Study Notes: Bacteria, Archaea, and Protists (Chapters 27 & 28)

Chapter 27 – Bacteria and Archaea

Structural Adaptations of Prokaryotes

Prokaryotes, which include Bacteria and Archaea, exhibit specialized structures that enable survival in diverse environments.

• Cell Walls: Most bacteria have cell walls containing peptidoglycan, a polymer of sugars and amino acids that provides structural support and shape.

• Capsules: Many prokaryotes secrete a sticky outer layer called a capsule that aids in adherence to surfaces and protection from the immune system.

• Fimbriae: Hair-like appendages that allow prokaryotes to attach to surfaces or other cells.

• Flagella: Long, whip-like structures used for movement (motility) in liquid environments.

Differences Between Bacteria and Archaea

Bacteria and Archaea are both prokaryotes but differ in several key structural and biochemical features:

• Cell Wall Composition: Bacterial cell walls contain peptidoglycan; archaeal cell walls lack peptidoglycan and may contain polysaccharides or proteins.

• Membrane Lipids: Bacteria have ester-linked membrane lipids, while Archaea have ether-linked lipids, often with branched hydrocarbon chains, enhancing stability in extreme environments.

Gram-Positive vs. Gram-Negative Bacteria

The Gram stain distinguishes bacteria based on cell wall structure:

• Gram-Positive: Thick peptidoglycan layer; stains purple. More susceptible to certain antibiotics.

• Gram-Negative: Thin peptidoglycan layer plus an outer membrane containing lipopolysaccharides; stains pink/red. Often more resistant to antibiotics due to the outer membrane acting as a barrier.

Genetic Diversity Mechanisms in Prokaryotes

Prokaryotes generate genetic diversity through several mechanisms:

• Mutation: Spontaneous changes in DNA sequence; rapid reproduction increases the impact of mutations.

• Transformation: Uptake of foreign DNA from the environment.

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

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

Metabolic Diversity in Prokaryotes

Prokaryotes display remarkable metabolic diversity, classified by energy and carbon sources:

• Photoautotrophs: Use light energy to synthesize organic compounds from CO2.

• Chemoautotrophs: Obtain energy from inorganic chemicals and carbon from CO2.

• Heterotrophs: Obtain both energy and carbon from organic compounds.

• Oxygen Requirements:

  • Obligate aerobes: Require oxygen for cellular respiration.

  • Obligate anaerobes: Poisoned by oxygen; use fermentation or anaerobic respiration.

  • Facultative anaerobes: Can survive with or without oxygen.

Major Clades of Bacteria and Archaea

Prokaryotes are classified into major clades based on genetic and physiological traits:

• Bacteria: Includes groups such as Proteobacteria, Chlamydias, Spirochetes, Cyanobacteria, and Gram-positive bacteria.

• Archaea: Includes extremophiles (organisms that thrive in extreme environments) such as:

  • Halophiles: Live in highly saline environments.

  • Thermophiles: Thrive in very hot environments.

  • Methanogens: Produce methane as a metabolic byproduct.

• Nitrogen-fixing species: Convert atmospheric nitrogen (N2) into ammonia (NH3), making nitrogen available to plants.

Ecological Roles of Prokaryotes

Prokaryotes are essential to ecosystem function:

• Decomposers: Break down dead organic matter, recycling nutrients.

• Symbionts: Live in close association with other organisms, often providing benefits (e.g., gut bacteria in humans).

• Pathogens: Cause diseases in plants, animals, and humans.

• Chemical Recyclers: Participate in biogeochemical cycles (e.g., carbon, nitrogen, sulfur cycles).

Chapter 28 – Protists

Overview of Protists

Protists are a diverse group of mostly unicellular eukaryotes. They exhibit a wide range of structural and functional diversity and are found in nearly all environments where water is present.

• Most eukaryotic lineages are protists.

• Protists can be autotrophic, heterotrophic, or mixotrophic.

Endosymbiosis and the Origin of Mitochondria and Plastids

The endosymbiotic theory explains the origin of mitochondria and plastids (e.g., chloroplasts) in eukaryotic cells:

• Primary Endosymbiosis: A eukaryotic cell engulfed a prokaryotic cell (e.g., an alpha-proteobacterium became the mitochondrion).

• Plastids: Originated when a eukaryote engulfed a photosynthetic cyanobacterium.

• Secondary Endosymbiosis: A eukaryote engulfed another eukaryotic cell that already contained plastids, leading to complex plastid structures (e.g., in some algae).

Major Supergroups of Eukaryotes

Protists are classified into five major supergroups based on molecular and morphological evidence:

• Excavata: Includes diplomonads and euglenozoans, often with modified mitochondria.

• SAR: Includes Stramenopiles (e.g., diatoms, brown algae), Alveolates (e.g., apicomplexans), and Rhizaria.

• Archaeplastida: Includes red algae, green algae, and land plants (the latter are not protists but are closely related).

• Unikonta: Includes amoebozoans and opisthokonts (animals and fungi).

• Rhizaria: Sometimes grouped within SAR; includes many amoeboid organisms with threadlike pseudopodia.

Key Protist Clades

• Excavata: Diplomonads (e.g., Giardia), euglenozoans (e.g., Euglena), often with reduced or modified mitochondria.

• SAR: Diatoms (photosynthetic algae with silica cell walls), brown algae (multicellular seaweeds), apicomplexans (parasitic, e.g., Plasmodium causes malaria).

• Archaeplastida: Red algae (phycoerythrin pigment), green algae (chlorophyll a and b), closest relatives of land plants.

• Unikonta: Amoebozoans (amoebas with lobe-shaped pseudopodia), opisthokonts (animals, fungi, and related protists).

Protist Nutrition

Protists display diverse nutritional strategies:

• Photoautotrophs: Use light energy to produce organic molecules from CO2.

• Heterotrophs: Ingest or absorb organic molecules.

• Mixotrophs: Combine photosynthesis and heterotrophic nutrition, depending on environmental conditions.

Protist Life Cycles

Protists exhibit a variety of life cycles, including:

• Alternation of Generations: Alternation between multicellular haploid (gametophyte) and diploid (sporophyte) forms, common in algae.

• Sexual and Asexual Reproduction: Many protists can reproduce both sexually (increasing genetic diversity) and asexually (rapid population growth).

Table: Comparison of Bacteria and Archaea

Feature	Bacteria	Archaea
Cell Wall	Contains peptidoglycan	No peptidoglycan; may have polysaccharides or proteins
Membrane Lipids	Ester-linked, unbranched fatty acids	Ether-linked, branched hydrocarbons
Antibiotic Sensitivity	Often sensitive	Usually resistant
Habitat	Wide range, including moderate environments	Many are extremophiles (high salt, temperature, etc.)

Table: Major Nutritional Modes in Prokaryotes and Protists

Mode	Energy Source	Carbon Source	Example Organisms
Photoautotroph	Light	CO2	Cyanobacteria, algae
Chemoautotroph	Inorganic chemicals	CO2	Nitrosomonas (soil bacteria)
Photoheterotroph	Light	Organic compounds	Some purple and green bacteria
Chemoheterotroph	Organic compounds	Organic compounds	Most prokaryotes, protists, animals, fungi

Additional info: Tables and some explanations have been expanded for clarity and completeness based on standard biology textbooks.