Functional Anatomy of Prokaryotic and Eukaryotic Cells: Microbiology Study Notes

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Overview: Comparing Prokaryotic and Eukaryotic Cells

This section introduces the fundamental differences between prokaryotic and eukaryotic cells, which are essential for understanding microbial structure and function.

• Prokaryotes: Cells without a true nucleus; genetic material is not enclosed by a membrane.

• Eukaryotes: Cells with a true nucleus; genetic material is enclosed within a nuclear membrane.

• Key differences: Prokaryotes lack membrane-bound organelles and histones, while eukaryotes possess both. Cell division in prokaryotes occurs by binary fission, whereas eukaryotes divide by mitosis.

Cell Shapes and Arrangements in Prokaryotes

Bacteria exhibit a variety of shapes and arrangements, which are often used for classification and identification.

• Common shapes:

  • Bacillus (rod-shaped)

  • Coccus (spherical-shaped)

  • Spiral (includes vibrio, spirillum, spirochete)

  • Other shapes: star-shaped, rectangular

• Arrangements:

  • Pairs: diplococci, diplobacilli

  • Clusters: staphylococci

  • Chains: streptococci, streptobacilli

  • Groups of four: tetrads

  • Cubelike groups of eight: sarcinae

• Scientific names often reflect shape: e.g., Bacillus (genus) is rod-shaped.

Structure of a Prokaryotic Cell

Prokaryotic cells have several key structures, both external and internal to the cell wall, that contribute to their function and pathogenicity.

• External structures:

  • Glycocalyx: Viscous, gelatinous layer outside the cell wall; can be a capsule (organized, firmly attached) or slime layer (unorganized, loose). Contributes to virulence by preventing phagocytosis and aiding in biofilm formation.

  • Flagella: Filamentous appendages for motility; composed of filament, hook, and basal body. Enable movement via "runs" and "tumbles" and are used for chemotaxis.

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

  • Fimbriae and Pili: Hairlike structures for attachment (fimbriae) and DNA transfer/motility (pili).

• Internal structures:

  • Cell wall: Provides shape and protection; made of peptidoglycan in bacteria.

  • Plasma membrane: Selectively permeable phospholipid bilayer; site of ATP production and transport.

  • Cytoplasm: Contains DNA (nucleoid), ribosomes, and inclusions.

  • Nucleoid: Region containing the bacterial chromosome (circular DNA).

  • Ribosomes: Sites of protein synthesis (70S type in prokaryotes).

  • Inclusions: Reserve deposits of nutrients.

  • Endospores: Highly resistant, dormant structures formed under stress.

Bacterial Cell Wall: Composition and Types

The cell wall is a critical structure for bacterial survival, classification, and antibiotic targeting.

• Peptidoglycan: Polymer of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by polypeptides, forming a lattice.

• Gram-positive cell walls: Thick peptidoglycan, teichoic acids, high susceptibility to penicillin, disrupted by lysozyme.

• Gram-negative cell walls: Thin peptidoglycan, outer membrane with lipopolysaccharide (LPS), low susceptibility to penicillin, produce endotoxins.

• Atypical cell walls: Acid-fast (waxy mycolic acid), mycoplasmas (no cell wall), archaea (pseudomurein or no wall).

Plasma (Cytoplasmic) Membrane: Structure and Function

The plasma membrane controls the movement of substances in and out of the cell and is essential for energy production and cellular integrity.

• Structure: Phospholipid bilayer with embedded proteins (integral, peripheral), glycoproteins, and glycolipids.

• Fluid mosaic model: Membrane components move freely; membrane is self-sealing.

• Function: Selective permeability, ATP production, photosynthetic pigments (chromatophores).

Movement of Materials Across Membranes

Cells use passive and active processes to transport substances across membranes.

• Passive processes:

  • Simple diffusion: Movement from high to low concentration.

  • Facilitated diffusion: Transport via membrane proteins; can be specific or nonspecific.

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

  • Osmotic pressure: Pressure needed to stop water movement.

  • Isotonic, hypotonic, hypertonic solutions: Affect water movement and cell integrity.

• Active processes:

  • Active transport: Requires ATP and transporter proteins; moves substances against gradient.

  • Group translocation: Substance is chemically altered during transport.

Internal Structures: Cytoplasm, Nucleoid, Ribosomes, Inclusions, Endospores

Prokaryotic cells contain several internal structures vital for their survival and function.

