Required Practical: Investigating Light Intensity and Photosynthesis

What the practical measures

Photosynthesis uses light energy to make glucose from carbon dioxide and water, releasing oxygen. An aquatic plant such as pondweed releases some of that oxygen as visible bubbles. Counting bubbles over a fixed time therefore gives an estimate of its rate of photosynthesis. The question in AQA required practical activity 6 is whether changing the light intensity reaching the pondweed changes that rate.

A lamp provides the light. Moving it farther from the pondweed reduces the light reaching the plant, so lamp–pondweed distance is a practical way to vary light intensity. Distance is measured directly with a ruler; light intensity is not directly measured in this arrangement. Count the bubbles produced in one minute and report the result in bubbles per minute. More generally,

$\text{rate of bubble production} = \frac{\text{number of bubbles counted}}{\text{time taken}}.$

Use minutes as the time unit if the answer is required in bubbles per minute.

Setting up a fair investigation

Place a freshly cut piece of pondweed in a boiling tube containing a fixed volume of sodium hydrogen carbonate solution. This solution supplies carbon dioxide. Keep its concentration and volume the same throughout. Position the cut end of the pondweed upwards so that the bubbles can be seen leaving it, and support the tube in a rack. Put the lamp at a measured distance from the pondweed; a ruler and stopwatch are needed to measure distance and counting time.

The independent variable is the light intensity changed by moving the lamp; the dependent variable is the measured bubble rate. Use the same piece of pondweed and lamp, the same counting interval and the same solution at each distance. Temperature also affects photosynthesis, and a lamp may warm the tube: keep temperature as steady as possible rather than assuming that moving the lamp changes only light. Keep water away from electrical wiring and avoid touching a hot lamp.

Collecting rates at different distances

One supported method begins with the tube 10 cm from the lamp. Allow the pondweed about five minutes to adjust, then count the bubbles released in one minute. Repeat the one-minute count twice more at that distance and calculate the mean of the three counts. Move the lamp to further measured distances, such as 20, 30 and 40 cm, allowing the plant to adjust and repeating the three counts each time. Repeats make an unusual count easier to spot and give a more dependable mean.

A suitable results table has distance between pondweed and light source (cm) in its first column. The heading number of bubbles per minute spans four columns: readings 1, 2 and 3, then their mean. Each row represents one distance. Because each reading lasts one minute, its bubble count is also its rate in bubbles per minute. If counts are taken over another length of time, divide the number of bubbles by the time in minutes before comparing rates.

Presenting and interpreting the results

Plot the independent variable on the horizontal axis and the mean rate, in bubbles per minute, on the vertical axis. Choose even scales that use the available graph space and plot each point accurately. If the horizontal axis is distance (cm), increasing distance would usually be associated with fewer bubbles per minute: less light reaches the pondweed. The graph below shows how such results could be displayed.

Data for Distance from lamp and estimated photosynthesis rate

Read a graph back into numbers by locating a distance on the horizontal axis and reading the corresponding rate from the vertical axis. You can also compare two plotted rates to describe how much the bubble count changes. At Higher Tier, relative light intensity may instead be calculated using $1/d^2$, where $d$ is the lamp–pondweed distance, and plotted on the horizontal axis. This changes the horizontal quantity, not the bubble measurements.

Bubble counting has a limitation: bubbles can have different volumes, and rapid bubbles may be missed. A greater bubble count therefore suggests a greater rate of oxygen production but is not an exact measurement of oxygen volume. Measuring the volume of gas collected over a fixed time is another way to estimate the rate.

Method and variables

Vary light intensity by changing the measured lamp–pondweed distance. Count oxygen bubbles from pondweed for a fixed time. Keep the plant, solution, counting time and temperature as constant as possible. Repeat at each distance and calculate a mean.

Rate and results

$\text{bubble rate} = \text{bubbles counted} \div \text{time in minutes}$, in bubbles per minute. Record distance (cm), three one-minute counts and their mean. Plot the independent variable on the horizontal axis and mean bubble rate on the vertical axis; label units and use an appropriate scale. Greater distance usually means lower and a lower rate. At Higher Tier, relative light intensity can be represented by .