Prokaryotic and Eukaryotic Cell Structures & Functions: Study Guide

Prokaryotic and Eukaryotic Cell Structures & Functions

Key Terminology

• Histones: Proteins that package and order DNA into structural units called nucleosomes in eukaryotes.

• Organelles: Specialized structures within a cell that perform distinct processes (e.g., mitochondria, Golgi apparatus).

• Monomorphic: Bacteria that maintain a single shape.

• Pleomorphic: Bacteria that can alter their shape or size in response to environmental conditions.

• Taxis: Directed movement of an organism in response to a stimulus (e.g., chemotaxis, phototaxis).

• Selectively Permeable: Property of cell membranes that allows some substances to pass while blocking others.

• Osmotic Pressure: Pressure required to prevent the movement of water across a semipermeable membrane.

• Inclusion: Reserve deposits found in prokaryotic and eukaryotic cells (e.g., granules, vesicles).

• Endospore: Highly resistant, dormant structure formed by certain bacteria for survival in harsh conditions.

• Sporulation: Process of endospore formation.

• Germination: Process by which an endospore returns to its vegetative state.

• Endocytosis: Cellular process of engulfing material from the environment into the cell.

• Phagocytosis: Type of endocytosis where large particles are engulfed ('cell eating').

• Pinocytosis: Type of endocytosis where liquids are engulfed ('cell drinking').

• Nucleolus: Dense region within the nucleus where ribosomal RNA is synthesized.

• Chromatin: Complex of DNA and proteins (mainly histones) in the nucleus.

• Chromosome: Thread-like structure of nucleic acids and proteins carrying genetic information.

• Nuclear Pores: Openings in the nuclear envelope that regulate transport between nucleus and cytoplasm.

• Nuclear Envelope: Double membrane surrounding the nucleus.

• Nucleosome: Structural unit of chromatin, consisting of DNA wound around histone proteins.

Similarities and Differences Between Prokaryotes and Eukaryotes

Overview

Prokaryotic and eukaryotic cells share some fundamental features but differ significantly in structure and complexity.

• Similarities:

  • Both have a plasma membrane, cytoplasm, ribosomes, and genetic material (DNA).

  • Both carry out essential life processes such as metabolism and reproduction.

• Differences:

  • Prokaryotes lack a true nucleus and membrane-bound organelles; eukaryotes possess both.

  • Prokaryotic DNA is typically circular and found in the nucleoid; eukaryotic DNA is linear and organized into chromosomes within the nucleus.

  • Prokaryotes are generally smaller and simpler; eukaryotes are larger and more complex.

Prokaryotic Cell Structures

Shapes and Arrangements

• Shapes: Cocci (spherical), bacilli (rod-shaped), spirilla (spiral), vibrio (comma-shaped), spirochete (flexible spiral).

• Arrangements: Single, pairs (diplo-), chains (strepto-), clusters (staphylo-), tetrads, sarcinae.

Glycocalyx

• Definition: Gelatinous, sticky substance surrounding the outside of some cells.

• Capsule: Organized, firmly attached layer; protects against phagocytosis.

• Slime Layer: Unorganized, loosely attached; aids in adherence to surfaces.

Flagella

• Function: Motility; enables bacteria to move toward or away from stimuli.

• Arrangements: Monotrichous (single), lophotrichous (tuft at one end), amphitrichous (one at each end), peritrichous (all over).

• Movement: "Run" (straight movement), "tumble" (change in direction).

• Taxis: Movement in response to stimuli; chemotaxis (chemicals), phototaxis (light).

• Positive taxis: Movement toward stimulus; negative taxis: movement away.

Axial Filaments

• Definition: Bundles of fibrils that spiral around the cell, found in spirochetes.

• Movement: Produces a corkscrew motion.

Fimbriae vs. Pili

• Fimbriae: Short, numerous; used for attachment to surfaces.

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

Cell Wall

• Functions: Maintains shape, protects from osmotic lysis, anchors flagella.

• Gram-Positive: Thick peptidoglycan layer, teichoic acids, stains purple.

• Gram-Negative: Thin peptidoglycan, outer membrane with lipopolysaccharides, stains pink.

• Mycoplasma: Lacks cell wall; has sterols in plasma membrane for stability.

• Archaea: May lack peptidoglycan; cell wall composed of pseudomurein or other polymers.

Protoplast vs. Spheroplast

• Protoplast: Gram-positive cell with cell wall removed.

• Spheroplast: Gram-negative cell with partial cell wall removal.

• L-form: Bacteria that have lost their cell wall, can arise naturally or by chemical treatment.

• Cell Wall Damage: Lysozyme, antibiotics (e.g., penicillin) can damage or inhibit cell wall synthesis.

Plasma Membrane

• Phospholipid Bilayer: Two layers of phospholipids with embedded proteins.

• Peripheral Proteins: Loosely attached to membrane surface.

