Biology · Simulation and virtual lab Free
Photosynthesis: bubbles from pondweed
Count the oxygen bubbles given off by pondweed and find out which factor limits the rate of photosynthesis.
- You will learn to
- Count oxygen bubbles to measure the rate of photosynthesis.
- Investigate light intensity, carbon dioxide, temperature and light colour.
- Identify the limiting factor from the shape of the graph.
- You will measure
- Bubbles per minute at different lamp distances; rate against 1/d².
- Time needed
- About 30–40 min
- Equipment
- Nothing extra. Real version: pondweed, boiling tube, clamp stand, lamp, metre rule, NaHCO₃, thermometer, stopwatch.
- Price
- Free: no payment, no account.
Virtual lab
Follow the steps with the simulation above (go back to the simulation ↑). Each time you tap Record, a row is added to the table and a point to the graph.
Aim
To investigate the effect of light intensity, carbon dioxide concentration, temperature and light colour on the rate of photosynthesis.
What to do
- Look at the apparatus: pondweed (cut end up) in a boiling tube of water held by a clamp, a lamp standing on a metre rule, a thermometer, a bubble counter and a stopwatch. Keep 1 % sodium hydrogencarbonate, 25 °C and white light.
- Put the lamp at 10 cm from the plant (read it on the metre rule). Tap Count bubbles for 1 minute, then Record.
- Repeat at 15, 20, 30, 40, 60 and 100 cm.
- Look at the graph of rate against light intensity (1/d²). Where does it level off?
- Repeat a few distances with no sodium hydrogencarbonate, and at 40 °C. Compare the curves.
- Keep the lamp at 20 cm and try red, blue and green light. Which colour gives the fewest bubbles?
Results
Questions
Write the word equation for photosynthesis.
carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll).
Why do we use 1/d² as a measure of light intensity?
The light from a small lamp spreads out, so its intensity is inversely proportional to the square of the distance.
What is a limiting factor?
The factor that is in shortest supply and so holds back the rate of a process. Increasing it increases the rate; increasing the other factors does not.
Why does the rate level off at high light intensity?
Light is no longer limiting. The rate is now limited by carbon dioxide concentration or temperature.
Why are there very few bubbles in green light?
Chlorophyll absorbs red and blue light well but reflects most green light, so little energy is available for photosynthesis.
Why does the rate fall at 40 °C and above?
Photosynthesis is controlled by enzymes. Above the optimum temperature the enzymes start to be denatured.
For teachers: expected results and how the model works
Rate (bubbles/min) = 60 × L/(L + 25) × C/(C + 0.4) × f(T), with L the relative light intensity (100 at 10 cm), C the hydrogencarbonate percentage + 0.05, and f(T) doubling every 10 °C up to an optimum of 30 °C, then falling. Bubble counts have natural random variation, as in real pondweed experiments: repeat and take averages.
Exam practice Exam set
National-exam style questions on this simulation (16 marks). Try each one on paper first, then open the marking scheme.
Write the word equation and the balanced chemical equation for photosynthesis. (3 marks)
Marking scheme: Carbon dioxide + water → glucose + oxygen (in light, with chlorophyll) (1). 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (1 for formulae, 1 for balancing).
A lamp was placed at different distances from pondweed. At 10 cm, 60 bubbles/min; at 20 cm, 16 bubbles/min. (a) Calculate 1/d² for each distance. (b) Explain the relationship shown. (5 marks)
Marking scheme: (a) 0.0100 cm⁻² (1) and 0.0025 cm⁻² (1). (b) Light intensity is proportional to 1/d² (1); halving the distance makes the light four times as intense (1), so the rate is about four times faster: light is the limiting factor (1).
Explain what is meant by a limiting factor, using the levelling of the graph of rate against light intensity. (3 marks)
Marking scheme: A limiting factor is the factor in shortest supply that controls the rate (1). At low light the rate rises with light, so light is limiting (1); where the graph levels off, another factor such as CO₂ concentration or temperature is now limiting (1).
Why is the pondweed placed in a solution of sodium hydrogencarbonate? (2 marks)
Marking scheme: It supplies carbon dioxide (1) so that CO₂ is not the limiting factor (1).
Suggest why counting bubbles is not a very accurate way to measure the rate, and give a better method. (3 marks)
Marking scheme: Bubbles are not all the same size (1) and some oxygen dissolves in the water (1); collect the gas in a capillary tube / gas syringe and measure its volume (1).
Donat Sciences and MathsBiology · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
Photosynthesis: bubbles from pondweed
Aim: To investigate the effect of light intensity, carbon dioxide concentration, temperature and light colour on the rate of photosynthesis.
Apparatus (real laboratory): pondweed, boiling tube, clamp stand, lamp, metre rule, NaHCO₃, thermometer, stopwatch.
