirochete

• SEM scale: 2 \,\mu\text{m} (as shown in figures)

• Note: diversity of morphology is highlighted in these images

Other Terms Describing Morphology

• Monomorphic – single shape

• Pleomorphic – shape varies, multiple shapes

References for Prokaryotic Taxonomy

• Bergey’s Manuals

  • Identification reference book: Bergey’s Manual of Determinative Bacteriology

  • Classification reference book: Bergey’s Manual of Systematic Bacteriology

Prokaryotic Cell Components (Overview)

• Key components/regions:

  • Capsule or glycocalyx (sugar coat)

  • Cytoplasm

  • Ribosomes

  • Cell wall

  • Plasma membrane (cell membrane)

  • Nucleoid (nuclear area) containing DNA

  • Plasmid

  • Flagella

  • Fimbriae (pili)

• Note: Inclusions (storage granules) are also discussed in later slides

Glycocalyx (Glycocalyx: sugar coat)

• A sticky layer surrounding the cell

• Composition: polysaccharide, polypeptide, or both

• Capsule:

  • Organized and firmly attached

  • Functions: resistance to phagocytosis (virulence factor), adherence, promoting colonization

• Slime layer:

  • Unorganized and loosely attached

  • Functions: attachment; formation of biofilms

• Capsule or slime layer collectively termed glycocalyx

Slime Layer vs Capsule (Glycocalyx visuals)

• Slime Layer:

  • First colonists create an organic surface coating

  • Cells stick to coating; as division occurs, a dense mat forms

  • Biofilm maturation involves additional microbes (e.g., at 3 h and 8 h in the diagram)

• Capsule:

  • More organized, dense glycocalyx contributing to adherence and protection

• Both contribute to biofilm formation and surface interactions

Flagella

• Function: locomotion

• Structure: protein flagellin

• Visual: TEM images show flagella length and structure (2 \,\mu\text{m} scale in figure)

• Swarming observation: Proteus in swarming stage may have > 1000 peritrichous flagella

Flagella Arrangements

• Monotrichous: single flagellum at one end

• Lophotrichous: tuft of flagella at one end

• Amphitrichous: flagella at both ends

• Peritrichous: flagella distributed over the entire cell surface

Bacterial Cell Envelope and Morphology (Gram stain context)

• Gram-positive vs. Gram-negative cell wall organization and features:

  • Gram-positive:

  • Thick peptidoglycan layer

  • Teichoic acids present (wall teichoic acid and lipoteichoic acid)

  • Outer membrane absent; periplasm absent or not equivalent to Gram-negative

  • Gram-negative:

  • Thin peptidoglycan layer

  • Outer membrane present

  • Periplasmic space present

  • Lipopolysaccharide (LPS) present in outer membrane; includes lipid A and O polysaccharide

  • Porin proteins, lipoproteins

• Diagrammatic reference: includes coverage of peptidoglycan layers, teichoic acids, outer membrane, periplasm, and associated proteins

Bacteria: Gram-Positive vs. Gram-Negative (Color, Structure, and Implications)

• Gram Reaction Colors:

  • Gram-positive: Purple

  • Gram-negative: Red

• Peptidoglycan layer thickness:

  • Thick in Gram-positive

  • Thin in Gram-negative

• Teichoic acids:

  • Present in Gram-positive

  • Absent in Gram-negative

• Periplasm:

  • Absent in Gram-positive

  • Present in Gram-negative

• Outer membrane:

  • Absent in Gram-positive

  • Present in Gram-negative

Endotoxin (LPS) and Antibiotic Susceptibility in Gram Types

• Endotoxin (LPS) presence: Present in Gram-negative; Absent in Gram-positive

• Penicillin susceptibility: Gram-positive generally more susceptible; Gram-negative less susceptible

• Lysozyme susceptibility: Yes in Gram-positive; No in Gram-negative (due to outer membrane barrier in Gram-negatives)