• Cytoplasm: Aqueous substance containing water, proteins, carbohydrates, lipids, ions, DNA, ribosomes, and inclusions.

• Nucleoid: Contains the bacterial chromosome (circular DNA); plasmids may carry additional genes.

• Ribosomes: 70S type; site of protein synthesis; target for certain antibiotics.

• Inclusions: Reserve deposits (e.g., phosphate, polysaccharide, lipid, sulfur, gas vacuoles, magnetosomes).

• Endospores: Dormant, highly resistant structures formed under stress; important in food safety and disease.

Eukaryotic Cell Structure

Overview of Eukaryotic Cell Structures

Eukaryotic cells are more complex than prokaryotic cells, containing numerous membrane-bound organelles.

• Flagella and cilia: Projections for locomotion or moving substances; flagella are long and few, cilia are short and numerous. Both have a 9+2 microtubule arrangement and move in a wavelike manner.

• Cell wall and glycocalyx: Cell walls found in plants, algae, fungi (cellulose, chitin, glucan, mannan); glycocalyx found in animal cells, strengthens surface and aids in cell recognition.

• Plasma membrane: Similar to prokaryotes but contains sterols and carbohydrates; capable of endocytosis (phagocytosis, pinocytosis, receptor-mediated).

• Cytoplasm: Contains cytosol, cytoskeleton (microfilaments, intermediate filaments, microtubules), and enables cytoplasmic streaming.

• Ribosomes: 80S type (60S + 40S subunits) in cytoplasm and on ER; 70S type in mitochondria and chloroplasts.

Organelles of Eukaryotic Cells

Eukaryotic organelles perform specialized functions essential for cell survival and activity.

• Nucleus: Double membrane (nuclear envelope) encloses DNA; DNA complexed with histones to form chromatin, which condenses into chromosomes during cell division.

• Endoplasmic reticulum (ER):

  • Rough ER: Studded with ribosomes; site of protein synthesis.

  • Smooth ER: Synthesizes membranes, fats, hormones.

• Golgi complex: Modifies and transports proteins from ER via secretory vesicles.

• Lysosomes: Contain digestive enzymes; formed in Golgi complex.

• Vacuoles: Storage and transport; formed by Golgi or endocytosis.

• Mitochondria: Double membrane, inner folds (cristae), site of ATP production; contains 70S ribosomes and circular DNA.

• Chloroplasts: Site of photosynthesis; contains thylakoids, chlorophyll, 70S ribosomes, circular DNA.

• Peroxisomes: Oxidize fatty acids, destroy hydrogen peroxide.

• Centrosomes: Organize mitotic spindle; critical for cell division.

Endosymbiotic Theory of Eukaryotic Evolution

The endosymbiotic theory explains the origin of eukaryotic cells from prokaryotic ancestors.

• Larger bacterial cells engulfed smaller ones, leading to the development of organelles such as mitochondria and chloroplasts.

• Evidence: Mitochondria and chloroplasts resemble bacteria in size and shape, have circular DNA, reproduce independently, possess 70S ribosomes, and have double membranes.

Summary Table: Gram-Positive vs. Gram-Negative Bacteria

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick	Thin
Teichoic acids	Present	Absent
Outer membrane	Absent	Present
Susceptibility to penicillin	High	Low
Exotoxins	Produced	Produced
Endotoxins	Absent	Present (Lipid A)
Flagella basal body rings	2	4

Key Terms

• Prokaryote: Cell lacking a nucleus and membrane-bound organelles.

• Eukaryote: Cell with a nucleus and membrane-bound organelles.

• Peptidoglycan: Polymer forming bacterial cell wall.

• Glycocalyx: External polysaccharide/polypeptide layer.

• Flagella: Motility appendages.

• Fimbriae/Pili: Attachment and DNA transfer structures.

• Endospore: Dormant, resistant bacterial cell.

• Osmosis: Water movement across membrane.

• Endosymbiotic theory: Origin of eukaryotic organelles from prokaryotes.

Important Equations

• Osmosis: $\text{Osmotic Pressure} = \text{Pressure required to prevent water movement across a membrane}$

• Facilitated Diffusion: $\text{Rate} = k \times (C_{\text{outside}} - C_{\text{inside}})$ where k is a constant, C is concentration