A LABORATORY EXERCISE WHICH ENCOURAGES 6TH GRADERS TO RECOGNIZE THE RELATIONSHIP BETWEEN HUMAN RESPIRATION AND PLANT PHOTOSYNTHSIS

Abstract

In order to develop a laboratory exercise suitable for 6th grade pupils of elementary schools, the Cookson and Price method (S. S. R. 64, 84-87, 1982), in which the photosynthetic rate is measured through the rate of leaf discs floating by means of accumulation of oxygen produced by their own photosynthesis, was re-examined. Using leaves of Elodea densa instead of leaf discs of terrestrial plants as in the Cookson and Price method, the time to exhaust the air from the leaf discs in order to immerse them can be omitted. More than half of the Elodea leaves used can float in distilled water (DW), which was aerated previously with expiratory air, when they are illuminated. But they cannot float at all in DW in which the CO₂ concentration has been equilibrated to the air. 1/ET₅₀ value, which is the reciprocal of the time required for half of the leaves used in the experiment to float, can be used as a relative photosynthetic rate. Spontaneous leaf floating was observed after a sudden rise in water temperature. Keeping the leaves at the experimental temperature for at least one hour before the experiment and keeping the water temperature constant throughout the experiment can help prevent spontaneous leaf floating. To shorten the time in which at least half of the leaves begin to float, some appropriate light and temperature conditions are necessary. The light intensity should be higher than 10klx and the temperature should be kept within the range from 25℃ to 35℃. From this experiment, pupils can notice that their expiratory air stimulates the photosynthesis of water plants.