Prokaryotes

Definition of Prokaryotes:
- Prokaryotes are unicellular organisms that do not contain a nucleus or membrane-bound organelles.
Characteristics of Prokaryotes:
- No Organelles: Prokaryotic cells lack membrane-bound organelles and instead have structures directly immersed in the cytoplasm.
- Circular DNA: The genetic material is in the form of a circular chromosome, which is not enclosed within a nuclear membrane.
- Ribosomes: Prokaryotic ribosomes are 70S, smaller compared to those found in eukaryotes.

Definition of Eukaryotes:
- Eukaryotes are organisms whose cells contain a nucleus and organelles.
Characteristics of Eukaryotes:
- Organelles: Eukaryotic cells contain various organelles, each with specific functions.
- Linear DNA: Eukaryotic DNA is linear and organized within a membrane-bound nucleus.
- Ribosomes: Eukaryotic ribosomes are 80S, larger than those in prokaryotes.
- Reproduction: Eukaryotes primarily reproduce sexually but can reproduce asexually through processes such as mitosis.
- Microtubules and Flagella: Eukaryotic cells may have complex microtubule structures and some possess flagella for movement.

Cellular Shapes & Sizes:
- Prokaryotic cell sizes range from 0.5 to 5 µm. Eukaryotic cells range from 16 to 100 µm.
- Shapes:
- Cocci: Spherical-shaped bacteria.
- Bacilli: Rod-shaped bacteria.
- Spirochaetes: Spiral-shaped bacteria.
- Most motile bacteria utilize flagella for movement and demonstrate taxis (the ability to sense and respond to environmental stimuli).
- Example: Escherichia coli (E. coli) features a flagellar structure that operates similarly to a biological wheel or turbine, using a proton gradient to rotate and generate movement.

Eukaryotic Flagellum:
- Comprised of microtubules arranged in a 9+2 pattern.
- Flagella are powered by ATP and are significantly different from prokaryotic flagella, which are built from solid proteins.

Size and Structure:
- Prokaryotic genomes are much smaller than eukaryotic genomes and consist of circular chromosomes.
- Some bacteria also contain plasmids, which are small, circular rings of DNA that can carry beneficial genes and move between DNA.

Methods of Genetic Diversity:
- Rapid Reproduction: Prokaryotes reproduce quickly, which contributes to genetic diversity.
- Mutation: Provides variation in the genetic material, with higher mutation rates observed in prokaryotes.
- Genetic Recombination: Occurs through non-sexual methods:
- Transformation: Some prokaryotes can absorb DNA from dead cells in their environment, which may stem from different species.
- Transduction: Bacteriophages (viruses that infect bacteria) can accidentally incorporate bacterial DNA and transfer it to new hosts.
- Conjugation: Involves the transfer of DNA between a donor and recipient cell via a pilus. The donor cell must have an F factor for this process.

Autotrophs:
- Prokaryotes that can synthesize their own food from inorganic substances.
- Photoautotrophs: Use sunlight for energy through photosynthesis, predominantly found among prokaryotes.
- Chemoautotrophs: Obtain energy from chemical processes, often involving inorganic compounds.
- Heterotrophs: Must consume organic carbon from other organisms to survive, typical of many bacteria including those in environments rich in nutrients.

Obligate Aerobes: Require oxygen for survival.
Obligate Anaerobes: Poisoned by oxygen; resort to fermentation or anaerobic respiration.

Unique Properties:
- Archaea are often found in extreme environments and are believed to share evolutionary history with eukaryotes.
Methanogens:
- A subset of Archaea that are obligate anaerobes, commonly found in swamps and marsh environments, where they produce methane and help break down cellulose.
- Not pathogenic to animals.

Microscopic Size:
- Bacteria are typically small and observed using electron microscopes.
Cell Wall Composition:
- All bacteria possess a cell wall composed mainly of peptidoglycan.
- Some bacteria have an additional outer membrane.
- Fimbriae: Hair-like appendages used for attachment to surfaces or other cells.
Gram Staining: A technique used to classify bacteria based on cell wall structure.
- Gram-positive bacteria: Retain the crystal violet stain, indicating a thick peptidoglycan layer.
- Gram-negative bacteria: Do not retain the stain, possessing a thinner peptidoglycan layer surrounded by an outer membrane, often stained with a different color.

Definition and Purpose:
- Endospores are highly resilient structures formed by some bacteria in response to environmental changes, allowing for preservation and longevity.
- They can survive extreme conditions and are dormant until conditions become favorable for growth.

Characteristics:
- Cyanobacteria (often referred to as blue-green algae) are notable for their ability to perform photosynthesis using inorganic compounds to produce organic materials.
- They include unicellular and multicellular forms, such as Anabaena, which can fix nitrogen.

Chemical Cycling:
- Prokaryotes play a vital role in decomposing organic matter and recycling nutrients in ecosystems.
- Exclusive ability to fix nitrogen is crucial for maintaining soil fertility.
Ecological Interactions:
- Prokaryotes engage in various symbiotic relationships:
- Mutualism: Both organisms benefit from the interaction (example: soybean and nitrogen-fixing bacteria).
- Commensalism: One organism benefits while the other remains unaffected (example: lichens).
- Parasitism: One organism (the parasite) benefits at the host's expense, potentially leading to disease.

Pathogenic Mechanisms:
- Pathogens can cause disease through toxins.
- Exotoxins: Released actively by living bacteria; can cause a variety of diseases.
- Endotoxins: Released when bacteria die, triggering immune responses that can lead to systemic effects in hosts.