Overall, photosynthesis is relatively inefficient, as only one to two percent of solar energy is used to form photosynthetic products. From that one to two percent, many plants undergo a process called photorespiration that causes them to waste much of the photosynthetic energy absorbed. By studying photosynthesis in detail and understanding processes like photorespiration, we can learn to control these processes so we can increase the productivity and efficiency of plants. “Research along these lines is critical, as recent studies show that agricultural production is leveling off at a time when demand for food and other agricultural products is increasing rapidly.” (Gust) Research into photosynthesis could lead to the production of more efficient crop strains or the development of more efficient, selective, and environmentally friendly herbicides that take effect by preventing steps of photosynthesis from occurring.
Developing our understanding of photosynthesis can help to deal with issues of global importance, the decreasing amounts of fossil fuels and coal, which are vital to our daily life, especially in developing countries such as China where most of the power plants rely on coal. By learning how to control and increase the efficiency of photosynthesis, we can increase the production of fuels derived from agriculture such as ethanol. A new experimental process carried out by scientists today involve making ethanol from trees, grasses, and crop wastes by breaking down the cellulose in woody fibers to form cellulosic ethanol. This is much more efficient as trees and grasses do not need to be replanted every year like grains. ("Ethanol made from," 2006) By increasing our understanding of photosynthesis, we can cut down on our dependence on fossil fuels, and on the production of greenhouse gases, by increasing the production of ethanol as an alternate source of fuel.
Photosynthesis can even help in the development of technology, as engineers strive to make transistors and circuit components smaller in order to make computers faster and more compact, similar to how thylakoid membranes are packed inside chlorophyll molecules. Learning how plants control the movement of light energy to the different reaction centers and convert the light energy into chemical energy, can allow for the development of molecular-scale computers
The reason why I chose this research question is that apart from my own personal interest in plants and my concern over the rising levels of pollution on earth, by understanding photosynthesis, the basis of life on earth, we can make medical and technological discoveries to improve our standards of living. Yet at the same time we can learn to control photosynthesis in order to increase the production of food and ethanol, and limit the amount of carbon dioxide in our atmosphere.
Equipment/Materials
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One Pygmy Chain Sword, Echinodorus tenellus
- One High wattage (50W) Lamp
- One GLX Data Logger
- One GLX Lux Light Intensity Probe
- One GLX Dissolved Oxygen Probe
- One 1L Beaker
- One Weighing Scale correct to 2 decimal Places
Method/Procedure:
- Set up the data logging equipment
- Cover or turn off all sources of light in the area the experiment is being held
- Putting the beaker on the scale, add exactly 500.00 grams of tap water into the beaker
- Place the Pygmy Chain Sword plant inside the beaker, with the dissolved oxygen probe soaked inside the beaker as well, next to the stem of the plant.
- Place the light intensity probe in front of the beaker facing light source, with the beaker containing the plant getting the full beam from the lamp, with a lux reading of 2.62 lux
- For the next seventy two hours, starting at 12 in the afternoon, in twenty four hour time periods, record the amount of dissolved oxygen in the water every sixty minutes and the light intensity every sixty minutes as well by reading off the data logger
- Average out the three sets of recorded results from the data logger and plot a graph with time as the x axis and the amount of dissolved oxygen present in the beaker as the y axis
- Using the graph, see if there are any patterns or correlation between the light intensity that the plant is exposed to and the amount of dissolved oxygen which is produced
- As 2.62 lux is the intensity of normal light, the results recorded would be results from the control experiment
- As done in step 5, place the dissolved oxygen probe into the beaker next to the plant again, however move the beaker further away from the light source, so that the light intensity which the plant is exposed to is reduced to 2.19 lux
- Record the amount of dissolved oxygen in the water every sixty minutes and the light intensity every sixty minutes using the data logger, over a period of three consecutive twenty four hour periods
- Look for patterns or correlations between the light intensity and the amount of dissolved oxygen present using a graph with the x axis as time and the y axis as the amount of dissolved oxygen produced
- Repeat steps 11 to 13 at lux light intensities of 1.75, 1.31, 0.88, and 0.44 lux.
