Microbiology · Cell Structure
Prokaryotic Cell Structure and Function
On this page 6 sections
In 30 seconds
Prokaryotic cells Cells without nucleus or organelles Full entry → — bacteria and archaea — lack a membrane-bound nucleus and organelles. Bacteria have characteristic shapes (Cocci Spherical cells Full entry →, Bacilli Rod-shaped cells Full entry →, spiral forms) and Arrangements How cells group after division Full entry →, a Plasma membrane Phospholipid bilayer controlling transport Full entry →, a Peptidoglycan Sugar-peptide mesh in the wall Full entry → wall (thick in Gram-positive, thin plus an Outer membrane Extra lipid membrane in Gram-negatives Full entry → in Gram-negative), and often a Glycocalyx Sticky outer coating Full entry →. DNA sits in a Nucleoid Region holding the chromosome Full entry → with extra genes on Plasmids Small circular extra DNA Full entry →, proteins are made on 70S ribosomes Bacterial protein factories Full entry →, and appendages (Fimbriae Short hair-like appendages Full entry →, pili, flagella) handle attachment, DNA transfer, and movement; some genera form endospores.
Why this matters
Cell-wall structure has clinical consequences. The Gram-negative outer membrane excludes many drugs that kill Gram-positive cells, and lipid A can drive sepsis-like inflammation when Gram-negative bacteria are destroyed. Peptidoglycan is the target of several drug classes, and its absence in human cells explains their selective toxicity. Clinical decisions belong to qualified clinicians following local protocols.
Process, Laboratory, or Clinical Foundation
The clinical laboratory reads these structures directly: the Gram stain reads the wall (thick peptidoglycan = purple; thin plus outer membrane = pink); shape and arrangement combine with the Gram reaction to begin identification. Structure also explains drug action: peptidoglycan-targeting drugs spare human cells, the Gram-negative outer membrane blocks many drugs, and plasmids carry resistance genes. Endospores survive disinfection and cause recurrent contamination. Specimen handling, staining, biosafety level, PPE, waste disposal, and infection-control practices vary by institution and must follow approved local policies; this material is conceptual.
The college version
1. Morphology and Arrangements
Cocci are spherical; bacilli are rods; coccobacilli are short oval rods; vibrios are curved, comma-shaped rods; spirilla are rigid spirals; spirochetes are flexible, corkscrew-shaped spirals. Cocci may form diplococci (pairs), streptococci (chains), staphylococci (clusters), or tetrads; bacilli may form diplobacilli (pairs) or streptobacilli (chains).
2. The Cell Envelope: Membrane and Wall
The plasma membrane is a phospholipid bilayer controlling what enters and leaves. Outside it, the cell wall of peptidoglycan — sugar chains cross-linked by peptides — gives shape and resists osmotic pressure. Gram-positive bacteria have a thick peptidoglycan layer and no outer membrane; Gram-negative bacteria have a thin layer between the plasma membrane and an outer membrane, with the periplasm between them. The outer membrane holds lipopolysaccharide (LPS); its toxic part, lipid A, is an endotoxin released when these cells die, and the membrane blocks many drugs.
3. Internal Structures
The nucleoid holds the single, usually circular chromosome. Plasmids are small circular extra DNA carrying genes such as antimicrobial resistance. Ribosomes are 70S (30S + 50S), smaller than eukaryotic 80S, so some drugs target them selectively. Inclusions are storage granules (glycogen, lipids).
4. Surface Structures and Endospores
The glycocalyx is a sticky outer coating. Firm and organized, it is a capsule (resists drying, evades immunity — a virulence factor); loose, it is a slime layer (adherence, biofilms). Fimbriae are short, numerous attachment appendages; pili are longer and fewer, with sex pili transferring DNA. Flagella are rotating propellers. Endospores are dormant, resistant structures made by certain Gram-positive genera (such as Bacillus and Clostridium) to survive heat and drying; they are not for reproduction.
How it works
- The plasma membrane encloses the cell and controls transport.
- Peptidoglycan forms a mesh wall that resists osmotic pressure and gives shape.
- Gram-positive walls are thick; Gram-negative walls are thin and covered by an outer membrane with LPS.
- DNA sits in the nucleoid with extra genes on plasmids; proteins are made on 70S ribosomes.
- Capsule, fimbriae, pili, and flagella mediate protection, attachment, DNA exchange, and movement.
- Under stress, some genera form endospores that survive harsh conditions and later germinate.
Common confusions
| Do not confuse | With | Difference |
|---|---|---|
| Capsule | Slime layer | Organized/firm vs loose |
| Fimbriae | Pili | Short/numerous vs longer, DNA transfer |
| Fimbriae | Flagella | Stick vs move |
| Endospore | Fungal spore | Survival vs reproduction |
| Nucleoid | Nucleus | Membrane-free DNA region vs membrane-bound |
| Spirilla | Spirochetes | Rigid vs flexible |
Memory aids
"Cocci are Circles, Bacilli are Bars, Vibrios are Very curved, Spirochetes Squirm."
