Cell Biology: Structure and Function of Prokaryotic and Eukaryotic Cells

Cell Biology

Introduction

This section provides an overview of the fundamental differences and similarities between prokaryotic and eukaryotic cells, focusing on their structure, function, and specialized components. Understanding these differences is essential for the study of microbiology, as it underpins the classification, physiology, and pathogenicity of microorganisms.

Prokaryotes

Definition and Characteristics

• Prokaryotes are organisms whose cells lack a membrane-bound nucleus.

• Their DNA is not enclosed within a membrane; instead, it is found in a single, circular chromosome located in a region called the nucleoid.

• They do not possess membrane-bound organelles such as mitochondria.

• Prokaryotes are exclusively single-celled organisms, including Bacteria and Archaea.

Eukaryotes

Definition and Characteristics

• Eukaryotes have a true nucleus, with DNA contained within a membrane-bound nuclear envelope.

• DNA is organized into multiple linear chromosomes.

• They possess membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

• Eukaryotes can be unicellular or multicellular. Most are multicellular, including protists, fungi, plants, and animals.

Bacterial Morphology

Shapes of Bacteria

• Coccus (plural: cocci): Spherical shape. Example: Streptococcus pyogenes

• Bacillus (plural: bacilli): Rod-shaped. Example: Escherichia coli

• Spirillum (plural: spirilla): Spiral-shaped. Example: Treponema pallidum

External Structures of Prokaryotes

Glycocalyx

• A glycocalyx is a sugar coat, a gelatinous, sticky polymer composed of polysaccharide, protein, or both.

• It is secreted from the prokaryote onto the outside of the cell wall.

• Two main forms:

  • Capsule: Firmly attached and organized.

  • Slime layer: Loosely attached and disorganized.

Capsules

• Enhance virulence (ability to cause disease) by protecting bacteria from phagocytosis.

• Essential for pathogenicity in some species (e.g., Bacillus anthracis).

• Enable adherence to surfaces and colonization of host tissues.

• Protect against dehydration and retain nutrients.

• Some bacteria (e.g., Streptococcus mutans) use capsules as an energy source when nutrients are scarce.

Flagella

• Long, filamentous appendages used for motility.

• Composed of three parts:

  1. Filament: Helical structure made of flagellin protein.

  2. Hook: Connects filament to basal body.

  3. Basal body: Anchors flagellum to cell wall and plasma membrane.

Flagellar Arrangements

• Peritrichous: Flagella distributed over the entire cell surface.

• Monotrichous: Single polar flagellum.

• Lophotrichous: Two or more flagella at one or both ends.

• Amphitrichous: Tuft of flagella at each end.

Bacterial Motility

• Flagellar proteins are used for bacterial strain identification (e.g., E. coli O157:H7).

• Flagella rotate either clockwise or counterclockwise, powered by energy production.

• Movement in one direction is called a run or swim; abrupt changes in direction are tumbles.

• Motility allows bacteria to move toward favorable environments (taxis):

  • Chemotaxis: Movement toward/away from chemical stimuli.

  • Phototaxis: Movement toward/away from light.

Pili and Fimbriae

• Found in many Gram-negative bacteria.

• Hair-like appendages made of pilin protein; shorter, thinner, and straighter than flagella.

• Fimbriae: Enable adherence to surfaces and other cells; essential for colonization (e.g., Neisseria gonorrhoeae).

• Pili: Longer than fimbriae; usually 1-2 per cell. Facilitate DNA transfer between cells via conjugation.

Bacterial Cell Wall

Structure and Function

• Semi-rigid, complex, and semi-permeable structure.

• Provides shape, protection from environmental changes, and prevents cell rupture.

• Composed primarily of peptidoglycan.

Peptidoglycan

• Polysaccharide composed of repeating disaccharides (N-acetyl glucosamine [NAG] and N-acetyl muramic acid [NAM]).

• Chains are cross-linked by short polypeptides, forming a strong lattice resistant to osmotic pressure.

Gram Positive Cell Wall

• Thick peptidoglycan layer outside the plasma membrane.

