The shape of the lesson
By the end of the lesson, learners can
- Against the gradient: In the sim there are already more nitrate ions inside the root hair cell than in the soil water, yet the cell keeps taking more in.
- Carriers and root hairs: Extended.
- Active transport: lower to higher concentration, through a membrane, using energy from respiration.
- Extended: protein carriers move the molecules or ions across.
- Root hairs: water in by osmosis, mineral ions in by active transport.
- Anything that stops respiration (no oxygen, a poison, low temperature) slows active transport.
One picture every idea returns to
Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (against a concentration gradient), using energy from respiration. Extended: protein carriers in the membrane move the molecules or ions across, which lets cells such as root hair cells take up mineral ions from very dilute soil water.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Uphill, paid for by respiration.. Diffusion and osmosis only ever go down a gradient. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Against the gradient | In the sim there are already more nitrate ions inside the root hair cell than in the soil water, yet the cell keeps taking more in. Project tab 7 of the lab (Active transport into a root hair cell) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Carriers and root hairs | Extended. | Mix-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 19 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Carriers and root hairs, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Active transport needs energy from respiration. | Do not say the energy comes from the kinetic energy of particles (that is diffusion), and do not say active transport "makes" energy: it uses it. |
| Root hair cells take up water by osmosis but mineral ions by active transport. | Keep the two processes separate in your answers. |
Active transport: worksheet
6 questions, 19 marks. Written in the style of the papers; no past paper question is reproduced.
Define active transport.
State three differences between diffusion and active transport.
Extended. Explain how a root hair cell absorbs nitrate ions from the soil, and why the plant needs them.
Barley roots were placed in a solution of potassium ions, with and without air bubbled through. The table shows the ion uptake after 3 hours.
| Condition | Potassium uptake / arbitrary units |
|---|---|
| air bubbled through | 48 |
| nitrogen bubbled through (no oxygen) | 6 |
(a) Calculate how many times greater the uptake was with air. (b) Explain the difference.
Farmers drain waterlogged fields. Suggest why crops grow poorly in waterlogged soil.
Extended. Glucose is absorbed from the small intestine by diffusion and by active transport. Explain why active transport is needed as well as diffusion.
Answer key and marking notes
- The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration.
- That is, against a concentration gradient.
- Using energy released by respiration.
- Diffusion is down a concentration gradient; active transport is against it.
- Diffusion uses the kinetic energy of the particles; active transport needs energy from respiration.
- Diffusion does not need carrier proteins (or a membrane); active transport needs protein carriers in a cell membrane.
- The concentration of nitrate ions in the soil water is lower than in the root hair cell, so they cannot be absorbed by diffusion.
- Protein carriers in the cell membrane bind nitrate ions and move them into the cell, against the concentration gradient.
- This uses energy released by respiration in the many mitochondria of the root hair cell.
- Nitrate is used to make amino acids, and so proteins, for growth.
- (a) 48 ÷ 6 = 8 times greater.
- (b) With air the root cells have oxygen for aerobic respiration, which releases energy.
- The energy is used by protein carriers to take up potassium ions by active transport, against the concentration gradient.
- Without oxygen, little energy is released, so active uptake almost stops; the small uptake may be by diffusion.
- Waterlogged soil has little air (oxygen) between the particles, so root cells cannot respire aerobically.
- Less energy is released for active transport, so the roots take up fewer mineral ions such as nitrate, and growth is reduced.
- Diffusion only moves glucose into the blood while its concentration in the gut is higher than in the blood.
- Once the concentration in the gut falls below that in the blood, glucose must be moved against the gradient.
- Active transport, using energy from respiration and protein carriers, absorbs this remaining glucose so none is wasted.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which feature is true of active transport but not of diffusion?
- AIt can move particles against a concentration gradient
- BIt depends on the random movement of particles
- CIt happens only in the cells of plants
- DIt moves only water molecules across membranes
Which is an example of active transport?
- AUptake of mineral ions from dilute soil water by root hair cells
- BMovement of oxygen from the alveoli into the blood
- CMovement of water from the soil into root hair cells
- DEntry of carbon dioxide into a leaf through the stomata
Roots were placed in a solution of potassium ions. The graph shows the potassium ion concentration inside the root cells and in the solution. Which observation shows that active transport is taking place?
- AThe concentration inside the cells rises above the concentration outside
- BThe concentration inside the cells rises during the first 10 minutes
- CThe concentration in the solution outside stays the same throughout
- DThe concentration inside the cells rises more slowly after 40 minutes
A poison that stops respiration is added to the solution around some roots. What happens to their uptake of nitrate ions?
