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Oxygen Production During Photosynthesis

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Oxygen Production During Photosynthesis

  • Meera Kapadia and Amirah Mohd Ariff

The purpose of this experiment was to investigate the role of light in the production of oxygen gas through photosynthesis. The independent variable for this investigation was the time elapsed between recordings of the location of the sodium bicarbonate edge; the dependent variable was the displacement of the sodium bicarbonate edge; and the control for this investigation was the height of the Elodea plant, which was four centimeters for each test tube, and the distance between the light source and the test tubes. Of the four test tubes used in this experiment, three contained Elodea in sodium bicarbonate solution; therefore, only one test tube without Elodea was prepared. The test tubes containing the Elodea were placed under three different conditions–receiving light from a white light bulb, receiving light from a white light bulb while being covered by aluminum foil, and receiving light from a blue light bulb. The distance traveled by the sodium bicarbonate from the initial point in the bent glass tubing was recorded for five, five minute intervals. The results indicated that the white light bulb set up had the highest rate of photosynthesis. However, some of the results obtained, including data which indicated that photosynthesis cannot occur in the presence of blue light, were not as expected, but occurred due to systematic errors in the vacuum seal breaking and the sodium bicarbonate edge moving toward the test tube. This would imply that photosynthesis consumes oxygen instead of produces it. Still, the alternative hypothesis which was light is a necessity in the production of oxygen during photosynthesis was supported.
In order for an entity to be considered alive it must meet five requirements–be able to metabolize, be composed of cells, process genetic information, work towards reaching the ultimate goal of self-replication, and continuously evolve (Freeman, 2). Focusing on energy metabolization raises the question, how do organisms that are alive, then, acquire energy? There must be a mechanism engineered by evolution to convert light energy into the chemical energy required by all organisms to survive. For living organisms, the primary process for initially acquiring energy and converting it to a useable form is known as photosynthesis which can be summarized as follows:
6CO2 + 6H2O+light energy→C6H12O6 + 6O2
where “plants harvest the kinetic energy in sunlight and store it in the bonds of carbohydrates” (Freeman, 80). Since only plants are able to photosynthesize, they are known as autotrophs that produce the energy that is cycled through the rest of the biosphere.
Through photosynthesis, plants are able to produce two necessities for survival required for humans–oxygen gas and usable energy in the form of food. Since photosynthesis provides humans with the essentials for survival, research is being conducted on how the rate of photosynthesis can be improved. In “Genetic modification of photosynthesis with E. coli genes for trehalose synthesis,” a 2004 study, Nicotiana tabacum was transformed with E. coli genes to improve the rate of photosynthesis per unit of leaf area (citation). It was found that changes in the photosynthesis levels were due to trehalose 6-phosphate content rather than trehalose. This result was analyzed for growth patterns, and it was found “a greater photosynthetic capacity did not translate into greater relative growth rate or biomass” since photosynthetic capacity was found to be negatively related to leaf area (citation). Overall, this experiment highlighted the complexities of photosynthesis regulation which affects the lives of all humans.
Even though the mechanism for energy conversion is known, several questions still arise regarding the location of photosynthesis, the extent of its efficiency, and most importantly, what conditions are preferred in order to maximize the rate of photosynthesis. Focusing on the final question posed, this investigation studied to what extent Elodea plants are able to photosynthesize in the presence of different types of light and conditions by measuring oxygen gas output. The null hypothesis was that light is not a required reactant for photosynthesis to occur. The alternative hypothesis was that the rate of photosynthesis is directly proportional to the amount of light available. From this, it can be predicted that the greater the amount of light available, the greater the oxygen output; ergo, the greater the rate of photosynthesis.
Materials and Methods
The materials needed for the apparatus of this experiment were four test tubes, a test tube rack, Elodea plants, bent glass tubing, a stopper, a beaker, water, a graduated pipette, aluminium foil, saturated sodium bicarbonate solution, a stopwatch, parafilm and clamp lights with two different colored bulbs.
The independent variable for this investigation was the time elapsed between recordings of the location of the water edge. The dependent variable was the displacement of the water edge. The control for this investigation was the height of the Elodea plant, which was four centimeters for each test tube, and the distance between the light source and the test tubes.
Four centimeters of Elodea were cut toward their stems, placed into two test tubes, and covered with saturated sodium bicarbonate solution. To test whether plants only photosynthesized in the presence of light, one of the test tubes were covered with aluminum. The third test tube was filled with only sodium bicarbonate solution; thus, it acted as a negative control. In order to ensure that no external oxygen would enter the test tube, bent glass tubing was inserted through a rubber stopper and covered with parafilm; thus, the apparatus was vacuum sealed. Some excess solution was displaced and visible in the bent tube when the stopper capped the test tube. This step was taken in order to measure the outward displacement of the oxygen that would indicate that photosynthesis had, indeed, occurred.
Before the test tubes were placed in front of the light source, a beaker filled with water was placed in between the light source and the empty test tube rack. The test tubes were then placed in the test tube rack about 30 cm away from the light source. These steps were taken to to ensure that the Elodea plants were not damaged due to direct light. The system was calibrated for five minutes. Next, the light was turned on and the displacement of the water edge after each five minute interval was recorded. This procedure was repeated for the final test tube with blue light as the light source.
Table 1:

