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Photosynthesis » Required Practical: Light Intensity and Photosynthesis

What you'll learn this session

Study time: 30 minutes

AQA spec: 8.2.6 (Required practical 6)

How to set up the pondweed investigation and measure the oxygen it gives off
Which variables to change, measure and control, including the heat shield
How to calculate rates and plot a graph of light intensity against rate
The errors, improvements and safety points examiners look for

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The aim of the practical

In this required practical you investigate the effect of light intensity on the rate of photosynthesis. You use an aquatic plant such as pondweed. Pondweed gives off bubbles of oxygen as it photosynthesises, so you can count or measure the gas to find the rate.

Start with a hypothesis built from what you already know: as light intensity increases, the rate of photosynthesis increases, until another factor becomes limiting. The practical tests that idea. The theory of rate and limiting factors is in the lessons Rate of Photosynthesis and Limiting Factors and Greenhouses, so here we focus on how to do the investigation well.

Key terms:

Heat shield: a large beaker of water placed between the lamp and the pondweed to soak up the heat from the lamp.
Repeat: doing the measurement again at the same light intensity so you can find a mean and spot odd results.

The apparatus and method

🔧 Apparatus

Pondweed, a boiling tube or small beaker, water containing dissolved carbon dioxide, a lamp, a metre ruler, a thermometer, a large beaker of water as a heat shield, a stopwatch.

💡 Variables

Independent: distance of the lamp (light intensity). Dependent: oxygen produced in a set time. Control: temperature, carbon dioxide, same pondweed, same time.

Method:

Cut a piece of pondweed and place it in a boiling tube of water with some sodium hydrogencarbonate dissolved in it. This gives the plant a supply of carbon dioxide.
Stand the boiling tube in the large beaker of water. This beaker is the heat shield. Place the lamp next to the beaker.
Measure the starting temperature of the water with a thermometer.
Set the lamp a measured distance from the pondweed, for example 10 cm.
Leave the pondweed for 2 minutes to settle at that light intensity.
Count the bubbles given off in 1 minute, or collect the gas and measure its volume.
Repeat the count at the same distance, then move the lamp to a new distance such as 20, 30, 40 and 50 cm and repeat.
Check the temperature each time. Change the water in the heat shield if it warms up.

Changing the distance of the lamp changes the light intensity. The closer the lamp, the brighter the light reaching the pondweed.

Controlling the other variables

Light intensity must be the only thing that changes. Otherwise you cannot be sure what caused any change in rate.

Temperature: a lamp gives out heat as well as light, and a warmer plant would photosynthesise faster. The heat shield and thermometer keep the temperature steady, so a change in rate is due to light and not warmth.
Carbon dioxide: using the same amount of sodium hydrogencarbonate in the water keeps the carbon dioxide supply the same each time.
The plant: use the same piece of pondweed every time, so the amount of chlorophyll stays the same.
Time: count for the same length of time at each distance.
Other light: block out room light as far as you can, because it adds to the lamp's light.

Measuring the rate

You can measure the oxygen in two ways.

⚪ Counting bubbles

Quick and simple, but bubbles vary in size and some are small enough to miss, so it is only a rough measure.

📏 Measuring volume

Collect the gas in an upturned measuring cylinder or a gas syringe and read its volume. This is more accurate than counting bubbles.

The rate is the amount of oxygen produced in a set amount of time:

rate = oxygen produced ÷ time taken

Worked example

At a distance of 20 cm, the pondweed produced 1.5 cm³ of oxygen in 5 minutes.
Rate = 1.5 ÷ 5 = 0.3 cm³ per minute.

Because the rate changes slightly from one count to the next, you need multiple samples at each light intensity. Take the readings several times, then work out the mean.

Worked example: finding the mean

At 30 cm the volumes of oxygen in 3 minutes were 0.9, 1.0 and 0.8 cm³.
Mean = (0.9 + 1.0 + 0.8) ÷ 3 = 0.9 cm³.
Rate = 0.9 ÷ 3 = 0.3 cm³ per minute.

Recording and presenting the results

Record your observations of gas production in a table with a column for each repeat, the mean, and the rate. Give units in the column headings, not in the body of the table.

Then plot a graph of light intensity against rate of photosynthesis:

Put the independent variable (light intensity or distance) on the horizontal axis.
Put the rate on the vertical axis.
Choose scales that use more than half of the graph paper, with equal steps.
Plot each point with a small cross and draw a smooth curve or line of best fit.

A typical graph rises as the light gets brighter, then levels off when another factor, such as carbon dioxide or temperature, becomes limiting. How to read these graphs is covered in Rate of Photosynthesis.

Higher tier only

Light intensity depends on the square of the distance from the lamp, so some investigations plot rate against 1 ÷ distance². The maths is in Limiting Factors and Greenhouses.

Errors and improvements

Counting bubbles is not accurate: bubbles vary in size and some are missed, so measure the volume of gas instead, or use a gas syringe.
Room light: do the experiment in a dim room or shield the apparatus.
Heating from the lamp: use a heat shield, or an LED lamp that gives out less heat.
Not enough time to settle: give the pondweed a few minutes at each new distance before counting.
Too few results: repeat at each distance and calculate a mean.
Anomalies: spot any result that does not fit the pattern and repeat it.

Safety and ethics

Keep electrical equipment and the lamp away from water, and take care that the lamp does not get hot enough to burn.
Handle glassware carefully and clear up spills straight away.
Wash your hands after touching pondweed and the water.
Use the living pondweed with care. Do not damage it more than you need to, and dispose of it as your teacher says rather than tipping it into a pond or drain, because it can spread.

Common mistakes

Writing that you "change the light" without saying how. Say you change the distance of the lamp and measure it with a ruler. Another mistake is forgetting to control temperature, which is the whole reason for the heat shield. Many students also forget that counting bubbles once is not enough: repeat and take a mean.

Exam-style question

A student investigated the effect of light intensity on the rate of photosynthesis of pondweed. The lamp was moved to different distances from the pondweed. The student counted the bubbles of gas given off in 2 minutes.

(a) Name the gas in the bubbles. [1 mark]

(b) The lamp gave out heat. Describe how the student could stop this from changing the results. [2 marks]

(d) Suggest one way to improve the accuracy of the gas measurement. [1 mark]

Model answer

(a) Oxygen (1).
(b) Place a large beaker of water between the lamp and the pondweed as a heat shield (1) and use a thermometer to check the temperature stays the same (1).
(c) 36 ÷ 2 = 18 bubbles per minute (1).
(d) Measure the volume of gas, for example with a gas syringe, instead of counting bubbles (1).

Exam tip

For (b), name the piece of apparatus and say what it does. Just writing "keep the temperature the same" repeats the question and earns no mark.

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