Prokaryotes and Eukaryotes: Cell Structure and Function

Prokaryotes

Prokaryotes are unicellular organisms lacking a nucleus and membrane-bound organelles. They include Bacteria and Archaea, and are generally smaller than eukaryotic cells.

• Lack nucleus

• Lack membrane-bound organelles

• Smaller size

• Examples: Bacteria, Archaea

External Structures

• Glycocalyx

  • Sticky layer of polysaccharides and polypeptides

  • Capsule: Firmly attached, prevents host recognition

  • Slime layer: Loosely attached, aids in attachment to surfaces

• Flagella

  • Allows for movement: rotation, motor, propels cell

  • Structure: filament, hook, basal cell

• Fimbriae

  • Sticky, bristle-like structures

  • Adhere to each other and surfaces, used to create biofilms

• Pili

  • Type IV fimbriae

  • Conjugation pili: genetic exchange

Cell Walls

• Provide structure and shape, attach to other cells

• Targeted by antibiotics

• Composed of peptidoglycan

• Gram positive: Thick peptidoglycan layer, purple stain

• Gram negative: Thin peptidoglycan, outer bilayer membrane with lipopolysaccharide, pink stain

Cytoplasmic Membrane

• Phospholipid bilayer

• Fluidity allows selective permeability

• Functions:

  • Store energy (proton gradient)

  • Harvest light energy (photosynthetic prokaryotes)

  • Protein movement into membrane

Eukaryotes

Eukaryotes possess a nucleus and membrane-bound organelles. They include algae, protozoa, fungi, animals, and plants.

• Have nucleus

• Membrane-bound organelles

• Examples: Algae, Protozoa, Fungi, Animals, Plants

External Structures

• Glycocalyx: Anchors animal cells, strengthens cell surface, protects against dehydration, cell recognition and communication

Cell Walls

• Fungi, algae, plants, and some protozoa have cell walls

• Animal cells do not have cell walls

• Plant cell walls: cellulose

• Fungal cell walls: cellulose, chitin, glucomannan

• Algal cell walls: polysaccharides

Cytoplasmic Membranes

• All eukaryotes have cytoplasmic membranes

• Fluid mosaic model: steroid lipids help fluidity

Cytoplasm and Organelles

• Endocytosis/Exocytosis: Use pseudopods to engulf cells; excrete waste via vesicles

• Flagella and Cilia: Movement; cilia are shorter and more numerous

• Ribosomes: 80S (60S + 40S subunits)

• Cytoskeleton: Microtubules, actin filaments, intermediate filaments

• Nucleus: Contains DNA, surrounded by nuclear envelope

• Endoplasmic Reticulum: Rough (protein synthesis), smooth (lipid synthesis)

• Golgi Body: Processes and packages molecules for export

• Lysosomes/Peroxisomes: Digestion and waste degradation

• Mitochondria: ATP production, contains own DNA

• Chloroplasts: Photosynthesis in plants and algae

Endosymbiotic Theory

• Large prokaryotes ingested small aerobic prokaryotes

• Smaller prokaryotes became mitochondria and chloroplasts

Microscopy and Staining

Principles of Microscopy

• Magnification: Mediated by refraction; image is inverted, reversed, and enlarged

• Resolution: Ability to distinguish two close points; limited by wavelength

• Contrast: Difference between objects and background; increased by staining and use of light

Types of Microscopes

• Bright Field: Simple (single lens) and compound (multiple lenses)

• Fluorescence: Uses UV light, increases resolution and contrast

• Electron Microscopy: Greater resolution and magnification; transmission and scanning types

Staining

• Principle: Color specimen with stain to increase contrast and resolution

• Simple Stain: Identifies size, shape, arrangement

• Differential Stain: Distinguishes cell types (e.g., Gram stain, Acid-fast stain)

• Special Stains: For specific structures (e.g., capsule, flagella)

Classification and Identification of Microorganisms

Taxonomy

• Classification, naming, and identification of organisms

• Organize large amount of information

• Relate organisms based on similarities

Linnaeus and Modern Taxonomy

• Classifies organisms based on common characteristics

• Two kingdoms: Animalia and Plantae (originally)

• Modern: Three domains (Eukarya, Bacteria, Archaea) based on rRNA sequences

Taxonomic Methods

• Physical characteristics: Morphology, color, groupings

• Biochemical tests: Ability to utilize/produce certain chemicals

• Serological tests: Study antigen-antibody reactions

• Phage typing: Use bacteriophages to identify bacteria

• Analysis of nucleic acids: DNA/RNA sequencing

• Taxonomy keys: Dichotomous keys for identification

Microbial Metabolism

Overview

Metabolism is the collection of controlled biochemical reactions that take place within a microbe, enabling reproduction and growth.

• Catabolism: Breaks larger molecules into smaller products; exergonic (releases energy)

• Anabolism: Synthesizes large molecules from smaller products; endergonic (requires energy)

• Oxidation and Reduction: Electron transfer reactions; use electron carriers (NAD+, NADP+, FAD)

• ATP Production: Energy storage and release; substrate-level phosphorylation, oxidative phosphorylation, photophosphorylation

• Enzymes: Catalysts that lower activation energy; affected by temperature, pH, concentration, inhibitors

Carbohydrate Catabolism

• Glycolysis: Oxidizes glucose to pyruvic acid; net gain: 2 ATP, 2 NADH, precursor metabolites

• Cellular Respiration: Complete oxidation of pyruvic acid via Krebs cycle and electron transport chain

  • Krebs cycle: per glucose

  • Electron transport chain: Produces proton gradient, ATP via chemiosmosis

  • Final electron acceptor: Oxygen (aerobic), other molecules (anaerobic)

• Fermentation: Incomplete oxidation; organic molecule as final electron acceptor

Photosynthesis

• Light energy used to synthesize carbohydrates from CO2 and H2O

• Chlorophylls: Pigments that capture light

• Photosystems: I and II; absorb light, use redox reactions to store energy

• Light-dependent reactions: Produce ATP and NADPH

• Light-independent reactions: Synthesize glucose via Calvin-Benson cycle

  • Steps: Fixation of CO2, reduction, regeneration of RuBP

Regulation of Metabolic Function

• Control of gene expression: timing and amount of protein production

• Control of metabolic expression: activity of proteins once made

Microbial Nutrition and Growth

Growth Requirements

• Microbial growth: Increase in population size

• Colony: Group of cells from a single parent cell

• Biofilm: Collection of microbes living on a surface in a complex community

Nutrients

• Used for energy and building organic molecules

• Major elements: carbon, oxygen, nitrogen, hydrogen

• Sources:

  • Autotrophs: Use CO2 as carbon source

  • Heterotrophs: Use organic carbon skeletons

  • Chemotrophs: Obtain energy from chemical bonds

  • Phototrophs: Obtain energy from light

Oxygen Requirements

• Essential for obligate aerobes

• Toxic for obligate anaerobes

• Four toxic forms: singlet oxygen, superoxide radicals, peroxide anion, hydroxyl radical

• Organisms classified by oxygen requirements:

  • Aerobes: Use oxygen

  • Anaerobes: Do not use oxygen

  • Facultative anaerobes: Can use both

  • Aerotolerant anaerobes: Can tolerate oxygen

Additional info:

• Some details inferred for completeness, such as the role of biofilms and the Calvin-Benson cycle steps.