Table of Contents
Overview of Prokaryotic Cells
Prokaryotic cells (procytes) are the simplest and usually smallest cellular life forms. Bacteria and Archaea are made of prokaryotic cells. In this chapter, we focus on what makes prokaryotes structurally and functionally distinct from eukaryotic cells, not on molecular details that are treated elsewhere.
Key general features:
- No membrane-bound nucleus; DNA lies free in the cytoplasm (nucleoid).
- Generally lack membrane-bound organelles (no mitochondria, chloroplasts, Golgi, ER, etc.).
- Typically very small (about 0.5–5 µm in length).
- Often surrounded by a rigid cell wall and sometimes additional outer layers.
- Very diverse in shape, metabolism, and habitats.
Size, Shape, and Arrangement
Prokaryotic cells show some characteristic shapes:
- Cocci: spherical cells (e.g., Streptococcus).
- Bacilli: rod-shaped cells (e.g., Escherichia coli).
- Vibrios: comma-shaped rods.
- Spirilla: rigid spiral-shaped cells.
- Spirochetes: flexible, corkscrew-shaped cells.
Cells can occur:
- Singly,
- In pairs (diplococci),
- In chains (streptococci, streptobacilli),
- In clusters (staphylococci).
The small size leads to a high surface-area-to-volume ratio, which:
- Facilitates rapid nutrient uptake and waste removal.
- Allows very fast growth and cell division under favorable conditions.
Basic Structural Organization
Although prokaryotes are “simple” compared to eukaryotes, they still have a clear internal organization. Most cells contain:
- Plasma membrane
- Cytoplasm with ribosomes
- Genetic material in a nucleoid
- Cell wall (in most species)
Many species also possess accessory structures like pili, flagella, capsules, or storage granules.
Below we consider each of these components in the specific context of prokaryotes.
The Prokaryotic Plasma Membrane
The plasma membrane of prokaryotes:
- Encloses the cytoplasm.
- Consists of a phospholipid bilayer with embedded proteins.
- Functions as a selective barrier for substances entering and leaving the cell.
Specific aspects in prokaryotes:
- Metabolic roles: Since they lack mitochondria and chloroplasts, many energy-converting processes take place in or on the plasma membrane:
- Components of the respiratory chain can be embedded in the membrane.
- Photosynthetic prokaryotes (e.g., cyanobacteria) often have extensive infoldings of the membrane bearing photosynthetic pigments.
- Transport systems: Specialized transport proteins import nutrients (sugars, amino acids, ions) and export waste or toxins.
- Structural differences in Archaea: Archaeal membranes often have:
- Different types of lipids (ether linkages instead of ester linkages),
- Isoprenoid chains instead of fatty acids,
- Sometimes a monolayer instead of a bilayer, which can increase stability in extreme environments.
Despite these differences, the core function—selective permeability and boundary of the cell—is the same.
Cytoplasm and Ribosomes
The cytoplasm of prokaryotes is a gel-like solution containing:
- Water, ions, small molecules.
- Enzymes that carry out metabolic reactions.
- Ribosomes and DNA.
Prokaryotic ribosomes:
- Are smaller than eukaryotic ribosomes (called 70S ribosomes; composed of 50S and 30S subunits).
- Are the sites of protein synthesis.
- Are targets of several antibiotics that selectively inhibit bacterial protein synthesis without directly affecting eukaryotic ribosomes.
The cytoplasm lacks internal membrane-bound compartments, but can be spatially organized through:
- Protein-based scaffolds,
- Localized enzyme complexes,
- Regions enriched in specific molecules (e.g., inclusion bodies).
Nucleoid: The Prokaryotic Genetic Region
Prokaryotes do not have a nucleus surrounded by a membrane. Instead, their DNA is located in a distinct region called the nucleoid.
Typical properties:
- Single, usually circular chromosome:
- Contains most of the genetic information.
- DNA is often supercoiled and compacted by special proteins.
- No nuclear envelope:
- Transcription (RNA synthesis) and translation (protein synthesis) can occur simultaneously, since ribosomes can interact with mRNA as it is being made.
- No histones in most Bacteria:
- Some Archaea have histone-like proteins.
The exact size and composition of the chromosome vary widely among species. Some bacteria also contain additional genetic elements, particularly plasmids.
Plasmids and Horizontal Gene Transfer
Plasmids are small, usually circular DNA molecules that:
- Exist independently of the chromosomal DNA.
- Replicate on their own.
- Often carry genes that:
- Provide antibiotic resistance,
- Allow utilization of unusual nutrients,
- Confer virulence factors important for causing disease.
Plasmids can be transferred between cells in several ways:
- Conjugation:
- Direct transfer from one cell to another through a specialized structure (pilus).
- Transformation:
- Uptake of free DNA from the environment.
- Transduction:
- Transfer via bacteriophages (viruses that infect bacteria).
These processes contribute to rapid genetic change and adaptation in prokaryotic populations.
The Prokaryotic Cell Wall
Most prokaryotes possess a cell wall outside the plasma membrane. Functions:
- Provides mechanical strength.
- Maintains cell shape.
- Protects against osmotic lysis (bursting due to water influx).
Bacterial Cell Walls and Peptidoglycan
In Bacteria, the cell wall is typically composed of peptidoglycan:
- A polymer consisting of:
- Sugar chains (glycan) made from repeating units of two modified sugars.
- Short peptide chains (amino acids) cross-linking the sugar chains.
- This mesh-like structure forms a strong but flexible layer around the cell.
