The shape of the lesson
By the end of the lesson, learners can
- Light, carbon dioxide, temperature: Drag the yellow dot along the graph to change the light intensity, then change CO 2 and temperature (5-45 °C).
- Redox indicators: DCPIP and chloroplasts: Choose light intensity (lamps at 10, 20, 30 and 40 cm, plus a tube in foil) or light wavelength (red, blue, green, white and foil), then switch on the lamps and read the time for each tube to decolourise.
- Whole plants: the pondweed practical: Move the lamp from 5 to 50 cm, change the NaHCO 3 concentration (0-2 %) or the water bath (10-45 °C), and press record after each change.
- On the slope: the factor on the x-axis is limiting.
- On the plateau: name another factor, with evidence.
- DCPIP / methylene blue: blue to colourless when reduced.
- Light intensity ∝ 1/d2; control temperature with a heat shield.
One picture every idea returns to
A limiting factor is the factor in shortest supply, which limits the rate of a process; increasing it increases the rate. Light intensity, carbon dioxide concentration and temperature are limiting factors of photosynthesis. More light raises the rate by supplying more ATP and reduced NADP from the light-dependent stage; more carbon dioxide raises the rate of fixation by rubisco; temperature affects the enzyme-controlled light-independent stage most, with a rise up to an optimum and a fall as enzymes denature. The rate stops rising when a different factor becomes limiting. In a chloroplast suspension, electrons from the light-dependent stage reduce a redox indicator such as DCPIP or methylene blue, turning it from blue to colourless, so the time taken measures the rate at different light intensities or wavelengths. In whole plants, such as pondweed in hydrogencarbonate solution, the rate is measured as oxygen given off.
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, The factor in shortest supply.. Light intensity, carbon dioxide concentration and temperature each set an upper limit on the rate of photosynthesis. 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: Light, carbon dioxide, temperature | Drag the yellow dot along the graph to change the light intensity, then change CO 2 and temperature (5-45 °C). Project tab 6 of the lab (Photosynthesis lab, limiting factors) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Redox indicators: DCPIP and chloroplasts | Choose light intensity (lamps at 10, 20, 30 and 40 cm, plus a tube in foil) or light wavelength (red, blue, green, white and foil), then switch on the lamps and read the time for each tube to decolourise. Project tab 7 of the lab (Photosynthesis lab, DCPIP and isolated chloroplasts) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Whole plants: the pondweed practical | Move the lamp from 5 to 50 cm, change the NaHCO 3 concentration (0-2 %) or the water bath (10-45 °C), and press record after each change. Project tab 8 of the lab (Photosynthesis lab, investigating with pondweed) and let a student drive it while the class predicts. | Frayer Model |
| 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, 18 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 Whole plants: the pondweed practical, 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.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
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 |
|---|---|
| On the plateau, do not say light is limiting. | Light is no longer limiting there: increasing it has no effect. Name another factor (carbon dioxide or temperature) and back it up from the graph. |
| DCPIP is reduced when it turns colourless, not oxidised. | It accepts the electrons that would otherwise reduce NADP, so its colour change measures the light-dependent stage only, not the whole of photosynthesis. |
| Moving the lamp closer also heats the water, so temperature becomes a second independent variable. | Use a water bath, an LED lamp or a heat shield (a tank of water between lamp and plant) to control it. |
Limiting factors and investigations: worksheet
6 questions, 18 marks. Written in the style of the papers; no past paper question is reproduced.
Define the term limiting factor and name three limiting factors of photosynthesis.
Explain the effects of increasing temperature from 10 °C to 45 °C on the rate of photosynthesis of a plant in bright light and high carbon dioxide concentration.
Pondweed was placed at different distances from a lamp. Bubbles per minute: 10 cm, 48; 20 cm, 13; 30 cm, 6; 40 cm, 3. Calculate the light intensity at 20 cm as a proportion of that at 10 cm, and comment on the relationship between rate and light intensity.
Describe how you would use a chloroplast suspension and DCPIP to investigate the effect of light wavelength on the rate of the light-dependent stage.
Explain why DCPIP changes colour when a chloroplast suspension is illuminated.
A student counts bubbles from pondweed to investigate the effect of carbon dioxide concentration. Suggest three ways to improve the reliability and validity of the results.
Answer key and marking notes
- The factor in shortest supply (furthest from its optimum), which limits the rate of the process; increasing it increases the rate.
- Light intensity, carbon dioxide concentration and temperature.
- From 10 °C up to the optimum (about 25-30 °C) the rate increases.
- The light-independent stage is enzyme-controlled; molecules gain kinetic energy, giving more collisions and more enzyme-substrate complexes, for example with rubisco.
- Above the optimum the rate falls, as hydrogen and ionic bonds in enzymes break and active sites change shape (denaturation).
- Membrane proteins in the thylakoids may also be damaged, so the rate falls towards zero by about 45 °C.
- Intensity ∝ 1/d2: (10 ÷ 20)2 = 0.25 of the intensity at 10 cm.
- The rate at 20 cm is 13 ÷ 48 = 0.27 of the rate at 10 cm.
- The rate is roughly proportional to light intensity, so light is the limiting factor over this range.
- Extract chloroplasts by grinding leaves in ice-cold isotonic buffer, filtering and centrifuging; keep the suspension on ice.
- Mix equal volumes of chloroplast suspension and DCPIP in tubes; place each behind a different coloured filter (for example red, blue, green) at the same distance from the same lamp.
- Record the time for the blue colour to disappear (or the change in absorbance with a colorimeter); rate = 1 ÷ time.
- Controls: a tube in the dark (foil) and DCPIP without chloroplasts; keep temperature constant and repeat for a mean.
