Prokaryotic and Eukaryotic Cells

Characteristics of Life

• Heredity: transmission of DNA
• Reproduction: sexual or asexual
• Growth: increase in population size or cell size
• Development: changes over the lifespan of an organism
• Metabolism: chemical reactions in organisms
• Responsiveness: cells interacting with the environment (stimulus/response)
• Transport: movement of materials in and out of the cell

Cells: Key Concepts

• Cells are the main determining factor in deciding if something is living vs nonliving
• All cells share core features: cell membrane, cytoplasm, genetic material, ribosomes
• Major cell groups: Prokaryotic and Eukaryotic

Prokaryotic vs Eukaryotic: Quick Overview

• Prokaryotic:
  • Lack a membrane-bound nucleus
  • Lack membrane-bound organelles
  • Generally smaller and simpler
  • Considered older in evolutionary terms
  • Include Archaea and Bacteria
• Eukaryotic:
  • Have a nucleus and membrane-bound organelles
  • More complex and larger
  • Include organisms in the group Eukarya (plants, fungi, animals)

Prokaryotic Cell Structure (Key Parts)

• Plasma membrane
• Ribosomes
• Cytoplasm
• Cell wall
• Plasmid
• Nucleoid (DNA)
• (In a generalized diagram, other features can include a capsule, slime layer, fimbriae, pili, flagellum, cytoplasmic matrix, chromosome, inclusion bodies, actin filaments, etc.)

Eukaryotic Animal Cell (Key Parts)

• Ribosomes
• Mitochondria
• Nucleus
• Plasma membrane
• Nuclear envelope
• Cytoplasm

Prokaryotic Cell Shapes

• Coccus: spherical
• Bacillus: rod-shaped
• Vibrio: curved rod
• Coccobacillus: short rod
• Spiral varieties:
  • Spirillum: rigid helices
  • Spirochete: helical and flexible

Prokaryotic Cell Arrangements

• Pairs: Diplococci, diplobacilli
• Clusters: Staphylococci
• Chains: Streptococci, streptobacilli
• Fours: Tetrads
• Cubical packets: Sarcina

Review Prompt

• How are prokaryotic cells and eukaryotic cells different?

Bacterial Cells: Key Features

• Prokaryotic cells
• Small and often appear 2D (they are not)
• Contain peptidoglycan in their cell walls
• Produce energy (ATP) in their membranes

Generalized Bacterial Cell: Core Structures (Descriptive List)

• Slime layer
• Fimbriae
• Ribosomes
• Cell wall
• Cell membrane
• Capsule
• Cytoplasmic matrix
• Actin filaments
• Chromosome (DNA)
• Pilus
• Inclusion body
• Flagellum

Building from the Outside In: Cell Surface to Interior

• Glycocalyx (slime layer or capsule)
• Cell wall
• Cell membrane
• Cytoplasm and internal structures

Glycocalyx: Details and Functions

• A viscous outer coating to protect the cell
• Not all prokaryotes have this structure
• Types:
  • Slime layer: loose, unorganized, easily removed
  • Capsule: organized, tightly bound, usually sticky and thicker
• Functions:
  • Prevents phagocytosis
  • Disguises bacteria from the immune system (pathogenicity)
  • Aids in adherence to surfaces
  • Provides protection from extreme environments

Bacterial Cell Wall: Purpose and Composition

• Helps determine the bacterial shape
• Maintains cell integrity and prevents osmotic lysis
• May contribute to disease-causing ability
• Made of macromolecules (carbohydrates and proteins)
• Core component: Peptidoglycan (murein)

Peptidoglycan: Structure Details

• Primary component of the cell wall
• Made of glycan (sugar) and short peptide fragments
• Glycan units: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)
• NAG and NAM alternate and are linked by peptide fragments