• Example organisms (as listed): Bacillus (Gram-positive), Staphylococcus (Gram-positive), Escherichia (Gram-negative), Neisseria (Gram-negative)

Bacteria That Lack Cell Walls

• Atypical bacteria:

  • Mycobacterium and Nocardia: cell wall with lipid mycolic acids; acid-fast stain used for diagnosis; pathogenicity and chemical/dye resistance high

• Some bacteria lack a cell wall entirely (Mycoplasma): cell wall stabilized by sterols; pleomorphic shape

Cell Membrane (Plasma Membrane)

• Composition: phospholipid bilayer with embedded proteins

• Functions:

  • Selective permeability

  • Senses environmental signals

  • Energy transformation (e.g., respiration processes)

Cytoplasm (Cytoplasmic Contents)

• Cytoplasm is primarily aqueous, about 80% water

• Contains:

  • Proteins and other macromolecules (enzymes)

  • Small molecules and ions

  • Ribosomes

• Overall: a gel-like matrix where metabolic reactions occur

Genetic Material: The Bacterial Chromosome

• Location: nucleoid region within the cytoplasm

• Function: contains the blueprint for the cell’s proteins

• Structure: double-stranded DNA; bacteria have a single, circular chromosome

• Visualization: uncoiled DNA representation shows DNA fibers within the cell

Plasmids

• Function: extra genetic information (eg, antibiotic resistance)

• Structure: small, circular DNA molecules; mini-chromosomes

• Transfer: plasmids can be transferred between bacteria (horizontal gene transfer)

Ribosomes

• Function: site of protein synthesis

• Structure: ribosomal RNA (rRNA) + proteins

• Prokaryotic ribosomes: 70S

Inclusions / Storage Granules

• Function: energy or resource storage

• Types (examples mentioned):

  • Volutin (inorganic phosphate, metachromatic granules)

  • Lipid bodies

  • Sulfur granules

  • Gas vacuoles

  • Magnetosomes (iron oxide)

• Visualization: storage granules depicted in images

Endospores

• Formed by: Bacillus and Clostridium (endospore-forming bacteria)

• Function: survival under adverse conditions; a resting state

• Terminology:

  • Sporulation = spore formation

  • Germination = spore to vegetative cell

• Note: 1 bacterium forms 1 spore; endospore formation is not replication

Formation of an Endospore (Stepwise)

1) DNA replication
2) DNA aligns along cell's long axis
3) Cytoplasmic membrane invaginates to form forespore
4) Cytoplasmic membranes surround forespore with a second membrane
5) Cortex forms: calcium and dipicolinic acid between membranes
6) Spore coat forms around endospore
7) Endospore matures: increased resistance
8) Endospore is released from the original cell

Cell Membrane Transport Overview

• Passive transport (no energy):

  • Diffusion: movement down the concentration gradient

  • Osmosis: diffusion of water

  • Facilitated diffusion: via carrier proteins

• Active transport (requires energy):

  • Carrier proteins drive transport irrespective of gradient

  • Group translocation: transported molecule chemically modified during transport

  • Bulk transport: endocytosis, exocytosis, pinocytosis

• Theme: Movement of chemicals across the cell membrane

Movement Across Cell Membranes

• Simple diffusion: high concentration to low concentration

• Facilitated diffusion: via transporter proteins

• Active transport: requires energy; transporter proteins

• Note: Encouraged to review related animations and textbook materials

Osmosis and Tonicity (Simple Diffusion / Osmosis)

• Isotonic: no net movement of water

• Hypotonic: water flows into the cell; risk of lysis if cell wall is weak or damaged

• Hypertonic: water flows out of the cell; plasmolysis occurs

• Terms:

  • Cell lysis (in hypotonic environments for cells without robust walls)

  • Plasmolysis (in hypertonic environments)

Facilitated Diffusion Example

• Example carried by a transporter: glucose moves from outside to inside the cell through a transporter protein