- For the last test, with the light intensity at 0 lux, turn off the high wattage lamp, so the plant containing the beaker is left in complete darkness
- Repeat steps twelve and thirteen
- Repeat step seven
- Using the averages to calculate the overall average for the amount of dissolved oxygen produced in the 24 hour period and plot those results against the respective light intensities to see if there are any general trends
Results: *(Data tables are in size ten font so that they fit on the page)
Table 1
Experiment at light intensity of 2.62 lux
Example of Calculations
*(4.8382 + 4.8426 + 4.8423) / 3 = 4.841033 mg/L
Graph 1
Table 2
Experiment at light intensity of 2.19 lux
Example of Calculations
*(4.5931+ 4.5651+ 4.5605) / 3 = 4.572900 mg/L
Graph 2
Table 3
Experiment at light intensity of 1.75 lux
Example of Calculations
*(3.6724+ 3.6794+ 3.6786) / 3 = 3.676800 mg/L
Graph 3
Table 4
Experiment at light intensity of 1.31 lux
Example of Calculations
*(2.7686+ 2.7698+ 2.7603) / 3 = 2.766233 mg/L
Graph 4
Table 5
Experiment at light intensity of 0.88 lux
Example of Calculations
*(2.1569 + 2.1414 + 2.1567) / 3 = 2.151667 mg/L
Graph 5
Table 6
Experiment at light intensity of 0.44 lux
Example of Calculations
*(1.2177 + 1.2323 + 1.2323) / 3 = 1.227433 mg/L
Graph 6
Table 7
Experiment at light intensity of 0.00 lux
Example of Calculations
*(0.2313 + 0.2318 + 0.2325) / 3 = 0.231866667 mg/L
Graph 7
Table 8
Average Results of Amount of Dissolved Oxygen at all Light Intensities
Example of Calculations for Overall Averages
*(4.841033 + 4.843633 + 4.843833 + 4.845467 + 4.845900 + 4.842800 + 4.845000 + 4.847767 + 4.836533 + 4.813067 + 4.823067 + 4.826600 + 4.800633 + 4.803800 + 4.804900 + 4.792667 + 4.818800 + 4.780733 + 4.834067 + 4.845267 + 4.846400 + 4.842600 + 4.847300 + 4.848967 + 4.842533) / 25 = 4.830534
Graph 8
Graph 9
Analysis and Evaluation
As Echinodorus tenellus, or the Pygmy Chain Sword Plants are photoautotrophs, which are organisms capable of using light as a energy source to produce their own organic food substances from inorganic materials, in the control experiment, light is an extremely important factor affecting and controlling the amount of photosynthesis that is carried out by the plant. Essential for allowing plants to carry out photosynthesis, light has three factors which affect the rate of photosynthesis, the light intensity, the wavelength of the light rays, and the duration of time that the plants are exposed to light for.
Due to this controlling factor, when the aquatic plant was exposed to a light intensity of 2.62 lux over a twenty four hour period, there was a high amount of dissolved oxygen detected by the dissolved oxygen probe in the beaker, with 4.8412 milligrams per liter at the end of the first day, and 4.8422 milligrams per liter recorded at the end of the second and third day that the control experiment was carried out. However when the aquatic plant was place in an environment devoid of light, there were minute amounts of dissolved oxygen found, with 0.2337 milligrams per liter at the end of the first day, 0.2333 milligrams per liter at the end of the second day, and finally decreasing to 0.2315 milligrams per liter at the end of the third day when the experiment was carried out at a light intensity of zero lux. The results show that, when placed in an environment with a high light intensity, high amounts of dissolved oxygen was found as chlorophyll molecules in photosystem II absorb the light energy allowing them to become excited. The energy produced by this part of the reaction after the chlorophyll molecule allows for water to be broken down into its elements, oxygen and hydrogen. The more light the plant is exposed to, the more energy the chloroplasts in the leaves are able to absorb, and therefore more oxygen is released as a waste gas as the energy the chloroplasts absorb breaks down the water molecules in the light dependent portion of photosynthesis, a process also known as photolysis, the splitting of water to replace the electrons lost from photosystem II. In photolysis, oxygen is released and the protons are combined with NADP+ or sent back to the thylakoids.
