The shape of the lesson
By the end of the lesson, learners can
- What diffusion is: Drop dye into still water.
- Four factors that change the rate: In the investigation tab particles pass from a left chamber through an opening in a wall into a right chamber.
- Why organisms need diffusion: Gases and dissolved substances (solutes) move into and out of cells by diffusion.
- Definition: net movement from higher to lower concentration, down a concentration gradient, by random movement.
- Energy: kinetic energy of the particles; no respiration needed.
- Faster with: larger surface area, higher temperature, steeper gradient, shorter distance.
- In organisms: say which side supplies the substance and which side removes it.
One picture every idea returns to
Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration, down a concentration gradient, as a result of their random movement. The energy comes from the kinetic energy of the moving molecules and ions themselves, so the cell spends no energy. Some substances, such as oxygen, carbon dioxide and glucose, move into and out of cells by diffusion through the cell membrane.
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, Random movement, net direction.. Particles in liquids and gases never stop moving. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: What diffusion is | Drop dye into still water. Project tab 1 of the lab (Dye diffusing through water) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Four factors that change the rate | In the investigation tab particles pass from a left chamber through an opening in a wall into a right chamber. Project tab 2 of the lab (Factors affecting the rate of diffusion) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Why organisms need diffusion | Gases and dissolved substances (solutes) move into and out of cells by diffusion. Project tab 3 of the lab (Diffusion in living organisms) and let a student drive it while the class predicts. | Cooperative Concept Mapping |
| 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 Why organisms need diffusion, 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.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
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 |
|---|---|
| Do not say particles stop moving when diffusion is complete. | They keep moving at random; it is the net movement that becomes zero because there is no longer a concentration gradient. |
| "Higher concentration" on its own is not a factor. | The rate depends on the difference in concentration between the two regions, the concentration gradient. |
Diffusion: worksheet
6 questions, 19 marks. Written in the style of the papers; no past paper question is reproduced.
Define diffusion and state where the energy for diffusion comes from.
Explain why oxygen diffuses from the air in an alveolus into the blood, and why this continues for as long as a person is alive.
Cubes of agar containing a pink indicator were put into acid. The indicator turns colourless in acid. The table shows the time for each cube to turn completely colourless.
| Side of cube / cm | Surface area / cm2 | Volume / cm3 | Time / s |
|---|---|---|---|
| 1 | 6 | 1 | 240 |
| 2 | 24 | 8 | 590 |
| 3 | 54 | 27 | 1100 |
(a) Calculate the surface area to volume ratio of each cube. (b) Describe and explain the pattern in the results.
Plan an investigation to find the effect of temperature on the rate of diffusion of a dye in water. Include the variables you would control.
Glucose diffuses from the blood into respiring muscle cells. Explain why the concentration gradient for glucose is maintained.
Name three substances that move into or out of living cells by diffusion and state the direction of movement for each.
Answer key and marking notes
- The net movement of particles from a region of their higher concentration to a region of their lower concentration.
- Down a concentration gradient, as a result of their random movement.
- The energy comes from the kinetic energy of the random movement of the molecules and ions.
- The oxygen concentration in the alveolus is higher than in the blood arriving in the capillary, so oxygen diffuses down the gradient into the blood.
- Breathing replaces the air, keeping the oxygen concentration in the alveolus high.
- Blood flow carries oxygenated blood away, keeping the concentration in the capillary low, so the gradient is maintained.
- (a) 1 cm: 6 : 1; 2 cm: 3 : 1; 3 cm: 2 : 1.
- (b) As the cube gets bigger, the time to turn colourless increases.
- Larger cubes have a smaller surface area to volume ratio, so less surface for acid to enter per unit volume.
- The acid also has a greater distance to diffuse to reach the centre.
- Set up beakers of water at several temperatures, for example 10, 20, 30, 40 and 50 °C, using water baths and a thermometer.
- Add the same volume of the same dye to each beaker and start a stop-clock.
- Record the time for the colour to spread evenly (or measure the distance spread in a fixed time).
- Control: volume of water, volume and concentration of dye, size and shape of beaker; do not stir; repeat and calculate a mean.
- Respiration inside the muscle cells uses up glucose, keeping its concentration in the cells low.
- Blood flow keeps bringing glucose, keeping its concentration in the blood high.
- Oxygen: into respiring cells (or out of photosynthesising leaf cells).
- Carbon dioxide: out of respiring cells (or into leaf cells in the light).
- Glucose / amino acids: from the small intestine into the blood, or from the blood into cells. (Also accept water vapour out of a leaf, urea from cells into the blood.)
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 is an example of diffusion?
- ACarbon dioxide moving from the air spaces into a palisade cell in the light
- BWater moving from the soil into a root hair cell
- CNitrate ions moving from dilute soil water into a root hair cell
- DBlood being pumped through the capillaries around an alveolus
A leaf is in bright light. In which direction is the net diffusion of oxygen, and why?
