# Cell Transport
Substances need to move in and out of cells for the organism to survive. There are three main transport mechanisms: diffusion, osmosis, and active transport.
1. Diffusion
The net movement of particles from an area of higher concentration to an area of lower concentration (down the concentration gradient).
- Passive (no energy required)
- Continues until equilibrium is reached
- Examples: oxygen into cells, carbon dioxide out of cells, glucose absorption in the gut
Factors Affecting Rate of Diffusion
- Concentration gradient: steeper → faster
- Temperature: higher → faster (more kinetic energy)
- Surface area: larger → faster
- Membrane thickness: thinner → faster
2. Osmosis
The movement of water molecules from a dilute solution (high water concentration) to a concentrated solution (low water concentration) through a partially permeable membrane.
- Special case of diffusion (water only)
- Passive process
Effects on Cells
| Solution | Animal Cell | Plant Cell |
|---|---|---|
| Hypotonic (dilute) | Swells, may burst (lysis) | Turgid (firm) |
| Isotonic | Normal | Normal |
| Hypertonic (concentrated) | Shrinks (crenation) | Plasmolysed (flaccid) |
3. Active Transport
The movement of particles from a lower concentration to a higher concentration (against the concentration gradient) using energy from respiration.
- Requires ATP (energy from mitochondria)
- Uses carrier proteins in the cell membrane
Examples:
- Root hair cells absorb mineral ions from soil
- Gut cells absorb glucose even when concentration inside is higher
4. Comparing Transport Methods
| Feature | Diffusion | Osmosis | Active Transport |
|---|---|---|---|
| Direction | High → Low | High water → Low water | Low → High |
| Energy needed? | No | No | Yes (ATP) |
| What moves? | Any small molecule | Water only | Specific molecules/ions |
| Carrier proteins? | No | No (through membrane) | Yes |
5. Required Practical: Osmosis
Investigate osmosis in potato cylinders:
- Cut potato into equal cylinders
- Place in different concentrations of sucrose solution
- Measure mass before and after (24 hours)
- Calculate percentage change in mass
- Plot graph of % change vs concentration
Where the line crosses 0% change = isotonic concentration.
6. Practice Questions
- Explain why osmosis is important for plants.
- Give two examples of active transport in the body.
- A plant cell is placed in pure water. Describe and explain what happens.
- Why do root hair cells have many mitochondria?
Want to check your answers and get step-by-step solutions?
Summary
- Diffusion: high → low concentration; passive; any molecule
- Osmosis: water through partially permeable membrane; dilute → concentrated; passive
- Active transport: low → high; requires ATP; uses carrier proteins
- Root hair cells and gut lining use active transport