The Distance Traveled of The Sodium Bicarbonate Solution From The Initial Point (cm)
Conditions Time 0 (0 mins) Time 1 (5 mins) Time 2 (10 mins) Time 3 (15 mins) Time 4 (20 mins) Time 5 (25mins)
Elodea and concentrated Sodium Bicarbonate Solution 0.00 0.10 0.15 0.15 0.20 0.30
Elodea and concentrated Sodium Bicarbonate Solution, covered with aluminium foil 0.00 0.05 0.05 0.05 -0.10 -0.10
Concentrated Sodium Bicarbonate Solution without Elodea (control) 0.00 0.00 0.00 0.00 0.00 0.00
Elodea and concentrated Sodium Bicarbonate Solution, with a blue light source 0.00 0.00 0.00 0.00 0.00 -0.15

Table 1: The table above displays the displacement of the sodium bicarbonate for each five minute interval.
Figure 1:
Figure 1: This figure displays the rate of photosynthesis under varying conditions for four different test tubes.
The data collected indicated that the test tube that contained Elodea and concentrated sodium bicarbonate solution placed under white light, which encompasses the entire visible light spectra, had the greatest displacement of the solution from the initial point. During the first five minute interval, the sodium bicarbonate solution displaced 0.10 cm from the initial point, 0.15 cm after 10 minutes, 0.15 cm after 15 minutes, 0.20 after 20 minutes, and 0.30 cm after 25 minutes. The test tube that did not contain Elodea indicated no change in sodium bicarbonate location in the 25 minute duration of the experiment. The third test tube placed under white light containing both elodea and concentrated sodium bicarbonate while being covered with aluminum foil showed a displacement of 0.05 cm during each of the first three five minute intervals. However, the displacement decreased to -0.10 cm during the fourth time interval interval, 20 minutes after the experiment began, and remained constantly at this value for the rest of the experiment. When the Elodea plant’s ability to photosynthesize in the presence of blue light (~500 nm) was tested in a test tube containing concentrated sodium bicarbonate, there was no evidence of displacement for 20 minutes (citation). After 20 minutes elapsed, however, the solution indicated that it had been displaced 0.15 cm in the negative direction.
Since oxygen was only visibly produced in the presence of light, the null hypothesis, that the rate of photosynthesis is unrelated to the availability of light was not supported. The alternative hypothesis, however, that the rate of photosynthesis is directly proportional to the amount of light available was supported. This is because light energy acts as a reactant in the reaction of photosynthesis which produces organic carbohydrates that store energy that is used to make ATP that provides energy to plants (Freeman, 81). Evidence of light being a requirement for photosynthesis was seen in the test tube that contained both the Elodea plant, sodium bicarbonate, and was exposed to the white light. In Figure 2, the slope of the Elodea and concentrated sodium bicarbonate solution stayed positive during the entirety of the experiment. This shows that the rate of oxygen production was increasing as time increased which gave evidence that photosynthesis was actively occurring. The importance of light was further emphasized by the results found in the test tube that contained the Elodea plant and sodium bicarbonate, the factory of photosynthesis and its raw materials, but was covered by aluminum foil. The light never reached the inside of the test tube where the Elodea and sodium bicarbonate were, so the plant did not have the energy to conduct photosynthesis. However, as evident from the trendline of the Elodea in the concentrated sodium bicarbonate solution covered by aluminum, it seems that there was an initial oxygen output before it decreased to a negative rate of oxygen production. This can be attributed to systematic errors in which the vacuum seal applied to the test tube was applied incorrectly. A dysfunctional seal could have caused the location of the sodium bicarbonate to vary in the bent glass pipette.
Freeman, Scott, et al. 2014. Biological Science, 5th ed.; Pearson Education, Inc.
Mclean. Laboratory Exercises for General Biology 1. 3rd ed. Plymouth: Hayden-McNeil, 2015. Print.

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