Prokaryotic and Eukaryotic Cells: MDCAT Biology notes
Prokaryotic and eukaryotic cells MDCAT notes: bacterial cell wall, Gram staining, mesosomes, plasmids, flagella, spores, shapes, nutrition and control.
Prokaryotes compared with eukaryotes
| Feature | Prokaryote (bacteria, e.g. E. coli) | Eukaryote |
|---|---|---|
| Nucleus | None; a single circular ds DNA lies in the nucleoid | True nucleus with a nuclear envelope |
| Membrane-bound organelles | Absent | Present |
| Ribosomes | 70S (50S + 30S) | 80S (60S + 40S) |
| Division | Binary fission, the closest equivalent to mitosis | Mitosis and meiosis |
| Gene expression | Translation starts while transcription is still going on | Transcription in the nucleus, translation in the cytoplasm |
Ribosomes, which are the protein factories, are the only organelle common to both groups. Bacteria are unicellular prokaryotes.
Bacterial cell structure
- Cell wall: this gives rigidity and shape and protects against osmotic lysis. Its unique component is peptidoglycan (murein), made of sugars cross-linked by short peptides, so chemically it is carbohydrate plus protein. Fungal walls are chitin and plant walls are cellulose.
- Gram-positive walls are a single thick peptidoglycan layer with little lipid. They lack lipopolysaccharide.
- Gram-negative walls have two layers: thin peptidoglycan plus a lipopolysaccharide outer membrane. This makes them more lipid-rich and more resistant to β-lactam antibiotics such as penicillin.
- Periplasmic space: the gap between the cell membrane and the cell wall.
- Cell membrane: delicate, and damage to it kills the cell. Its infoldings are mesosomes, which take part in respiration (acting like mitochondria), DNA replication and cell division.
- Capsule and slime: outer layers. A capsule increases pathogenicity: it resists dehydration, helps the cell stick to surfaces and hides it from host defences.
- Plasmids: small circular ds DNA that replicate on their own. They carry fewer genes, such as antibiotic resistance, and the chromosome does not depend on them.
- Flagella provide motility; pili attach the cell to surfaces.
| Flagella arrangement | Meaning |
|---|---|
| Atrichous | No flagella |
| Monotrichous | One flagellum at one pole |
| Lophotrichous | A tuft of two or more at one pole |
| Amphitrichous | Flagella at both poles |
| Peritrichous | All over the surface |
Endospores are formed mainly by Gram-positive bacteria (Bacillus, Clostridium). They are dehydrated, metabolically dormant for long periods, and resist heat and extreme conditions.
Shapes and arrangements
- Cocci are spherical and bacilli are rod-shaped. Spirilla are spiral.
- Cocci can be arranged as a chain (streptococci), a grape-like bunch (staphylococci), or a cube of 8 formed by division in three planes (sarcina).
- Mycoplasma are the smallest known bacteria.
Nutrition, growth and control
- Photosynthetic bacteria include purple and green sulphur bacteria, which release sulphur rather than oxygen, and purple non-sulphur bacteria. Chemosynthetic bacteria, such as nitrifying bacteria, get energy by oxidizing inorganic substances. Saprotrophs are nature's recyclers.
- Oxygen needs: aerobes (Pseudomonas), anaerobes, and facultative anaerobes, which grow with or without oxygen.
- Growth phases: lag, log (exponential), stationary, decline.
- Gene transfer: conjugation, transformation, and transduction (via a bacteriophage).
- Physical control: heat and sterilization (dry heat oxidizes cell compounds), radiation (X-rays cause mutations), filtration. Chemical control: antiseptics on living surfaces, disinfectants on non-living surfaces. Antibiotics work against bacteria such as Salmonella typhi, not viruses.
- Robert Koch identified the tuberculosis bacterium and developed pure-culture techniques.
Common MDCAT traps
- Mesosomes are infoldings of the cell membrane, not the cell wall.
- Gram-negative walls have two layers; Gram-positive walls have one.
- No lipopolysaccharide means Gram-positive.
- Antiseptics are for living tissue; disinfectants are for non-living surfaces.
- Peptidoglycan contains sugar and peptide, not carbohydrate alone.
Quick revision
- Prokaryotes lack membrane-bound organelles.
- Ribosomes are 70S, with a 50S large subunit.
- Endospores endure extreme conditions.
- Staphylococci form bunches; streptococci form chains.
- Bacteria divide exponentially in the log phase.