Smith, W. “Leaf Form and Photosynthesis”


Lynn Dang

University of Colorado @ Boulder, Fall 2008

In our experiment, we tested whether or not yellow leaves photosynthesized. From a previous experiment, we determined that green leaves contained a great amount of chlorophyll a, chlorophyll b, and carotenoids. Since healthy green leaves contained these pigments, we believed that yellow fall leaves were dying and therefore did not contain any chlorophyll, which lead us to hypothesized that yellow leaves do not engage in photosynthesis.

To test our hypothesis, we collected three yellow leaves similar in size from an oak tree. First, we had to confirm that no chlorophyll was present in the leaves, so we cut each leaf in half and used each half to make a different pigment. To make the pigments, we cut each half into fine pieces and crushed them in a mortar containing petroleum ether. Once the leaves were crushed for a 10 to 15 minutes, 50 ml of each pigment was transferred to a strip of chromatography paper using a mircopippetor. The strips of chromatography paper were then placed in a graduated cylinder with 5mL of petroleum ether. After allowing the pigments to separate, we removed the chromatography paper and confirmed that the leaves did not contain any chlorophyll. Next, we used the other half of the leaves to measure respiration rates. One at a time, we placed the leaves into the chamber and secured the CO2 gas sensor on top. We ran a light or “daytime” trial with where we measured the respiration rate when there was one light shining on the chamber for 5 minutes and a dark or “night time” trial where we measured the respiration rate when the chamber was covered in aluminum foil for 5 minutes. This procedure was repeated each leaf.

Our results indicated that there was not a significant difference between the rate of respiration in the light (mean =4.925333333 ppm/min) and the rate of respiration in the dark (mean=4.410666667 ppm/min) since our P value was 0.399162203, which is greater, that 0.05. These results supported out hypothesis because since the rates of respiration for dark and light were not different, photosynthesis was not taking place.

            From our experiment, we concluded that photosynthesis does not occur in yellow leaves because theoretically, the rate of CO2 would be much lower during photosynthesis than during cellular respiration, which our results contradicted. Our hypothesis was consistent with our results because we hypothesized that yellow leaves would not engage in photosynthesis because of the absence of chlorophyll.

A problem within our experiment includes cutting the leaves in half, which could have potentially dehydrated the leaves and altered our results. Another problem that could have skewed our results was not giving the CO2 sensor time to equilibrate between trials, which could have given us false respiration rates.

If we were to follow up on this experiment we would test red leaves for chlorophyll, measure the respiration rates and then compare those rates to the yellow leaves to investigate any differences.