Prokaryotic cells (from the Greek pro meaning "before" and karyon meaning "nut" or "kernel")
are the oldest and most widespread forms of life on Earth. They are defined by their simplicity:
they lack a membrane-bound nucleus and complex organelles like mitochondria or chloroplasts.
Despite their simple appearance, they are biochemically diverse and highly efficient.
1. The Anatomy of a Prokaryote
A prokaryotic cell is typically much smaller than a eukaryotic cell (ranging from 0.1 to 5.0 \mum).
Its structure is divided into three main regions:
, External Appendages
* Flagella: Long, tail-like structures used for locomotion. They rotate like a propeller to "swim."
* Pili (or Fimbriae): Hair-like projections. Fimbriae help the cell stick to surfaces, while
specialized "sex pili" are used to exchange DNA with other cells.
* Capsule: A sticky, sugar-rich outer layer that protects the cell from being eaten by immune
cells and helps it stick to surfaces.
The Cell Envelope
* Cell Wall: A rigid layer (usually made of peptidoglycan) that maintains the cell's shape and
prevents it from bursting.
* Plasma Membrane: A phospholipid bilayer that regulates what enters and leaves the cell.
The Internal Region (Cytoplasm)
* Nucleoid: Since there is no nucleus, the DNA—usually a single, circular chromosome—is
bunched together in this central region.
* Plasmids: Small, extra loops of DNA that often carry "bonus" genes, such as antibiotic
resistance.
* Ribosomes (70S): These are smaller than eukaryotic ribosomes but perform the same vital
task: protein synthesis.
2. Reproduction and Genetic Exchange
Prokaryotes do not undergo mitosis. Instead, they reproduce via binary fission, a process where
the cell duplicates its DNA and splits into two identical daughter cells. This can happen
incredibly fast—E. coli can double every 20 minutes!
Because they reproduce asexually, they use three "tricks" to maintain genetic diversity:
* Transformation: Taking up DNA "scraps" from their environment.
* Transduction: DNA being moved from one bacterium to another by a virus.
* Conjugation: Directly transferring DNA to another cell via a pilus bridge.
3. Prokaryotes vs. Eukaryotes
To understand them "deeply," it helps to see how they differ from the cells in your body:
Feature Prokaryotes (Bacteria/Archaea) Eukaryotes (Animals/Plants/Fungi)
Nucleus Absent (Nucleoid region) Present (Membrane-bound)
DNA Shape Circular Linear
Organelles None (no mitochondria/ER) Many (mitochondria, Golgi, etc.)
Cell Size Small (0.1–5 \mum) Large (10–100 \mum)
Ribosomes Smaller (70S) Larger (80S)
4. Why Their Small Size Matters
Prokaryotes rely on simple diffusion to move nutrients and waste. Because they are so small,
the distance from the surface to the center is tiny, allowing molecules to travel quickly without
the need for complex internal transport systems (like the "highways" in our cells).
are the oldest and most widespread forms of life on Earth. They are defined by their simplicity:
they lack a membrane-bound nucleus and complex organelles like mitochondria or chloroplasts.
Despite their simple appearance, they are biochemically diverse and highly efficient.
1. The Anatomy of a Prokaryote
A prokaryotic cell is typically much smaller than a eukaryotic cell (ranging from 0.1 to 5.0 \mum).
Its structure is divided into three main regions:
, External Appendages
* Flagella: Long, tail-like structures used for locomotion. They rotate like a propeller to "swim."
* Pili (or Fimbriae): Hair-like projections. Fimbriae help the cell stick to surfaces, while
specialized "sex pili" are used to exchange DNA with other cells.
* Capsule: A sticky, sugar-rich outer layer that protects the cell from being eaten by immune
cells and helps it stick to surfaces.
The Cell Envelope
* Cell Wall: A rigid layer (usually made of peptidoglycan) that maintains the cell's shape and
prevents it from bursting.
* Plasma Membrane: A phospholipid bilayer that regulates what enters and leaves the cell.
The Internal Region (Cytoplasm)
* Nucleoid: Since there is no nucleus, the DNA—usually a single, circular chromosome—is
bunched together in this central region.
* Plasmids: Small, extra loops of DNA that often carry "bonus" genes, such as antibiotic
resistance.
* Ribosomes (70S): These are smaller than eukaryotic ribosomes but perform the same vital
task: protein synthesis.
2. Reproduction and Genetic Exchange
Prokaryotes do not undergo mitosis. Instead, they reproduce via binary fission, a process where
the cell duplicates its DNA and splits into two identical daughter cells. This can happen
incredibly fast—E. coli can double every 20 minutes!
Because they reproduce asexually, they use three "tricks" to maintain genetic diversity:
* Transformation: Taking up DNA "scraps" from their environment.
* Transduction: DNA being moved from one bacterium to another by a virus.
* Conjugation: Directly transferring DNA to another cell via a pilus bridge.
3. Prokaryotes vs. Eukaryotes
To understand them "deeply," it helps to see how they differ from the cells in your body:
Feature Prokaryotes (Bacteria/Archaea) Eukaryotes (Animals/Plants/Fungi)
Nucleus Absent (Nucleoid region) Present (Membrane-bound)
DNA Shape Circular Linear
Organelles None (no mitochondria/ER) Many (mitochondria, Golgi, etc.)
Cell Size Small (0.1–5 \mum) Large (10–100 \mum)
Ribosomes Smaller (70S) Larger (80S)
4. Why Their Small Size Matters
Prokaryotes rely on simple diffusion to move nutrients and waste. Because they are so small,
the distance from the surface to the center is tiny, allowing molecules to travel quickly without
the need for complex internal transport systems (like the "highways" in our cells).
