top of page

B8 RP Required Practical: Rate of Photosynthesis

GCSE Combined Science · AQA · Chapter B8: Photosynthesis

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: aQA required practical: investigating the effect of light intensity on the rate of photosynthesis using pondweed. 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

  • Describe the pondweed method, including how temperature and carbon dioxide are controlled
  • Identify the independent, dependent and control variables, and the hazards
  • Calculate means and rates, and relative light intensity 1/d² (Higher tier)
  • Plot and interpret a graph of the results, and evaluate the method

Key words

  • Independent variable: The variable you change: light intensity (by changing the lamp distance).
  • Dependent variable: The variable you measure: rate of oxygen production.
  • Control variable: Kept the same so the test is fair, e.g. temperature, CO₂ concentration.
  • Anomaly: A result that does not fit the pattern. Leave it out of the mean.
  • Repeatable: You get similar results when you repeat the experiment yourself.
  • Resolution: The smallest change an instrument can show, e.g. 0.1 cm³ on a gas syringe.

Aim and set-up

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

Apparatus: pondweed, boiling tube and clamp, paper clip, sodium hydrogencarbonate solution, beaker of water, thermometer, LED lamp, metre ruler, stopclock. A gas syringe or measuring cylinder can be used to collect the gas.

Method

  • 1. Cut a 6–8 cm piece of pondweed under water, with the stem cut at an angle. Attach a paper clip to the other end so it stays down.
  • 2. Put it, cut end up, in a boiling tube of sodium hydrogencarbonate solution and clamp the tube.
  • 3. Place a beaker of water between the lamp and the tube as a heat shield. Turn off the room lights.
  • 4. Put the tube 50 cm from the lamp. Switch on and wait 2 minutes for the pondweed to adjust.
  • 5. Count the bubbles in 1 minute. Repeat twice more and check the temperature with the thermometer.
  • 6. Repeat at 40, 30, 20 and 10 cm. Record results in a table and calculate means.
  • Independent; In this practical Light intensity (changed by moving the lamp: distance d)
  • Dependent; In this practical Rate of photosynthesis: bubbles per minute, or cm3 of oxygen per minute
  • Control; In this practical Temperature (heat shield, LED lamp, thermometer); CO₂ concentration (same NaHCO₃ solution); same piece of pondweed; same counting time; no other light
  • Hot lamp; Risk Burns; Control Use an LED lamp; do not touch the bulb
  • Mains electricity near water; Risk Electric shock; Control Keep lamp and plug away from water; mop spills; dry hands
  • Glassware / scalpel; Risk Cuts; Control Handle with care; cut on a tile away from your hand

Processing results

Worked example: means and an anomaly

Bubbles per minute (made-up data):
10 cm: 62, 58, 60 20 cm: 41, 44, 26 30 cm: 27, 25, 26 40 cm: 17, 19, 18 50 cm: 12, 11, 13
At 20 cm the 26 is anomalous, so mean = (41 + 44) ÷ 2 = 42.5 ≈ 42.
At 10 cm: mean = (62 + 58 + 60) ÷ 3 = 60.

Answer: Means: 60, 42, 26, 18 and 12 bubbles per minute

Worked example: rate from a volume

In 4 minutes, 1.6 cm3 of gas collects in a gas syringe. Rate = volume ÷ time = 1.6 ÷ 4 = 0.40 cm3/min.

Answer: 0.40 cm3 per minute

Relative light intensity (Higher tier)

Light intensity is inversely proportional to distance squared, so you can work out a relative light intensity = 1 / d² and plot rate against it. Use the same units for every distance (metres are used below).

Worked example: relative light intensities

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)

Evaluating, and another method

  • Bubbles vary in size; fast bubbles are hard to count; Improvement Collect the gas in a gas syringe and measure its volume
  • Lamp warms the water; Improvement LED lamp; heat shield; check the temperature
  • Light from windows or room lights; Improvement Work in a darkened room
  • CO₂ may run out; Improvement Use fresh, excess sodium hydrogencarbonate solution
  • Pondweed not adjusted to new light; Improvement Wait 2 minutes or more before each count

Leaf discs: small discs cut from spinach leaves are made to sink in sodium hydrogencarbonate solution in a syringe. In light, oxygen made by photosynthesis collects in the air spaces and the discs float. The time taken to float is a measure of the rate.

Common mistakes

  • Giving “distance” as the IV without linking it to light intensity.
  • Forgetting to wait after moving the lamp.
  • Including an anomalous result in a mean.
  • Saying light intensity halves when the distance doubles. It quarters.

Revision questions

Try these, then check your answers below each one.

Question 1. The diagram shows a set-up for investigating the effect of light intensity on the rate of photosynthesis. What is the job of part B, a beaker of water between the lamp and the pondweed? (Choose from: It supplies carbon dioxide to the pondweed, It absorbs heat from the lamp so the temperature of the pondweed stays constant, It makes the light brighter, It collects the oxygen given off.)

Answer: It absorbs heat from the lamp so the temperature of the pondweed stays constant

Working: The water acts as a heat shield. It stops the lamp warming the pondweed, so temperature is a control variable.

Question 2. In this investigation, what is the independent variable? (Choose from: The number of bubbles per minute, Light intensity, changed by moving the lamp, The temperature of the water, The length of the pondweed.)

Answer: Light intensity, changed by moving the lamp

Working: Light intensity is changed by changing the distance between the lamp and the pondweed. Bubbles per minute is the dependent variable.

Question 3. Name the gas that forms the bubbles given off by the pondweed.

Answer: oxygen

Working: Oxygen is made in photosynthesis and is given off as bubbles from the cut stem.

Related lessons

Continue in the full course

This lesson is section B8 RP (AGCSC0805) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

bottom of page