(9) Measuring the rate of photosynthesis - experimental method 1 measuring the volume of oxygen produced with a gas syringe

• Possible practical work you may have encountered - methods of measuring the rate of photosynthesis

  • You can investigate the need for chlorophyll for photosynthesis with variegated leaves

  • Taking thin slices of potato and apple and adding iodine to observe under the microscope - test for starch, which gives a blue colour with iodine.

  • Investigating the effects of light, temperature and carbon dioxide levels (using Canadian pondweed, Cabomba, algal balls or leaf discs from brassicas) on the rate of photosynthesis.

  • You can use computer simulations to model the rate of photosynthesis in different conditions

  • You can use sensors to investigate the effect of carbon dioxide and light levels on the rate of photosynthesis and the release of oxygen.

• You may have done/seen experiments on the rate of photosynthesis in which the volume of oxygen formed is measured with a gas syringe connected to a flask of sodium hydrogen carbonate solution (to supply the carbon dioxide) and Canadian pondweed immersed in it.

• All experimental methods depend on measuring the rate of oxygen production as a measure of the rate of photosynthesis.

  • The faster the oxygen production the faster the photosynthesis.

  • It is assumed that the rate of oxygen production is proportional to the rate of photosynthesis.

  • So, how can we measure the rate of photosynthesis?

• Next, methods of measuring the rate of photosynthesis

You can use this gas syringe system to measure the effects of changing temperature, light intensity and carbon dioxide level (via different concentrations of sodium hydrogencarbonate solution).

• 

• Method 1. Gas syringe system

  • A lamp and thermostated water bath are not shown in this diagram, but they are in the apparatus diagram for method 2.

  • There are several aquatic plants you can use, the most popular seems to Canadian pondweed (elodea canadensis), but this is regarded as an invasive species, so perhaps some other oxygenated aquatic plant should be used!

  • In this 'set-up' you measure the rate of photosynthesis by measuring the rate of oxygen production as the gas is collected in the gas syringe.

    • From the graph of volume of oxygen versus time you measure the initial gradient to calculate the rate of production of oxygen as a measure of the rate of photosynthesis.

    • The graph should be reasonably linear at first e.g. rate of photosynthesis in cm³/min.

  • You can use sodium hydrogencarbonate (NaHCO₃) as source of carbon dioxide and vary its concentration to vary the carbon dioxide concentration. You can use from 0.1% to 5% of NaHCO₃ ie 0.1g to 5g per 100 cm³ of water.

    • With increasing concentration you should see an increase in the rate of oxygen bubbles (eg cm³/min), but you must keep the temperature constant eg lab. temp. 20-25^oC, and the light intensity constant by keeping the lamp (not shown in the diagram) the same distance from the flask.

    • The light from the laboratory itself will contribute, but the total light should be constant.

    • You need to use the same quantity and batch of pondweed (or other oxygenating aquatic plant).

    • You use the same volume of water/sodium hydrogencarbonate solution.

    • Using the set-up described in the diagram, at constant temperature, constant light intensity - by using same lamp at the same distance from the flask, you can investigate the effect of the concentration of carbonate/carbon dioxide on the rate of photosynthesis.

  • To vary temperature you need to immerse the conical flask in a thermostated water bath (not shown here, see method 2. Part 10 diagram) of different, but carefully controlled constant temperatures.

    • You should be able to demonstrate a maximum ~35-40^oC i.e. the rate should be significantly lower at ~20^oC and 50^oC.

    • The concentration of NaHCO₃ and the light intensity should be both kept constant.

  • Varying the light intensity is quite difficult, you need to position a lamp at different measured distances away from the flask, but for accurate results you must take a light meter reading by the flask in the direction of the lamp - but you can still use the basic set-up of apparatus described in method 1. above.

    • A lamp in position is shown in method 2. and see the discussion on the inverse square law in method 2.

  • This simple experiment can readily show in principle the effect of changing the three controlling factors of the rate of photosynthesis.

  • Problems and errors with the method

    • Ideally the experiments should be done in the dark, with the lamp the only source of light, not very convenient in a classroom situation but it is particularly important when varying the light intensity - I don't see how you can get accurate results for light intensity though using a light meter might just ok?

    • Do you swirl the flask so the NaHCO₃ concentration remains reasonably constant?, but will the same leaf area be exposed to the light in the direction of the lamp?

    • When varying the temperature it is not easy to maintain a constant temperature - if it falls a little, you could use the average temperature, not as accurate, but better than nothing! A thermostated water bath would be ideal.

  • The above apparatus is typical of that used in rate of reaction experiments in chemistry.

  • How can we measure the speed or rate of a chemical reaction?

  • See graphs in photosynthesis Part 6.

• You can use other experiment designs to look more conveniently, and hopefully more accurately at the three factors that influence the rate of photosynthesis.

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