Exam 2 Review: Bacteria, Horizontal Gene Transfer, Gut Microbiome, Plant Evolution, and Endosymbiosis

Bacteria: Gram-Positive vs. Gram-Negative

Cell Wall Structure and Gram Stain Results

Bacteria are classified based on their cell wall structure, which affects their staining properties and response to antibiotics.

• Gram-Positive Bacteria:

  • Thick peptidoglycan cell wall.

  • Retain crystal violet stain; appear purple under microscope.

  • More susceptible to penicillin due to disruption of cell wall synthesis.

• Gram-Negative Bacteria:

  • Thin peptidoglycan layer and an outer membrane containing lipopolysaccharides (LPS).

  • Lose crystal violet stain; take up pink counterstain.

  • More resistant to penicillin; outer membrane acts as a barrier, often requiring different antibiotics.

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

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Outer Membrane	Absent	Present (with LPS)
Gram Stain Color	Purple	Pink
Penicillin Susceptibility	High	Low

Horizontal Gene Transfer (HGT) in Prokaryotes

Definition and Evidence

Horizontal gene transfer is the movement of genetic material between organisms without reproduction, playing a major role in prokaryotic evolution.

• About 17% of E. coli genes originated from other bacteria via HGT.

• Approximately 80% of prokaryotic genes have experienced HGT.

• HGT contributes to traits like antibiotic resistance and adaptation.

Mechanisms of HGT

• Transformation: Uptake of free DNA from the environment.

• Transduction: Transfer of DNA via bacteriophages (viruses).

• Conjugation: Direct transfer of DNA between cells through a pilus.

The Gut Microbiome: Benefits and Dysbiosis

Functions and Health Implications

The gut microbiome consists of diverse microorganisms that inhabit the digestive tract, providing essential benefits to host health.

• Protection against pathogens by competing for resources and space.

• Supports immune system development and proper immune responses.

• Produces vitamins and aids in digestion.

Dysbiosis and Associated Diseases

• Dysbiosis: Microbial imbalance linked to diseases such as obesity, inflammatory bowel disease (IBD), diabetes, and allergies.

• Evidence links dysbiosis to mental health conditions, including mood disorders.

• The gut-brain axis describes communication between gut microbiota and the brain, influencing behavior and mood regulation.

Evolutionary Trends in Plants: From Algae to Angiosperms

Transition from Water Dependence to Terrestrial Adaptations

Plants evolved from aquatic algae to fully terrestrial angiosperms, developing adaptations to reduce reliance on water and improve survival on land.

• Algae:

  • Advantage: Live in water, no risk of desiccation.

  • Limitation: No adaptations for land (no cuticle or vascular tissue).

• Bryophytes:

  • Advantage: First land plants; developed cuticle to reduce water loss.

  • Limitation: Lack vascular tissue; require water for reproduction.

• Ferns:

  • Advantage: Possess vascular tissue for transport and increased height.

  • Limitation: Still require water for fertilization.

• Gymnosperms:

  • Advantage: Seeds and pollen reduce dependence on water.

  • Limitation: No fruit; less efficient reproduction than angiosperms.

• Angiosperms:

  • Advantage: Flowers and fruits enhance reproduction and dispersal.

  • Limitation: High energy cost and reliance on pollinators in many species.

Summary Table: Plant Evolutionary Groups

Group	Key Limitation	Evolutionary Advantage
Algae	No land adaptations	Live in water
Bryophytes	No vascular tissue	Cuticle for water loss prevention
Ferns	Require water for fertilization	Vascular tissue
Gymnosperms	No fruit	Seeds and pollen
Angiosperms	High energy cost, pollinator reliance	Flowers and fruits

Endosymbiosis: Evolution of Plastids

Primary vs. Secondary Endosymbiosis

Endosymbiosis describes the origin of plastids (chloroplasts) through the engulfment of other cells, leading to photosynthetic abilities in eukaryotes.

• Primary Endosymbiosis:

  • A heterotrophic eukaryotic cell engulfed a cyanobacterium.

  • The cyanobacterium was not digested and became a primary plastid (chloroplast).

  • Resulted in photosynthetic ability and new metabolic pathways.

• Secondary Endosymbiosis:

  • A eukaryotic host cell engulfed another photosynthetic eukaryote (e.g., red or green alga) already containing primary plastids.

  • Resulted in secondary plastids, often surrounded by more than two membranes.

Example: Many protists (e.g., Euglena, dinoflagellates) possess plastids derived from secondary endosymbiosis.