Transport in Cells: Osmosis and Active Transport
Diffusion (Recap)
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration — down a concentration gradient. It is a passive process (no energy required).
Examples in biology:
- Oxygen diffusing into blood in the lungs
- Carbon dioxide diffusing out of blood into the alveoli
- Glucose diffusing into cells from the blood
Osmosis
Osmosis is the movement of water molecules across a partially permeable membrane from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution).
It is a special case of diffusion — only water moves, and only through a membrane.
Osmosis in Animal Cells
| Solution type | Water concentration compared to cell | What happens | Effect |
|---|---|---|---|
| Hypotonic (dilute) | Higher outside | Water moves into the cell | Cell swells and may burst (lysis) |
| Isotonic (same) | Equal | No net movement | Cell stays normal |
| Hypertonic (concentrated) | Lower outside | Water moves out of the cell | Cell shrinks (crenation) |
Osmosis in Plant Cells
Plant cells have a cell wall that prevents them from bursting:
| Solution type | What happens | Effect |
|---|---|---|
| Hypotonic (dilute) | Water enters by osmosis | Cell becomes turgid (firm) — the vacuole pushes the membrane against the cell wall |
| Isotonic | No net movement | Cell is flaccid (limp) |
| Hypertonic (concentrated) | Water leaves by osmosis | Cell becomes plasmolysed — the membrane pulls away from the cell wall |
Turgor pressure keeps plants upright. When plants lack water, cells become flaccid and the plant wilts.
Required Practical: Osmosis in Potato Cylinders
Aim: Investigate the effect of different sugar solution concentrations on the mass of potato cylinders.
Method:
1. Cut potato cylinders of equal length and diameter using a cork borer
2. Weigh and record the mass of each cylinder
3. Place cylinders into beakers with different concentrations of sugar solution (e.g. 0.0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M)
4. Leave for a set time (e.g. 24 hours)
5. Remove, blot dry, and reweigh
6. Calculate percentage change in mass: % change = (change in mass ÷ original mass) × 100
Expected results:
- In dilute solutions: potato gains mass (water enters by osmosis)
- In concentrated solutions: potato loses mass (water leaves by osmosis)
- At the isotonic point: no change in mass — the graph crosses the x-axis
Control variables: potato type, cylinder size, temperature, volume of solution, time left in solution.
Active Transport
Active transport is the movement of particles from an area of lower concentration to an area of higher concentration — against the concentration gradient. It requires energy from respiration (ATP).
This is the opposite direction to diffusion.
Examples of active transport:
- Root hair cells absorb mineral ions from the soil (soil has a lower concentration of minerals than the cell)
- Gut epithelial cells absorb glucose and amino acids from the intestine into the blood, even when the blood already has a higher concentration
- Root hair cells can absorb water against the gradient when the soil is relatively dry
Comparing Transport Methods
| Feature | Diffusion | Osmosis | Active transport |
|---|---|---|---|
| What moves | Any particles | Water only | Specific particles |
| Direction | High → low concentration | High → low water conc. | Low → high concentration |
| Membrane needed? | Not always | Yes (partially permeable) | Yes (carrier proteins) |
| Energy required? | No (passive) | No (passive) | Yes (from respiration) |
Factors Affecting Rate of Diffusion
- Concentration gradient — steeper gradient = faster diffusion
- Temperature — higher temperature = more kinetic energy = faster diffusion
- Surface area — larger surface area = faster diffusion
- Diffusion distance — thinner membrane/shorter distance = faster diffusion
Exam Tips
- Osmosis is about water moving through a partially permeable membrane — always include both key terms
- In the potato practical, always calculate percentage change in mass, not just change in mass, because starting masses may differ
- Active transport needs energy from respiration — cells with lots of mitochondria carry out more active transport