From Seed to Seed:
Plant Science for K-8 Educators

Activity 15: Floating Leaves?

Grades: 5-8

Associated Lesson Topics:

• Photosynthesis
• Light requirements

National Standards:

• SCIENCE: Standard A, Standard C

Planting the seed...

What do you think plants need to survive? Where do you think they get these much-needed resources? What do you think they do with these resources once they have obtained them?

Teacher Information:

We recommend conducting this demonstration after students have been introduced to the basics of photosynthesis. The purpose is to help solidify students' understanding that during photosynthesis, plants consume carbon dioxide and produce oxygen. By exposing leaf "discs," in a carbon-rich solution, to light and dark, students will observe photosynthesis and further understand what plants require to photosynthesize.

Necessary Materials:

• Carbon-rich solution. This solution can be made by mixing a small amount of baking soda and warm water.
• Small plastic syringe with the needle removed.
• A single leaf from a plant growing in your garden or classroom.
• Metal single-hole punch or plastic straw.
• Fluorescent lights.

Procedure:

• Pour the carbon-rich solution into the syringe.
• Remove a leaf from a plant.
• Immediately, use a metal hole punch or a plastic straw to punch holes in the leaf and create leaf discs.
• Place the leaf discs in the syringe with the carbon-rich solution.
• Replace the plunger on the syringe and hold with the tip pointing up.
• Push the plunger to force out all of the air.
• Put a finger over the nozzle and gently pull the plunger down. This action will release oxygen from the leaf discs, causing them to sink.
• Tap the syringe to force the air bubbles to rise. Continue doing this until all leaf discs sink.
• Place the syringe with the tip pointing up under fluorescent lights. As the leaf discs photosynthesize, they will produce oxygen and rise to the surface.
• When all of the leaf disks have reached the surface, place the syringe, pointing upwards in a dark spot. Check the syringe occasionally. In the dark, photosynthesis ceases, and the effects of respiration become clear. The leaf discs begin to consume oxygen, causing them to sink.

Harvesting the Crop...

What happened when the leaf discs were placed under lights? In the dark? Based on what you know about photosynthesis, why do you think this happened? Why do you think the leaf discs were placed in a solution instead of just in water? Again, knowing what you do about photosynthesis, what can you infer that the leaf discs were getting out of the solution? Can you make suggestions about where they get this normally? Through this demonstration, students can also learn about respiration. Respiration is, essentially, the reverse of photosynthesis in that oxygen is consumed and carbon dioxide is produced.

Sources

• Plastic syringe
  Carolina Biological Supply www.carolina.com

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