The shape of the lesson
By the end of the lesson, learners can
- Starch tests with a control: Choose an experiment in the sim and press Next step to work through the method.
- Pondweed and limiting factors: Pondweed gives off bubbles of oxygen in the light, so counting bubbles per minute measures the rate.
- Gas exchange in light and dark: Hydrogencarbonate indicator is red at normal carbon dioxide levels, yellow when carbon dioxide rises and purple when it falls.
- Starch test: destarch, expose, boil, hot ethanol (water bath), rinse, iodine: blue-black = starch.
- Every experiment needs a control that differs only in the factor being tested.
- Indicator: purple = less carbon dioxide, red = normal, yellow = more carbon dioxide.
- Extended: on a plateau, the x-axis factor is no longer limiting; name another factor.
One picture every idea returns to
A leaf that has photosynthesised stores starch, so testing a destarched leaf for starch with iodine shows where photosynthesis happened; experiments with controls show that chlorophyll, light and carbon dioxide are all needed. The rate of photosynthesis increases with light intensity, carbon dioxide concentration and temperature (up to an optimum). Extended: the factor in shortest supply is the limiting factor: only increasing that factor raises the rate.
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, Starch, bubbles and a colour change.. You cannot watch glucose being made, so biologists look for evidence: starch in a leaf, oxygen bubbles from pondweed, a change in the colour of an indicator. 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: Starch tests with a control | Choose an experiment in the sim and press Next step to work through the method. Project tab 4 of the lab (Testing leaves for starch) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Pondweed and limiting factors | Pondweed gives off bubbles of oxygen in the light, so counting bubbles per minute measures the rate. Project tab 5 of the lab (Rate of photosynthesis and limiting factors) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Gas exchange in light and dark | Hydrogencarbonate indicator is red at normal carbon dioxide levels, yellow when carbon dioxide rises and purple when it falls. Project tab 6 of the lab (Hydrogencarbonate indicator) 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, 24 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 Gas exchange in light and dark, 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 |
|---|---|
| Ethanol is flammable, so it is heated in a water bath, never over a flame. | And the iodine test detects starch, not glucose: you are using stored starch as evidence that photosynthesis happened. |
| On the plateau of a light intensity graph, light is no longer the limiting factor. | Something else (carbon dioxide or temperature) is limiting there. Students often name light because it is the variable on the x-axis. |
| Plants respire all the time, in the light as well as the dark. | In the light the indicator turns purple because photosynthesis uses carbon dioxide faster than respiration produces it, not because respiration stops. |
Investigating photosynthesis: worksheet
6 questions, 24 marks. Written in the style of the papers; no past paper question is reproduced.
Describe how to test a leaf for starch, giving the reason for each step.
A destarched plant with variegated leaves was left in bright light for six hours. A leaf was then tested for starch. Describe the expected result and explain what it shows. Identify the control.
Plan an investigation into the effect of light intensity on the rate of photosynthesis of pondweed.
The table shows the rate of oxygen production by pondweed at two carbon dioxide concentrations.
| Light intensity / arbitrary units | 1 | 2 | 4 | 6 | 8 |
|---|---|---|---|---|---|
| Rate at 0.01% carbon dioxide | 8 | 15 | 20 | 20 | 20 |
| Rate at 0.04% carbon dioxide | 8 | 16 | 30 | 38 | 38 |
Extended. Identify the limiting factor at light intensity 1 and at light intensity 8 for 0.01% carbon dioxide, and explain your answers.
Three sealed tubes of red hydrogencarbonate indicator were set up: A with an aquatic plant in light, B with the plant in a light-proof cover, C with no plant in light. Predict the colour of each after several hours and explain.
Extended. A tomato grower heats a greenhouse in winter but finds the crop still grows slowly on dull days. Explain why, and suggest what the grower could do.
Answer key and marking notes
- Boil the leaf in water for about a minute to kill the cells and stop reactions, making it permeable to iodine.
- Place it in hot ethanol in a water bath to remove the chlorophyll (ethanol is flammable, so no flame).
- Rinse in water to soften the brittle leaf, then spread it on a white tile.
- Add iodine solution: blue-black shows starch is present, orange-brown shows it is absent.
- The parts of the leaf that were green turn blue-black; the white parts stay orange-brown.
- Only the green parts contain chlorophyll, so only they could photosynthesise and make starch.
- This shows chlorophyll is needed for photosynthesis.
- The white parts of the same leaf (which had the same light and carbon dioxide) act as the comparison / control.
- Place a piece of pondweed in a test-tube of water containing sodium hydrogencarbonate (a source of carbon dioxide).
- Put a lamp at a measured distance, for example 10, 20, 30, 40 and 50 cm.
- Allow a few minutes to adjust, then count the bubbles of oxygen released in one minute (or collect the gas and measure its volume).
- Keep carbon dioxide concentration and temperature constant (use a water bath or a heat shield between lamp and tube); use the same piece of pondweed.
- Repeat each distance and calculate a mean; work in a dark room so the lamp is the only light source.
