What you need to know
- Describe diffusion, osmosis and active transport precisely.
- Predict net movement from concentration differences.
- Explain factors affecting diffusion rate.
- Explain the effects of osmosis on animal and plant cells.
- Plan and interpret an osmosis investigation.
Diffusion
The net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient.
Particles move randomly in all directions. If more particles begin on one side, more will cross away from that side than return to it. This produces net movement until the particles become evenly distributed. Particles still move at equilibrium, but there is no overall change in concentration.
Examples include oxygen diffusing from alveoli into blood, carbon dioxide diffusing from blood into alveoli, and dissolved products of digestion diffusing into blood.
Factors that increase diffusion rate
- A steeper concentration gradient: a larger difference produces faster net movement.
- A higher temperature: particles have more kinetic energy and move faster.
- A larger surface area: more particles can cross at the same time.
- A shorter diffusion distance or thinner membrane.
Diffusion rate is proportional to surface area × concentration difference ÷ membrane thickness. You may be asked to apply this relationship to an unfamiliar exchange surface.
Osmosis
The net movement of water molecules through a partially permeable membrane from a dilute solution to a more concentrated solution.
A dilute solution contains a high concentration of water molecules. A concentrated solution contains a lower concentration of water molecules because more solute is present. The partially permeable membrane allows small water molecules through but prevents some larger solute particles from crossing.
Misconception AlertOsmosis is the movement of dissolved solute particles.Select to reveal the correctionSelect to hide the correction
Osmosis is the net movement of water molecules through a partially permeable membrane, from a dilute solution to a more concentrated solution.
Effects of osmosis on cells
| Surrounding solution | Animal cell | Plant cell |
|---|---|---|
| More dilute than cell contents | Water enters; the cell swells and may burst because it has no cell wall. | Water enters; the vacuole swells and the cell becomes turgid. The wall prevents bursting. |
| Same water concentration | No net movement; size remains stable. | No net movement; cell may be flaccid rather than fully supported. |
| More concentrated than cell contents | Water leaves; the cell shrinks. | Water leaves; the cell becomes flaccid. Severe loss pulls the membrane from the wall, called plasmolysis. |
Turgor pressure helps support leaves and young stems. If plant cells lose water, tissues wilt because the cells are no longer pressing firmly against their walls.
Active transport
The movement of substances from a lower concentration to a higher concentration, against the concentration gradient, using energy transferred by respiration.
Carrier proteins in the cell membrane move specific particles. Because movement is against the gradient, active transport requires energy. Cells performing much active transport often contain many mitochondria.
- Root hair cells absorb mineral ions from dilute soil solution even when the ion concentration is higher inside the cell.
- Cells in the small intestine absorb glucose into blood when the glucose concentration in the intestine is lower than in the blood.
“Against the concentration gradient” means from low concentration to high concentration. Always mention energy from respiration when explaining active transport.
Comparing the three processes
| Process | Substance | Direction | Membrane needed? | Energy from respiration? |
|---|---|---|---|---|
| Diffusion | Any particles able to move | High to low concentration | Not always | No |
| Osmosis | Water only | Dilute to concentrated solution | Yes, partially permeable | No |
| Active transport | Specific dissolved substances or ions | Low to high concentration | Yes, cell membrane | Yes |
Required practical: osmosis in plant tissue
Investigate the effect of solution concentration on the mass of plant tissue, commonly potato cylinders.
- Prepare a range of known sucrose or salt concentrations, including distilled water.
- Cut equal-sized potato cylinders. Remove skin and keep length or starting mass similar.
- Blot each cylinder and measure its initial mass.
- Place cylinders into equal volumes of the solutions for the same time at the same temperature.
- Remove, blot in the same way and measure final mass.
- Calculate percentage change in mass and plot it against concentration.
Independent variable: solution concentration. Dependent variable: percentage change in mass. Control the tissue source, cylinder dimensions, solution volume, temperature and time.
A positive percentage means water entered the tissue. A negative percentage means water left. Where the graph crosses 0% change, the solution has approximately the same water concentration as the cell contents.
Blot before both mass measurements, use several cylinders per concentration, calculate a mean and cut cylinders with a cork borer to improve consistency.
Quick retrieval check
1. Define diffusion.
2. What two features make osmosis different from ordinary diffusion?
3. Why does active transport require respiration?
4. Why must potato cylinders be blotted before weighing?
Exam connection
A potato cylinder has an initial mass of 4.80 g and a final mass of 4.20 g. Calculate its percentage change in mass and explain the result.
Show the answer
Percentage change = (4.20 − 4.80) ÷ 4.80 × 100 = −12.5%. The surrounding solution was more concentrated than the cell contents, so water moved out through partially permeable cell membranes by osmosis.