Quick review
Topic Recap
Prokaryotic cells lack a nucleus and organelles. Shape (cocci, bacilli, spiral forms) and arrangement aid identification. The envelope includes a plasma membrane and a peptidoglycan wall — thick in Gram-positive, thin plus an outer membrane with LPS in Gram-negative bacteria. Inside, DNA resides in the nucleoid with plasmids, proteins are made on 70S ribosomes, and inclusions store reserves. Surface structures (capsule, slime layer, fimbriae, pili, flagella) handle protection, attachment, DNA transfer, and movement; endospores enable survival.
Knowledge Check
- What structural feature defines a prokaryotic cell?
- Name the three main shapes and the terms for pairs and chains of cocci.
- What is the toxic component (endotoxin) of the Gram-negative outer membrane?
- Why can some drugs target bacterial 70S ribosomes but not human ribosomes?
- What is the purpose of a bacterial endospore?
Answers and Rationales
- The absence of a membrane-bound nucleus and organelles; DNA sits in a nucleoid.
- Cocci (spheres), bacilli (rods), and spiral forms (vibrios, spirilla, spirochetes). Pairs = diplococci; chains = streptococci.
- Lipid A, the toxic part of LPS, which triggers fever and inflammation when Gram-negative cells break apart.
- Human ribosomes are 80S and bacterial ribosomes 70S; the difference lets drugs inhibit bacterial protein synthesis without blocking human synthesis.
- Survival — dormant, resistant structures for withstanding heat, drying, and disinfectants.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Picture a bacterium as a tiny one-room house: DNA, protein factories, and food stores share one open space, wrapped by a protective wall. That open plan is what "prokaryotic" means — no separate room (nucleus) for DNA and no walled-off organelles like our cells have.
The outer wall powers a famous lab trick. Bacteria build it from a tough mesh called peptidoglycan — Gram-positive bacteria build a thick wall, Gram-negative bacteria a thin wall wrapped in an oily outer membrane. The Gram stain detects that difference.
Where it stops being exact: "one-room house" is a simplification. Prokaryotes still have a defined plasma membrane, a DNA-rich nucleoid, and specialized surface structures.
Simple Example
A lab report says "Gram-negative diplococci" (pairs of spheres). Gram-negative means thin peptidoglycan plus an outer membrane; the coccus shape and pair arrangement narrow the possibilities.
Key takeaways
- High yield: Prokaryotes lack a nucleus and membrane-bound organelles.
- High yield: Gram-positive = thick peptidoglycan (purple); Gram-negative = thin peptidoglycan + outer membrane with LPS (pink).
- High yield: Lipid A is the endotoxin in LPS, triggering fever and inflammation when Gram-negative cells die.
- High yield: Bacterial ribosomes are 70S, so some drugs target them selectively.
- Plasmids carry resistance genes and transfer between cells.
- Endospores are for survival, not reproduction, and resist heat and disinfectants.
Study toolsYou’ll learn to · Key vocabulary
You’ll learn to
- Describe prokaryotic cells and the common shapes (morphology) and arrangements of bacteria.
- Compare Gram-positive and Gram-negative cell walls, including peptidoglycan, the outer membrane, and lipopolysaccharide.
- Explain the plasma membrane, nucleoid, plasmids, 70S ribosomes, inclusions, and the glycocalyx (capsule and slime layer).
- Describe fimbriae, pili, and flagella, and the structure and significance of endospores.
Key vocabulary
- Prokaryotic cells
- Cells without nucleus or organelles
- Morphology
- Cell shape
- Cocci
- Spherical cells
- Bacilli
- Rod-shaped cells
- Coccobacilli
- Short, oval rods
- Vibrios
- Curved, comma-shaped rods
- Spirilla
- Rigid spiral cells
- Spirochetes
- Flexible, corkscrew cells
- Arrangements
- How cells group after division
- Plasma membrane
- Phospholipid bilayer controlling transport
- Cell wall
- Peptidoglycan layer giving shape
- Peptidoglycan
- Sugar-peptide mesh in the wall
- Gram-positive wall
- Thick peptidoglycan, no outer membrane
- Gram-negative wall
- Thin peptidoglycan + outer membrane
- Outer membrane
- Extra lipid membrane in Gram-negatives
- Periplasm
- Space between the membranes
- Lipopolysaccharide (LPS)
- Outer-membrane molecule
- Lipid A
- Toxic part of LPS
- Nucleoid
- Region holding the chromosome
- Plasmids
- Small circular extra DNA
- 70S ribosomes
- Bacterial protein factories
- Inclusions
- Storage granules
- Glycocalyx
- Sticky outer coating
- Capsule
- Organized, firm glycocalyx
- Slime layer
- Loose glycocalyx
- Fimbriae
- Short hair-like appendages
- Pili
- Longer appendages (incl. sex pilus)
- Flagella
- Rotating whip-like propellers
- Endospores
- Dormant, resistant structures
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