Prokaryotes

Shapes of Prokaryotes

Prokaryotes exhibit several distinct shapes, which are important for identification and classification.

• Coccus: Spherical shape (e.g., Streptococcus).

• Bacillus: Rod-shaped (e.g., Escherichia coli).

• Spirillum: Spiral-shaped (e.g., Spirillum volutans).

• Vibrio: Comma-shaped.

• Additional info: Some prokaryotes form clusters or chains, such as staphylococci (clusters) or streptococci (chains).

Cell Wall Composition: Bacteria vs. Archaea

The cell wall provides structural support and protection. Its composition differs between bacteria and archaea.

• Bacteria: Cell wall contains peptidoglycan, a polymer of sugars and amino acids.

• Archaea: Cell wall lacks peptidoglycan; may contain pseudopeptidoglycan or other unique compounds.

• Additional info: The cell wall is a target for antibiotics, such as penicillin, which inhibits peptidoglycan synthesis.

Gram Stain Technique

The Gram stain is a differential staining method used to classify bacteria based on cell wall structure.

1. Apply crystal violet dye.

2. Add iodine (mordant).

3. Wash with alcohol (decolorizer).

4. Counterstain with safranin.

• Gram-positive: Retain crystal violet, appear purple.

• Gram-negative: Lose crystal violet, take up safranin, appear pink/red.

Cell Wall Differences: Gram Positive vs. Gram Negative

• Gram-positive: Thick peptidoglycan layer, no outer membrane.

• Gram-negative: Thin peptidoglycan layer, outer membrane containing lipopolysaccharides.

Cell Surface Structures

• Capsule: Sticky layer outside cell wall; protects against desiccation and immune response.

• Endospores: Dormant, resistant structures formed under stress.

• Fimbriae: Short, hair-like appendages for attachment.

• Pili: Longer appendages; involved in DNA transfer (conjugation).

Specialized Membranes

Some prokaryotes have internal membranes for metabolic functions, such as photosynthesis or respiration.

Chromosome Arrangement

• Nucleoid: Region containing the single, circular chromosome.

• Plasmids: Small, circular DNA molecules; carry accessory genes.

Binary Fission

Prokaryotes reproduce by binary fission, a simple form of asexual reproduction.

1. DNA replication.

2. Cell elongation.

3. Septum formation.

4. Division into two identical cells.

Genetic Diversity in Prokaryotes

• Transformation: Uptake of naked DNA from environment.

• Transduction: DNA transfer via bacteriophages (viruses).

• Conjugation: Direct transfer of DNA between cells via pilus.

Metabolism Relative to Oxygen

• Obligate aerobes: Require oxygen for respiration.

• Obligate anaerobes: Cannot tolerate oxygen; use fermentation or anaerobic respiration.

• Facultative anaerobes: Can use oxygen or not, depending on availability.

Heterocysts

Specialized cells in some cyanobacteria for nitrogen fixation.

• Who has them: Cyanobacteria.

• Function: Provide anaerobic environment for nitrogenase enzyme.

Groups of Bacteria and Archaea

Major groups are classified based on characteristics and metabolic capabilities.

Symbiotic Relationships

• Mutualism: Both partners benefit.

• Commensalism: One benefits, other unaffected.

• Parasitism: One benefits, other harmed.

Endotoxins vs. Exotoxins

• Endotoxins: Components of outer membrane (lipopolysaccharides); released when cell dies.

• Exotoxins: Secreted proteins; cause disease symptoms.

Protists

Endosymbiosis

The theory of endosymbiosis explains the origin of mitochondria and chloroplasts in eukaryotic cells.

• Basic idea: Ancestral eukaryote engulfed prokaryotes, which became organelles.

• Evidence: Organelles have their own DNA, double membranes, and reproduce independently.

Protist Groups

Fungi

Basic Structure of Fungi

Fungi are eukaryotic organisms with unique structural features.

• Hyphae: Filamentous structures; form the body of fungi.

• Mycelium: Network of hyphae; increases surface area for absorption.

Types of Hyphae

• Septate hyphae: Divided by cross-walls (septa).

• Coenocytic hyphae: Lack septa; multinucleate.

Types of Mycorrhizal Fungi

• Ectomycorrhizal: Form sheaths around roots; do not penetrate cells.

• Arbuscular (Endomycorrhizal): Penetrate root cell walls; form arbuscules.

Fungal Life Cycle

Fungi reproduce sexually and asexually. Key stages include plasmogamy and karyogamy.

• Plasmogamy: Fusion of cytoplasm from two parent cells; creates heterokaryotic stage (cells with two or more nuclei).

• Karyogamy: Fusion of nuclei; creates diploid stage.

• Heterokaryotic/Dikaryotic: Cells contain two genetically distinct nuclei.

Fungi Groups and Characteristics

Roles of Fungi

• Decomposers: Break down organic matter.

• Mutualists: Form symbiotic relationships (e.g., mycorrhizae, lichens).

• Pathogens: Cause diseases in plants and animals.

Lichens

Lichens are symbiotic associations between fungi and photosynthetic partners (algae or cyanobacteria).

• Make-up: Fungal hyphae provide structure; photosynthetic partner provides food.

• Reproduction: Can reproduce sexually (fungal partner) or asexually (fragmentation or soredia).

Example: Fungal Life Cycle Equation

Sexual reproduction in fungi involves the following stages:

• Plasmogamy: (heterokaryotic)

• Karyogamy: (diploid)

• Meiosis: (haploid spores)

Additional info: Fungi are more closely related to animals than to plants, based on molecular evidence.