Exploration of the Effect of Light Intensity, Action Spectrum, andAbsorption Spectrum on Photosynthesic Processes Madison McVeyOctober 25, 2016Biology 111-541TA: Marko BalohAbstract:In exercise 7, Photosynthesis, students determined the relationship between cellular respiration and photosynthesis. This was done by conducting experiments that affected the light intensity, action spectrum, and absorption spectrum and observing the effect it had on the processes of photosynthesis. Students completed these experiments in groups of eight and simultaneously conducted the different experiments. Removing the gasses from spinach leaves and observing the results when exposed to different colored lights tested the effect of the action spectrum on photosynthesis. The effect of the absorption spectrum was noted when four pigments from chlorophyll were analyzed at various wavelengths using a spectrophotometer. The effect of light intensity on photosynthesis was observed by analyzing the reaction rate of tubes filled with a chloroplast solution exposed to various amounts of light. In general, it was found that photosynthesis occurs most effectively when exposed to blue light, results can best be analyzed at 400-500 nm, and both the absorbance value and rate of the reaction decreased, as the intensity of light increased. Introduction: Out of all the biological processes, photosynthesis is arguably the most important. For starters, all organisms depend on plants in some way, so it ultimatelyis the basis of the entire food web. As if food wasn’t important enough to survive, all animals and humans depend on oxygen to survive. Without a constant cycle of photosynthesis, the supply of oxygen, and ultimately life, would diminish (NGA). 1Photosynthesis is a 100% efficient process that removes the carbon dioxide in the air and replaces it with carbohydrates and oxygen (Harris-Haller 2014). Photosynthesis is represented by the chemical reaction of 6 CO2 + 6 H2O + 686 kilocalories C6H12O6 + 6 O2. As outlined in the equation, carbon dioxide and water react with 686 kilocalories of light energy to form a molecule of glucose and six molecues of oxygen (Harris-Haller 2014). This reaction is driven by two main stages, one being photochemical, which is responsible for releasing the oxygen molecules, and the other biochemical, responsible for fixing carbon into sugars (Harris-Haller 2014). The carbon dioxide used as a reactant in the photosynthesis reaction is a by-product of cellular respiration, which is responsible for driving cellular metabolism. Cellular respiration is a process that is ultilized by all eukaryotic organisms, including those that are also photosynthetic (Harris-Haller 2014). This lab will attempt to confirm the relationship between photosynthesis andcellular respiration through experimental data. For the three experiments conducted in this lab, there were three separate hypotheses and null hypotheses. For the light intensity experiment, the null hypothesis was that the amount of light each tube was exposed to would not effect the rate of reaction and all would perform at an equal rate. The hypothesis was that the tube wrapped in foil and kept in a dark space would have the fastest rate of reaction and highest absorbance value. The hypotesis created for the absorption spectrum lab hypotesized that the lowest absorbance of the pigments would be recorded at the lowest wavelength, whereas the null hypothesis was the varying wavelengths would have no effect on the absorbance levels of the four pigments. 2Methods: For the light intensity experiment, students began by setting the spectrophotometer to 605 nm and making and labling five tubes of solution. Four of the tubes were labled experimental tubes, and contained 0.5 mL of chloroplasts, 3 mL of 0.1 M PO4 buffer, 100 L of water and 400 L of DPIP. μ μ One tube was marked asthe blank, and only contained 0.5 mL of chloroplasts, 3 mL of 0.1 M PO4 buffer, and 500 L of waterμ . An initial absorbance reading was taken immediately after the DPIPwas added, then each tube was placed in its designated experimental climate. The “dark” tube was wrapped in foil and placed in the lab bench cabinet and the other three experimental tubes were placed at various distances from a light source. Tubes were inverted and the absorbance values were recorded every five minutes for thirty minutes, where each time the data was recorded. For the action spectrum lab, students began by using a cork borer to cut out 60 spinach leaf disks. The leaves were transferred to a syringe and the syringe filled with enough sodium bicarbonate to cover the leaves. The tip of the syringe was covered with parafilm and the plunger pulled on for approximately 15 seconds. The plunger was released for approximately 45 seconds and the aspirated disks began tofall. This process was repeated until all of the disks had sunk. Next, 40 mL of sodium bicarbonate and 10 spinach disks were added to five 50 mL beakers. Beakers were imemediatly placed in the designated light box at or below the level of light. The “dark” beaker went into the lab bench cabinet. The light boxes were arranged in a square and the light bulb hung directly over the open space between the boxes. The 3light was turned on and every five minutes, the number of disks floating was recorded. For the absorbption spectrum experiment, students began by turning on the spectrophotometer and obtaining a set of pigments and blanks. The pigments were identified by colored caps and marked so they could be oriented the same during each reading. Readings of each pigment were taken from 400 nm to 700 nm in 50 nm increments. The spectrophotometer was blanked each time a new wavelength was taken and a new pigment was used. The absorbance value for each measurement was recorded in the data table. At times, the spectrophotometer would read a flashing 1.999 signaling the pigments may be “off scale”, in the data table, the absorbance is noted as infinite. Results: The light intensity experiment reported the slowest rate of photosynthetic reaction belonged to the “dark” tube and the fastest rate of reaction belonged to the tube placed 49 cm away from the light source. The action spectrum experiment reported the white light had the fastest rate of photosynthtic reaction whereas the dark box had no activity observed. The absorbance spectrum reaction showed the widest range of absorbance readings belonged to Xanthophyll and
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