Prokaryotes: Structure, Function, and Diversity

Prokaryotes: Introduction and Early Evolution

Definition and Domains

Prokaryotes are unicellular organisms lacking a membrane-bound nucleus and organelles. They include two major domains: Bacteria and Archaea. Prokaryotes originated approximately 3.5 billion years ago and are known for surviving in extreme environments.

• Single-celled organisms

• Can survive extreme conditions (e.g., high temperature, salinity, acidity)

Sequence of Events for First Cells to Develop

The origin of life involved several key steps leading to the formation of the first cells:

1. Synthesis of amino acids and nitrogenous bases

2. Joining of these monomers to make macromolecules (proteins or nucleic acids)

3. Packaging these molecules into membranes to make the first protocell

4. Nucleic acids and cells begin to self-replicate

Abiotic Synthesis of Amino Acids

Experiments by Stanley Miller and Harold Urey in the 1950s simulated early Earth conditions to demonstrate the abiotic synthesis of organic molecules.

• Used a spark discharge in a closed flask simulating lightning in the early atmosphere

• Produced amino acids and other organic compounds

• Showed that organic molecules could form under prebiotic conditions

Joining of Monomers into Macromolecules

Macromolecules such as proteins and nucleic acids could form spontaneously under certain conditions, such as on hot sand or clay surfaces.

• Heated water can join amino acids without enzymes

• Possible formation near hot springs or hydrothermal vents

Packaging Macromolecules into Protocells

Protocells are simple cell-like structures that can form spontaneously from lipids.

• Lipid vesicles (bubbles) can encapsulate organic molecules

• Protocells could have provided a microenvironment for chemical reactions

Self-Replication of RNA

RNA is hypothesized to be the first genetic material due to its ability to self-replicate and catalyze reactions.

• RNA can act as both genetic material and as a catalyst (ribozyme)

• RNA templates could have enabled the replication of protocells

Properties and Structure of Prokaryotes

General Features

• Very small size: typically 0.5 – 5 μm

• Three main shapes: spherical (coccus), rod-shaped (bacillus), and spiral

Cell Wall Structure

Prokaryotic cell walls are primarily composed of peptidoglycan, a polymer of sugars and amino acids.

• Provides structural support and protection

Types of Cell Walls in Bacteria

• Gram-positive: Thick peptidoglycan layer, stains purple in Gram stain

• Gram-negative: Thin peptidoglycan layer, outer membrane present, stains pink

Capsules and Surface Structures

• Capsules: Made of polysaccharides; help bacteria adhere to surfaces and evade immune responses

• Endospores: Highly durable, dormant structures that protect DNA in harsh conditions

• Fimbriae: Hair-like appendages for attachment to surfaces

• Pili: Longer appendages used for DNA exchange (conjugation)

Motility

• Movement via flagella, which rotate to propel the cell

• Flagella are composed of a filament, hook, and motor protein

• Prokaryotes can move toward or away from stimuli (chemotaxis)

Internal Organization

• No membrane-bound nucleus or organelles

• DNA is located in a single, circular chromosome

Prokaryotic Diversity and Nutrition

Oxygen Requirements

• Anaerobic: Do not use oxygen

• Aerobic: Use oxygen

Modes of Nutrition

Prokaryotes exhibit diverse metabolic strategies for obtaining energy and carbon.

Mode	Energy Source	Carbon Source	Examples
Photoautotroph	Light	Carbon dioxide	Algae, photosynthetic protists, photosynthetic prokaryotes, plants
Chemoautotroph	Inorganic compounds (e.g., H2S, NH3)	Carbon dioxide	Unique to prokaryotes
Photoheterotroph	Light	Organic compounds	Unique to prokaryotes
Chemoheterotroph	Organic compounds	Organic compounds	Many prokaryotes, animals, fungi

Additional Features and Applications

• Some antibiotics target prokaryotic ribosomes, affecting bacteria but not human cells

• Prokaryotes play essential roles in nutrient cycling, decomposition, and biotechnology

Example: Escherichia coli is a Gram-negative bacterium commonly found in the human gut and used extensively in genetic engineering.

Additional info: Prokaryotes are foundational to the tree of life and are critical for understanding cellular evolution and metabolic diversity.