Lesson focus: Diffusion, osmosisThe net movement of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane. and active transportMoving particles against a concentration gradient, from a low concentration to a high concentration, using energy from respiration. move substances across membranes by different mechanisms and in different directions relative to gradients.
Compare diffusion, osmosis and active transport and predict their effects on plant and animal cells.
Build on earlier learning
Diffusion in living systemsThe previous lesson established diffusion and gradients. This lesson distinguishes water movement and ATP-driven transport.
You will be successful when you can…
- Define osmosis precisely.
- Explain active transport.
- Compare all three mechanisms.
- Predict effects on plant and animal cells.
Words that carry the science
- osmosis
- Net movement of water molecules through a partially permeable membrane from higher to lower water concentration.
- partially permeable membrane
- A membrane that allows some particles through more readily than others.
- active transport
- Movement of substances against a concentration gradient using energy released by respiration.
- turgid
- Firm plant-cell state produced when water entry presses contents against the cell wall.
- plasmolysed
- Plant-cell state in which water loss causes the membrane to pull away from the wall.
- transport protein
- A membrane protein that assists movement of particular substances.
Osmosis is diffusion of water through a selective barrier
01Osmosis is diffusion of water through a selective barrier
osmosisThe net movement of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane. is the net movement of water molecules through a partially permeable membraneA thin layer that lets some substances, such as water, through but not others. from a region of higher water concentration to lower water concentration. In dilute solutions, water concentration is relatively high; adding dissolved solute lowers the proportion of water molecules.
Water moves both ways, but unequal crossing produces the net movement. Describing “water moving from high to low concentration” is incomplete unless water concentration and the membrane are specified.
02Walls change the outcome for plant cells
In a dilute external solution, water enters a plant cell and the vacuole expands. The cell becomes turgidDescribes a plant cell that is swollen and firm because it is full of water. as the wall resists further expansion. In a concentrated solution, water leaves; the cell becomes flaccid and may become plasmolysedDescribes a plant cell that has lost so much water that its insides pull away from the cell wall..
Animal cells lack a rigid cell wall. Excessive water entry may cause lysis, while water loss makes cells shrink. Medical solutions are formulated to avoid damaging net water movement.
03Active transport moves against a gradient
transport proteinA protein in a cell membrane that helps particular substances get across it. can move ions or molecules from lower to higher concentration using energy released by respiration, commonly via ATP. This is active transportMoving particles against a concentration gradient, from a low concentration to a high concentration, using energy from respiration..
Root hair cells can take up mineral ions even when soil concentration is lower than cell concentration. Intestinal cells can absorb some nutrients against gradients. active transportMoving particles against a concentration gradient, from a low concentration to a high concentration, using energy from respiration. is selective and can be limited by oxygen supply, temperature and metabolic inhibitors.
Osmosis in cells
Water moves across a partially permeable membrane from a dilute solution to a more concentrated one. Sugar particles are too big to pass.
Water Sugar
The dashed circle is the cell membrane.
Controls
Particle numbers are illustrative. Water still moves both ways; osmosis is the net movement.
Choose the transport mechanism
Nitrate ions enter a root hair cell even though nitrate concentration is lower in soil than inside the cell.
- The ions move from lower to higher concentration.
- This is against the concentration gradient.
- The membrane must use specific transport proteins.
- Energy released by respiration is required, so the mechanism is active transport.
How to make the reasoning visible: Do not call every membrane movement diffusion; direction, particle type and energy requirement distinguish the processes.
Compare membrane transport
Classify each movement from the stated evidence.
| Situation | Particle | Direction | Mechanism |
|---|---|---|---|
| oxygen enters respiring cell | oxygen | high to low concentration | diffusion |
| water enters root hair cell | water | high to low water concentration through membrane | osmosis |
| nitrate enters root against gradient | nitrate ion | low to high concentration | active transport |
| glucose spreads through cytoplasm | glucose | high to low concentration | diffusion |
Which process is stopped most directly by loss of ATP supply?
Why does a turgid plant cell not keep swelling indefinitely?
Check your understanding
Answer from memory, then check against the suggested answer.
Define osmosis.
Which process can move a substance against its gradient?
What does turgid mean?
Why can waterlogged soil reduce mineral-ion uptake by roots?
Why is “osmosis moves water to equal concentrations” not always a sufficient prediction in a plant cell?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Substances cross cell membranes by diffusion, osmosis and active transport.
Compare diffusion, osmosis and active transport.
(6)Total for this question: 6 marks
Command word: For compareA command word: say how two or more things are similar and how they are different., organise the answer by direction, particle, membrane requirement and energy use.