However, when the Pygmy Chain sword plant was placed in an closed environment with not sources of natural or artificial light, the amount of dissolved oxygen per milligram per liter found in the beaker containing the plant decreased with each consecutive day, with 0.2337 milligrams per liter found the first day, before decreasing to 0.2333 milligrams per liter found at the end of the second day, and finally plummeting to 0.2315 milligrams per liter at the end of the final day of the experiment. The reason for the decreasing amounts of dissolved oxygen found in the beaker when the plant was subjected to no light at all is that all living things need energy, and go through a process called cellular respiration to make it. Cellular respiration is the controlled release of energy in the form of ATP from organic compounds like glucose inside the cells. When the Pygmy Chain Sword plant was deprived from light, as the chloroplasts in the leaves were no longer absorbing light energy, photolysis no longer occurred to replace the electrons lost from photosystem II in the light dependent reactions of photosynthesis. As a result, no more oxygen was released as a waste gas, therefore the amounts of dissolved oxygen measured by the dissolved oxygen probe in the beaker decreased as the Pygmy Chain Sword plant carried out aerobic respiration. In aerobic respiration, the six-carbon glucose molecule produced in photosynthesis is broken down into three carbon pyruvate molecules, in the process releasing two molecules of Adenosine Triphosphate. The pyruvate molecules are then completely broken down into carbon dioxide and water, with the release of another thirty six molecules of ATP. As oxygen is used in aerobic respiration, when placed in a dark environment with no photosynthesis taking place, the amount of dissolved oxygen slowly decreased as no more was being produced, but the existing amounts present were being used up.
Overall, my results show that with increasing light intensity, there was an increasing trend of amounts of dissolved oxygen found in the water showing that light intensity plays a key role in controlling the rate of photosynthesis because of the light energy needed to excite the chlorophyll molecules and starting off a chain of reactions which lead to the production of glucose, the breaking down of carbon dioxide, and the release of oxygen as a waste gas. However, with decreasing light intensity, when there was no light at all for the plant to use in photosynthesis, instead of the amounts of dissolved oxygen staying the same and maintaining its level, the data showed that there was a decreasing trend in the amount of dissolved oxygen present in the beaker containing the Pygmy Chain Sword plant due to the plant carrying out respiration.
However, this is only the theoretical explanation for the events, which took place and the trends that the data show. In the actual experiment, there were many sources of error that affected the results significantly. Throughout the experiment, only one Pygmy Chain Sword plant was used, in order to keep variables such as the age of the plant and the size of the leaves constant, however by using the same plant throughout the entire experiment, although the other variables were kept constant, this only gave me a limited range of data. Although my data showed a general increasing trend of amounts of dissolved oxygen found in the beaker containing the Pygmy Chain Sword plant with increasing light intensity, in most trials within the experiment, there were many fluctuations, sudden rises or falls in the amounts of dissolved oxygen found and with the data showing irregular patterns. Initially, all experiments were supposed to be started off at twelve in the afternoon, with no breaks between any of the tests, but time was needed to reset the data loggers and the lux intensity and dissolved oxygen probes, which caused for lost time, which make have caused inaccuracies in the data. Another reason was that time was needed to move the plant further or closer to the high wattage lamp in order to expose it to the light intensity that was desired, for example, 0.44 lux, thus throwing the schedule for the experiments off.
It is very likely that some of the inaccuracies were caused by factors other than light. The amount of dissolved oxygen produced which signified the rate of photosynthesis could have been affected by many other external other than light, such as changes in temperature, humidity, or pH levels. As the experiments did not take place at a laboratory prepared specifically for the experiment, large sounds, floods of light coming in with doors opening and closing when cleaners entered despite instructions indicating otherwise, or increasing room temperature as the experiment was carried out during the summer could have possibly resulted in slight inaccuracies in the results.