- AIt stops, because no energy is released for active transport
- BIt increases, because the ions now enter faster by diffusion
- CIt is unchanged, because active transport uses the kinetic energy of the ions
- DIt stops, because the poison makes the cell membrane fully permeable
The cell sap of a pond plant contains potassium ions at 90 mmol/dm3. The pond water contains 1.5 mmol/dm3. How many times greater is the concentration in the cell sap?
- A60 times
- B88.5 times
- C0.017 times
- D135 times
The rate of ion uptake by roots rises as the temperature increases from 10 °C to 35 °C, then falls sharply above 40 °C. What is the best explanation for the fall?
- ARespiratory enzymes and protein carriers are denatured, so less energy and fewer working carriers
- BThe ions have less kinetic energy at high temperature, so they move more slowly
- CThe concentration gradient against which the ions move becomes less steep
- DThe root cells take in more water by osmosis, which dilutes the ions inside
Root cells take up far more potassium ions than sodium ions, even when the soil water contains more sodium. What is the best explanation?
- AThe protein carriers in the membrane are specific to particular ions
- BPotassium ions have more kinetic energy than sodium ions
- CSodium ions can only cross the membrane by osmosis
- DThe cell wall is partially permeable and keeps sodium out
The graph shows how the rate of ion uptake by active transport changes as the concentration of ions outside the cells increases. Why does the rate level off?
- AAll the protein carriers are in use, so extra ions cannot be moved any faster
- BThe concentration gradient becomes too steep for the ions to diffuse
- CThe ions outside have used up all of their kinetic energy
- DThe cells stop respiring once they contain enough ions
Quiz key
Active transport uses energy from respiration to move particles from a lower to a higher concentration, against the gradient; diffusion can only go down the gradient. Random movement of particles is what drives diffusion, not active transport. Active transport happens in animal cells too, for example in the small intestine. The movement of water across membranes is osmosis; active transport moves ions and molecules such as glucose.
Mineral ions are often more concentrated inside root hair cells than in the soil water, so the cells must take them in against the gradient using energy from respiration. Oxygen moves into the blood down its concentration gradient by diffusion. Water enters root hair cells by osmosis. Carbon dioxide enters a leaf by diffusion because photosynthesis keeps its concentration inside low.
Diffusion could only raise the inside concentration until it equalled the outside concentration (10 units). The inside concentration keeps rising to about 29 units, three times the outside value, so ions are being moved against the gradient by active transport. A rise in the first 10 minutes, while the inside is still below 10, could be diffusion. A constant outside concentration says nothing about the direction of movement. Slowing after 40 minutes does not show the gradient is being reversed.
Active transport needs energy released by respiration. If respiration stops, the protein carriers cannot move nitrate ions against the gradient, so uptake stops. Ions do not start entering faster by diffusion: the poison does not change the concentration gradient, and root cells usually hold more nitrate than the soil water, so diffusion cannot bring them in. Kinetic energy powers diffusion, not active transport. A fully permeable membrane is not what this poison does; it acts on respiration.
90 ÷ 1.5 = 60, so the cell sap is 60 times more concentrated. The cells must take in potassium against this steep gradient by active transport. 88.5 is the difference (90 − 1.5), not how many times greater. 0.017 is 1.5 ÷ 90, the division the wrong way round. 135 multiplies the two values instead of dividing.
Above about 40 °C the enzymes that control respiration and the protein carriers in the membrane change shape (denature). Less energy is released and fewer carriers work, so active uptake falls. Ions gain kinetic energy, not lose it, as the temperature rises. A less steep gradient would make active transport easier, not harder. Temperature does not make cells take in extra water and dilute the ions, and dilution would not reduce the rate of uptake by carriers.
Each type of protein carrier binds only particular ions, so a root cell with many potassium carriers takes up potassium selectively, using energy from respiration, whatever the sodium concentration. Ions of similar size at the same temperature do not differ enough in kinetic energy to explain this, and kinetic energy drives diffusion, not active uptake. Osmosis is the movement of water, not ions. The cell wall is fully permeable; it is the cell membrane that controls entry.
Ions are moved by protein carriers in the membrane, and there is a limited number of them. At high concentrations every carrier is already busy, so adding more ions outside cannot increase the rate. A steeper gradient from outside to inside would speed diffusion, not stop it, and this uptake is not diffusion. Ions do not run out of kinetic energy at a constant temperature. Cells keep respiring; if respiration stopped, uptake would fall to zero rather than level off.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.