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Photosynthesis demonstration


When we (humans) breath, we use oxygen and give off carbon dioxide in a process called respiration. Plants, trees and algae use carbon dioxide and give off oxygen in a process called photosynthesis. These two cycles create what is known as an interdependent relationship; plants rely on animals for carbon dioxide and animals rely on plants for oxygen. In a closed environment such as the space shuttle, humans use oxygen at a rate that is difficult to replace with plants alone given limited space.


This purpose of this science demonstration is to observe photosynthesis in action using the freshwater plant Elodea and compute how much Elodea it would take to generate enough oxygen for one human breath. Photosynthesis converts carbon dioxide, water and energy (in the form of light) to sugar, water and oxygen. The chemical equation looks like this:

equation for photosynthesis


In the Scott Carpenter Station experiment hood, you will see a jar wrapped in aluminum foil labeled "Photosynthesis demonstration". This jar contains Elodea underneath an inverted funnel with a graduated test tube (marked for volume measurement) over the narrow end of the funnel. As the Elodea photosynthesizes, it gives off oxygen that is captured in the funnel and rises into the test tube. This allows us to measure the amount of oxygen generated by the Elodea.
  1. Before you begin your webcast or webchat, unwrap the foil and record (on the measurement collection sheet) the volume of oxygen in the top (narrow end) of the test tube (the test tube is marked in milliliters) and the time you make this measurement. Look closely at the Elodea, you should see little bubbles forming on the leaves and rising to the test tube. Leave the jar unwrapped and under the plant lights during your webcast or chat.
  2. After your webcast or webchat, look at the test tube again and record the volume of oxygen generated since your last measurement and the time of the measurement.
  3. The difference between the two volumes is the volume of oxygen generated by the Elodea, compute and record this on the measurement sheet. Now, given the fact that the volume of the average human breath at rest (tidal volume) is 500 ml and it contains 21% oxygen (about 1/5th ), how much more Elodea would it take to make enough oxygen for one breath? Record your answer on the Collection Sheet.


Is the amount of Elodea required practical to use on a space station?

What other ways can you think of to provide O2 and remove CO2 from an enclosed environment?

Measurement Collection Sheet

Your name Date Start time End time Start
Volume 02

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