Prokaryotes and Pathogens
Prokaryotes vs Eukaryotes
Prokaryotic organisms have cells that lack a true membrane-bound nucleus. Their DNA is not enclosed. They are typically much smaller than eukaryotic cells.
Bacteria
Bacteria are microscopic single-celled prokaryotes.
Structure
┌────────────────────────────────┐
│ Cell wall (peptidoglycan) │
│ ┌──────────────────────────┐ │
│ │ Cell membrane │ │
│ │ Cytoplasm │ │
│ │ Circular DNA chromosome │ │
│ │ Plasmids (small rings │ │
│ │ of extra DNA) │ │
│ └──────────────────────────┘ │
└────────────────────────────────┘
| Structure | Present in bacteria? |
|---|---|
| Cell wall | ✓ (made of peptidoglycan/murein) |
| Cell membrane | ✓ |
| Cytoplasm | ✓ |
| Plasmids | ✓ (small extra rings of DNA) |
| Circular DNA chromosome | ✓ (not enclosed in a nucleus) |
| True nucleus | ✗ |
| Mitochondria | ✗ |
| Chloroplasts | ✗ (some can photosynthesise but lack chloroplasts) |
Nutrition
- Most bacteria feed off other living or dead organisms.
- Some can carry out photosynthesis (but this is not via chloroplasts).
Examples
- Lactobacillus bulgaricus: used to make yoghurt
- Pneumococcus: causes pneumonia
Viruses
Viruses are not living organisms.
| Feature | Detail |
|---|---|
| Size | Smaller than bacteria |
| Structure | Protein coat (capsid) surrounding genetic material (DNA or RNA) |
| Reproduction | Parasitic: can only reproduce inside living host cells |
| Host range | Infect every type of living organism |
| Cell wall | ✗ |
| Metabolism | None: no independent metabolism |
Viruses Are Not Cells
Viruses are not made of cells. They have no cytoplasm, no membrane-bound organelles, and cannot carry out any metabolic processes independently. They are not classified as living organisms.
Pathogens
A pathogen is any microorganism that causes disease.
Pathogens may be:
- Fungi (e.g. athlete’s foot caused by a fungus)
- Bacteria (e.g. Pneumococcus causing pneumonia)
- Protoctists (e.g. Plasmodium causing malaria)
- Viruses (e.g. influenza, HIV)
Exam Questions
Q: State three ways in which bacterial cells differ from human cells.
- Bacterial cells have no true nucleus; human cells do. 2. Bacterial cells have a cell wall; human cells do not. 3. Bacterial cells contain plasmids (small rings of DNA); human cells do not. 4. Bacteria have a single circular chromosome; human cells have linear chromosomes.
Q: Explain why viruses are not considered to be living organisms.
Viruses cannot carry out any of the characteristics of living organisms independently (e.g. they do not respire, grow, or reproduce on their own). They can only reproduce by invading and taking over the machinery of living host cells.
Q: Define the term pathogen.
A pathogen is a microorganism that causes disease. Pathogens may be fungi, bacteria, protoctists or viruses.
Q: Give one similarity and one difference between bacteria and viruses.
Similarity: both are microscopic and can cause disease (both can be pathogens). Difference: bacteria are living cells with their own metabolism; viruses are not living cells and can only reproduce inside a host cell.
Exam Tip
Remember the key distinction: bacteria are living cells (prokaryotes); viruses are not living and are not cells. Bacteria have ribosomes and can carry out metabolism; viruses cannot.