- AOut of the leaf, because photosynthesis makes the oxygen concentration higher inside
- BInto the leaf, because photosynthesis uses up the oxygen inside
- COut of the leaf, because respiration makes the oxygen concentration higher inside
- DInto the leaf, because the leaf cells respire faster than they photosynthesise
Glucose is diffusing into a cell from the surrounding solution. Which change would make glucose enter the cell faster?
- AIncreasing the glucose concentration in the surrounding solution
- BIncreasing the glucose concentration inside the cell
- CCooling the cell and the solution by 10 °C
- DReducing the surface area of the cell membrane
Blocks of agar containing a pink indicator were placed in acid at different temperatures. The indicator turns colourless when acid reaches it. What does the table show?
- AAs temperature increases, the rate of diffusion of the acid increases
- BAs temperature increases, the rate of diffusion of the acid decreases
- CTemperature has no effect on the rate of diffusion of the acid
- DThe rate of diffusion doubles for every 10 °C rise in temperature
Which substance leaves a respiring muscle cell by diffusion?
- ACarbon dioxide
- BOxygen
- CGlucose
- DStarch
An agar block measures 1 cm × 1 cm × 4 cm. What is its surface area to volume ratio?
- A4.5 : 1
- B4 : 1
- C6 : 1
- D0.22 : 1
A single-celled organism gets all its oxygen by diffusion through its surface, but a large animal also needs lungs and a blood system. What is the best reason?
- AA large animal has a smaller surface area to volume ratio and longer diffusion distances
- BA large animal has a larger surface area to volume ratio, so diffusion would be too fast
- CEach cell in a large animal needs much less oxygen than a single-celled organism
- DOxygen molecules are unable to diffuse through the cells of large animals
A 2 cm agar cube containing indicator is placed in acid, which diffuses in from every face. The cube is completely colourless after 500 s. What is the mean rate of diffusion of the acid?
- A0.020 mm/s
- B0.040 mm/s
- C0.0020 mm/s
- D50 mm/s
Quiz key
In the light, photosynthesis uses up carbon dioxide inside the palisade cell, so its concentration in the air spaces is higher and it diffuses into the cell down its gradient. Water entering a root hair cell through the membrane is the special case called osmosis. Nitrate ions entering from dilute soil water move against their gradient, which is active transport. Blood pumped through capillaries is mass flow driven by the heart, not random movement of particles.
In bright light photosynthesis is faster than respiration, so oxygen is produced faster than it is used. Its concentration in the leaf becomes higher than in the air and it diffuses out through the stomata. Photosynthesis produces oxygen rather than using it, and respiration uses oxygen rather than producing it. Leaf cells respire faster than they photosynthesise only in dim light or darkness.
A higher glucose concentration outside makes the concentration gradient into the cell steeper, so the net rate of diffusion increases. Raising the concentration inside the cell makes the gradient less steep and slows entry. Cooling lowers the kinetic energy of the molecules, so they move more slowly. A smaller surface area gives less membrane for molecules to cross at the same time.
The time to turn colourless falls from 400 s to 140 s as the temperature rises, so the acid diffuses into the blocks faster: the rate increases. Saying the rate decreases confuses the falling time with a falling rate. The times clearly change, so temperature does have an effect. The rate does not double for each 10 °C: from 20 to 30 °C the time only falls from 400 s to 280 s, which would need to halve to 200 s.
Respiration produces carbon dioxide, so its concentration inside the muscle cell is higher than in the blood, and it diffuses out. Oxygen and glucose are used up by respiration, so their concentrations are lower inside the cell and they diffuse in. Starch is a large, insoluble molecule that is not found in animal cells and cannot cross the cell membrane.
Surface area = 2 × (1 × 1) + 4 × (1 × 4) = 2 + 16 = 18 cm2. Volume = 1 × 1 × 4 = 4 cm3. Ratio = 18 ÷ 4 = 4.5 : 1. The value 4 : 1 comes from forgetting the two square ends (16 ÷ 4). 6 : 1 is the ratio for a 1 cm cube, not this block. 0.22 : 1 is volume divided by surface area, the ratio the wrong way up.
As an organism gets bigger its volume grows faster than its surface area, so its surface area to volume ratio becomes smaller, and the cells in the middle are a long way from the surface. Diffusion alone would be far too slow, so gas exchange surfaces and a transport system are needed. Large animals have a smaller ratio, not a larger one. Their cells still need oxygen for respiration. Oxygen can diffuse through cells; the problem is distance, not a barrier.
Acid enters from every face, so it only has to travel half the side, 1 cm = 10 mm, to reach the centre. Rate = distance ÷ time = 10 mm ÷ 500 s = 0.020 mm/s. 0.040 mm/s uses the whole 20 mm side as the distance. 0.0020 is the rate in cm/s (1 cm ÷ 500 s) with the wrong unit. 50 mm/s divides time by distance, the wrong way up.
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.