- At light intensity 1: light is limiting.
- Increasing carbon dioxide makes no difference there (both rates are 8), but increasing light increases the rate.
- At light intensity 8 (0.01%): carbon dioxide is limiting.
- Increasing light from 4 to 8 has no effect (rate stays 20), but raising carbon dioxide to 0.04% increases the rate to 38.
- A turns purple: photosynthesis is faster than respiration, so the plant removes carbon dioxide from the water.
- B turns yellow: no light, so no photosynthesis; respiration releases carbon dioxide.
- C stays red: there is no organism to change the carbon dioxide concentration.
- C is the control, showing that the colour changes in A and B are caused by the plant.
- On dull days light intensity is low, so light is the limiting factor.
- Raising the temperature does not increase the rate when another factor is limiting.
- The grower could use artificial lighting to raise light intensity (and add carbon dioxide once light is no longer limiting).
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.
A leaf that has been photosynthesising is boiled, decolourised in ethanol and tested with iodine solution. What colour shows that starch is present?
- Ablue-black
- Borange-brown
- Cbrick red
- Dpurple
A destarched plant has a strip of foil fixed across one leaf and is left in bright light. Which part of the investigation acts as the control?
- Athe uncovered part of the same leaf
- Bthe part of the leaf under the foil
- Ca leaf from a plant kept in the dark
- Dthe strip of foil removed after testing
To show that carbon dioxide is needed for photosynthesis, a plant is sealed in a clear bag with soda lime. What is the soda lime for?
- AIt absorbs carbon dioxide from the air in the bag
- BIt releases carbon dioxide into the air in the bag
- CIt removes starch from the leaves before testing
- DIt keeps the air in the bag at a constant temperature
The table shows the number of oxygen bubbles per minute released by pondweed with a lamp at different distances. What is the mean rate at 20 cm?
- A21 bubbles per minute
- B22 bubbles per minute
- C19 bubbles per minute
- D63 bubbles per minute
Tubes of red hydrogencarbonate indicator were sealed and left for 6 hours. Which tube shows that the plant's photosynthesis and respiration were happening at the same rate?
- AZ
- BY
- CW
- DX
Extended: the graph shows the rate of photosynthesis against light intensity at two carbon dioxide concentrations. What is the limiting factor at point Q?
- Acarbon dioxide concentration
- Blight intensity
- Cthe amount of chlorophyll
- Dthe amount of water
Extended: a grower adds extra carbon dioxide to a greenhouse on a very dull winter day, but the rate of photosynthesis of the crop does not increase. What is the best explanation?
- ALight intensity is the limiting factor
- BCarbon dioxide is the limiting factor
- CThe extra carbon dioxide closes the stomata
- DPhotosynthesis does not happen in winter
Pondweed is kept at a high light intensity and plenty of carbon dioxide. The rate of photosynthesis rises from 15 °C to 35 °C, then falls sharply at 45 °C. Why does it fall?
- AThe enzymes controlling photosynthesis are denatured
- BThe light intensity falls at high temperatures
- CCarbon dioxide becomes the limiting factor
- DThe chlorophyll absorbs too much light energy
Quiz key
Iodine turns blue-black with starch. Orange-brown is the colour of iodine when no starch is present. Brick red is a positive Benedict's test for reducing sugar, and purple is a positive biuret test for protein.
The uncovered part had light but was otherwise identical, so comparing it with the covered part shows the effect of light. The covered part is the test, not the control. A leaf from another plant in the dark differs in more than one way, and the foil itself is not a comparison.
Soda lime absorbs carbon dioxide, so the plant in the bag has none and makes no starch. A substance releasing carbon dioxide would be used for the control, if anything. Starch is removed by destarching in the dark, and soda lime does not control temperature.
(22 + 19 + 22) ÷ 3 = 63 ÷ 3 = 21. 22 is the most common value (the mode), not the mean. 19 is just the lowest trial, and 63 is the total before dividing by three.
In dim light (Z) photosynthesis uses carbon dioxide as fast as respiration releases it, so the level and the colour stay red. Y is also red, but it has no plant, so it is the control. W turned purple because photosynthesis was faster than respiration, and X turned yellow because the plant only respired in the dark.
At Q more light gives no increase in rate, so light is not limiting; raising the carbon dioxide concentration raises the rate to R, so carbon dioxide is limiting. Light intensity is limiting only on the rising part, such as P. Chlorophyll and water are not varied here, so the graph gives no evidence that they limit the rate.
On a dull day light is in shortest supply, so it limits the rate and adding carbon dioxide has no effect. If carbon dioxide were limiting, adding it would increase the rate. Extra carbon dioxide does not close stomata in this way, and plants still photosynthesise in winter whenever there is light.
Photosynthesis is controlled by enzymes. Above the optimum their active sites change shape and they are denatured, so the rate falls. The lamp gives the same light intensity at any water temperature. A limiting factor gives a plateau, not a sharp fall, and chlorophyll cannot absorb too much light in this way.
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.