Chapter 4: Functional Anatomy of Prokaryotic and Eukaryotic Cells: Structure and Function

Prokaryotic and Eukaryotic Cells

Similarities and Differences

Both prokaryotic and eukaryotic cells contain nucleic acids, proteins, lipids, and carbohydrates, and utilize similar chemical reactions for metabolism, protein synthesis, and energy storage. However, they differ significantly in cellular organization and complexity.

• Prokaryotic cells lack a true nucleus and membrane-bound organelles.

• Eukaryotic cells possess a nucleus and various membrane-bound organelles.

Shapes and Arrangements of Prokaryotic Cells

Bacterial Morphology

Bacteria exhibit a variety of shapes and arrangements, determined by their genetics. The three primary shapes are:

• Bacillus (bacilli): Rod-shaped

• Coccus (cocci): Spherical

• Spiral: Corkscrew-shaped (includes vibrio, spirillum, and spirochete)

Arrangements

• Staphylo-: Grape-like clusters (e.g., Staphylococcus)

• Strepto-: Chains (e.g., Streptococcus)

Other Arrangements

Bacteria can also be found in pairs (diplococci, diplobacilli), tetrads, sarcinae (cubical packets), and palisades (side-by-side arrangement).

Prokaryotic Cell Structure

Major Components

• Cell wall: Provides shape and protection

• Plasma membrane: Regulates entry and exit of substances

• Cytoplasm: Gel-like substance inside the cell

• Nucleoid: Region containing DNA

• Ribosomes: Sites of protein synthesis

• Plasmids: Small, circular DNA molecules

• Capsule: Outer protective layer (if present)

• Flagella, fimbriae, pili: Appendages for movement and attachment

Glycocalyxb

Structure and Types

The glycocalyx is a viscous, gelatinous layer external to the cell wall, composed of polysaccharides and/or polypeptides. It exists in two forms:

• Capsule: Neatly organized and firmly attached

• Slime layer: Unorganized and loosely attached

Functions of Glycocalyx

• Adherence: Helps bacteria attach to surfaces and each other (biofilm formation)

• Virulence: Capsules prevent phagocytosis by host cells

• Protection: Shields against dehydration and toxins

• Nutrition: May serve as a nutrient reserve

Flagella and Motility

Flagella Structure

Flagella are long, whip-like appendages used for movement. They differ structurally between Gram-positive and Gram-negative bacteria.

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

Flagella Movement

Bacteria move by rotating their flagella, a process called taxis. Movement can be toward (positive taxis) or away from (negative taxis) a stimulus, such as chemicals (chemotaxis).

Other Appendages

• Axial filaments (endoflagella): Found in spirochetes, these structures wrap around the cell and enable corkscrew movement.

Fimbriae and Pili

• Fimbriae: Short, hairlike structures for attachment

• Pili: Longer, involved in motility and DNA transfer (conjugation)

Bacterial Cell Wall

Structure and Function

The bacterial cell wall maintains cell shape, prevents osmotic lysis, and anchors flagella. It is primarily composed of peptidoglycan, a polymer of sugars and amino acids.

• Contains both D- and L-amino acids

Gram-Positive vs. Gram-Negative Cell Walls

Bacteria are classified based on their cell wall structure, as revealed by the Gram stain:

• Gram-positive: Thick peptidoglycan layer, teichoic acids

• Gram-negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharides, porins, and a periplasmic space

Plasma Membrane

Structure and Function

The plasma membrane is a phospholipid bilayer with embedded proteins, separating the cell wall from the cytoplasm. It is selectively permeable and involved in nutrient breakdown and energy generation.

• Phosphate heads are hydrophilic; fatty acid tails are hydrophobic

Movement of Substances Across Membranes

• Passive processes: Simple diffusion, facilitated diffusion, osmosis (no ATP required)

• Active processes: Active transport, group translocation (require ATP)

Internal Structures of Prokaryotic Cells

Inclusions

• Metachromatic granules: Inorganic phosphate storage

• Polysaccharide and lipid inclusions: Energy reserves

• Sulfur granules: Energy source for sulfur bacteria

• Carboxysomes: Contain enzymes for photosynthesis

• Gas vacuoles: Buoyancy in aquatic bacteria

• Magnetosomes: Contain iron oxide, act as magnets

Endospores

Endospores are highly durable, dehydrated cells formed by some Gram-positive bacteria. They are resistant to heat, dehydration, chemicals, and radiation, and can survive for millions of years.

Eukaryotic Cell Structure

Major Components

• Nucleus: Contains genetic material

• Endoplasmic reticulum (ER): Rough ER (protein synthesis), Smooth ER (lipid synthesis)

• Golgi complex: Protein modification and sorting

• Lysosomes: Digestive enzymes

• Vacuoles: Storage

• Mitochondria: ATP production

• Chloroplasts (plants/algae): Photosynthesis

• Peroxisomes: Oxidation of fatty acids and detoxification

• Centrosomes: Cell division

Flagella and Cilia

Eukaryotic flagella and cilia are projections for motility or moving substances along the cell surface. They have a 9+2 arrangement of microtubules and move in a wave-like manner.

Cell Wall and Glycocalyx

• Cell wall: Present in plants, algae, fungi; made of cellulose, chitin, glucan, or mannan

• Glycocalyx: Sticky carbohydrate layer for protection and attachment

Endosymbiotic Theory

The endosymbiotic theory proposes that eukaryotic cells evolved when larger prokaryotic cells engulfed smaller ones, which became organelles such as mitochondria and chloroplasts.