Prokaryotic vs. Eukaryotic Cells
NSO – Biology – Science Olympiad
1. Simple Explanation
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Definition | Cells that lack a true nucleus and membrane‑bound organelles (e.g., bacteria, archaea). | Cells that contain a membrane‑bound nucleus and many specialized organelles (e.g., plants, animals, fungi, protists). |
| Size | 0.1–5 µm in diameter. | 10–100 µm in diameter. |
| DNA | Circular chromosome in the nucleoid; often plasmids. | Linear chromosomes inside the nucleus; often multiple. |
| Cell Division | Binary fission (simple, rapid). | Mitosis (complex, regulated). |
| Ribosomes | 70S (small). | 80S (larger). |
| Cell Wall | Peptidoglycan (bacteria) or pseudo‑peptidoglycan (archaea). | Cellulose (plants), chitin (fungi), or no wall (animals). |
| Energy Production | Can be aerobic or anaerobic; many use fermentation. | Typically aerobic; mitochondria produce ATP via oxidative phosphorylation. |
| Cytoskeleton | Minimal; mainly actin‑like proteins. | Extensive, with microtubules, actin filaments, intermediate filaments. |
| Examples | Escherichia coli, Staphylococcus aureus. | Human cells, plant cells, yeast cells. |
Bottom line:
- Prokaryotes are small, simple, single‑chromosome organisms without membrane‑bound organelles.
- Eukaryotes are larger, complex, and compartmentalized with a true nucleus and many organelles.
2. Key Points (Cheat‑Sheet)
| Category | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | None (DNA in nucleoid). | True nucleus encased in nuclear envelope. |
| Chromosome | 1 circular plasmid + plasmids. | Multiple linear chromosomes. |
| Membrane‑Bound Organelles | None (except ribosomes). | Mitochondria, chloroplasts, ER, Golgi, lysosomes, peroxisomes, etc. |
| Cell Wall | Peptidoglycan (bacteria) / pseudo‑peptidoglycan (archaea). | Cellulose (plants), chitin (fungi), or no wall. |
| Ribosomes | 70S. | 80S. |
| Flagella | Type‑IV (tubular) or Type‑VI (protein). | 9+2 microtubule axoneme. |
| Cytoskeleton | Minimal actin‑like. | Extensive microtubules, actin filaments, intermediate filaments. |
| Reproduction | Binary fission, conjugation, transformation, transduction. | Mitosis, meiosis, sexual reproduction. |
| Metabolism | Diverse: photosynthesis, chemosynthesis, fermentation. | Mostly aerobic respiration; plants also photosynthesis. |
| Examples | E. coli, Bacillus subtilis, Methanobacterium. | Human, mouse, wheat, yeast. |
3. Worked Example
Problem:
You are given a microscope slide with a stained cell sample. Identify whether the cell is prokaryotic or eukaryotic and list at least three organelles you can observe.
Solution Steps
-
Check for a nucleus
- Observation: A distinct, dark‑stained region surrounded by a clear halo.
- Conclusion: Presence of a nucleus → eukaryotic.
-
Look for other organelles
- Mitochondria – small, oval bodies with double membranes.
- Rough endoplasmic reticulum (RER) – sheet‑like structures studded with ribosomes.
- Golgi apparatus – stacked cisternae, darker than surrounding cytoplasm.
-
Size comparison
- Cell diameter ≈ 20 µm → within eukaryotic range.
-
Answer
- The cell is eukaryotic.
- Observed organelles: nucleus, mitochondria, RER (or Golgi).
- (Optional) The presence of ribosomes on the RER confirms protein synthesis machinery.
4. Quick Revision Summary
-
Prokaryotes
- No nucleus, 1 circular chromosome, peptidoglycan wall, 70S ribosomes, binary fission.
- Example: E. coli.
-
Eukaryotes
- True nucleus, multiple linear chromosomes, diverse organelles, 80S ribosomes, mitosis.
- Example: Human muscle cell.
-
Key Differences to Recall
- Nucleus (present vs. absent).
- Chromosome shape (linear vs. circular).
- Organelle presence (many vs. none).
- Cell size (small vs. large).
- Division type (binary fission vs. mitosis).
-
Mnemonic:
- Pro: Peptidoglycan, Plasmid, Primary replication.
- Euk: Envelope (nucleus), Extra organelles, Exponential mitotic growth.
Quick Self‑Check Questions
- What type of cell has a peptidoglycan cell wall?
- Which organelle is responsible for ATP production in eukaryotes?
- How does the ribosome size differ between prokaryotes and eukaryotes?
- Name one organelle that is only found in eukaryotic cells.
Answer these without looking at the notes to test your retention.