Investigating the Rate of Photosynthesis

Investigating the Rate of Photosynthesis Introduction Photosynthesis is the process by which plants use the energy from sunlight to produce sugar, which cellular respiration converts into ATP, the "fuel" used by all living things. The conversion of sunlight energy into usable chemical energy is associated with the actions of the green pigment chlorophyll. We can write the overall reaction of photosynthesis (Figure 1) as: 6H2O + 6CO2 + Light = C6H12O6 + 6O2 The above chemical equation can be written in words as: Water + Carbon Dioxide + Light = Sugar + Oxygen Some of the sugar produced during photosynthesis is used by the plant for its life processes (such as growing and reproducing); plants release the energy stored the sugars through cellular respiration (Figure 2). The excess sugar is converted mainly to starch and stored in various plant parts which may be used as food by animals and humans. Figure 1 The reaction of respiration: C6H12O6 + 6O2 = 6CO2 + 6H2O + ATP (energy) Sugar + Oxygen = Carbon dioxide + Water + Energy Oxygen produced during photosynthesis replenishes the oxygen that was used up by living things during respiration. This cycle of photosynthesis and respiration (Figure 2) maintains the balance of carbon dioxide and oxygen on earth. Rate of Photosynthesis The rate of photosynthesis means how fast photosynthesis takes place. This can be measured by the amount of glucose or oxygen produced by a plant over a given time. By understanding the factors that affect the rate of photosynthesis scientists can do work to try and increase the rate of photosynthesis to increase the yield of a crop. The three main things affecting the rate of photosynthesis Figure 2 are the amounts of available, light, temperature and carbon dioxide. Experimental Question: How does changing the concentration of carbon dioxide affect the rate of photosynthesis? Hypothesis: Procedure 1. Obtain a sprig of the aquatic plant by cutting the stem at an angle. Gently crush the stem around the cut to open up the plants stem in case the scissors pinched the stem shut. 2. Weigh the sprig of elodea and record the mass in grams in your data. 3. Setup the apparatus pictured in Figure 3. Each table will be using a different solution of water. Carbonated water has been used to manipulate the concentration of carbon dioxide in each solution. 4. Be sure that the test tube inverted on top of the funnel is filled with water. You should build your apparatus submerged in the containers of the solution. 5. Try not to trap any air in the top of the test tube. If you do trap air in the tube, draw a line at the bottom of the bubble with a dry erase marker. 6. Label your apparatus with your names and class periods. Place your setup under a light source and wait to record your data. Data Construct a table and graph to illustrate your data. An example of a data table has been provided below. Use the graph provided by your instructor to determine the “Volume of O2 produced by the plant. Group Solution 1 2 3 4 5 6 7 Plant Mass (g) Time O2 (mm) 0% Carbonated Water 2.5% Carbonated Water 5% Carbonated Water 7.5% Carbonated Water 10% Carbonated Water 12.5% Carbonated Water 15% Carbonated Water Volume of O2 Volume of O2 / Mass Figure 3 Conclusions 1. Describe the relationship between the amount of carbon dioxide in the water and the rate of photosynthesis. Reference your data to support your response. Additionally, discuss if the data supports your hypothesis or not. 2. List the inputs and outputs of photosynthesis. 3. Name three factors that may affect the rate of photosynthesis 4. Explain any sources of experimental error and predict how they may have impacted your results. Extension 1. At the maximum rate of photosynthesis, how many molecules of glucose did each gram of Elodea produce in one day? Use the following equation and conversion factors to make your calculations. For each 6 molecules of oxygen produced by photosynthesis 1 molecule of glucose is made. PV = NRT P = Pressure = 1 Atmosphere V = Volume (Convert your measured volume in mL to L by dividing by 1000) N = Number of Molecules (1 Mole = 6.02X1023) R = Gas Constant = 0.82 Liters (Atmospheres)/Moles (Degrees Kelvin) T = Temperature = 293 Degrees Kelvin Figure 4: The curve shows the volume held by the test tube used in the experiment with zero representing the bottom (closed end) of the test tube.