Another significant weakness of the experiment is the fact that the Pygmy Chain Sword plant was that when I began carrying out the experiment, I decided to carry out the tests at the different light intensities consecutively, so that after I completed the experiment at a lux intensity of 2.62, I would immediately carry out the next test at the lux intensity of 2.19 lux. Therefore, the lack of breaks or gaps between the different parts of the experiment may have affected the plant’s ability to photosynthesize properly, and in turn affected the amount of dissolved oxygen released by the Pygmy Chain Sword plant.
By solving such problems and minimizing errors, the experiment can be improved and more precise results may be obtained. First of all, conducting the experiment in a laboratory, where the experiment could be isolated from other sounds or human induced stimuli would prevent any errors in the data collected. Keeping the temperature constant by using an air conditioner and using a thermometer to collect temperature data every time the results are recorded would lead to more accurate data. Using a lamp which could automatically change the intensity of the light every 72 hours would give more accurate and precise data, without having me to enter the lab and tinker with the experimental set up to change the light intensity, thus reducing the extra variables caused with my entering of the room. Giving sufficient time for the plant to reset its photosynthetic systems between tests at the different light intensities and increasing the number of plants and trails they undergo would give far more accurate results
Conclusion
In conclusion, having looked at the process of photosynthesis in detail, and the aerobic and anaerobic respiration cycles, the process of varying the light intensity that the Pygmy Chain Sword plant is exposed to that was investigated in the experiment can be summarized as follows. When the Pygmy Chain sword plant was exposed to high intensities of light, e.g. 2.62 lux or 2.19 lux, over a twenty four time period, large amounts of dissolved oxygen was found in the water, with an average of 4.83 mg per liter of dissolved oxygen detected by the probe when the plant was exposed to a light intensity of 2.62 lux. On the other hand, when the Pygmy Chain sword plant was exposed to low intensities of light, e.g. 0 lux or 0.44 lux, minute amounts of dissolved oxygen were found in the water, with an average of 0.32 mg per liter of dissolved oxygen detected by the probe when the plant was exposed to no light at all, in an environment with a light of intensity of 0 lux.
The amount of dissolved oxygen registered by the probe correlates to the intensity of the light the plant is exposed to, so that if there is an increase in the intensity of the light the plant is exposed to, then there would be an increase in the amount of dissolved oxygen detected, while if there was a decrease in the light intensity, there would also be a decrease in the amount of dissolved oxygen detected by the dissolved oxygen probe. This is because, with increasing light intensity, more light would be accepted by the thylakoid membranes within the chloroplasts in the leaves of plants, as the leaves absorb the red and blue spectrums of white light. The light energy harnessed from the sun, or an artificial light source, can then be used to reduce substances such as water and convert carbon dioxide into organic compounds, form Adenosine Triphosphate (ATP), and as a consequence, have oxygen released as a waste gas. Therefore, since with increasing light intensity, there is more energy for the plant to harness and use to convert inorganic compounds into organic compounds and oxygen, so the amount of dissolved oxygen produced by the Pygmy Chain Sword Plant increases when the light intensity it is exposed to increases as well.
Although the experiment did have some significant weaknesses, there is room for some improvements and modifications in the method in which the experiment was carried out, such as by increasing the number of experimental Pygmy Chain Sword plants, the number of times the experiments were carried out, and by testing the plant at a wider range of light intensities. These improvements and modifications in the method could allow for more accurate and reliable results to be obtained in order to confirm my conclusion that increasing light intensity over a twenty-four hour time period increases the rate of photosynthesis. Further development of this basic experiment on a more in-depth, professional level, such as an investigation into the other factors affecting the rate of photosynthesis like temperature and the color of light, would give us a better understanding of the process of photosynthesis and the factors which control and determine the rate of which photosynthesis occurs. More advanced research on the effects of light on rate of photosynthesis would especially aid scientists in developing ways to help control the production of the greenhouse gas carbon dioxide by increasing the rate at which photosynthesis occurs.
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