Prokaryotes: Structure, Function, and Diversity

Prokaryotes

Introduction to Microbiology

Microbiology is the study of microorganisms, which are found in virtually every environment on Earth. While some microorganisms cause disease, the vast majority are either harmless or beneficial to humans and ecosystems.

• Ubiquity: Microorganisms are present everywhere, including extreme environments.

• Beneficial Roles: Many microbes aid in food production, digestion, fermentation, antibiotic synthesis, and genetic engineering.

• Pathogenicity: Less than 1% of microorganisms cause disease.

Major Groups of Prokaryotes

• Bacteria: Typically unicellular organisms lacking a nucleus and membrane-bound organelles. Some exhibit rudimentary multicellularity.

• Archaea: Also unicellular and lacking a nucleus and organelles. Often inhabit extreme environments (e.g., hot springs, salt lakes).

Prokaryotic Cell Structure

Internal and External Structures

• Cell Membrane: Composed of a phospholipid bilayer, described by the fluid mosaic model. Proteins and phospholipids move within the membrane, allowing dynamic function. Sites for photosynthesis and respiration in prokaryotes.

• Cell Wall: Provides structural support and determines cell shape. In bacteria, primarily composed of peptidoglycan (alternating N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) sugars linked by peptide bridges). Most bacteria have cell walls, with some exceptions.

• Surface Layers: Many prokaryotes possess capsules (dense, well-defined polysaccharide/protein layers) or slime layers (loose, unorganized). These structures aid in protection and attachment.

• Filamentous Appendages: Includes fimbriae (for attachment) and pili (for DNA transfer during conjugation).

DNA Organization

• Nucleoid Region: Prokaryotic DNA is located in a non-membrane-bound region called the nucleoid.

• Chromosomes: Most prokaryotes have a single, circular chromosome.

• Plasmids: Small, circular DNA molecules that can carry genes for antibiotic resistance and other functions.

External Structures

• Fimbriae: Short, hair-like structures for attachment to surfaces.

• Pili: Longer appendages that can form bridges between cells for the transfer of plasmid DNA (conjugation).

Endospores

Some bacteria can form highly resistant structures called endospores under adverse conditions.

• Not reproductive cells; serve as resting structures.

• Contain very little water and are highly resistant to heat, chemicals, drying, and radiation.

• Endospore-forming bacteria can cause diseases such as botulism and tetanus.

Motility

• Taxis: Directed movement toward or away from a stimulus (e.g., chemotaxis—movement in response to chemicals).

• Flagella: Common motility structures in prokaryotes. Prokaryotic flagella are smaller and move by rotation, unlike the whip-like motion of eukaryotic flagella.

Microbial Growth and Nutrition

Binary Fission

Prokaryotes reproduce asexually by binary fission, a process distinct from mitosis.

• Each cell divides to form two genetically identical daughter cells.

• Population growth is measured at the community level, not by individual cell division.

Microbial Nutrition

• Bacteria exhibit diverse energy and oxygen requirements.

• Energy Sources:

  • Phototrophs: Obtain energy from light.

  • Chemotrophs: Obtain energy from chemical compounds (organic or inorganic).

• Some bacteria can utilize energy from sources other than light, such as hot springs (thermophiles).

Role of Oxygen

• Aerobes: Require oxygen for survival.

• Anaerobes: Cannot survive in the presence of oxygen.

• Facultative Anaerobes: Can survive with or without oxygen.

Summary Table: Key Features of Prokaryotes

Feature	Bacteria	Archaea
Cell Type	Unicellular, prokaryotic	Unicellular, prokaryotic
Nucleus	Absent	Absent
Cell Wall Composition	Peptidoglycan	Varied (no peptidoglycan)
Habitat	Ubiquitous	Often extreme environments
DNA Structure	Circular chromosome, plasmids	Circular chromosome, plasmids
Motility	Flagella (rotary motion)	Flagella (distinct structure)

Example: Escherichia coli

Escherichia coli is a common bacterium found in the intestines of humans and animals. It is used extensively in research and biotechnology due to its rapid growth and well-understood genetics.

Additional info: Prokaryotes are essential for nutrient cycling, environmental stability, and biotechnology applications. Their diversity in metabolism and habitat allows them to colonize nearly every environment on Earth.