Contents
1. Cell Structure
| Organelle | Function |
|---|---|
| Cell membrane | Controls what enters and leaves the cell (partially permeable) |
| Cytoplasm | Site of many chemical reactions; contains organelles |
| Nucleus | Contains DNA; controls cell activities and protein synthesis |
| Mitochondria | Site of aerobic respiration; produces ATP energy |
| Ribosomes | Site of protein synthesis |
| Endoplasmic reticulum | Transport network within cell; rough ER has ribosomes attached |
2. Plant vs Animal Cells
| Feature | Animal cell | Plant cell |
|---|---|---|
| Cell wall | Absent | Present (cellulose) |
| Chloroplasts | Absent | Present (in green parts) |
| Large permanent vacuole | Absent (small temporary) | Present |
| Cell membrane | Present | Present |
| Nucleus | Present | Present |
| Mitochondria | Present | Present |
Cell Wall, Chloroplasts, large permanent Vacuole — WWV. Animal cells have none of these three.
3. Levels of Organisation
Cells → Tissues → Organs → Organ systems → Organism
| Level | Definition | Example |
|---|---|---|
| Cell | Basic unit of life | Red blood cell, muscle cell |
| Tissue | Group of similar cells with a common function | Muscle tissue, epithelial tissue |
| Organ | Group of tissues working together | Heart, lungs, stomach |
| Organ system | Group of organs with a shared function | Circulatory system, digestive system |
Specialised cells — adaptations
| Cell | Adaptation | Reason |
|---|---|---|
| Red blood cell | Biconcave disc; no nucleus; haemoglobin | Large surface area; more space for haemoglobin; carries O₂ |
| Root hair cell | Long extension (hair) | Large surface area for absorption of water and ions |
| Palisade cell | Many chloroplasts; tall shape | Maximises light absorption for photosynthesis |
| Sperm cell | Flagellum; many mitochondria; acrosome | Motility; energy; penetrating egg |
4. Movement of Substances
Net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. No energy required (passive).
Net movement of water molecules from a region of higher water potential (more dilute solution) to a region of lower water potential (more concentrated solution), through a partially permeable membrane. No energy required (passive).
Movement of particles against a concentration gradient (from low to high concentration), through a partially permeable membrane, using energy (ATP) from respiration. Requires carrier proteins.
| Diffusion | Osmosis | Active transport | |
|---|---|---|---|
| Direction | High → low concentration | High water potential → low water potential | Low → high concentration |
| Energy needed | No | No | Yes (ATP) |
| Substance moved | Any small particles | Water only | Specific ions/molecules |
A plant cell placed in a dilute solution (high water potential): water enters by osmosis → cell becomes turgid (swells, presses against cell wall). This gives plants their rigidity.
A plant cell placed in a concentrated solution (low water potential): water leaves → cell becomes plasmolysed (membrane pulls away from cell wall). Plant wilts.
- Diffusion: passive, any small particle, no membrane required, high to low concentration
- Osmosis: passive, water molecules ONLY, requires partially permeable membrane
- Active transport: requires ATP, goes AGAINST concentration gradient, uses carrier proteins
- Never say osmosis moves solutes - it ONLY moves water
- Rate of diffusion increases with: larger surface area, steeper concentration gradient, thinner membrane
5. Common Exam Traps
Osmosis involves the movement of water molecules ONLY through a partially permeable membrane. Dissolved solutes do not move by osmosis. "Glucose moves by osmosis" is always wrong.
Active transport requires ATP produced by respiration — not directly from food. If a question asks why active transport stops when oxygen is removed, the answer is: aerobic respiration cannot occur → no ATP → no active transport.
Diffusion is NET movement — particles move in both directions, but the NET flow is from high to low concentration. Never say "all particles move from high to low" — say "net movement is from high to low".
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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.