# Cell Structure and Ultrastructure
A-Level Biology requires detailed knowledge of cell organelles, their ultrastructure as revealed by electron microscopy, and techniques for studying cells.
1. Eukaryotic Cell Organelles
| Organelle | Structure | Function |
|---|---|---|
| Nucleus | Double membrane (nuclear envelope) with pores; contains chromatin/chromosomes | Houses DNA; controls cell activity |
| Nucleolus | Dense region within nucleus | Produces rRNA and assembles ribosomes |
| Rough ER | Membrane-bound sacs with ribosomes | Protein synthesis and transport |
| Smooth ER | Membrane-bound sacs without ribosomes | Lipid synthesis; detoxification |
| Golgi apparatus | Stack of flattened membrane sacs | Modifies, packages, and secretes proteins |
| Mitochondria | Double membrane; inner membrane folded into cristae; matrix | Aerobic respiration; ATP production |
| Chloroplasts | Double membrane; thylakoids (grana); stroma | Photosynthesis (plants only) |
| Lysosomes | Membrane-bound vesicles with hydrolytic enzymes | Digestion of unwanted material |
| Ribosomes | Two subunits (large + small); 80S (eukaryotic), 70S (prokaryotic) | Protein synthesis |
| Centrioles | Cylindrical; made of microtubules | Spindle formation during cell division |
| Cell membrane | Phospholipid bilayer with proteins | Controls entry/exit of substances |
2. Prokaryotic vs Eukaryotic
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Size | 1-5 μm | 10-100 μm |
| DNA | Circular; in nucleoid | Linear; in nucleus |
| Ribosomes | 70S | 80S |
| Membrane-bound organelles | No | Yes |
| Cell wall | Peptidoglycan | Cellulose (plants) / chitin (fungi) |
| Plasmids | Yes | Rarely |
3. Microscopy
Resolution: minimum distance between two distinguishable points.
| Microscope | Magnification | Resolution |
|---|---|---|
| Light | ×1500 | 200 nm |
| TEM | ×500,000 | 0.5 nm |
| SEM | ×100,000 | 3-10 nm |
4. Cell Fractionation
- Homogenisation: break cells open in cold, isotonic, buffered solution
- Filtration: remove debris
- Differential centrifugation: spin at increasing speeds; heaviest organelles sediment first (nuclei → mitochondria → ER → ribosomes)
5. Practice Questions
- Describe the structure and function of the Golgi apparatus.
- Compare rough and smooth ER.
- Why must the homogenisation solution be cold, isotonic, and buffered?
- Explain why mitochondria sediment before ribosomes.
- Compare prokaryotic and eukaryotic cells.
Want to check your answers and get step-by-step solutions?
Summary
- Eukaryotic: membrane-bound organelles; 80S ribosomes; linear DNA in nucleus
- Prokaryotic: no membrane-bound organelles; 70S ribosomes; circular DNA
- Electron microscopy: higher resolution than light microscopy
- Cell fractionation: homogenise → filter → differential centrifugation