|MadSci Network: Botany|
The classic technique uses the aquarium plant Elodea (Elodea canadensis or Egeria densa), also called anacharis. Freshly cut the stem ends of the Elodea right before doing the experiment. The Elodea shoots are placed underwater in a beaker beneath a funnel with a water-filled test tube inverted on the stem of the funnel. An inverted test tube supported by a clamp and ringstand can also be used without a funnel. The cut stem ends of the Elodea are placed inside the opening of the test tube. Add a pinch of sodium bicarbonate (baking soda) to the water to provide sufficient carbon dioxide in the form of bicarbonate ion. The oxygen bubbles formed during photosynthesis displace water from the test tube. The gas collected will not be pure oxygen but it is higher in oxygen than the 21% oxygen in the atmosphere. The last website cited below states that one sample of collected gas was about 50% oxygen, 6% carbon dioxide and 44% nitrogen gas. The reason for that is because the water starts out saturated with air, which is about 79% nitrogen gas and 21% oxygen gas. The water also contains an unnaturally high amount of carbon dioxide because of the sodium bicarbonate added. Elodea also produce carbon dioxide continuously via cellular respiration. If the Elodea had been in the water overnight, they could have enriched the water with carbon dioxide naturally. As the Elodea produces oxygen in photosynthesis, some of the oxygen dissolves in the water and displaces nitrogen gas and carbon dioxide gas. You can use a burning splint to show that the gas collected is enriched in oxygen. The flame becomes more intense when thrust into a test tube full of the collected gas. To measure the oxygen more exactly, you would need an oxygen gas analyser. Plant scientists usually measure photosynthesis by measuring the carbon dioxide consumed or by the dry matter produced. Carbon dioxide gas is usually measured using an infrared gas analyser. References Elodea Photosynthesis Photosynthesis Demonstration Elodea Photosynthesis LICOR Portable Photosynthesis System What is the gas produced by Elodea?
Try the links in the MadSci Library for more information on Botany.