Biology HL IA - Photosynthesis of elodea lab report

Tools:

1. Photosynthometer:

        syringe, plastic tube,

        measuring pipette,

        funnel, test tube,         beaker;

2. Clamps and Stand;
3. Elodea Canadensis;
4. Razor blade;
5. NaHCO3  powder;
6. Scales;
7. Distilled water;
8. Beaker;
9. Thermometer;
10. Lamp (20W);
11. Stopwatch.

Procedure

The experiment was done in the dark room. 5g of NaHCO3 was measured using scales and put into 250mL of distilled water. Then, 10 centimeters of Elodea was cut using razor blade and immediately put into the carbonate-filled test tube with the cut end kept up. Test tube with the Elodea was put into the beaker filled with water in order to prevent the change in temperature due to the heat of light lamp. Thermometer was placed into beaker to measure changes in temperature. The photosynthometer was set up like shown in Figure 1 (below). Whole mechanism of it was filled with carbonate water using syringe by removing the plunger of syringe. At first trial, the lamp was positioned 10 cm away from the beaker containing the plant. The photosynthesis was let to take place for 10 minutes, using a stop-watch. The bubbles that collected on the highest point of photosynthometer were pulled to the measuring pipette by pulling the plunge of syringe gently. The volume of the gas bubbles released was recorded (mL) and experiment was repeated 4 more times using the same position of lamp in order to obtain reliable results. Finally, experiment was repeated with the lamp 20, 30, 40 and 50 cm away.

Figure 1 Laboratory set.

Data collection and processing

Table 1 Raw data; volume of gas bubbles released per 10 minutes in different light intensity.

As the rate of photosynthesis increases, the volume of oxygen will also increase as it is a product of photosynthesis and vice versa. As the chart 1 shows only the changes in volume of oxygen released, therefore, the rate of photosynthesis can be calculated by dividing recordings from minutes, for example:

The rate of photosynthesis when lamp is 10 cm away is: 0.728:10 = 0.0728 mL/min.

Table 2 Processed data; average rate of photosynthesis in different conditions.

Conclusion and evaluation

From the results that I have gathered I can state that an increase in light intensity certainly does increase the rate of photosynthesis. Therefore, my hypothesis is proved to be right. The tendency is that light intensity decreases proportionally to the increase in length between a plant and a light source. However, this tendency does not fit the scientific explanation that light intensity is inversely proportional to the distance squared because the light energy spreads out as it travels further and further from its source (light energy travels along the circumference of an expanding circle. When light energy is released from a point, the energy is dispersed equally along the circumference. But since the circle is expanding, the circumference increases and the same light energy is distributed along a greater surface1). There are some weaknesses and limitations of a method that could explain the fact that results are not exactly true as, for example, that I didn’t find the optimum light intensity of this plant. I will discuss the reasons in the following passage.

First, there are more factors that alter the rate of photosynthesis such as temperature and carbon dioxide amount in the solution. As I did not change the solution after every trial it can be presumed that a plant photosynthesized continually and going to latter the amount of carbon dioxide was decreasing and might have become the limiting factor of photosynthesis. This limitation could be reduced if solution would be changed after every trial. Second, the apparatus that I used might not have fit the method. The lamp that I used had a cover that directed the light energy somewhat. The light energy did not spread out as much as a plain light bulb with no cover. The distribution of the light energy was more concentrated, changing the gradient of the graph. The improvement would be to try to construct another source of light such as light bulb with no cover. Third, the plastic tube used in the experiment was not transparent; hence it was difficult to find out whether it contained any air bubbles before the experiment or during it. As a result, some recording might be unfair. Although there aren’t any transparent tubes in our laboratory, I might have bought one. Furthermore, sometimes the air bubbles released from elodea did not go straight to funnel and were on the leaves until bigger bubbles collected, hence those smaller bubbles volume was not recorded at a time it had to be. Also, data obtained might not be accurate due to a human error, such as precision. The range of length between a plant and a lamp were too distant to obtain a real-life graph in order to see in the optimum light intensity. To improve the accuracy of the results, the readings would have to be taken several more times. The entire experiment could have been performed again, and the new results could be combined

1 Advanced Biology by Michael Kent, Oxford University Press

if the same plant is used. But the photosynthetic rate of the same piece of pond weed would eventually decrease over time anyway. Repetitions would, however, improve the overall reliability of the results. Finally, results of an experiment might be affected by improper control of controlled variables as well. Some of controlled variables might not have been considered before the experiment, for example, while performing the experiment, some of the oxygen produced from photosynthesis may have dissolved into the water. Some oxygen may have even been used by micro-organisms  on the pond weed. Some oxygen is also used during the respiration of the plant.

As I discussed minor improvements in the above passage, major suggestions for improvements of investigation could also be outlined. Investigating the effects of the quality of light on the rate of photosynthesis, for example, different colored light, could be used. This could have commercial benefit, as finding out the effect of the color of light on the rate of photosynthesis could aid plant rowers to find out which type(s) of light make(s) plants grow quickest. If not the color of light, other factors that alter the rate of photosynthesis, such as temperature or pH could be also investigated.