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Photosynthesis: Key skills

The Hill reaction

Robert Hill discovered that isolated chloroplasts were capable of carrying out photosynthesis and releasing oxygen if an electron acceptor (oxidising agent) is present. This is now called the Hill reaction. There are a number of chemicals that can substitute for the naturally occurring acceptor of photosynthesis, which is called NADP. As an example there is a blue dye called DCPIP (2,6 dichlorophenolindophenol) that will turn colourless when reduced. The progress of the reaction can either be followed by timing how long the dye takes to change colour or measuring the decrease in absorbance in a colorimeter. Isolated chloroplasts can be used to measure the rate of photosynthesis.

Measuring the effect of light intensity

There are a number of different ways light intensity can be changed. The distance of a lamp from the plant can be changed, (light intensity is proportional to 1/ `d^2` where `d` is the distance). Neutral stop filters can be obtained from photographic shops to decrease the light intensity. The voltage supplied to the lamp can be varied to alter its intensity. It is important to note that 40W bench lamps are not bright enough to carry out this investigation. Tungsten or halogen lamps of 150W are recommended. Low energy bulbs emit a restricted number of wavelengths and are therefore not useful for photosynthesis experiments. The room that the experiment is carried out in should also be in darkness to exclude external light sources. Most lamps emit heat and this can alter the temperature of the experiment. For this reason a heat shield such as a tank of water is often placed next to the plant to prevent an unwanted rise in temperature. Test tubes can also be stood in beakers of water to reduce temperature fluctuations. The temperature should be measured at intervals during the experiment.

It is important that carbon dioxide does not become a limiting factor so it is usual to increase the carbon dioxide concentration in the water by adding sodium hydrogen carbonate to the water.

Measuring the effect of carbon dioxide concentration

The carbon dioxide concentration can be altered in a solution, by making a range of sodium hydrogenate solutions e.g. 0-0.4% sodium hydrogen carbonate and then measuring the rate of oxygen production from an aquatic plant in these solutions. Isolated chloroplasts or immobilised algae could also be used. All other experimental variables such as light intensity, wavelength of light, temperature, and mass of plant material must remain fixed.

Measuring the effect of temperature on the rate of photosynthesis.

The temperature of the water in which the aquatic plants, immobilized algae or isolated chloroplasts can be altered and the rate of oxygen evolution measured. The range 5-40℃ is appropriate. The plant must be allowed to equilibrate at each new temperature before the rate is measured. All other control variables must remain constant.

Measuring the effect of light quality (wavelength)

Coloured filters can be obtained and used around test tubes or in front of lamps. The transmission qualities of filters can be obtained. It is likely that filters will also affect light intensity.