Types of Cell Walls: Gram-Positive vs Gram-Negative

• Gram-Positive:
  • Thick peptidoglycan layer
  • Periplasmic space between cell wall and cell membrane
  • Teichoic acid and lipoteichoic acid present
  • Roles: cell wall maintenance, growth, binding pathogens to tissues
• Gram-Negative:
  • More complex, multiple layers
  • Outer membrane and thin peptidoglycan layer
  • Periplasmic space and inner cell membrane
  • Outer membrane contains lipopolysaccharides (LPS) and lipoproteins
  • Lipid A (endotoxin component of LPS) can be released upon cell death
  • Porins regulate entry/exit; outer membrane provides barrier to detergents, dyes, enzymes, antibiotics

Gram-Positive vs Gram-Negative: Key Differences (Summary)

• Major layers: One (Gram-positive) vs Two (Gram-negative)
• Major chemical components: Peptidoglycan; Teichoic/Lipoteichoic acids (Gram-positive); LPS, lipoproteins, outer membrane (Gram-negative)
• Thickness: Thicker peptidoglycan in Gram-positive (≈20–80 nm) vs thinner in Gram-negative (≈8–11 nm)
• Periplasmic space: Narrow (Gram-positive) vs Extensive (Gram-negative)
• Outer membrane: No (Gram-positive) vs Yes (Gram-negative)
• Permeability: More penetrable in Gram-positive vs less penetrable in Gram-negative

Gram Stain Procedure: Visual Outcome

• Step 1: Flood with crystal violet for 1 minute; rinse
• Step 2: Flood with iodine for 1 minute; rinse
• Step 3: Decolorize with alcohol until no more purple comes off; rinse
• Step 4: Flood with safranin; rinse and dry
• Result: Gram-positive retain crystal violet (purple); Gram-negative lose the purple and appear pink/red after safranin

Gram Stain: What You See

• Gram-positive: crystal violet retained, appears purple
• Gram-negative: crystal violet washed out; safranin stains pink/red

Atypical Prokaryotic Cell Walls

• Acid-fast cell walls:
  • Bulk of wall contains waxy lipids (mycolic acids); contributes to pathogenicity
  • Acid-fast stain used for identification (e.g., Mycobacterium: TB and leprosy)
• No cell wall:
  • Some bacteria lack a cell wall entirely; natural sterols confer resistance to lysis
  • Examples: Mycoplasmas (e.g., M. pneumoniae)

Moving from Wall to Membrane: Plasma Membrane Basics

• Lipid bilayer with hydrophilic heads and hydrophobic tails
• Fluid mosaic model: dynamic, fluid phospholipid bilayer
• Proteins: transmembrane, integral, peripheral
• Extracellular and intracellular proteins

Functions of the Prokaryotic Cell Membrane

• Regulates transport in/out of the cell; selective permeability
• Prevents large, charged molecules from passing; small, uncharged molecules diffuse
• Site of metabolic activities (e.g., ATP synthesis in some prokaryotes)
• Secretion of toxins and enzymes

Inside the Prokaryotic Cell: Core Components

• Cytoplasm: mostly water (≈70–80%), dissolved sugars, amino acids, organic molecules
• Genetic information: DNA in a circular chromosome located in the nucleoid
• Ribosomes: protein synthesis; 70S type
• Inclusions: storage granules and other reserves
• Additional features often present: plasmids, cytoskeletal elements (actin-like), etc.

Genetic Information in Prokaryotes

• DNA is circular and located in the nucleoid
• Essential genes vs. plasmids (nonessential but advantageous)
• Plasmids confer traits such as drug resistance, toxin/enzyme production
• Plasmids are valuable tools in genetic engineering and biotechnology for inserting new genes
• Plasmids replicate independently and can be manipulated in the lab

Ribosomes in Prokaryotes

• Size: 70S ribosomes
• Subunit composition: Large subunit 50S, Small subunit 30S
• Function: site of protein synthesis
• Appearance: granular in most preparations

Inclusions and Other Internal Structures

• Inclusions: storage compartments for nutrients
• Gas vacuoles: provide buoyancy in aquatic bacteria
• Granules: inorganic compounds, metachromatic granules (metachromasia)
• Microcompartments: enzyme-rich microreactors
• Magnetosomes: magnetite crystals guiding orientation to Earth’s magnetic field