• Contains teichoic acids (wall and lipoteichoic acids) unique to Gram-positive bacteria.

• Only one membrane: the plasma membrane.

Gram Negative Cell Wall

• Thin peptidoglycan layer.

• Contains both a plasma membrane and an outer membrane.

• Outer membrane includes:

  • Lipids (phospholipids)

  • Proteins

  • Lipopolysaccharides (LPS):

    • Lipid portion (endotoxin): toxic

    • Polysaccharide portion: O sugars, used for identification

The Gram Stain

• Gram-positive cells retain crystal violet stain (appear purple) due to thick peptidoglycan.

• Gram-negative cells do not retain crystal violet; alcohol wash disrupts outer membrane, and cells take up safranin counterstain (appear pink).

Importance of Peptidoglycan

• Unique to bacteria; absent in eukaryotes.

• Target for host defenses (e.g., lysozyme) and antibiotics (e.g., penicillin inhibits peptidoglycan synthesis).

The Plasma Membrane

• Composed of a phospholipid bilayer.

• Acts as a semi-permeable barrier, selectively allowing the inflow and outflow of materials.

• Exists in a semi-fluid state, allowing membrane proteins to move and function.

• Alcohol can disrupt the plasma membrane.

Cytoplasm

• Material within the plasma membrane (~80% water).

• Contains amino acids, carbohydrates, nucleotides, enzymes, and inorganic ions.

• Houses major cellular structures: nucleoid, ribosomes, inclusion bodies, and sometimes endospores.

Nucleoid

• Region containing the bacterial chromosome (not membrane-bound).

• May also contain plasmids: small, circular DNA molecules with non-essential genes (e.g., antibiotic resistance).

Ribosomes

• Sites of protein synthesis, composed of protein and rRNA.

• Prokaryotic ribosomes: 70S (50S large + 30S small subunit).

• Eukaryotic ribosomes: 80S (60S large + 40S small subunit).

• Antibiotics (e.g., streptomycin, erythromycin) can selectively target bacterial ribosomes.

Inclusion Bodies

• Deposits of nutrient granules for storage and later use.

• Types include sulfur granules, polysaccharide granules, lipid inclusions, and enzymes.

• Serve as a basis for bacterial identification.

Endospores

• Formed only by Gram-positive bacteria (e.g., Bacillus anthracis, Clostridium botulinum).

• Highly resistant to heat, desiccation, chemicals, and radiation.

• Allow bacteria to remain dormant until favorable conditions return.

Sporulation Process

1. DNA replication

2. Septum formation divides the cell

3. Larger compartment engulfs the smaller, forming a forespore

4. Peptidoglycan and protective materials form the spore coat

5. Spore is released from the mother cell

Eukaryotic Cells

• Include both unicellular and multicellular organisms.

• Larger and more complex than prokaryotes.

• Simple eukaryotes: protozoa (unicellular), fungi (multicellular except yeasts), algae (some unicellular, some multicellular).

• Higher eukaryotes: plants and animals.

Eukaryotic Flagella and Cilia

• Long, flexible structures containing protein and cytoplasm.

• Move in a whip-like fashion (prokaryotes move in a corkscrew motion).

• Both are used for motility.

Eukaryotic Cell Wall

• Absent in animal cells.

• Simpler than bacterial peptidoglycan; composed of a single polysaccharide:

  • Cellulose: algae and plants

  • Chitin: fungi

Eukaryotic Plasma Membrane

• Similar structure to prokaryotic cells but contains sterols, making it more rigid.

• Capable of endocytosis (engulfing particles from outside the cell).

Eukaryotic Cytoplasm

• Located within the plasma membrane but outside the nuclear membrane.

• Contains a complex internal structure called the cytoskeleton:

  • Provides support and shape

  • Transports substances through the cell

  • Composed of protein filaments

Membrane Bound Organelles

• Absent in bacteria; present in eukaryotes.

• Examples:

  • Nucleus: Contains genetic material

  • Mitochondria: Site of ATP synthesis (powerhouse of the cell)

  • Chloroplasts: Site of photosynthesis (in plants and algae)