| MadSci Network: Botany |
In a classic classroom demonstration of oxygen production in photosynthesis using Elodea, a little bit of sodium bicarbonate (baking soda) is added to the solution to provide a ready supply of carbon for photosynthesis. The amount of dissolved carbon dioxide in a small beaker will quickly run out when Elodea is rapidly photosynthesizing so the sodium bicarbonate provides an abundant supply. The behavior of dissolved carbon dioxide is complex because dissolved carbon dioxide reacts with water to form carbonic acid (H2CO3), which then can convert to bicarbonate ion (HCO3-) or carbonate ion (CO3-2) as the pH changes. The graph at the bottom of the first webpage shows how the relative amount of those three change as the pH changes. Another complicating factor is that plants absorb mineral nutrient cations and anions and can either raise or lower solution pH depending on the dissolved ions present. Elodea photosynthesis or cellular respiration also can cause a shift in solution pH. The usual technique when working with Elodea photosynthesis is to capture the gas evolved in an inverted, water-filled test tube. The volume of the gas can then be measured. The amount of gas is estimated to be directly proportional to the amount of photosynthesis. The gas will not be pure oxygen so your O2 sensor could be used to determine the exact amount of O2 produced. General recommendations for an experiment are as follows: 1. Use replication and statistics, e.g. five beakers of Elodea per pH, at a minimum determine means and standard deviations, graph results 2. Use an electric light source, such as fluorescent, that will be more constant than sunlight. Incandescent lamps might heat up the solution too much and cause dissolved gas to come out of solution. 3. Standardize your methods including the distance of the container with the Elodea from the light source, the Elodea fresh weight per beaker, the water volume per beaker, the type of water, the amount of sodium bicarbonate per beaker and the water temperature. 4. Calibrate your O2 sensor and pH meter with appropriate standards. 5. Gently blot the Elodea dry with paper towels before determining the fresh weight. Express your rate of photosynthesis on a fresh weight basis. 6. Probably the best way to adjust pH is to use a pH buffer. A pH buffer reduces the changes in pH caused by the plant. You might try a potassium phosphate buffer at a concentration no more than 5 millimolar. If the buffer concentration is too high the Elodea will lose water by osmosis. If extremely high, plasmolysis occurs. Your chemistry teacher might be able to help you prepare buffers. References The Carbonate System Carbon Dioxide and Carbonic Acid Re: what and how can i measure the amount of oxygen plants create? How do I prepare a phosphate buffer solution with a specific pH? Photosynthesis in Elodea Re: How do plants alter the pH of a fish tank? Re: When is a cell flaccid compared to turgid compared to plasmolysed?
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