Investigating Photosynthesis

This lesson covers the practical investigation of photosynthesis, including Core Practical 5 from the Edexcel GCSE Biology (1BI0) specification. You need to understand the method, variables, analysis and evaluation of the pondweed experiment, as well as the starch test and the variegated leaf experiment.

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Core Practical 5: Investigating the Effect of Light Intensity on the Rate of Photosynthesis

Aim

To investigate how light intensity affects the rate of photosynthesis in an aquatic plant (such as Elodea or Cabomba — commonly called pondweed).

Principle

When an aquatic plant photosynthesises, it releases oxygen gas as a by-product. By measuring the rate of oxygen production (either by counting bubbles or collecting the gas), we can determine the rate of photosynthesis at different light intensities.

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Equipment

• A beaker of water containing sodium hydrogen carbonate (NaHCO₃) solution — this provides a constant supply of dissolved CO₂
• A piece of Elodea (pondweed) — freshly cut at an angle to expose the xylem
• A bench lamp (with a known wattage bulb)
• A ruler (to measure the distance between the lamp and the plant)
• A stopwatch
• A thermometer
• Optional: a syringe or gas collection apparatus to collect oxygen

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Method

1. Set up the apparatus: Place a piece of freshly cut Elodea in a beaker of sodium hydrogen carbonate solution. The cut end should face upwards so bubbles can be seen rising.
2. Position the lamp at a measured distance from the beaker (e.g., 10 cm).
3. Wait 2 minutes for the plant to acclimatise to the light conditions.
4. Count the number of oxygen bubbles produced in one minute. Record the result.
5. Repeat the count two more times at the same distance and calculate a mean.
6. Move the lamp to a new distance (e.g., 20 cm, 30 cm, 40 cm, 50 cm) and repeat steps 3–5 for each distance.
7. Calculate the relative light intensity at each distance using the inverse square law: light intensity = 1/d².

Exam Tip: Always state that you allow the plant to acclimatise before starting measurements. This ensures the plant has reached a steady rate of photosynthesis at the new light level.

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Variables

Variable type	Variable	Details
Independent	Light intensity (distance from lamp)	Changed by moving the lamp to different distances
Dependent	Rate of photosynthesis	Measured by counting oxygen bubbles per minute or collecting gas volume
Controlled	Temperature	Use a heat shield (glass screen) or large water bath to absorb heat from the lamp
Controlled	CO₂ concentration	Add sodium hydrogen carbonate (NaHCO₃) to the water to keep CO₂ constant
Controlled	Type and length of plant	Use the same species and similar-sized pieces of pondweed
Controlled	Colour/wavelength of light	Use the same lamp throughout
Controlled	Time period	Count bubbles over the same duration each time (e.g., 1 minute)

Exam Tip: A common error is to say the independent variable is "distance". While you change the distance, what you are really investigating is light intensity, which you calculate using 1/d². State the independent variable as "light intensity" but explain that you change it by altering the distance.

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Using 1/d² to Calculate Relative Light Intensity

Since we cannot easily measure light intensity in lux without specialist equipment, we use the inverse square law as an approximation:

Distance from lamp (cm)	d²	Relative light intensity (1/d²)
10	100	0.0100
15	225	0.0044
20	400	0.0025
30	900	0.0011
40	1600	0.000625
50	2500	0.0004

When plotting a graph, plot 1/d² (relative light intensity) on the x-axis and rate of photosynthesis (bubbles per minute) on the y-axis.

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Expected Results

• As the lamp is moved closer to the plant (higher light intensity), the rate of oxygen bubble production increases.
• Beyond a certain point, the rate levels off — another factor (e.g., CO₂ concentration or temperature) has become limiting.

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Alternative Method: Measuring Gas Volume

Instead of counting bubbles (which can be inaccurate because bubbles vary in size), you can:

1. Attach an inverted measuring cylinder or syringe to collect the gas produced.
2. Measure the volume of gas collected in a set time period (e.g., 5 minutes).
3. This is more accurate because it measures total volume rather than counting individual, potentially different-sized bubbles.

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Sources of Error and Improvements

Source of error	Why it is a problem	How to improve
Bubbles vary in size	Each bubble may contain a different volume of gas, making counting inaccurate	Collect gas in a syringe and measure total volume instead
Heat from the lamp	The lamp may raise the temperature of the water, changing another variable	Place a glass heat shield (beaker of water) between the lamp and the plant
Background light	Light from windows or room lights adds to the experimental light	Conduct the experiment in a darkened room
Air bubbles vs oxygen	Some bubbles may be air from the cut stem, not oxygen from photosynthesis	Allow the plant to acclimatise and discard the first set of bubbles
Counting errors	Difficult to count fast bubbles accurately	Use a video camera and count bubbles from the recording, or collect gas volumetrically

Exam Tip: When evaluating practical work, always suggest improvements that are specific and explain why they improve accuracy or reliability. Saying "repeat the experiment" is not enough — explain that repeating and calculating a mean reduces the effect of anomalous results and improves reliability.

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Testing a Leaf for Starch

Starch is a product of photosynthesis (glucose is converted to starch for storage). Testing for starch confirms that photosynthesis has occurred.

Method