Do Plants Really Make Air?

 Introduction:

Photosynthesis is the process of capturing sunlight and transforming the energy of that light into a chemical form.  Photosynthesis takes place in specialized cell structures known as chloroplasts.  Chloroplasts contain the green pigment chlorophyll which plants use to trap light energy.  It is found in the chloroplasts and appears green because it reflects green light. The actual biochemistry of the process has been studied extensively and is fairly well understood, but it is more important that students understand the basic aspects of the process rather than the intricate details.  This activity will use a common aquatic plant, Elodea canadensis, commonly known as Elodea to demonstrate the conversion of one gas to another in the process.

National Standard Addressed: 

One of the most important national standards addressed by this lesson is evidence, models, and explanation.  This activity allows students to directly see the evidence of oxygen produced during photosynthesis and provides a model for them to look at.  You can explain to students the process that the plant undergoes to produce the oxygen.

The life science standard, structure and function in living systems, is also given some time in this lab.  As part of the lesson you could explain to the students the structure of plants and use this lab to illustrate the different functions.  As a living system plants are very interesting and also are relatively easy to study.

As a teacher, guiding and facilitating learning is a big part of this lab.  It is a hands-on lab and students will need some assistance with the various steps.  For the most part you will need to facilitate the lab to be sure the students understand what they are seeing.

Objectives and Benchmarks:

At the end of this lab students will be able to describe the process of photosynthesis, at a basic level, and the products it makes.

Materials:

Large Test Tubes & Rack            Deionized Water            Elodea Plants              

Matches                                               Funnel            (short stem)    Slides and Coverslips

Light Source (150 Watt Bulb)            Large Beaker                         Splints  

Sodium Bicarbonate in a 2% Solution                                          

Advance Preparation:

Obtain Elodea and Sodium Bicarbonate solution

Procedure:

Students should have a long section of Elodea.  Then they will wind the stem of Elodea up under the open wide end of a funnel.  Have them fill the beaker with the bicarbonate solution until the level is at least 2 cm above the top of the funnel stem.  Next they should fill a test tube to the top with tap water and place their finger over the end.  They need to invert the tube and move it into the beaker so that the opening is in the solution.  They can now remove their finger and place the inverted tube over the open stem of the funnel.  There should be no air in the top of the tube when they finish.  Now the students should place this beaker near a very bright light source for the next 48 hours.

At the end of 2 days they can note whether any changes have occurred in the tube?  In the lab period, each group will individually remove their inverted test tube from the funnel and attempt to ignite that gas with a glowing splint.  This will only work if they are very careful.  One person should light the splint and allow another to hold it as it burns.  Another person should remove the tube, holding it upside down to keep the gas trapped inside.  As the water drains out they should extinguish the flame and quickly place the glowing splint in the tube.  If the glow of the splint increases or the splint re-ignites this can be considered an indication of the presence of oxygen in the tube.

Things to Consider:

Why do some leaves appear darker or different colors than others?

Did you have an indication that the Elodea plant produced oxygen under the funnel over the 2-day period? 

Possible Integration: 

The chemistry of the gases produced and chemistry of photosynthesis is a good way to incorporate this into a chemistry activity.  Not only is the plant biology covered but also a good lesson in chemistry is produced.

Also the topic of environmental science can be covered with the question of why are plants necessary.

Critical Concepts: