Light intensity, carbon dioxide concentration and temperature each set an upper limit on the rate of photosynthesis. Whichever is furthest from ideal limits the rate, and practical investigations with chloroplasts, redox indicators and pondweed let you measure each effect.
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.
Drag the yellow dot along the graph to change the light intensity, then change CO2 and temperature (5-45 °C). The panel names the limiting factor. Start at the default settings, where CO2 is limiting and the graph has levelled off, then compare the blue high-CO2 curve and the orange warmer curve.
| Factor | Effect of increasing it | Why |
|---|---|---|
| light intensity | rate rises in proportion at low intensity, then levels off | more photoactivation, so more ATP and reduced NADP for the Calvin cycle; the plateau shows another factor has become limiting |
| carbon dioxide concentration | rate rises, then levels off | more CO2 for rubisco to fix with RuBP, so more GP and TP; at about 0.04 % in air CO2 is often the limiting factor on a bright day |
| temperature | rate rises up to an optimum, then falls | the light-independent stage is controlled by enzymes such as rubisco: more kinetic energy means more enzyme-substrate complexes, but above the optimum the enzymes denature |
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.
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. In the lab, the 10 cm tube clears in about 67 s, 20 cm in about 267 s, 30 cm in about 607 s and 40 cm in about 1112 s: doubling the distance roughly quarters the light and the rate. With colours, blue (about 69 s) and white (about 83 s) are fastest, red takes about 134 s, green is by far the slowest (about 833 s) and the dark tube shows no change.
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.
Move the lamp from 5 to 50 cm, change the NaHCO3 concentration (0-2 %) or the water bath (10-45 °C), and press record after each change. At 25 °C and 1 % NaHCO3 the count is only about 6 bubbles per minute at 20 cm, about 25 at 10 cm, and about 32 at 5 cm, where CO2 starts to limit. Use the 1/d2 column to plot rate against light intensity.
| Variable investigated | How it is changed | Key variables to control |
|---|---|---|
| light intensity | distance of the lamp, d; intensity ∝ 1/d2 | temperature (water bath, LED or heat shield), CO2 (same NaHCO3 concentration) |
| carbon dioxide concentration | concentration of sodium hydrogencarbonate solution | light intensity (fixed distance), temperature |
| temperature | thermostatically controlled water bath | light intensity, NaHCO3 concentration |
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.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the limiting factors simulator at the starting settings, the curve has levelled off and the panel says carbon dioxide is limiting. What happens if you drag the light dot further right?
In the DCPIP simulator, why does the tube under green light take by far the longest to decolourise?
In the DCPIP simulator, what is the purpose of the tube wrapped in foil?
In the pondweed simulator the lamp moves from 20 cm to 10 cm. Why does the bubble count rise about four times?
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
A plant is in bright light at 0.04 % carbon dioxide and 25 °C. Raising the temperature to 30 °C has no effect on the rate, but raising the carbon dioxide to 0.1 % doubles it. What was the limiting factor?
A lamp is moved from 40 cm to 10 cm from a pondweed. By what factor does the light intensity increase?
Why are chloroplasts for a DCPIP investigation extracted in ice-cold isotonic buffer solution?
At low light intensity, raising the temperature from 15 °C to 25 °C hardly changes the rate of photosynthesis. Which is the best explanation?
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