- Light causes photoactivation of chlorophyll, releasing electrons in the light-dependent stage (photolysis supplies electrons too).
- DCPIP accepts these electrons in place of NADP and is reduced, changing from blue to colourless.
- Collect the gas in a capillary tube / photosynthometer and measure its volume rather than counting bubbles of different sizes.
- Keep the light intensity and temperature constant (fixed lamp distance, water bath or heat shield, LED lamp).
- Allow the plant to acclimatise at each concentration before measuring; repeat each reading at least three times and calculate a mean.
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.
Look at the graph. What is the main factor limiting the rate of photosynthesis at point A and at point B?
- AA: light intensity; B: carbon dioxide concentration
- BA: carbon dioxide concentration; B: light intensity
- CA: temperature; B: light intensity
- DA: light intensity; B: temperature
In an investigation with pondweed, what is the purpose of adding sodium hydrogencarbonate to the water?
- Ato supply carbon dioxide to the plant
- Bto supply oxygen for the plant to respire
- Cto keep the water at a constant temperature
- Dto act as a redox indicator that changes colour
DCPIP is added to a chloroplast suspension in the light. Which change shows the light-dependent stage is happening?
- Athe DCPIP is reduced and changes from blue to colourless
- Bthe DCPIP is oxidised and changes from colourless to blue
- Cthe DCPIP is reduced and changes from colourless to blue
- Dthe DCPIP is oxidised and changes from blue to colourless
A lamp is 15 cm from a pondweed. To what distance must it be moved to give one quarter of the light intensity?
- A30 cm
- B60 cm
- C7.5 cm
- D22.5 cm
Why does a grower who adds extra carbon dioxide to a glasshouse often also heat it?
- Aonce carbon dioxide is no longer limiting, temperature may become the limiting factor
- Bcarbon dioxide only dissolves in leaf cells when the air is warm
- Cheating removes the extra carbon dioxide before it can harm the plants
- Dcarbon dioxide raises the optimum temperature of photosynthesis to 45 °C
Chloroplast suspensions with DCPIP were lit with different colours. Time to decolourise: blue 70 s, red 130 s, green no change after 600 s, dark no change. What is the rate in blue light, relative to red light?
- A1.9 times as fast
- B0.54 times as fast
- C60 times as fast
- Dthe same, as both decolourised
Pondweed at 25 °C gives 30 bubbles per minute with the lamp at 10 cm. When the lamp is moved to 5 cm the count rises only to 34, though light intensity has increased four times. Which is the best explanation?
- Aanother factor, such as carbon dioxide concentration, is now limiting
- Bthe plant is now saturated with oxygen and cannot release more
- Cthe lamp has cooled the water, slowing the enzymes
- Dlight intensity is proportional to d, not 1/d2
A student uses the pondweed method to investigate the effect of temperature from 10 °C to 40 °C. Which change would most improve the validity of the results?
- Ameasure the volume of gas in a capillary tube rather than counting bubbles
- Buse a brighter lamp closer to the plant at the higher temperatures
- Cuse fresh pondweed for each temperature without acclimatising it
- Dadd more sodium hydrogencarbonate as the temperature rises
Quiz key
At A both curves are rising and lie together, so more light raises the rate and extra carbon dioxide makes no difference: light limits. At B the 0.04 % curve is flat, so more light has no effect, but the 0.10 % curve is higher, so carbon dioxide limits. Swapping them misreads the slope and plateau. Temperature is the same (25 °C) for both curves, so the graph gives no evidence that it limits at either point.
Hydrogencarbonate ions release carbon dioxide in solution, so the concentration of NaHCO3 controls the carbon dioxide available. The plant produces oxygen in photosynthesis rather than needing it supplied. Temperature is controlled with a water bath, not by a solute. Redox indicators such as DCPIP are used with chloroplast suspensions, not as a carbon dioxide source.
DCPIP accepts electrons from the light-dependent stage, just as NADP would, so it is reduced; reduced DCPIP is colourless, so the blue disappears. The options with oxidation have the redox change the wrong way round. Colourless to blue is the reverse of the real colour change.
Intensity ∝ 1/d2, so doubling the distance (15 → 30 cm) gives 1/22 = 1/4 of the intensity. 60 cm is four times the distance, giving 1/16 of the intensity. 7.5 cm halves the distance, giving four times the intensity. 22.5 cm treats intensity as falling linearly with distance.
Raising carbon dioxide increases the rate only until another factor, such as temperature or light intensity, becomes limiting; raising that too gives a further increase. Carbon dioxide dissolves in the moist cell walls at any normal temperature. Heating does not remove carbon dioxide, and the grower wants it present. The optimum temperature is set by enzymes like rubisco and is not pushed up to 45 °C, where they denature.
Rate ∝ 1/time: (1/70) ÷ (1/130) = 130 ÷ 70 = 1.9. 0.54 is 70 ÷ 130, which treats time itself as the rate. 60 is the difference in seconds, not a ratio. Both tubes decolourised, but at different rates, which is what the times measure. The result matches the action spectrum: blue and red are absorbed, green is not.
At 5 cm light is no longer limiting, so the rate is held back by the supply of carbon dioxide (or by temperature). A plant does not become saturated with oxygen; it diffuses away. A closer lamp warms the water if anything, which would speed up enzymes, not slow them. Light intensity does follow the inverse square law, so a fourfold increase was supplied.
Bubbles differ in size, so measuring the volume of gas collected is a more accurate measure of the rate. Changing the lamp or the hydrogencarbonate concentration with temperature introduces a second independent variable, so the effect of temperature could not be isolated. Different pieces of pondweed vary in size and leaf area, and skipping acclimatisation lets the rate change while being measured.
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.