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Estimating Rates of Photosynthesis INTRODUCTION Photosynthesis is the process by which organisms that contain the pigment chlorophyll convert light energy into chemical energy which can be stored in the molecular bonds of organic molecules (e.g., sugars). Photosynthesis powers almost all trophic chains and food webs on the Earth. The process involves the use of light energy to convert carbon dioxide and water into sugar and oxygen. In eukaryotes, photosynthesis happens in organelles called chloroplasts. All photosynthesis uses as the primary light trapping pigment chlorophyll a. Different photosynthetic organisms like plants and algae use a variety of secondary pigments to increase the wavelengths of light they can absorb. The net process of photosynthesis is described by the following equation: 6 CO2 + 6 H2O + light energy C6H12O6 + 6 O2 This equation simply shows that carbon dioxide and water combine in the presence of sunlight to form sugars; oxygen is released as a by-product of this reaction. Cellular Respiration is the general process by which organisms oxidize organic molecules (e.g., sugars) and derive energy (ATP) from the molecular bonds that are broken. It is the process of converting the chemical energy of organic molecules into an energy form immediately usable by organisms. Glucose may be oxidized completely if sufficient oxygen is available. All organisms, including plants and animals, oxidize glucose for energy. In eukaryotic organisms, aerobic respiration occurs in organelles called mitochondria. Respiration is the opposite of photosynthesis, and is described by the equation: C6H12O6 + 6 O2 6 CO2 + 6 H2 O + 36 ATP Simply stated, this equation means that oxygen combines with sugars to break molecular bonds, releasing the energy (in the form of ATP) contained in those bonds. In addition to the energy released, the products of the reaction are carbon dioxide and water. In eukaryotic cells, cellular respiration begins with the product of glycolysis (pyruvic acid) being transported into the mitochondria. A series of metabolic pathways (the Krebs cycle and others) in the mitochondria result in the further breaking of chemical bonds and the liberation of ATP. CO2 and H2O are end products of these reactions. The theoretical maximum yield of cellular respiration is 36 ATP per molecule of glucose metabolized. Photosynthesis Light 6 CO2 + 6 H2O + light energy C6H12O6 + 6 O2 Energy NADPH Light H+ and Calvin Reactions ATP Cycle Sugar (C6H12O6) containing stored energy CHLOROPLAST H2O O2 CO2 Electron NADH Transport Krebs Pyruvic Glycolysis Chain H+ Cycle Acid MITOCHONDRION ATP Energy Aerobic Respiration C6H12O6 + 6 O2 6 CO2 + 6 H2O + ATP FIGURE 12-1 In order to measure the rate of photosynthesis or respiration you can measure the production rate of any product or the consumption rate of any reactant. In this lab, we are going to focus on oxygen. As oxygen is liberated as a waste product of photosynthesis, we can measure the amount of oxygen produced and use that value to estimate the rate at which photosynthesis is taking place. Similarly, since oxygen is required for aerobic respiration, measuring the amount of oxygen consumed by an organism that is respiring aerobically will give us a value representing the rate of respiration. MATERIALS test tubes with stands 2% potassium bicarbonate solution Elodea shoots plastic tubing ruler forceps meter stick LED lamps PROCEDURE The Color of Light and the Rate of Photosynthesis Because white light consists of a spectrum of colors and because chlorophyll is green, different colors of light (different wavelengths) affect the rate of photosynthesis. Plants should photosynthesize at higher rates when exposed to light of wavelengths that chlorophyll absorbs well and at lower rates when exposed to wavelengths that are not as well absorbed by chlorophyll. We will test this idea with four different setups. In each setup, you will have a test tube filled with 70 ml of 2% potassium bicarbonate (NaHCO3) solution which will provide an adequate concentration of CO2 for photosynthesis. You will use only one Elodea shoot to control for the differences that might exist between shoots. The LED lamps that we are using emit very little heat. If we used incandescent bulbs, the heat from the lamp would cause the gas to expand and make it look like more gas was being produced than actually is produced by photosynthesis. There is a ruler attached to a pipette with a submerged tip so that you can measure the movement of the water over time. The water will be ‘pushed’ through the pipette by the pressure of the oxygen gas produced by the Elodea. The summary equation for photosynthesis shows a molecule of oxygen is produced for every molecule of carbon dioxide converted into glucose. If oxygen gas simply replaced carbon dioxide gas molecule for molecule, you would expect no change in the total amount of gas in the system. But in our system, the carbon dioxide is coming out of the liquid solution. Its removal from the system does not affect the partial pressure of the gas in the tube – only the oxygen being liberated which is in the form of gas. The amount of oxygen liberated is a good way to measure the rate of photosynthesis. For each setup you will measure the amount of oxygen liberated at 3 minute intervals for a total of 15 minutes. After turning on the light, you should wait 5 minutes before taking your first reading so that the system equilibrates. You will have four different colored LED lamps. You will test the photosynthetic rate for each of the four different colors of light at 15 cm distance between the light source and the Elodea. You should pose specific hypotheses with your group as to the expected outcome in each case. Your prediction should be based on your knowledge of the absorption spectra for chlorophyll a and chlorophyll b (the two principle photosynthetic pigments in Elodea) from the spectrophotometry lab. Light Color Prediction for Relative Photosynthetic Rate by color WHITE BLUE RED GREEN Light Color (mm of O2) TIME (min) WHITE BLUE RED GREEN 0 0 0 0 0 3 6 9 12 15 Graph the “Amount of Oxygen Liberated” (Rate of Photosynthesis) versus “Time” for all four light colors on one graph. DISCUSSION QUESTIONS 1. Why is measuring the liberation of oxygen a good way to measure photosynthetic rate? Why not just measure the amount of sugar produced? 2. List five factors that might influence the rate of oxygen production or consumption in Elodea. Explain how you think each will affect the rate?
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