# Cell Structure
All living organisms are made of cells. Understanding cell structure is fundamental to GCSE Biology. You need to know the differences between plant, animal, and bacterial cells, and the functions of each organelle.
1. Animal Cells
| Organelle | Function |
|---|---|
| Nucleus | Contains DNA; controls cell activities |
| Cell membrane | Controls what enters and leaves the cell |
| Cytoplasm | Where chemical reactions take place |
| Mitochondria | Site of aerobic respiration; releases energy |
| Ribosomes | Site of protein synthesis |
2. Plant Cells
Plant cells have all the organelles of animal cells PLUS:
| Organelle | Function |
|---|---|
| Cell wall | Made of cellulose; provides structural support |
| Chloroplasts | Contain chlorophyll; site of photosynthesis |
| Permanent vacuole | Contains cell sap; maintains turgor pressure |
3. Prokaryotic Cells (Bacteria)
| Feature | Detail |
|---|---|
| DNA | Free in cytoplasm (not in nucleus); circular |
| Plasmids | Small extra rings of DNA |
| Cell membrane | Present |
| Cell wall | Present (not cellulose) |
| Flagellum | For movement (some bacteria) |
| Ribosomes | Present (smaller than eukaryotic) |
| NO nucleus | DNA is free-floating |
| NO mitochondria | No membrane-bound organelles |
4. Eukaryotic vs Prokaryotic
| Feature | Eukaryotic | Prokaryotic |
|---|---|---|
| Size | Larger (10-100 μm) | Smaller (1-5 μm) |
| Nucleus | Yes (membrane-bound) | No |
| DNA | Linear, in chromosomes | Circular |
| Organelles | Membrane-bound | No membrane-bound |
| Examples | Animal, plant, fungal cells | Bacteria |
5. Microscopy
- Light microscope: magnification up to ×1500; resolution ~200 nm
- Electron microscope: magnification up to ×500,000; resolution ~0.2 nm
- SEM: 3D surface images
- TEM: 2D internal structure
Unit Conversions
- 1 mm = 1000 μm
- 1 μm = 1000 nm
6. Required Practical: Microscopy
- Place specimen on slide with a drop of water
- Lower coverslip carefully (avoid air bubbles)
- Add stain (e.g. iodine for plant cells, methylene blue for animal)
- Start on lowest magnification
- Focus with coarse then fine adjustment
- Move to higher magnification if needed
7. Practice Questions
- Name three structures found in plant cells but NOT animal cells.
- What is the function of mitochondria?
- Calculate magnification if image size = 5 mm and actual size = 10 μm.
- Compare and contrast eukaryotic and prokaryotic cells.
- Why do electron microscopes have better resolution than light microscopes?
Want to check your answers and get step-by-step solutions?
Summary
- Animal cells: nucleus, cell membrane, cytoplasm, mitochondria, ribosomes
- Plant cells: all above + cell wall, chloroplasts, permanent vacuole
- Prokaryotic: no nucleus, circular DNA, plasmids, smaller
- Magnification = image size / actual size