A novel approach to photosynthesis practicals

The topic of photosynthesis is rarely greeted with enthusiasm by students. This complex process requires a mature understanding of many abstract scientific ideas and it is not easy to illustrate in a practical way. In order to try to change students’ attitudes to photosynthesis, a new practical procedure is proposed for key stage 4. It involves the immobilisation of algae and measurement of the colour changes that take place as the algae use up carbon dioxide from hydrogencarbonate indicator. All students involved in school trials were able to obtain reliable data. Overall impressions of student motivation towards this activity were encouraging. Traditional school science experiments on photosynthesis focus on starch tests, usually in Pelargonium. Classic practicals largely take the form of demonstrations. The presence of starch is considered proof of photosynthetic activity, and a series of controlled experiments can be set up where one variable is excluded to demonstrate the essential requirements for this process. Barker and Carr (1989) showed that many students did not understand the starch test. Students are distracted by the complex mechanics involved in starch testing and do not link starch production with the photosynthetic process. As an example, one student thought that when iodine and chlorophyll were mixed together they produced starch. Kinchin (2000) also criticises the idea of starch production as proof of photosynthesis because of the many examples that seem to conflict with the idea that starch production is dependent on light. He makes the point that students know that a potato contains starch but also that a potato is produced in the dark and without the need for chlorophyll. Some other problems of these practical activities arise because students do not understand the concept of a controlled experiment. The removal of carbon dioxide from the air surrounding a plant cannot be seen and therefore does little to promote the idea of it being an essential requirement for photosynthesis. Eisen and Stavy (1993) have suggested that there is insufficient emphasis on the basic chemistry of the process. Students cannot make the link between the carbon from carbon dioxide in the air and the carbon present in carbohydrates. To date there is little practical work that can be done to reinforce this. Other experimental work may focus on the production of oxygen as a product of photosynthesis. Oxygen evolution by Elodea and other aquatic plants can be used as proof of photosynthesis, and if the oxygen is collected the rate can be determined under different conditions. Such experiments, often used for investigations at key stage 4, can produce good data but they are notoriously difficult to complete on a regular basis. Although they may serve a function in terms of assessment, they do little to improve student understanding of the whole process. Plant nutrition has always been an area of student misconception (Driver et al., 1994). Students come to science with some deeply entrenched ideas, and the need for students to restructure their ‘working’ knowledge of plant nutrition as they develop higher level concepts is widely accepted. Much of the recent Novel approach to photosynthesis practicals Eldridge 38 School Science Review, March 2004, 85(312) work involved in trying to help students overcome their difficulties has been based on challenging them to construct theoretical models or become involved in confrontational exchanges with others to help clarify their ideas (Lumpe and Staver, 1995; Kinchin, 2000). While this is very important, there is also a need to motivate students in this area of biology. Photosynthesis is such a central scientific idea that it appears throughout the National Curriculum. If teachers are not careful, students may feel that they are repeating the same topic several times. Experience shows that by the time that students get to key stage 4 they may be very ‘turned off’ plants, and there is a need to introduce something stimulating to challenge them. One way of keeping their interest is to provide them with a novel practical procedure. This article describes such a procedure which, while demanding, allows students to develop new skills and use apparatus that they may not have met before. It suggests the use of immobilised green algae as photosynthetic organisms which are easy to manipulate. Students may then determine the rate of carbon dioxide absorption by the algae using hydrogencarbonate indicator and a colorimeter or colour scale. In addition, the importance of carbon dioxide as a requirement for photosynthesis is emphasised in an implicit way, as students are in effect taking measurements of how much carbon dioxide is taken up by the algae, which causes the change in the colour of the indicator. A major aim of this work was to make the topic of photosynthesis more interesting from a practical viewpoint. Growing a healthy culture of algae A healthy culture of Scenedesmus quadricauda (or other fast-growing green algae) is needed in order to carry out the practical. The starter culture used here was obtained from Sciento (see Apparatus) and the culture initially grown on for four weeks in order to supply enough material for several classes of pupils. The culture was maintained using an enrichment medium (see Apparatus) in a 2 litre clear plastic bottle. In order to allow circulation and aeration with carbon dioxide, a lift pump was used. A 15 mm diameter plastic pipe, just shorter than the total height of the bottle, was inserted into the neck of the bottle. The pipe had small holes at the bottom and one just above the water surface. An air lead without a diffuser was Figure 1b A simple algal generator showing the operation of a lift pump. Figure 1a Algal generator. 15 mm diameter plastic pipe with small hole above water surface and a number of holes towards the bottom forming lift pump air supply fluorescent light constant temperature 18 °C – 20 °C 2 l clear plastic bottle with algal culture and enrichment medium