MadSci Network: Botany

Re: what and how can i measure the amount of oxygen plants create?

Date: Mon Oct 1 21:15:08 2001
Posted By: David Hershey, Faculty, Botany, NA
Area of science: Botany
ID: 1001963025.Bt

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.


Elodea Photosynthesis

Photosynthesis Demonstration

Elodea Photosynthesis

LICOR Portable Photosynthesis System

What is the gas produced by Elodea?

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