Investigating Light Intensity and Measuring Photosynthesis

$$\text{Rate of oxygen production} = \frac{\text{volume of oxygen collected}}{\text{time}}$$

For example, measuring volume in cm³ and time in minutes gives a rate in cm³/min.

Setting up the light-intensity investigation

AQA required practical activity 5 investigates the effect of light intensity on photosynthesis using an aquatic organism such as pondweed. Moving a lamp changes the light reaching the plant: a nearer lamp usually provides greater light intensity.

Use a fresh piece of pondweed, a boiling tube held in a rack, sodium hydrogencarbonate solution, an LED lamp, a ruler and a stopwatch. Sodium hydrogencarbonate supplies carbon dioxide for photosynthesis. A thermometer can be used to check that the solution stays at the same temperature.

Place the pondweed in the solution and position the lamp a measured distance from it. Measure consistently from the lamp to the pondweed, rather than changing the reference point between readings.

Allow the plant time to adjust to the light level before counting. For example, wait five minutes, then count the bubbles produced in 60 seconds. Repeat the count twice more at the same distance, giving three readings.

Move the lamp to a new distance, allow the plant to adjust again and repeat the measurements. Suitable distances are 10, 20, 30 and 40 cm. Record all readings in a table and calculate the mean at each distance.

Making it a fair comparison

The independent variable is lamp distance, used to change light intensity. The dependent variable is the measured rate of oxygen production.

Keep other conditions constant so that differences in rate can be attributed to the change in light:

• Use the same piece of pondweed throughout, keeping its size and amount of chlorophyll unchanged.
• Keep the concentration and volume of sodium hydrogencarbonate solution the same, so carbon dioxide availability is comparable.
• Keep temperature constant. An LED lamp releases relatively little heat; alternatively, a glass tank of water between a lamp and the plant can reduce heating.
• Use the same lamp and the same counting time, with other room lighting kept unchanged.

Temperature control is particularly important. A lamp that heats the water as it moves closer would change both light intensity and temperature, making the effect of light alone harder to identify.

Repeats reveal how consistent the readings are. Calculate a mean to reduce the influence of random variation. If one reading is very different from the others, investigate and take further readings rather than simply ignoring it.

Turning measurements into rates

Each count was made over one minute.

At 20 cm, the mean count is $(24 + 25 + 26)/3 = 25$ bubbles. Because the counting period was one minute, the mean rate is 25 bubbles/min. For a different counting period, divide the mean count by that time rather than treating the count itself as the rate.

Plotting and reading a graph

Plot lamp distance on the horizontal axis and mean bubbling rate on the vertical axis. Label both axes with units and choose evenly spaced scales that use a substantial part of the available graph area. For these data, horizontal intervals of 10 cm and vertical intervals of 10 bubbles/min are convenient.

Each table row becomes a coordinate pair. For example, the 20 cm row becomes the point $(20, 25)$. Plot all the points accurately. For continuous measurements such as distance, draw an appropriate smooth curve of best fit rather than a bar chart or a dot-to-dot line that follows every small variation.

Data for Lamp distance and mean bubbling rate

To translate the graph back into numbers, locate a distance on the horizontal axis, move vertically to the plotted point or fitted curve, then read across to the rate axis. At 30 cm, these practice data give 12 bubbles/min. You can also work backwards: a rate of 25 bubbles/min corresponds to a distance of 20 cm.

The rate falls as the lamp moves further away. Less light reaches the pondweed, so photosynthesis slows and fewer oxygen bubbles are released per minute. Describe the trend using values as well as words: increasing distance from 10 to 20 cm reduces the rate from 50 to 25 bubbles/min in this example.

On a graph with light intensity, rather than distance, on the horizontal axis, the rate usually rises as light intensity increases. If the curve levels off, increasing light intensity is no longer increasing the rate: another factor is restricting photosynthesis. The horizontal-axis label therefore matters when interpreting the direction of a trend.

Higher Tier: distance and the inverse square law

For the inverse square model, light intensity is proportional to the reciprocal of the square of the distance from the light source:

$$I \propto \frac{1}{d^2}$$

Here, $I$ is light intensity and $d$ is distance. The symbol $\propto$ means ‘is proportional to’. Doubling the distance makes the intensity one-quarter as large; tripling the distance makes it one-ninth as large. Halving the distance makes the intensity four times as large.

You can process distance readings by calculating $1/d^2$:

At 20 cm, $1/20^2 = 1/400 = 0.0025$. This is a relative intensity value, not an absolute measurement in lux. Use the same distance unit throughout a comparison.

These calculated values can replace distance on the horizontal axis of a photosynthesis-rate graph. They allow you to examine the relationship between rate and relative light intensity more directly. However, the inverse square law describes light intensity, not automatically photosynthesis rate: the rate need not keep increasing if another factor becomes limiting.

• Change lamp distance; measure oxygen bubbles produced in a fixed time.
• Allow adjustment after each distance change.
• Repeat readings and calculate a mean.
• Keep pondweed, carbon dioxide availability, temperature, lamp and counting time constant.
• Use an LED lamp or reduce lamp heating with a water-filled glass tank.

Measuring rate

$$\text{Rate} = \frac{\text{oxygen volume or bubble count}}{\text{time}}$$

Typical units: cm³/min or bubbles/min. Collecting gas in a gas syringe improves on bubble counting because bubble volumes vary.

Graphs

• Horizontal axis: distance (cm), or relative light intensity.
• Vertical axis: mean photosynthesis rate, with units.
• Use even scales, plot accurately and draw an appropriate best-fit curve.
• Read coordinates to convert graphs into numerical values.
• Greater distance usually means lower light intensity and a lower rate.
• A plateau on a rate–light intensity graph means more light is no longer increasing the rate.

Higher Tier: inverse square law

$$I \propto \frac{1}{d^2}$$

Double distance → quarter intensity. Triple distance → one-ninth intensity.

Calculate $1/d^2$ using consistent distance units; report relative intensity in arbitrary units. Photosynthesis rate does not necessarily follow the same relationship.