Cell Structure
Cell Structures and Their Functions
| Structure | Function | Plant? | Animal? |
|---|---|---|---|
| Nucleus | Contains DNA; controls cell activities and protein synthesis | ✓ | ✓ |
| Cytoplasm | Jelly-like fluid where most chemical reactions occur | ✓ | ✓ |
| Cell membrane | Controls what enters and leaves the cell (selectively permeable) | ✓ | ✓ |
| Mitochondria | Site of aerobic respiration; produces ATP | ✓ | ✓ |
| Ribosomes | Site of protein synthesis | ✓ | ✓ |
| Cell wall | Rigid outer layer; provides support and shape; made of cellulose | ✓ | ✗ |
| Chloroplasts | Site of photosynthesis; contain chlorophyll to absorb light | ✓ | ✗ |
| Vacuole | Stores cell sap; maintains turgor pressure; supports the cell | Large, permanent ✓ | Small, temporary ✓ |
Plant vs Animal Cell
ANIMAL CELL PLANT CELL
┌─────────────────┐ ══════════════════════
│ Cell membrane │ ║ Cell wall ║
│ ┌───────────┐ │ ║ ┌──────────────┐ ║
│ │ Nucleus │ │ ║ │ Nucleus │ ║
│ └───────────┘ │ ║ └──────────────┘ ║
│ Cytoplasm │ ║ Cytoplasm ║
│ Mitochondria │ ║ Mitochondria ║
│ Ribosomes │ ║ Ribosomes ║
│ │ ║ Chloroplasts ║
└─────────────────┘ ║ Large vacuole ║
══════════════════════
Key Differences
| Feature | Animal cell | Plant cell |
|---|---|---|
| Cell wall | ✗ | ✓ (cellulose) |
| Chloroplasts | ✗ | ✓ (in green parts) |
| Vacuole | Small, temporary | Large, permanent, contains cell sap |
| Shape | Irregular | Regular (box-like) |
| Carbohydrate store | Glycogen | Starch |
Not All Plant Cells Have Chloroplasts
Only cells exposed to light (e.g. palisade cells, guard cells) contain chloroplasts. Root cells, for example, are underground and have no chloroplasts.
Specialised Cells
Cells are adapted for their specific function:
| Cell | Adaptations | Function |
|---|---|---|
| Red blood cell | Biconcave shape (large surface area); no nucleus (more room for haemoglobin); flexible membrane | Carry oxygen |
| Sperm cell | Long tail (flagellum) for movement; many mitochondria; acrosome (enzymes to penetrate egg) | Fertilise egg |
| Palisade cell | Packed with chloroplasts; near top of leaf for maximum light | Photosynthesis |
| Root hair cell | Large surface area extension; no chloroplasts; thin walls | Absorb water and minerals |
| Nerve cell (neurone) | Long axon; myelin sheath; dendrites | Transmit electrical signals |
Exam Questions
Q: State the function of the mitochondria.
Mitochondria are the site of aerobic respiration. They produce ATP, which provides energy for the cell.
Q: State three structures present in a plant cell but not in an animal cell.
Cell wall (cellulose), chloroplasts, and a large permanent vacuole.
Q: Explain why red blood cells do not have a nucleus.
The absence of a nucleus leaves more space inside the cell for haemoglobin, allowing more oxygen to be carried per cell.
Q: Explain why ribosomes are found in all cells.
All cells need to synthesise proteins for growth, repair and to make enzymes. Ribosomes are the site of protein synthesis, so every cell requires them.
Exam Tip
When asked to compare plant and animal cells, always give specific differences: not just “plant cells have a cell wall.” State what it is made of (cellulose) and what it does (provides support and shape).