Photosynthesis required practical

In the photosynthesis required practical, you shine a lamp on a piece of pondweed and measure how much oxygen it gives off at different distances from the lamp. The closer the lamp, the higher the light intensity, and the faster the pondweed photosynthesises, so the more oxygen bubbles it produces each minute. It is required practical 6 in AQA's GCSE Biology and required practical 5 in Combined Science Trilogy.

Aim

AQA's specification states the aim as: investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed. Photosynthesis makes oxygen, so measuring the oxygen given off is a way to measure the rate. Our guide to photosynthesis explains the reaction and the other factors that limit its rate.

Equipment

AQA's handbook lists:

• a beaker of water and a filter funnel, with plasticine to raise the funnel;
• a measuring cylinder (1 cm³ or 10 cm³);
• a freshly cut 10 cm piece of pondweed;
• a light source;
• a metre rule and a stopwatch.

Its technical notes add that Cabomba is no longer available to schools because it is classed as an invasive species, so other pondweeds such as Elodea are used instead. It recommends a high-intensity light source, of at least 1000 lumens, because low-energy bulbs may not give enough light for measurable photosynthesis.

Method

These steps follow the student sheet in AQA's handbook.

1. Put the 10 cm piece of pondweed, cut end at the top, into a beaker of water.
2. Cover it with an upside-down filter funnel, raised off the bottom of the beaker with plasticine.
3. Fill the measuring cylinder with water and place it upside down over the end of the funnel, so it collects the gas.
4. Use the metre rule to place the beaker 1 m (100 cm) from the light source.
5. Start the stopwatch. In three minutes, count the bubbles released and record the volume of gas collected in the measuring cylinder.
6. Move the light so the beaker is 80 cm away, refill the measuring cylinder, and repeat.
7. Repeat at 60, 40 and 20 cm.

If no bubbles appear even close to the light, AQA's handbook suggests cutting a few millimetres off the end of the pondweed, or using a freshly cut piece.

Variables

Temperature matters because photosynthesis is controlled by enzymes. AQA's handbook notes that standing the pondweed in at least 250 cm³ of water usually means no heat shield is needed, and that the water temperature can be checked with a thermometer.

Results table

How to analyse the results

Work out the rate at each distance:

For example, if 1.8 cm³ of gas is collected in 3 minutes, the rate is 1.8 ÷ 3 = 0.60 cm³/min. Plot the rate against the distance, or better, against the light intensity. If you repeat each distance, the Practical Data Lab averages the repeats and draws error bars for you.

Light intensity and the inverse square law

Moving the lamp changes the light intensity, but not in proportion to the distance. It follows an inverse square law: halve the distance and the light is four times as bright, so a relative light intensity can be worked out as 1 ÷ d². AQA's specification asks Higher tier students to understand and use this inverse square law.

Moving the lamp from 40 cm to 20 cm makes the light (40 ÷ 20)² = 4 times as intense. A graph of rate against relative light intensity usually rises steeply at first, then levels off when another factor, such as carbon dioxide concentration or temperature, becomes the limiting factor.

Here is a quick check.

Risks

Errors and improvements

• Counting bubbles is unreliable. Bubbles vary in size and can be missed when they come quickly. Collecting the gas and measuring its volume is more accurate, which is the evaluation question AQA's worksheet asks.
• Temperature changes. A lamp close to the beaker can warm the water. Use a large volume of water, check it with a thermometer, or use an LED light, which gives out less heat.
• Background light. Light from windows or ceiling lights adds to the lamp. Darken the room.
• Carbon dioxide running out. Over a long experiment the water can run short of carbon dioxide. Adding a little sodium hydrogencarbonate is a common way to keep it high.
• Small volumes. AQA's handbook warns that measurable volumes of oxygen are hard to achieve in a lesson. Repeats and a sensitive measuring cylinder help.
• Not all the gas is oxygen. The bubbles also contain some dissolved gases from the water, so the volume slightly overestimates the oxygen produced.

Common mistakes

• Saying light intensity is proportional to distance. It is proportional to 1 ÷ d².
• Forgetting to control temperature. A warmer lamp at shorter distances would also speed up photosynthesis.
• Calling the gas carbon dioxide. Photosynthesis gives off oxygen.
• Comparing totals over different times. Divide by the time to get a rate.

Practise biology in a short timed paper

GCSE Biology Higher and A-level Biology: Year 2 each give you a short timed paper every day, with 8 questions worth 25 to 35 marks across the course, so photosynthesis and required practicals turn up regularly but not in every paper. The Cell Biology course has an interactive photosynthesis model.

Read next

Biology · CellsWhere does photosynthesis take place? Equation and stagesWhere photosynthesis takes place, from leaf to chloroplast, the equation, reactants and products, both stages, the rate, and how it links to respiration.