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Sheet 23  PDF
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Photosynthesis...
using algae wrapped in jelly balls
(from Osmosis 23, Autumn 2002) Photosynthesis . . . using algae wrapped in jelly ballsAlgae can be considered as one-celled plants, and they usually live in water. You are going to use algae to look at the rate of photosynthesis. The algae are tiny and are difficult to work with directly in the water so the first part of the practical involves ‘immobilising’ the algae. This effectively traps large numbers of algal cells in ‘jelly like’ balls so that we can keep them in one place and not lose them. We use sodium alginate to help make the jelly. Sodium alginate is not harmful to the algae. When these algae are ‘wrapped up’ in the jelly balls they are excellent to use in experiments on photosynthesis. These algal balls are:
When you have made your algal balls you can use them to determine the rate of carbon dioxide absorption - which indicates how fast photosynthesis is taking place. You can detect carbon dioxide absorption using hydrogencarbonate indicator. Hydrogencarbonate indicator is very sensitive to changes in carbon dioxide level. The indicator is orange/red in colour when equilibrated with atmospheric air. It changes to yellow when more carbon dioxide is added and changes through red to a deep purple colour when carbon dioxide is removed. The diagram below shows these colours.
Doing investigations with algal balls When you place the algal balls in hydrogencarbonate indicator solution, the colour of the indicator changes from orange / red to purple. This is because the algae are taking carbon dioxide out of the indicator thereby lowering the concentration in the indicator as they use carbon dioxide in photosynthesis. Here is an outline
of how you could investigate the effect of light intensity on the
rate of photosynthesis. You will need to decide on details of quantities
and how to vary the light intensity.
A colorimeter can be used to measure the amount of light absorbed by the coloured solution. Different colours absorb different wavelengths of light to a different extent, so we shine light of a single wavelength through the mixture. It is best to use green light (wavelength 550 nm), because the colours of the indicator (purples, reds and yellows) absorb green light quite well.
The experiments described here have several aims:
Possible
investigations
These instructions to students for making the algal balls are quite prescriptive but the way that the pupil takes this forward into investigations of photosynthesis has been presented in a deliberately ‘open ended’ way. This is so that students have the opportunity to plan and carry out their own investigation. There are a number of variables that they can consider and different ways that the method can be adapted to investigate each variable. Here are some suggestions: 1. Light intensity
2. Wavelength of light - use coloured filters from photographic shops or gels from theatre lighting companies. (If students are to analyse the results well they will need to be aware of different light sources producing light of different wavelengths and they will need to know what wavelengths of light each of the filters lets through.) 3. Temperature - set up water baths under a bank of lights and float the sealed containers in the water with light shining from above. 4. Number
of algal cells - make several sets of algal balls with different
numbers of algae in them. This
Use of the colorimeter - A suitable colorimeter for pupils is the CO7500 from Biochrom Ltd. (22 Cambridge Science Park, Milton Road, Cambridge CB4 OFJ. Tel: 01223 423723. enquiries@biochrom.co.uk) This colorimeter has a digital readout and is simple to use. It is reliable, robust and the results are repeatable. It also remains standardised for 10 minutes at a time. It is important to measure absorbance (not transmission) in these experiments since there is a linear response between absorbance and pH of the indicator over the range studied.* Algal enrichment medium can be purchased from Sciento. Sciento are at:
An article, which describes in more detail experiments that were carried out with Scenedesmus algal balls, was published in 2004 in School Science Review, 85 (312), 37-45 and a copy is available here. Dr Eldridge has produced some additional technical notes to support this protocol and these are available here. Notes for technicians are available here. Resources which may be useful when designing experiments to investigate the colour of light, and light intensity on the rate of photosynthesis are available here. Dr Deborah Eldridge is Head of Science at King Ecgbert School, Sheffield. Dr Eldridge carried out this work during her SAPS-Robinson College Schoolteacher Fellowship in Plant Science. SAPS is very grateful to King Ecgbert School, Sheffield for releasing her.
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