Defining Characteristics and Morphology of Prokaryotic Cells
Prokaryotic cells are defined by their structural simplicity compared to eukaryotic cells. They lack membrane-bound organelles entirely, which means all cellular processes happen within a single shared compartment. Despite this simplicity, prokaryotes are the most abundant and metabolically diverse organisms on Earth.
Key Features of Prokaryotic Cells
No membrane-enclosed organelles. This is the defining trait. DNA sits directly in the cytoplasm in a region called the nucleoid, rather than being enclosed in a nucleus.
Smaller ribosomes. Prokaryotic ribosomes are 70S, compared to the 80S ribosomes found in eukaryotic cells. Both carry out protein synthesis, but this size difference is clinically significant because many antibiotics specifically target 70S ribosomes without harming human cells.
A single, circular chromosome. Most prokaryotes carry one circular DNA molecule with a relatively small genome. E. coli, for example, has about 4.6 million base pairs, compared to roughly 3 billion in the human genome.
Plasmids. Many prokaryotes also carry plasmids, which are small, circular DNA molecules that replicate independently of the main chromosome. Plasmids often carry genes for antibiotic resistance or other survival advantages, and they can be transferred between cells.
Cell wall. External to the cell membrane, most prokaryotes have a rigid cell wall that provides structural support and protection.
- In bacteria, the wall contains peptidoglycan (also called murein), a mesh-like polymer unique to bacteria.
- In archaea, cell walls lack peptidoglycan and instead may contain pseudomurein, an S-layer (a crystalline protein coat), or other distinct structures.
Small size. Prokaryotic cells typically range from 0.1 to 5 μm in diameter. For reference, most eukaryotic cells are 10–100 μm. Mycoplasma species are among the smallest known prokaryotes, while E. coli is a more typical ~1–2 μm rod.
Shapes and Arrangements of Prokaryotes
Prokaryotic cell shape is one of the first things used to identify an organism under the microscope. The four major shapes are:
- Cocci — spherical (Streptococcus)
- Bacilli — rod-shaped (Escherichia coli)
- Spirilla — rigid spiral or helical (Spirillum)
- Vibrios — comma-shaped or curved rod (Vibrio cholerae)
After cell division, some species remain attached to each other in characteristic arrangements, which are especially useful for identifying cocci:
| Arrangement | Description | Example |
|---|---|---|
| Diplococci | Pairs | Neisseria |
| Streptococci | Chains | Streptococcus pyogenes |
| Staphylococci | Grape-like clusters | Staphylococcus aureus |
| Tetrads | Groups of four in a square | Micrococcus |
| Sarcinae | Cube-like packets of eight | Sarcina |
| Streptobacilli | Chains of rods | Streptobacillus moniliformis |
The arrangement depends on the plane of cell division and whether daughter cells separate or stay connected.
Structure and Function of Prokaryotic Cell Components
The Cell Envelope
The cell envelope is everything from the cell membrane outward. It's the prokaryote's interface with the environment.
Cell membrane. A phospholipid bilayer that acts as a selective barrier, controlling what enters and exits the cell. It also houses proteins involved in energy production (since prokaryotes lack mitochondria, the membrane takes on this role).
Cell wall. Maintains cell shape and prevents the cell from bursting due to osmotic pressure. The structure of the cell wall is the basis for the Gram stain, one of the most important classification tools in microbiology:
- Gram-positive bacteria (e.g., Bacillus, Staphylococcus) have a thick peptidoglycan layer (20–80 nm). They retain the crystal violet stain and appear purple.
- Gram-negative bacteria (e.g., Salmonella, E. coli) have a thin peptidoglycan layer (5–10 nm) sandwiched between the inner membrane and an outer membrane. They lose the crystal violet during decolorization and pick up the safranin counterstain, appearing pink.
The outer membrane of Gram-negative bacteria contains lipopolysaccharide (LPS), which can trigger strong immune responses and is also called endotoxin.
Capsule. Some prokaryotes produce a polysaccharide capsule (or glycocalyx) outside the cell wall. The capsule helps the cell adhere to surfaces and evade the host immune system. Streptococcus pneumoniae with a capsule is virulent; without one, it's easily destroyed by immune cells.
Internal Structures
- Cytoplasm — the gel-like interior where metabolic reactions occur.
- Ribosomes (70S) — free in the cytoplasm, these carry out protein synthesis. They consist of a 30S small subunit and a 50S large subunit.
- Inclusions — storage granules that stockpile nutrients like glycogen, polyphosphate, or sulfur for later use.
- Nucleoid — the region (not a membrane-bound compartment) where the circular chromosome is concentrated. The DNA is typically supercoiled to fit within the small cell.
- Plasmids — independently replicating DNA circles carrying accessory genes. During conjugation, plasmids (particularly the F plasmid) can be transferred from one cell to another through a structure called the F pilus.
External Appendages
Pili and fimbriae are short, hair-like surface projections. Fimbriae help the cell attach to surfaces (important for colonization and biofilm formation). The F pilus (sex pilus) is a specialized structure used to transfer DNA between bacterial cells during conjugation.
Flagella are long, whip-like structures made of the protein flagellin that provide motility. Flagellar arrangement varies by species (e.g., a single polar flagellum, tufts at one or both ends, or flagella covering the entire surface). Bacteria use flagella to swim toward nutrients or away from toxins, a behavior called chemotaxis.
Bacterial vs. Archaeal Cell Characteristics
Bacteria and archaea are both prokaryotes, so they share the fundamental features: no membrane-bound organelles, 70S ribosomes, and a single circular chromosome in the cytoplasm. However, they differ in several important ways.
| Feature | Bacteria | Archaea |
|---|---|---|
| Cell wall | Contains peptidoglycan | No peptidoglycan; may have pseudomurein, S-layer, or other structures |
| Membrane lipids | Ester-linked fatty acids | Ether-linked isoprenoid chains (more stable under extreme conditions) |
| RNA polymerase | One type (relatively simple) | Multiple types, more similar to eukaryotic RNA polymerase |
| Unique metabolism | Standard aerobic/anaerobic pathways | Includes methanogenesis (methanogens produce methane) |
| Extreme environments | Some extremophiles, but less common | Many are extremophiles (hyperthermophiles, halophiles, acidophiles) |
The fact that archaeal RNA polymerase resembles the eukaryotic version more than the bacterial one is part of the evidence that archaea are more closely related to eukaryotes on the tree of life.
Prokaryotic Cell Processes
Binary fission is how prokaryotes reproduce. The chromosome replicates, the cell elongates, and a septum forms at the midpoint, dividing the cell into two genetically identical daughter cells. This is simpler and faster than eukaryotic mitosis, which is why bacterial populations can double in as little as 20 minutes under ideal conditions.
Metabolism in prokaryotes is remarkably diverse. Different species can use aerobic respiration, anaerobic respiration, fermentation, or photosynthesis for energy production. Some archaea carry out methanogenesis, a metabolic pathway found nowhere else in biology. This metabolic flexibility is a major reason prokaryotes thrive in virtually every environment on Earth.
Motility takes several forms beyond flagella. Some bacteria use gliding motility (movement along a surface without flagella) or twitching motility (using type IV pili to pull the cell forward in short jerks). These different strategies allow prokaryotes to navigate toward favorable conditions through processes like chemotaxis.