Endospores: Survival Strategy in Some G+ Genera

• Genera: Clostridium, Bacillus
• Two-phase life cycle:
  • Vegetative cell: metabolically active
  • Endospore: dormant, highly resistant to heat, desiccation, radiation, chemicals
• Endospores germinate when conditions are favorable
• Important note: Endospores are not a reproductive structure
• Sporulation is the process of endospore formation

Sporulation: Endospore Formation Steps (Outline)

1) Spore septum isolates newly replicated DNA and a portion of cytoplasm
2) Plasma membrane starts to surround DNA and cytoplasm
3) Spore septum surrounds isolated portion to form forespore
4) Peptidoglycan layer forms between membranes
5) Spore coat forms
6) Endospore is freed from cell

Filamentous Appendages: Flagella, Pili, Fimbriae

• Flagella: locomotion in certain bacteria; not all cells have them
  • Outside the cell wall; anchored by basal body; rotate to produce movement
  • Structure: hollow chain of flagellin protein
  • Movement: tumble (clockwise) and run (counterclockwise)
• Pili: elongated tubular structures; often involved in DNA transfer (conjugation)
• Fimbriae: short, hairlike projections used for attachment and biofilm formation

Eukaryotic Cells: Overview

• Eukaryotic cells include algae, helminths, arthropods, protozoa, fungi
• Can be unicellular or multicellular

Evolution of Eukaryotes: Endosymbiosis Theory

• Eukaryotic cells evolved ~2.5 billion years ago
• Endosymbiosis: large prokaryotes engulfed smaller prokaryotes
• Smaller cells became organelles (notably mitochondria and chloroplasts)
• Key events:
  • Aerobic bacteria became mitochondria
  • Photosynthetic bacteria became chloroplasts
• Evidence: mitochondria and chloroplasts have their own circular DNA and 70S-like ribosomes; double membranes; similar ribosomal and genetic features to bacteria

Generalized Eukaryotic Cell: Major Organelles

• Nucleus: membrane-bound; contains cellular DNA as linear chromosomes; nuclear envelope with pores
• Endoplasmic reticulum (ER): rough ER with ribosomes; smooth ER without ribosomes
• Golgi apparatus: stack of cisternae; protein modification and packaging into vesicles
• Mitochondria: powerhouse; ATP production; inner membrane folds (cristae); own DNA and 70S ribosomes
• Chloroplasts (in algae/plants): photosynthesis; bound by membranes; chlorophyll; own DNA and 70S ribosomes
• Lysosomes: digestive enzymes; degrade macromolecules and damaged organelles/microbes
• Peroxisomes: lipid and hydrogen peroxide metabolism; degrade uric acid, fatty acids; contain enzymes
• Cytoskeleton: microfilaments, intermediate filaments, microtubules; structural support and transport

The Nucleus: Structure and Function

• Nucleus is the most prominent organelle (not present in prokaryotes)
• Shape: spherical or oval
• Nuclear envelope contains pores for transport
• Functions: housing cellular DNA; DNA replication and transcription occur here; cell division via mitosis and meiosis

Endoplasmic Reticulum (ER)

• Network of flattened membranes surrounding the nucleus
• Two types:
  • Rough ER: studded with ribosomes; protein processing and modification; attaching carbohydrates or lipids to proteins
  • Smooth ER: lacks ribosomes; enzymes for lipid, steroid, and phospholipid synthesis

Golgi Apparatus

• Stack of flattened sacs called cisternae near the ER
• Functions: protein modification; protein storage; packaging for transport via secretory and transfer vesicles

Mitochondria and Chloroplasts: Origins and Features

• Mitochondria:
  • Generate ATP
  • Inner and outer membranes; inner folds (cristae) increase surface area
  • Contain their own DNA and 70S ribosomes
  • Supports endosymbiotic theory
• Chloroplasts:
  • Found in algae and plant cells
  • Carry out photosynthesis; convert light energy to chemical energy
  • Bound by membranes; contain chlorophyll; own DNA and 70S ribosomes
• Both organelles support the endosymbiotic origin of eukaryotic organelles

