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. Each investigation needs a control so you know the result was caused by the factor you changed. Identifying and explaining limiting factors is Extended content.
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.
Choose an experiment in the sim and press Next step to work through the method. Each experiment compares part of a leaf, or a whole plant, that lacks one factor with a control that has 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.
Pondweed gives off bubbles of oxygen in the light, so counting bubbles per minute measures the rate. In the sim you can move the lamp (10 to 60 cm), change the carbon dioxide level and the temperature, and drag the dot along the graph.
| Factor | How to vary it | Effect on rate |
|---|---|---|
| Light intensity | move the lamp closer or further away | increases, then levels off |
| Carbon dioxide | add different amounts of sodium hydrogencarbonate to the water | increases, then levels off |
| Temperature | water bath at set temperatures | increases to an optimum (about 35 °C in the sim), then falls as enzymes denature |
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.
Hydrogencarbonate indicator is red at normal carbon dioxide levels, yellow when carbon dioxide rises and purple when it falls. In the sim four sealed tubes are left for 6 hours: A (plant, light), B (plant, dark), C (no plant, light) and D (plant, dim light under gauze).
| Tube | Final colour | Why |
|---|---|---|
| A: plant in light | purple | photosynthesis faster than respiration, so carbon dioxide is taken in |
| B: plant in dark | yellow | no photosynthesis; respiration gives out carbon dioxide |
| C: no plant (control) | red | nothing changes the carbon dioxide level |
| D: plant in dim light | red | photosynthesis and respiration at about the same rate |
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.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the starch test sim, why is the leaf put in hot ethanol?
In the carbon dioxide experiment, which plant is the control?
With the sim's starting settings (lamp 20 cm, carbon dioxide 0.5, 25 °C), which factor does it show as limiting?
In the indicator sim, what colour is tube B (plant in the dark) after 6 hours?
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Why is a plant kept in the dark for 48 hours before a starch test investigation?
Extended: on a graph of rate against light intensity, the curve levels off at high light intensity. What does this show?
Pondweed gives 40 bubbles per minute with a lamp at 10 cm. Assuming light is limiting and rate is proportional to light intensity, what is the expected rate at 20 cm?
Tubes of hydrogencarbonate indicator, each with pondweed, are left in bright light, dim light and darkness. Which result shows photosynthesis and respiration at equal rates?
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