Prokaryotes

Types of Prokaryotic Cells
- Bacteria and Archaea are different types of prokaryotic cells.
Origin of Life
- Life on Earth likely originated as prokaryotic cells approximately 3.8 billion years ago.
Classification of Prokaryotes
- Prokaryotic cells are classified into two separate domains: Archaea and Bacteria.
- Eukaryotic cells make up the domain of Eukarya.

General Features
- Unicellular organisms.
- Lack a membrane-bound nucleus and organelles.
Unique Sizes, Shapes, and Arrangements
- The shapes and arrangements of prokaryotic cells aid in identifying certain pathogens.
- Monomorphic bacteria have one shape, while pleomorphic bacteria can take on multiple forms.

Sizes

Prokaryotic cell sizes vary significantly, ranging from 0.2 µm to 750 µm in diameter.
- Mycoplasma species are among the smallest.
- Cells acquire nutrients mainly through diffusion, which limits their size due to the surface area-to-volume ratio.

Shapes

Arrangements

Division

Binary Fission
- Prokaryotic cells reproduce asexually through a process called binary fission, which involves the following steps:
1. DNA is copied.
2. The cell grows.
3. The copied chromosome is pulled towards opposite ends of the cell.
4. A septum (partition) forms at the midpoint.
5. The septum eventually separates the two daughter cells.
- Frequency of Binary Fission is variable and influenced by environmental factors such as temperature, pH, and nutrient availability.

Prokaryotic cells depend on their plasma membrane and cell wall as protective barriers.
- All cells are encapsulated by an outer lining known as the plasma membrane.
- Structures outside the plasma membrane are classified as extracellular structures, while those within are termed intracellular structures.
- In most prokaryotic cells, a cell wall exists just outside the plasma membrane.

Plasma Membrane: Structure & Function

The plasma membrane consists of a thin, flexible phospholipid bilayer acting as a selective barrier.
Membrane components:
- Proteins may constitute nearly half of the plasma membrane mass and serve various functions including:
- Transporters: Facilitate the movement of substances.
- Anchors: Stabilize membrane structure.
- Receptors: Detect signals from the environment.
- Enzymes: Catalyze biochemical reactions.
The plasma membrane also serves as the site for metabolic reactions where prokaryotes generate ATP.
- Mesosomes are structures found within the plasma membrane.
- The membrane exhibits selective permeability that allows certain substances, such as gases, water, and small, noncharged molecules, to diffuse easily, while ions and larger polar substances necessitate protein transporters for entry or exit.

Cell Wall: Structure & Function

The cell wall provides a rigid structure that serves as an additional protective layer.
Bacteria utilize peptidoglycan as a fundamental component of their cell walls, making it a common target for drugs.
Gram Staining is a technique that classifies cells based on cell wall architecture, resulting in two main categories:
- Gram-negative bacteria:
- Characterized by a thin peptidoglycan layer (2–8 nm).
- Possess a periplasmic space and an outer membrane that contains lipopolysaccharide.
- Stains pink in gram staining.
- Gram-positive bacteria:
- Feature a thick peptidoglycan layer (20–80 nm).
- Lack an outer membrane.
- Stains purple.

Clinical Implications of Gram Status

Gram-negative species are generally more resistant to chemical agents compared to Gram-positive species.
The outer membrane serves as a selective barrier that protects against various damaging agents including lysozyme, certain medications, and some detergents and disinfectants.

Pros and Cons of Gram-positive Cell Walls

Pros:
- Retain moisture longer.
- Provide protection against mechanical stresses.
- Presence of teichoic acids aids in stabilizing the cell wall and regulating division.
Cons:
- More sensitive to compounds that interfere with peptidoglycan synthesis, such as penicillin.

Acid-Fast Staining

This staining technique identifies a waxy lipid called mycolic acid in the cell walls of certain bacteria.
Acid-fast cells appear red/pink after the staining process, e.g., Mycobacterium tuberculosis.
The waxy cell wall impedes diffusion, resulting in a slow growth rate for acid-fast bacteria, necessitating long and rigorous multidrug therapies for infected patients.

Prokaryotes lack membrane-bound organelles but still possess distinct intracellular structures.
Cytoplasm:
- Located within the plasma membrane and serves as the site for most biochemical reactions.
Nucleoid:
- The region where DNA is organized into a single, circular chromosome.
Ribosomes:
- Composed of RNA and protein, responsible for protein synthesis by linking amino acids.
Cytoskeleton:
- Consists of long protein filaments that provide structure and support to the cell.

Endospores

Some bacterial species form endospores to withstand harsh environmental conditions.
Endospores are metabolically inactive structures allowing bacteria to enter a dormant state that is highly resistant to environmental stresses such as starvation, heat, drying, freezing, and radiation.
When conditions improve, endospores can germinate into vegetative cells (actively growing cells).

Clinically Significant Spore-Forming Bacteria

Examples of important bacteria that can form endospores include:
- Bacillus anthracis (anthrax)
- Clostridium tetani (tetanus)
- Clostridium botulinum (botulism)
- Clostridium perfringens (gas gangrene)
- Clostridioides difficile (severe diarrhea).
Endospores can persist for extended durations on surfaces, including in healthcare settings, posing a significant challenge for infection control measures.

Sporulation Process

Sporulation is the process by which an endospore is formed. Key steps in sporulation include:
1. Copying of DNA.
2. Packaging of DNA, ribosomes, and specific enzymes into the spore coat.
3. Surrounding the spore core with several layers that are resistant to heat and chemicals.
4. Release of the spore once complete.