Cytoskeleton and Cytoplasmic Components

• Cytoskeleton: microfilaments, intermediate filaments, microtubules
• Provides structural support, facilitates organelle movement, and anchors organelles

Extracellular Matrix and Cell Walls in Eukaryotes

• Extracellular matrix: in some eukaryotes; composed of carbohydrates and proteins
  • Enables adherence to surfaces; mediates communication via receptors; provides protection
• Eukaryotic cell wall (in fungi, most algae, plants):
  • Made of polysaccharides; structural polysaccharides include:
  • Chitin (fungi)
  • Cellulose (plants and some algae)
  • Mannan (yeast)
  • Provides rigidity, structural support, and shape

Eukaryotic Cell Membrane and Transport

• Plasma membrane: phospholipid bilayer; selectively permeable
• Sterols present to modulate fluidity and stability

Cytoplasm in Eukaryotes

• Fluid-like interior containing organelles and cytoskeleton
• Cytoplasm anchors and facilitates movement of organelles

The Nucleus: Detailed Roles

• Contains DNA as linear chromosomes
• Nuclear envelope with nuclear pores regulates traffic
• DNA replication and transcription occur in the nucleus
• Cell division occurs via mitosis (and meiosis in reproductive cells)

Endomembrane System and Protein Trafficking

• ER, Golgi, vesicles coordinate synthesis, modification, and transport of proteins

Mitochondria and Chloroplasts: Details

• Mitochondria: ATP production; energy metabolism
• Chloroplasts: photosynthesis; light energy capture
• Endosymbiotic evidence: 70S ribosomes, circular DNA, double membranes

Cytoskeleton: Roles in Eukaryotic Cells

• Structural support; intracellular transport; organelle positioning; cell movement

Specialized Structures Outside the Cell (Eukaryotic Context)

• Flagella (in some eukaryotic cells): longer, thicker than prokaryotic flagella; not hollow; composed of microtubules arranged as 9+2; move in a wave-like manner
• Cilia: numerous short projections; used for motility, feeding, and filtering; beat in oar-like motions; also covered by plasma membrane

Connections to Foundational Principles and Real-World Relevance

• Endosymbiotic theory explains the origin of mitochondria and chloroplasts and highlights evolutionary relationships between cells
• Plasmids as tools in genetic engineering enable gene insertion and biotechnology applications
• LPS in Gram-negative bacteria functions as an endotoxin and is a key factor in pathogenicity and immune response
• Cell wall differences inform antibiotic targets (e.g., cell wall synthesis inhibitors affect Gram-positive and Gram-negative bacteria differently)
• Understanding cell membranes and transport underpins drug delivery, Nutrition, and cellular metabolism

Quick Reference: Key Numerical and Specific Facts

• Ribosomes: Prokaryotic 70S (Large subunit $50S$, Small subunit $30S$)
• Bacterial cell wall components: Peptidoglycan (murein); N-acetylglucosamine (NAG); N-acetylmuramic acid (NAM)
• Gram-positive wall thickness: approx. $20-80\text{ nm}$; thicker than Gram-negative
• Gram-negative wall thickness: approx. $8-11\text{ nm}$ for peptidoglycan layer
• Gram-negative outer membrane contains LPS (lipopolysaccharide)
• Endospores: extremely resistant; not reproductive; formed by certain G+ genera (Clostridium, Bacillus)
• Eukaryotic organelles with their own genomes and ribosomes: mitochondria and chloroplasts possess circular DNA and 70S ribosomes
• Ciliary and flagellar structure in eukaryotes: 9+2 arrangement of microtubules
• Endosymbiotic origin years: eukaryotes evolved about 2.5 billion years ago

Connections to Practical Applications and Ethics

• Genetic engineering uses bacterial plasmids to insert new genes; raises biosafety and ethical considerations in gene editing and synthetic biology
• Understanding Gram staining informs diagnostic microbiology and antibiotic selection
• Endotoxins (LPS) and bacterial cell wall components influence vaccine design and immune response considerations
• The endosymbiotic origins have philosophical implications about the unity of life and evolutionary processes