Functional Anatomy of Prokaryotic and Eukaryotic Cells: Study Notes

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Comparison of Prokaryotic and Eukaryotic Cell Structure

Cells are classified as either prokaryotic or eukaryotic based on structural and functional differences. Understanding these differences is fundamental to microbiology.

• Prokaryotic cells lack a true nucleus and membrane-bound organelles. Their genetic material is located in a nucleoid region.

• Eukaryotic cells possess a true nucleus enclosed by a nuclear membrane and contain various membrane-bound organelles.

• Both cell types have plasma membranes, ribosomes, and cytoplasm, but their organization and complexity differ.

Basic Shapes of Bacteria

Bacteria exhibit three primary shapes, which are important for identification and classification.

• Coccus: Spherical-shaped bacteria (e.g., Staphylococcus).

• Bacillus: Rod-shaped bacteria (e.g., Bacillus subtilis).

• Spiral: Includes vibrio (comma-shaped), spirillum (rigid spiral), and spirochete (flexible spiral).

Glycocalyx: Structure and Function

The glycocalyx is a gelatinous, sticky substance surrounding the outside of some bacterial cells.

• Structure: Composed of polysaccharides, polypeptides, or both.

• Types: Capsule (organized and firmly attached) and slime layer (unorganized and loosely attached).

• Functions: Protects against desiccation, aids in adherence to surfaces, and protects against phagocytosis.

Surface Appendages: Flagella, Axial Filaments, Fimbriae, and Pili

Bacteria possess various surface structures for motility and attachment.

• Flagella: Long, whip-like structures for motility.

• Axial filaments: Found in spirochetes; enable corkscrew movement.

• Fimbriae: Short, hair-like structures for attachment to surfaces.

• Pili: Longer than fimbriae; involved in conjugation (DNA transfer) and attachment.

Cell Walls: Gram-Positive, Gram-Negative, Acid-Fast, Archaea, and Mycoplasmas

Bacterial cell walls differ in composition and structure, influencing staining properties and susceptibility to antibiotics.

Type	Main Features
Gram-Positive	Thick peptidoglycan layer, teichoic acids, stains purple
Gram-Negative	Thin peptidoglycan, outer membrane with lipopolysaccharide (LPS), stains pink
Acid-Fast	Mycolic acid in cell wall, resists decolorization (e.g., Mycobacterium)
Archaea	No peptidoglycan; may have pseudopeptidoglycan or protein layers
Mycoplasmas	Lack cell walls; have sterols in plasma membrane

Comparison: Archaea and Mycoplasmas

• Archaea: Cell walls lack peptidoglycan; unique membrane lipids; often live in extreme environments.

• Mycoplasmas: Lack cell walls entirely; smallest free-living organisms; plasma membrane contains sterols.

Protoplast, Spheroplast, and L Form

• Protoplast: Gram-positive cell with cell wall removed; susceptible to osmotic lysis.

• Spheroplast: Gram-negative cell with partial cell wall removal; retains some outer membrane.

• L Form: Bacteria that have lost their cell wall naturally or artificially; can revert to walled state.

Prokaryotic Plasma Membrane: Structure, Chemistry, and Functions

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins.

• Functions: Controls entry and exit of substances, site of energy generation (electron transport chain), and contains enzymes for metabolic reactions.

• Chemistry: Lacks sterols (except in mycoplasmas); contains hopanoids for stability.

Transport Mechanisms Across Membranes

• Simple diffusion: Movement of molecules from high to low concentration without energy input.

• Facilitated diffusion: Movement via membrane proteins; no energy required.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Active transport: Movement against concentration gradient using energy (ATP).

• Group translocation: Substance is chemically modified during transport (e.g., glucose phosphorylation in bacteria).

Nucleoid and Ribosomes: Functions

• Nucleoid: Region containing the bacterial chromosome (DNA); not membrane-bound.

• Ribosomes: Sites of protein synthesis; prokaryotic ribosomes are 70S (50S + 30S subunits).

Inclusions: Types and Functions

Bacteria store nutrients and other substances in cytoplasmic inclusions.

• Metachromatic granules: Store inorganic phosphate.

• Polysaccharide granules: Store glycogen and starch.

• Lipid inclusions: Store polyhydroxybutyrate and other lipids.

• Sulfur granules: Store sulfur for energy metabolism.

Endospores: Functions, Sporulation, and Germination

• Endospores: Highly resistant, dormant structures formed by some bacteria (e.g., Bacillus, Clostridium).

• Sporulation: Process of endospore formation under adverse conditions.

• Germination: Return of endospore to vegetative state when conditions improve.

Flagella: Prokaryotic vs. Eukaryotic

• Prokaryotic flagella: Composed of flagellin; rotate like propellers; anchored by basal body.

• Eukaryotic flagella: Composed of microtubules (9+2 arrangement); move in a wave-like motion; covered by plasma membrane.

Cell Walls and Glycocalyxes: Prokaryotic vs. Eukaryotic

• Prokaryotic cell walls: Usually peptidoglycan (bacteria); variable in archaea.

• Eukaryotic cell walls: Cellulose (plants, algae), chitin (fungi), or absent (animals, protozoa).

• Glycocalyx: Present in some eukaryotes (animal cells) as a carbohydrate-rich layer for protection and cell recognition.

Plasma Membranes: Prokaryotic vs. Eukaryotic

• Prokaryotic: Lacks sterols (except mycoplasmas); site of metabolic processes.

• Eukaryotic: Contains sterols (e.g., cholesterol); involved in endocytosis and exocytosis.

Cytoplasm: Prokaryotic vs. Eukaryotic

• Prokaryotic cytoplasm: Lacks cytoskeleton and membrane-bound organelles.

• Eukaryotic cytoplasm: Contains cytoskeleton (microtubules, microfilaments) and organelles.

Ribosomes: Structure and Function

• Prokaryotic ribosomes: 70S (50S + 30S subunits).

• Eukaryotic ribosomes: 80S (60S + 40S subunits); 70S ribosomes in mitochondria and chloroplasts.

• Function: Protein synthesis in both cell types.

Organelles: Definition and Functions

An organelle is a specialized structure within a cell that performs a specific function.

• Nucleus: Contains genetic material; controls cell activities.

• Endoplasmic reticulum (ER): Synthesis of proteins (rough ER) and lipids (smooth ER).

• Golgi complex: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Contain digestive enzymes for breakdown of macromolecules.

• Vacuoles: Storage of substances; large in plant cells.

• Mitochondria: Site of ATP production via aerobic respiration.

• Chloroplasts: Site of photosynthesis in plants and algae.

• Peroxisomes: Breakdown of fatty acids and detoxification of hydrogen peroxide.

• Centrosomes: Organize microtubules during cell division.

Endosymbiotic Theory of Eukaryotic Evolution

The endosymbiotic theory proposes that certain organelles (mitochondria and chloroplasts) originated as free-living bacteria that were engulfed by ancestral eukaryotic cells.

• Evidence:

  • Mitochondria and chloroplasts have their own circular DNA, similar to bacteria.

  • They possess 70S ribosomes, like prokaryotes.

  • They replicate independently of the cell by binary fission.

Example: The similarity between Rickettsia (a bacterium) and mitochondria supports the endosymbiotic origin of mitochondria.

Additional info: These notes expand on the learning objectives by providing definitions, comparisons, and examples relevant to the functional anatomy of prokaryotic and eukaryotic cells.