8.5 Required Practicals: Photosynthesis

A free GCSE Biology lesson for AQA (8461) from OBA School: required practical 6: investigating the effect of light intensity on the rate of photosynthesis. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Plan and describe the pondweed practical, including how to control variables
• Process results: mean, rate and relative light intensity using the inverse square law
• Plot and interpret a graph of rate against distance or light intensity
• Evaluate the method and suggest improvements

Key words

• Independent variable: The variable you change: here, light intensity (by changing distance from the lamp).
• Dependent variable: The variable you measure: here, the rate of oxygen production.
• Control variable: A variable kept the same so the test is fair, e.g. temperature, CO₂ concentration.
• Repeatable: Similar results are obtained when the same person repeats the experiment.
• Anomaly: A result that does not fit the pattern. Leave it out when calculating a mean.
• Precision: How close repeat readings are to each other (and to the mean).

Aim and equipment

Aim: to find out how light intensity affects the rate of photosynthesis in pondweed (such as Elodea or Cabomba). The oxygen given off is used to measure the rate.

You need: pondweed, boiling tube, sodium hydrogencarbonate solution, beaker of water, thermometer, LED lamp, metre ruler, stopclock. To collect the gas you could use an upturned measuring cylinder, a gas syringe or a capillary tube.

Method

• 1. Fill a boiling tube with sodium hydrogencarbonate solution and stand it in a beaker of water at room temperature.
• 2. Cut a fresh 8–10 cm piece of pondweed. Put it in the tube, cut end up, so bubbles come from the cut.
• 3. Place the lamp 10 cm from the pondweed. Leave for 5 minutes so the pondweed adjusts to the new light level.
• 4. Count the bubbles given off in 1 minute (or collect the gas for 5 minutes and measure its volume).
• 5. Repeat the count twice more at this distance.
• 6. Move the lamp to 20, 30, 40 and 50 cm and repeat steps 3 to 5. Check the water temperature each time.

• Independent; In this practical Light intensity (changed by moving the lamp: distance from lamp)
• Dependent; In this practical Rate of photosynthesis (bubbles per minute, or cm3 of oxygen per minute)
• Control; In this practical Temperature (water bath; LED lamps give out little heat), CO₂ concentration (same NaHCO₃ solution), same piece / type / length of pondweed, time for each count, room lights off

Risk assessment

• Hot lamp can cause burns: use an LED lamp and do not touch the bulb.
• Mains electricity near water can cause a shock: keep the lamp and plug away from water and mop up spills.
• Glassware can break and cut: handle with care and report breakages.

Processing the results

Worked example: mean and anomaly

Bubbles per minute at each distance (made-up class data):
10 cm: 48, 51, 50 20 cm: 33, 31, 35 30 cm: 20, 22, 9 40 cm: 13, 12, 14 50 cm: 8, 9, 7
At 30 cm the 9 is anomalous (far from 20 and 22), so leave it out: mean = (20 + 22) ÷ 2 = 21.
At 10 cm: mean = (48 + 51 + 50) ÷ 3 = 49.7 ≈ 50.

Answer: Means: 50, 33, 21, 13, 8 bubbles per minute

Worked example: rate from a volume

In 5 minutes, 1.2 cm3 of gas collects. Rate = volume ÷ time = 1.2 ÷ 5 = 0.24 cm3/min.

Answer: 0.24 cm3 per minute

Light intensity and the inverse square law (Higher tier)

Light intensity is not proportional to distance. It is inversely proportional to distance squared, so you can work out a relative light intensity = 1 / d² and plot rate against that instead.

Worked example: relative light intensities

Convert each distance to metres, square it, then find 1 divided by the answer.
10 cm: 1 / 0.10² = 100 20 cm: 1 / 0.20² = 25 30 cm: 1 / 0.30² = 11.1
40 cm: 1 / 0.40² = 6.25 50 cm: 1 / 0.50² = 4

Answer: 100, 25, 11.1, 6.25 and 4 (arbitrary units)

Tip: Moving the lamp from 10 cm to 20 cm does not halve the light. It cuts it to one quarter.

Evaluating the method

• Bubbles vary in size, so counting is not accurate; Improvement Collect the gas and measure its volume (gas syringe or capillary tube)
• Fast bubbling is hard to count; Improvement Use a longer time, or a tally counter
• Lamp warms the water; Improvement Use an LED lamp; check temperature; use a water bath
• Light from the room or window; Improvement Do the experiment in a darkened room
• CO₂ may run out over time; Improvement Use fresh, excess sodium hydrogencarbonate solution
• Some gas dissolves, or is not pure oxygen; Improvement Accept as a systematic error; compare results only within the experiment

Common mistakes

• Stating “distance” as the independent variable without linking it to light intensity. Say “light intensity (distance from the lamp)”.
• Forgetting the waiting time after moving the lamp, so the rate has not settled.
• Including an anomalous result in the mean.
• Saying the rate is proportional to distance. It decreases as distance increases, and light intensity follows 1/d².

Revision questions

Try these, then check your answers below each one.

Question 1. In the pondweed practical, what is the independent variable? (Choose from: Temperature of the water, Type of pondweed, Number of bubbles, Light intensity (distance from the lamp).)

Answer: Light intensity (distance from the lamp)

Working: You change the light intensity by moving the lamp.

Question 2. Why is the pondweed placed in sodium hydrogencarbonate solution? (Choose from: To keep it warm, To supply carbon dioxide so it is not limiting, To kill bacteria, To make the bubbles bigger.)

Answer: To supply carbon dioxide so it is not limiting

Working: Sodium hydrogencarbonate releases CO₂, so CO₂ is not a limiting factor.

Question 3. True or false? You should start counting bubbles straight after moving the lamp.

Answer: False

Working: Wait about 5 minutes so the rate settles at the new light intensity.

Related lessons

• Previous lesson: 8.4 Making the Most of Photosynthesis
• Next lesson: 9.1 Aerobic Respiration
• In this chapter: 8.1 Photosynthesis
• In this chapter: 8.2 The Rate of Photosynthesis
• In this chapter: 8.3 How Plants Use Glucose

Continue in the full course

This lesson is section 8.5 (ARPBH0805) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.