Prokaryotic Cell Structure
Laura Armstrong
Teacher
Recall Questions
This topic requires prior knowledge of eukaryotic cell structures to enable comparison of similarities and differences. You can test your knowledge on this below.
What is the key structural difference between prokaryotic and eukaryotic cells?
Prokaryotic cells lack a nucleus and membrane-bound organelles, whereas eukaryotic cells have them.
What is the function of the cell wall in prokaryotic cells?
It provides structural support and prevents the cell from bursting due to osmotic pressure.
What is a plasmid?
Plasmids are small, circular DNA molecules that can carry antibiotic resistance genes.
Topic Explainer Videos
Check out this @LauraDoesBiology video that explains prokaryotic cell structure or read the full notes below. Once you've gone through the whole note, try out the practice questions!
The Major Organelles in Prokaryotic Cells and their Functions
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Prokaryotic cells are unicellular organisms that lack a nucleus and membrane-bound organelles, such as mitochondria.
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They are smaller (0.1–5µm) than eukaryotic cells and include bacteria and archaea.
Prokaryotic Cell Organelles
| Organelle | Functions | Present in all or some prokaryotes? |
| Cell surface membrane | Partially permeable- controls movement of substances in and out of the cell. | All prokaryotes |
| Cytoplasm | Contains enzymes and ribosomes for metabolic reactions. | All prokaryotes |
| Ribosomes (70S) | Site of protein synthesis (smaller than eukaryotic 80S ribosomes). | All prokaryotes |
| Cell Wall | Provides structural support and prevents osmotic lysis. Made of murein (peptidoglycan). | All prokaryotes |
| Circular DNA (Nucleoid) | Contains genetic material that codes for polypeptides. Not enclosed in a nucleus. | All prokaryotes |
| Plasmids | Small, circular DNA that carries extra genes (e.g., antibiotic resistance). | Only some prokaryotes |
| Capsule (Slime Layer) | Protects against desiccation and phagocytosis. Helps bacteria stick to surfaces. | Only some prokaryotes |
| Flagella | Enables movement via a rotating motion. | Only some prokaryotes |
| Pili (Fimbriae) | Hair-like structures that help in attachment to surfaces and attachment of bacteria to one another. | Only some prokaryotes |
Key Terms
- Prokaryote: A unicellular organism that lacks a nucleus and membrane-bound organelles.
- Plasmid: A small, circular DNA molecule that carries additional genes.
- Capsule: A protective slime layer that prevents desiccation and phagocytosis.
- Flagellum: A whip-like structure used for movement.
- Murein: A polysaccharide-peptide polymer that forms bacterial cell walls.
- 70S Ribosomes: Smaller ribosomes found in prokaryotic cells for protein synthesis.
- Desiccation: Drying out.
No answer provided.
Exam Tips
Students often confuse prokaryotic and eukaryotic ribosomes.
Prokaryotic ribosomes are smaller 70S ribosomes.
Eukaryotic ribosomes are larger 80S ribosomes, however they also have 70S ribosomes in their mitochondria / chloroplasts (if a plant cell)
Not all bacteria have plasmids, capsules, or flagella – only some do!
Capsules are found in some prokaryotes, don’t confuse this with a capsid which is found in viruses!
No answer provided.
Describe and explain the differences between prokaryotic and eukaryotic cells. (6 marks)
1. Prokaryotic cells do not have a nucleus, whereas eukaryotic cells have a membrane-bound nucleus.
2. Prokaryotic cells contain circular DNA (nucleoid), while eukaryotic cells contain linear DNA associated with histone proteins.
3. Prokaryotes have only 70S ribosomes, while eukaryotic cells have larger 80S ribosomes for protein synthesis (and 70S ribosomes in their mitochondria/ chloroplasts).
4. Prokaryotic cell walls are made of murein (peptidoglycan), whereas plant cell walls in eukaryotes are made of cellulose and fungal cell walls of chitin.
5. Prokaryotes do not contain membrane-bound organelles (e.g., mitochondria, Golgi apparatus), whereas eukaryotic cells do.
6. Some prokaryotes have additional structures such as plasmids, capsules, pili, and flagella, which are not found in most eukaryotic cells.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!