Classification and Structures of Prokaryotic Cells
High-Yield Summary
- Prokaryotic cells lack a nucleus and membrane-bound organelles; DNA sits free in the cytoplasm as a single circular chromosome (nucleoid).
- All life splits into 3 domains: Bacteria and Archaea (both prokaryotic), and Eukarya. Archaea favor extreme environments and share more machinery with eukaryotes; Bacteria are widespread and include the human microbiome.
- Bacteria classified by shape (cocci, bacilli, spirilli) and oxygen use (obligate aerobe/anaerobe, facultative anaerobe, aerotolerant anaerobe).
- Gram-positive: thick peptidoglycan + teichoic acids. Gram-negative: thin peptidoglycan + outer membrane + periplasmic space + LPS.
- Prokaryotic ribosomes are 70S (30S + 50S) vs. eukaryotic 80S (40S + 60S) — the basis for selective antibiotics.
- Flagella (filament/flagellin, hook, basal body) drive chemotaxis, powered by a proton gradient.
Key Terms
- Nucleoid
- Region where prokaryotic DNA sits, unenclosed by a nuclear membrane; usually one circular chromosome.
- Plasmid
- Small circular DNA, replicates independently of the chromosome; often carries survival genes (e.g. antibiotic resistance); transferable between bacteria.
- Peptidoglycan
- Sugar-chain polymer cross-linked by peptides forming the bacterial cell wall's rigid, flexible mesh.
- Periplasmic space
- Space between inner and outer membrane in Gram-negative bacteria; holds enzymes for nutrient processing and defense.
- Lipopolysaccharide (LPS)
- Outer-membrane molecule in Gram-negative bacteria; structural role + triggers immune/inflammatory response.
- Chemotaxis
- Movement toward/away from chemical stimuli, directed via flagella.
Gram-Positive vs. Gram-Negative
| Gram-Positive | Gram-Negative |
|---|---|
| Thick peptidoglycan layer | Thin peptidoglycan layer |
| No outer membrane | Has outer membrane |
| No periplasmic space | Periplasmic space present |
| Teichoic acids (incl. lipoteichoic acid, LTA) trigger immune response | LPS triggers immune/inflammatory response |
Ribosomes: Prokaryotic vs. Eukaryotic
| Prokaryotic | Eukaryotic |
|---|---|
| 30S + 50S subunits | 40S + 60S subunits |
| 70S ribosome | 80S ribosome |
| Selective antibiotic target | Not targeted by ribosome-selective antibiotics |
Bacterial Flagellum Structure
- 1Basal body — embedded in cell membrane, acts as motor; rotation powered by a proton gradient across the membrane.
- 2Hook — connects the basal body to the filament.
- 3Filament — long helical tail made of flagellin protein; spins to propel the bacterium (chemotaxis).
Common MCAT Trap
- Obligate anaerobe vs. aerotolerant anaerobe: both rely entirely on anaerobic metabolism, but obligate anaerobes are killed by oxygen while aerotolerant anaerobes just don't use it (and survive fine in it).
- Archaea are prokaryotic (no nucleus) but their membrane/gene-expression machinery resembles eukaryotes more than bacteria — don't lump them with Bacteria functionally.
Quick Recall
What are the 3 domains of life, and which are prokaryotic?
What distinguishes an obligate anaerobe from an aerotolerant anaerobe?
What immune-triggering molecule is unique to Gram-negative bacteria?
Why do certain antibiotics target bacteria selectively?