AQA Biology
Demonstrating photosynthesis using isolated chloroplasts
Specification references
- 3.5.1
- 8.1, 8.2
- MS 3.2
Learning outcomes
After completing the practical you will be able to:
- measure oxygen production from chloroplasts trapped in beads of sodium
- understand the process of photosynthesis
- evaluate the reliability of your results.
Background
The aim of this activity is to measure oxygen production from chloroplasts trapped in beads of sodium alginate. Oxygen is a product of photosynthesis and the rate at which it is released from leaves is an indication of the rate at which photosynthesis is occurring. It is difficult to collect the oxygen given off from intact leaves without interfering with the process of photosynthesis, so using the chloroplasts enables us to investigate the process more easily.
Method
1Put the stinging nettle or spinach leaves into the blender with the sucrose solution and liquidise them. The sucrose solution is used instead of water so that the chloroplasts do not burst as a result of osmosis.
2Take 10 cm3 of the mixture and put it into the 100 cm3 beaker. Add the sodium alginate solution and stir with the glass rod.
3Draw up some of this mixture into the plastic syringe and position the syringe vertically above the beaker of calcium chloride solution. The nozzle of the syringe should be about 10 cm above the level of the solution in the beaker. Pushing down gently on the plunger, allow a steady stream of drops to fall into the calcium chloride solution. The drops will form greencoloured beads.
4When you have made about 50 to 75 beads, leave them to harden for about 15 minutes. Remove them from the beaker with the spoon, place them in a tea strainer, and rinse with distilled water before tipping them into the petri dish.
5Fill the measuring cylinder up to the 100 cm3 mark with water. Arrange the bench lamp at a measured distance from the measuring cylinder.
6Put 5–10 beads into the measuring cylinder, switch on the bench lamp, and record the time it takes for the beads to rise to the surface. The number of beads you use will depend on the size of the beads and the diameter of the measuring cylinder.
7Repeat using fresh beads (the same number as before) and varying the distance of the bench lamp from the measuring cylinder.
Questions
1Were your beads of a uniform size? Did you find the syringe difficult to control?
2Do you think the experiment should be carried out in a darkened room? Explain your answer.
Results
Tabulate your results to show the time taken for a known number of beads to rise to the surface with the light source at measured distances from the measuring cylinder.
Light intensity can be calculated by , where d is the distance, in cm, of the lamp from the measuring cylinder. Multiplying by 1000 will give figures which are easier to plot. Plot a graph of light intensity against time taken by the beads.
Discussion
1 What are the sources of error in this experiment? Suggest ways in which these errors can be overcome.
2 How could you modify the experiment to investigate how factors other than light affect the rate of photosynthesis?
3Photosynthesis requires carbon dioxide. Suggest how you could show that carbon dioxide was being used by the chloroplasts in the alginate beads.
Safety
Use gloves when handling stinging nettles.
Use eye protection when handling chemicals.
Calcium Chloride, IRRITANT - CLEAPSS HAZCARD 19A
Equipment and materials
- About 50 freshly picked young stinging nettle leaves, or the equivalent quantity of fresh spinach
- 50 cm3 10% sucrose solution
- Blender
- 10 cm3 3% sodium alginate solution in a test tube with a stopper in it
- 100 cm3 beaker
- 5 cm3 plastic syringe
- 100 cm3 glass measuring cylinder
- Petri dish
- Glass rod
- Spoon
- Tea strainer
- Distilled water
- Bench lamp with 100 W bulb
- Stopwatch or stopclock
- Ruler
- 100 cm3 beaker containing 50 cm3
3% calcium chloride solution
IRRITANTCLEAPSS HAZCARD 19A
© Oxford University Press 2016
This resource sheet may have been changed from the original1