Lecture #5 Cellular structure

Cellular Structure

Prokaryotes Vs. Eukaryotes

• Fundamental differences:

  • Presence of nucleus

  • Genetic material packaging and overall structure

  • Complexity of cellular processes

DNA Packaging

Prokaryotic Folding

• Genetic material is circular and not enclosed in a nucleus

Eukaryotic Folding

• DNA is linear and enclosed within a nuclear envelope

Genetic Material: Prokaryotes Vs. Eukaryotes

• Prokaryotic Chromosome

  • Located in the nucleoid region

  • Viscous due to high DNA and protein concentration

  • Plasmids: Extrachromosomal DNA, often beneficial but nonessential

• Characteristics of Prokaryotes

  • Lack of nucleus and membrane-bound organelles (mitochondria, chloroplasts, ER, Golgi apparatus)

  • Divide by Binary Fission (no mitosis)

Genetic Material: Plasmids

• Extrachromosomal DNA structures

  • Enable independent mRNA and protein production

  • Smaller than chromosomes, with specific purposes

  • Capable of horizontal gene transfer

The Cell Envelope

• Defined as structures external to the cell but bound to the plasma membrane

Classifications

1. Gram Positive (+)

2. Gram Negative (-)

Cell Wall Components

• Peptidoglycans:

  • Composed of amino acids and sugar monomers (NAG and NAM)

  • Intricate structure involving peptide interbridges targeted by penicillin

The Cell Envelope: Gram Positive (+)

• Structure:

  • Inner cell membrane (phospholipid bilayer)

  • Thick peptidoglycan layer (up to 30 layers)

  • Teichoic acid present

The Cell Envelope: Gram Negative (-)

• Contains two membranes:

  • Inner phospholipid bilayer

  • Outer membrane with lipopolysaccharides (LPS), contributing to endotoxin properties

  • Periplasmic space with a thin peptidoglycan layer

Summary of the Cell Envelopes of Prokaryotic Organisms

Flagellum

• Function: Cellular movement via spinning motion

• Distinction from Eukaryotes: Prokaryotic flagellum uses Proton Motive Force compared to Eukaryotic '9 x 2' microtubule arrangement

Taxis

• Definition: Stimuli that produce flagellar movement

Types of Taxis:

• Chemotaxis: Chemicals

• Aerotaxis: Oxygen

• Thermotaxis: Heat

• Phototaxis: Light

• Behavior: Movement includes forward movement (running) and directional changes (tumbling)

Flagellar Arrangements

• Important for immune evasion, identification, and virulence

Pili & Fimbriae

Fimbriae

• Short, bristle-like fibers for adherence and recognition

Pili

• Elongated tubules for material transfer (conjugation, toxin delivery)

• Often called secretion systems (Sec Sys)

Capsules and Slime Layers

Functions:

1. Adherence

2. Immune avoidance

3. Prevention of desiccation

4. Pathogenicity

• Structure:

  • Gel-like layers outside cell walls

  • Composed of glycocalyx (sugar shell) or proteins

  • Capsules: distinct, gelatinous

  • Slime layer: diffuse, irregular

Biofilms

• Definition: Polymer-encased communities, often beneficial or harmful

Formation and Structure:

• Free cells adhere to surfaces, multiply, and communicate via chemical signals

Importance of Biofilms

• Infections (80% involve biofilms)

• Resistance to immune response and antibiotics

• Examples: Dental plaque from Streptococcus mutans, beneficial uses in bioremediation and wastewater treatment

Endospores

• Definition: Durable dormant states of bacteria, resistant to harsh conditions

Sporulation

• Triggered by starvation (limited resources)

• Protects DNA and prevents damage

Germination

• Activation into a vegetative organism, requires nutrients

• Sporulation is not reproduction (1:1 ratio of spores to mother cells)

Storage Granules

• Inclusions: Non-membrane-bound storage structures for nutrition and other resources

• Examples: Glycogen, iron oxide, phosphate

• Gas vesicles for buoyancy are the only membrane-bound storage

Ribosomes

• Found in both Prokaryotes and Eukaryotes, essential for protein synthesis

• Ribosomal sizes: 70S (prokaryotes) and 80S (eukaryotes)

• Medical significance: Antibiotics targeting 70S ribosomes do not affect 80S

Endosymbiotic Theory

• Proposes ancestors of mitochondria/chloroplasts were bacteria

Evidence for Endosymbiotic Theory

• Mitochondria/chloroplasts contain their DNA

• Possess ribosomes similar to bacterial ribosomes

• Surrounded by double membranes

• Multiply by binary fission, showing a genetic similarity to bacteria

Eukaryotic Cells

• Diverse in structure and function:

Types:

1. Animals: Multicellular, lack cell walls

2. Protozoa: Mostly unicellular, usually motile

3. Plants: Cells with polysaccharide cell walls, capable of photosynthesis

4. Fungi: Cell walls of chitin, non-motile

Organelles

Membranous Organelles

1. Nucleus

2. Mitochondria

3. Endoplasmic reticulum

4. Golgi apparatus

5. Vesicles

6. Chloroplasts

Non-Membranous Organelles

1. Ribosomes

2. Cytoskeleton

3. Centrioles