What to do (with the simulation on www.donatstem.co.rw)
- Look at the apparatus: pondweed (cut end up) in a boiling tube of water held by a clamp, a lamp standing on a metre rule, a thermometer, a bubble counter and a stopwatch. Keep 1 % sodium hydrogencarbonate, 25 °C and white light.
- Put the lamp at 10 cm from the plant (read it on the metre rule). Tap Count bubbles for 1 minute, then Record.
- Repeat at 15, 20, 30, 40, 60 and 100 cm.
- Look at the graph of rate against light intensity (1/d²). Where does it level off?
- Repeat a few distances with no sodium hydrogencarbonate, and at 40 °C. Compare the curves.
- Keep the lamp at 20 cm and try red, blue and green light. Which colour gives the fewest bubbles?
Results
| # | d (cm) | 1/d² (cm⁻²) | Bubbles in 1 min (1) | Bubbles in 1 min (2) | Mean bubbles/min |
|---|
| 1 | | | | | |
| 2 | | | | | |
| 3 | | | | | |
| 4 | | | | | |
| 5 | | | | | |
| 6 | | | | | |
| 7 | | | | | |
Graph
Plot the rate (bubbles per minute) on the vertical axis against light intensity 1/d². Mark where the graph starts to level off.
Questions
- Write the word equation for photosynthesis. [2]
- Why do we use 1/d² as a measure of light intensity? [2]
- What is a limiting factor? [2]
- Why does the rate level off at high light intensity? [2]
- Why are there very few bubbles in green light? [2]
- Why does the rate fall at 40 °C and above? [2]
- Write the word equation and the balanced chemical equation for photosynthesis. [3]
- A lamp was placed at different distances from pondweed. At 10 cm, 60 bubbles/min; at 20 cm, 16 bubbles/min. (a) Calculate 1/d² for each distance. (b) Explain the relationship shown. [5]
- Explain what is meant by a limiting factor, using the levelling of the graph of rate against light intensity. [3]
- Why is the pondweed placed in a solution of sodium hydrogencarbonate? [2]
- Suggest why counting bubbles is not a very accurate way to measure the rate, and give a better method. [3]
Donat Sciences and MathsBiology · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
Marking scheme: Photosynthesis: bubbles from pondweed
Expected results
Rate (bubbles/min) = 60 × L/(L + 25) × C/(C + 0.4) × f(T), with L the relative light intensity (100 at 10 cm), C the hydrogencarbonate percentage + 0.05, and f(T) doubling every 10 °C up to an optimum of 30 °C, then falling. Bubble counts have natural random variation, as in real pondweed experiments: repeat and take averages.
Answers
- Write the word equation for photosynthesis. [2]
carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll).
- Why do we use 1/d² as a measure of light intensity? [2]
The light from a small lamp spreads out, so its intensity is inversely proportional to the square of the distance.
- What is a limiting factor? [2]
The factor that is in shortest supply and so holds back the rate of a process. Increasing it increases the rate; increasing the other factors does not.
- Why does the rate level off at high light intensity? [2]
Light is no longer limiting. The rate is now limited by carbon dioxide concentration or temperature.
- Why are there very few bubbles in green light? [2]
Chlorophyll absorbs red and blue light well but reflects most green light, so little energy is available for photosynthesis.
- Why does the rate fall at 40 °C and above? [2]
Photosynthesis is controlled by enzymes. Above the optimum temperature the enzymes start to be denatured.
- Write the word equation and the balanced chemical equation for photosynthesis. [3]
Carbon dioxide + water → glucose + oxygen (in light, with chlorophyll) (1). 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (1 for formulae, 1 for balancing).
- A lamp was placed at different distances from pondweed. At 10 cm, 60 bubbles/min; at 20 cm, 16 bubbles/min. (a) Calculate 1/d² for each distance. (b) Explain the relationship shown. [5]
(a) 0.0100 cm⁻² (1) and 0.0025 cm⁻² (1). (b) Light intensity is proportional to 1/d² (1); halving the distance makes the light four times as intense (1), so the rate is about four times faster: light is the limiting factor (1).
- Explain what is meant by a limiting factor, using the levelling of the graph of rate against light intensity. [3]
A limiting factor is the factor in shortest supply that controls the rate (1). At low light the rate rises with light, so light is limiting (1); where the graph levels off, another factor such as CO₂ concentration or temperature is now limiting (1).
- Why is the pondweed placed in a solution of sodium hydrogencarbonate? [2]
It supplies carbon dioxide (1) so that CO₂ is not the limiting factor (1).
- Suggest why counting bubbles is not a very accurate way to measure the rate, and give a better method. [3]
Bubbles are not all the same size (1) and some oxygen dissolves in the water (1); collect the gas in a capillary tube / gas syringe and measure its volume (1).