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This lesson covers the required core practical for investigating how light intensity affects the rate of photosynthesis, as required by the Edexcel GCSE Combined Science specification (1SC0). You need to describe the method using pondweed, explain how the inverse square law applies, identify variables and describe how to present and interpret the results.
To investigate the effect of light intensity on the rate of photosynthesis by measuring the volume of oxygen produced (or the number of bubbles released) by an aquatic plant (pondweed) at different distances from a light source.
As light intensity increases (i.e. the lamp is moved closer to the pondweed), the rate of photosynthesis will increase. This is because light provides the energy for photosynthesis. Since light intensity follows the inverse square law (intensity ∝ 1/d²), halving the distance should quadruple the light intensity.
| Equipment | Purpose |
|---|---|
| Pondweed (e.g. Elodea or Cabomba) | The photosynthesising organism |
| Beaker of water | Keeps the pondweed submerged and collects bubbles |
| Lamp (bench lamp with known wattage) | Provides the light source |
| Ruler or metre rule | Measures the distance between the lamp and the pondweed |
| Stopwatch | Times each measurement period (e.g. 1 minute) |
| Sodium hydrogencarbonate (NaHCO₃) added to the water | Provides a constant supply of dissolved CO₂ so it is not a limiting factor |
| Thermometer | Monitors water temperature |
| Screen or cardboard (optional) | Blocks ambient light from the surroundings |
| Gas syringe or inverted measuring cylinder (optional, for volume method) | Collects and measures the volume of gas produced |
graph TD
A["Set up pondweed in beaker with NaHCO₃ solution"] --> B["Place lamp at measured distance (e.g. 10 cm)"]
B --> C["Wait 2 min for acclimatisation"]
C --> D["Count bubbles for 1 minute"]
D --> E["Repeat twice more → calculate mean"]
E --> F["Move lamp to next distance"]
F --> C
E -->|"All distances tested"| G["Plot graph of results"]
| Variable | Type | Details |
|---|---|---|
| Distance of lamp from pondweed (and therefore light intensity) | Independent variable (the one you change) | Measured in cm; converted to light intensity using 1/d² |
| Number of bubbles per minute (or volume of gas) | Dependent variable (the one you measure) | Count bubbles or measure gas volume in cm³ |
| Temperature of water | Control variable | Monitor with a thermometer; use a heat shield (e.g. a beaker of water between the lamp and the pondweed) to absorb heat from the lamp |
| CO₂ concentration | Control variable | Add the same amount of NaHCO₃ to the water in each test |
| Type and length of pondweed | Control variable | Use the same piece of pondweed (or same species and similar size) for all tests |
| Time for each count | Control variable | Always count for exactly 1 minute |
| Colour/wavelength of light | Control variable | Use the same lamp throughout |
Exam Tip: The most important control variable is temperature. The lamp produces heat as well as light. Place a transparent container of water (a "heat shield") between the lamp and the beaker to absorb infrared radiation without blocking visible light.
| Distance (cm) | 1/d² (light intensity, arbitrary units) | Bubbles — trial 1 | Bubbles — trial 2 | Bubbles — trial 3 | Mean bubbles per minute |
|---|---|---|---|---|---|
| 10 | 0.0100 | ||||
| 15 | 0.0044 | ||||
| 20 | 0.0025 | ||||
| 25 | 0.0016 | ||||
| 30 | 0.0011 | ||||
| 35 | 0.00082 | ||||
| 40 | 0.000625 |
Light intensity is not directly proportional to distance. It follows the inverse square law:
Light intensity∝d21
To plot a graph that tests for a directly proportional relationship between light intensity and the rate of photosynthesis, plot:
If light intensity is the limiting factor, you should get a straight line through the origin when plotting rate against 1/d².
If the graph curves and levels off, another factor has become limiting (e.g. temperature or CO₂ concentration).
| Rule | Details |
|---|---|
| Axes labelled with variable name and unit | e.g. "Light intensity (1/d² in cm⁻²)" and "Mean bubbles per minute" |
| Suitable scale | Use at least half the graph paper for the data range |
| Points plotted accurately | Use small crosses (×) or dots with circles |
| Line of best fit | Draw a smooth curve or straight line that follows the trend; do not join dot-to-dot |
| Title | Descriptive title: "Graph showing the effect of light intensity on the rate of photosynthesis" |
| Observation | Interpretation |
|---|---|
| Straight line through the origin (rate vs 1/d²) | Rate is directly proportional to light intensity → light is the limiting factor |
| Line curves and plateaus at high light intensity | Another factor (CO₂ or temperature) has become the limiting factor |
| Anomalous result (one point far from the line) | Possible error — could be caused by miscounting bubbles or temperature fluctuation |
Exam Tip: When describing the trend, always use the correct scientific language: "As light intensity increases, the rate of photosynthesis increases because more light energy is available for the light-dependent reactions in the chloroplasts." Then explain why it plateaus: "At higher light intensities, another factor such as CO₂ concentration becomes the limiting factor."
| Source of error | How to improve |
|---|---|
| Counting bubbles is subjective — bubbles vary in size | Use a gas syringe to measure the actual volume of gas produced (more accurate) |
| Lamp produces heat — temperature rises at closer distances | Place a transparent heat shield (beaker of water) between the lamp and the pondweed |
| Ambient light from the room affects results | Carry out the experiment in a dark room or use a cardboard screen around the setup |
| Bubbles may stick to the pondweed or beaker wall | Gently tap the beaker or use a capillary tube to guide bubbles |
| CO₂ may be used up over time | Use excess NaHCO₃ and replace the solution between trials |
| Pondweed may vary in health | Use a fresh, healthy piece of pondweed; allow it to photosynthesise for a few minutes before starting |
| Hazard | Risk | Precaution |
|---|---|---|
| Hot lamp | Burns | Do not touch the bulb; allow to cool before moving |
| Water near electrical equipment | Electrical shock | Keep water away from the plug and cable; dry hands before handling the lamp |
| Broken glass (beaker) | Cuts | Handle glassware carefully; clear up broken glass immediately |
| NaHCO₃ solution | Low hazard — irritant to eyes | Wear eye protection; wash hands after use |
A student places a lamp 20 cm from pondweed and counts 30 bubbles per minute. The lamp is then moved to 10 cm.
Predict the new rate if light is the limiting factor:
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