• Integral Proteins: Penetrate or span the membrane.

• Fluid Mosaic Model: Describes the dynamic and flexible nature of the membrane.

• Functions: Selective barrier, site for metabolic processes, cell communication.

Transport Across Membranes

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

• Facilitated Diffusion: Movement via transport proteins; no energy required.

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

• Active Transport: Movement against concentration gradient; requires energy (ATP).

• Group Translocation: Substance is chemically modified during transport (unique to prokaryotes).

Osmotic Solutions

• Hypotonic: Lower solute concentration outside; water enters cell, may cause lysis.

• Hypertonic: Higher solute concentration outside; water leaves cell, causing plasmolysis.

• Isotonic: Equal solute concentration; no net water movement.

Internal Structures

• Ribosomes: Sites of protein synthesis; 70S in prokaryotes (50S + 30S subunits).

• Cytoplasm: Gel-like substance inside the cell membrane, contains enzymes, nutrients, and cell structures.

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

• Plasmid: Small, circular DNA molecules; carry non-essential genes (e.g., antibiotic resistance).

Endospores

• Stimulus for Sporulation: Nutrient depletion or harsh conditions.

• Release: Endospore is released when the mother cell lyses.

• Germination: Endospore returns to vegetative state when conditions improve.

Eukaryotic Cell Structures

Flagella vs. Cilia

• Flagella: Long, few; used for cell movement.

• Cilia: Short, numerous; move substances along cell surface or propel cell.

• Microtubules: Hollow tubes of tubulin; form the structural basis of cilia and flagella.

• 9+2 Array: Arrangement of microtubules in cilia/flagella (9 pairs surrounding 2 central microtubules).

Cell Wall and Membrane

• Composition: Plants (cellulose), fungi (chitin), algae (varied polysaccharides).

• Sterols: Lipids (e.g., cholesterol) found in eukaryotic membranes; provide stability.

Cytoplasm, Cytosol, and Cytoskeleton

• Cytosol: Fluid portion of cytoplasm.

• Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments) for shape and movement.

• Cytoplasmic Streaming: Movement of cytoplasm to distribute materials within cells.

Ribosomes

• Free Ribosomes: Float in cytosol; synthesize proteins for use within the cell.

• Fixed Ribosomes: Attached to endoplasmic reticulum; synthesize proteins for export or membranes.

• Size: Eukaryotic ribosomes are 80S (60S + 40S subunits); prokaryotic are 70S.

Centrosome and Centrioles

• Centrosome: Region near nucleus; microtubule organizing center.

• Centrioles: Paired structures within centrosome; help organize mitotic spindle during cell division.

Membranous Organelles

• Nucleus: Contains genetic material; site of DNA replication and transcription.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids; has cis (receiving) and trans (shipping) faces.

• Vesicles: Membrane-bound sacs for transport and storage.

• Lysosomes: Contain digestive enzymes; break down waste and cellular debris.

• Peroxisomes: Contain enzymes for oxidation reactions; break down fatty acids and detoxify hydrogen peroxide.

• Vacuoles: Storage organelles; large in plant cells for water and nutrients.

• Mitochondria: Site of cellular respiration; produces ATP.

• Chloroplasts: Site of photosynthesis; found in plants and algae.

Vesicle Destinations and Membrane Flow

• Three Destinations: Plasma membrane (exocytosis), lysosomes, or other organelles.

• Membrane Flow: Order: ER → Golgi apparatus → vesicles → destination.

Endocytosis Types

• Phagocytosis: Engulfment of large particles.

• Pinocytosis: Engulfment of fluids.

Nucleus Structure

• Nuclear Envelope: Double membrane surrounding nucleus.

• Nuclear Pores: Allow exchange of materials between nucleus and cytoplasm.

• Nucleolus: Site of ribosomal RNA synthesis.

• Chromatin: DNA-protein complex; condenses to form chromosomes during cell division.

• Nucleosome: DNA wrapped around histone proteins.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotes	Eukaryotes
Nucleus	No true nucleus; nucleoid region	True nucleus with nuclear envelope
Organelles	Absent	Present (mitochondria, ER, etc.)
Cell Wall	Peptidoglycan (bacteria), varied in Archaea	Cellulose (plants), chitin (fungi), absent in animals
Ribosome Size	70S	80S (cytoplasm), 70S (mitochondria/chloroplasts)
DNA Structure	Circular, single chromosome	Linear, multiple chromosomes
Division	Binary fission	Mitosis/meiosis

Example: Bacterial Endospore Formation

Bacillus and Clostridium species form endospores when nutrients are scarce, allowing survival in extreme conditions. When conditions improve, the endospore germinates into a vegetative cell.

Additional info:

• Some details (e.g., specific examples of group translocation or membrane flow) were inferred for completeness.

• Table entries and definitions were expanded for clarity and academic context.