Based on cell wall structure, bacteria are often classified into:
- Gram-positive bacteria:
- Thick peptidoglycan layer.
- Often contain additional molecules (e.g., teichoic acids).
- Stain dark purple in the Gram stain method.
- Gram-negative bacteria:
- Thin peptidoglycan layer located between two membranes:
- Inner plasma membrane.
- Outer membrane containing lipopolysaccharides (LPS).
- Stain pink/red in the Gram stain.
- The outer membrane can act as an additional barrier to antibiotics and harmful substances.
The Gram distinction is important for:
- Medical diagnosis,
- Choice of antibiotics,
- Understanding cell envelope structure.
Archaeal Cell Walls
In Archaea:
- There is no peptidoglycan.
- Alternative materials (e.g., pseudopeptidoglycan, proteins, or polysaccharides) form the cell wall.
- Many Archaea possess an S-layer (surface layer) made of regularly arranged protein or glycoprotein subunits.
These adaptations contribute to the extraordinary resistance of many Archaea to extremes of temperature, pH, or salinity.
Outer Layers: Capsule and Slime Layer
Many prokaryotes produce additional layers outside the cell wall:
- Capsule:
- Thick, well-organized, tightly attached layer, often made of polysaccharides.
- Slime layer:
- Looser, more diffuse, easily washed-off material.
Functions of capsules and slime layers:
- Protection against desiccation (drying out).
- Protection from phagocytosis by immune cells in pathogenic bacteria.
- Attachment to surfaces and other cells (important in forming biofilms).
- Trapping of nutrients or toxins outside the cell.
Biofilms—communities of microorganisms embedded in a self-produced matrix—are common on natural and artificial surfaces (rocks, pipes, teeth).
Surface Appendages: Pili and Fimbriae
Many prokaryotic cells have thin, hair-like projections:
- Fimbriae:
- Short, numerous.
- Involved mainly in adhesion to surfaces or host tissues.
- Pili (singular: pilus):
- Often fewer and longer.
- Specialized sex pili are used in conjugation (DNA transfer between cells).
These structures are important for:
- Colonization of surfaces and hosts,
- Biofilm formation,
- Exchange of genetic material.
Prokaryotic Flagella and Motility
Many prokaryotes are motile and move using flagella (singular: flagellum). Prokaryotic flagella are:
- Long, whip-like filaments extending from the cell surface.
- Built from a protein called flagellin.
- Anchored in the cell envelope by a complex structure functioning as a rotary motor.
Key features:
- The flagellar motor is driven by an ion gradient (often protons) across the membrane.
- Rotation of the flagellum propels the cell through liquids.
- Arrangement varies:
- Single flagellum at one end (monotrichous),
- Cluster at one or both ends (lophotrichous, amphitrichous),
- Many flagella surrounding the cell (peritrichous).
Prokaryotes can show taxis, i.e., directed movement toward or away from stimuli:
- Chemotaxis: response to chemical gradients (nutrients, toxins).
- Phototaxis: response to light.
- Aerotaxis: response to oxygen concentration.
Bacteria often move in a “run-and-tumble” pattern:
- Run: flagella rotate in one direction, forming a bundle and moving the cell straight.
- Tumble: flagella rotation reverses, bundle falls apart, and the cell reorients randomly.
By adjusting the frequency of tumbles in response to stimuli, cells bias their movement toward favorable conditions.
Internal Inclusions and Storage Granules
Prokaryotic cells often contain inclusion bodies or storage granules:
- Carbon storage:
- Glycogen granules,
- Polyhydroxyalkanoate (PHA) granules.
- Phosphate storage:
- Volutin or metachromatic granules (polyphosphate deposits).
- Sulfur granules in some sulfur-oxidizing bacteria.
- Gas vesicles in some aquatic prokaryotes:
- Protein-bound structures that provide buoyancy and help cells position themselves optimally in the water column.
These inclusions allow cells to store resources and adapt to fluctuating environmental conditions.
Endospores: Extreme Survival Forms in Some Bacteria
Certain Gram-positive bacteria (e.g., Bacillus, Clostridium) can form endospores, which are highly resistant dormant structures. Key points:
- Formed inside the mother cell under unfavorable conditions (nutrient depletion, extreme temperatures).
- Contain:
- A copy of the cell’s DNA,
- Minimal cytoplasm,
- Thick protective layers, including a tough coat.
- Extremely resistant to:
- Heat,
- Radiation,
- Drying,
- Chemicals.
When conditions become favorable again, the endospore germinates into a metabolically active bacterial cell.
Endospores are important in:
- Ecology (long-term survival and dispersal),
- Medicine and food safety (persistence of pathogens and spoilage organisms).
Diversity of Prokaryotic Lifestyles
While this chapter focuses on structure, it is worth noting that prokaryotic structural simplicity supports an enormous diversity of lifestyles:
- Metabolic diversity:
- Using light (photosynthesis) or chemical compounds (chemolithotrophy, chemoorganotrophy) as energy sources.
- Using oxygen or alternative electron acceptors (nitrate, sulfate, even metals).
- Ecological roles:
- Decomposers,
- Nitrogen fixers,
- Symbionts of plants and animals,
- Pathogens,
- Extremophiles in hot springs, salt lakes, deep-sea vents, and more.
Many of these aspects are explored in later chapters; here it is important to recognize that the core prokaryotic cell structure enables rapid growth, high adaptability, and colonization of nearly all environments on Earth.