id 3 - MetaBlast

Reconstructed Photosynthesis Lab

Katherine Larson, Biology teacher and RET master teacher, Des Moines Public Schools

A s teachers, at times we find ourselves losing the forest through the trees and when dealing with photosynthesis we quite literally need the trees. Photosynthesis is often taught in the classroom almost as a separate entity to plants. However, students in high school still need actual plant references in order to fully understand how plants and their systems and processes fit into the larger ecosystem. One way a teacher can do this is by starting their photosynthesis unit by doing macro scale labs with students to reinforce their understanding of how plants work. Some of these labs are restructured from Wisconsin Fast Plant labs. The Wisconsin Fast Plant is a curriculum set out to help teach students about plants. You can visit their website for even more activities, This lab provides a supplement for the Meta!Blast level: Light Reactions.

Lab 1

Studies have shown that even adults do not fully understand what plants need to function. Students often do not understand what the plant receives from the soil, air sunlight, and water. Misconceptions are often strengthened by statements such as "going to the store to pick up plant food" which is really a fertilizer that the plant cannot live on exclusively. It might be surprising to teachers, especially at the high school and college levels, about the level of knowledge that the students have about plants; teachers often skim over the basics because they assume that the students have this knowledge and the basics seem too elementary. Taking extra time to do basic experiments with plants in order to reinforce what plants need from the sun, soil, and air, can ultimately strengthen students' understanding of more complex ideas about photosynthesis.

Questions to ask students

  • What do plants need to grow?
    While this may seem like a very easy question, and students after thinking a bit are able to list soil, water, sunlight and air, many students and even adults are confused about what the plant gets from each of these items. For example, it is a common misconception that plants obtain their food from the soil.
  • How can we test to provide evidence that plants need these things?
    The class themselves can discuss how to format each test. This will enable them to come up with experimental designs, and also to actively incorporate the concept of controls.

Some ideas the class can try

  • Light vs. dark: the class can both grow plants from seeds, one in the light and one in the dark, and they can take mature plants and place some in the light and some in the dark, keeping all other factors the same.
  • Water vs. no water: same as above test this idea on both germinating plant seeds as well as mature plants.
  • Soil vs. no soil- plants can be grown or transferred into a medium such as sand, vermiculite, or paper towel to observe the changes that occur over time.
  • Air vs. no air- the class could restrict air flow by placing plants in a sealed jam jar, versus left in the air.
  • Fertilizer vs. no fertilizer- this lab can be done either with soil or hydroponically.

These labs work very well using the Wisconsin fast plants, students can see results five days after planting. What is interesting is that the plants grown for the light vs. dark experiments initially will actually grow faster and taller in the dark. This is a great opportunity to have students draw guesses about what will happen to their plants and why, before the lab starts. By keeping them longer under these conditions, students will observe that they indeed do need light.

Follow-up questions

  • What do plants need to grow?
  • What do plants need to grow?

This can lead the students directly into the next lab.

Lab 2

This lab could be a teacher directed inquiry to have students notice how the plant is affected by sunlight/darkness and that two different processes are occurring. A teacher could do this lab before introducing photosynthesis and respiration so the students are able to see the phenomenon before learning about the processes.

This lab is to help students actually see photosynthesis and respiration at work in the plant. The activity uses low cost, simple materials and seed leaves (cotyledons) from 3 or 4 day old Fast Plants which the seeds can be ordered on-line.

When plants photosynthesize they release oxygen into the atmosphere. This oxygen comes from water in the cells of leaves and is initially released into spaces inside leaves. The oxygen then moves from leaves into the atmosphere through small holes on the leaf surface called stomata. In this activity, the production of oxygen is used as a measure of the rate of photosynthesis. Plants also need carbon dioxide for photosynthesis. For this investigation carbon dioxide is provided by a baking soda solution.

Students from the previous Lab 1 now have an understanding that plants need air, water, light, and soil in order to sustain life. They should also be knowledgeable about what the plant receives from each of these items although this knowledge could be sparse (i.e. they know the plants need sunlight to grow and live but not sure what it actually receives from the sunlight.


  • three or four day old Fast Plants seedlings
  • baking soda
  • small straw
  • 35 mm film can
  • 5 ml syringe

Questions to ask

  • Why are plants so green?
  • What does it mean if some plants are more green than others?
  • How do plants "breathe"? and what does this mean for the plant?
  • What does sunlight actual do for the plant?

These ideas can go on the board and the teacher can use these ideas to help structure activities in the classroom so they hit the misconceptions the students may have.


  1. Add enough baking soda to barely cover the bottom of a film can. Fill can with water, add lid and shake to dissolve baking soda.
  2. Using the straw, cut four leaf discs from the cotyledons of 3- to 4-day old Fast Plants. Have them describe the plant thoroughly in a notebook looking at the color and size of the plants.
  3. Remove cap from the tip of syringe. Pull the plunger out of the syringe. Blow the leaf discs out of the straw into the syringe. Replace the plunger. If they can, have students remember which disc came from which plant.
  4. Draw 4 cc of baking soda solution into the syringe. Invert syringe, tip end up. Gently push the plunger to remove all the air.
  5. Put your finger over the syringe tip and pull the plunger. This will create a vacuum which will pull the air and oxygen from the leaf discs.
  6. Tip the end of the syringe down so that the leaf discs are in the solution. Release plunger; remove your finger. Turn syringe back up and tap the side repeatedly until all (or most) of the discs sink.
  7. Place the syringe narrow-end up about 5 em from the light source or in bright sunlight.
    1. What observations do you see happening over time?
    2. Why do you think this is occurring?
    3. When you look closely at the disks what do you observe?
  8. Discuss with student their results and why they think this occurred. As a secondary part, the students can then put the syringe in a dark room or cover the syringe partly.
  9. What do you notice happening after the syringe is in the dark area? Why do you thinkthis is happening?


Do not use too much baking soda! Use just enough to barely cover the bottom of the film can. If you use too much, bubbling will occur. The resulting bubbles will stick to the leaf disks and keep them from sinking. Add a drop of liquid soap or detergent to the baking soda solution to reduce static. After you have created a vacuum in the syringe, some of the leaf disks may still float. This is frequently caused by bubbles stuck to the disks.

These bubbles can usually be removed by sharply rapping the syringe on the edge of a desk or with your finger. While running your experiment, tap the syringe with your finger every 20-30 seconds to dislodge disks which are ready to float but stuck to the syringe. Remember that photosynthesis is dependent on light. For your initial experiments you may want to have the disks rise quickly (3-5 minutes). This will require that your syringes be several centimeters from the light source.


This investigation can also allow you to explore respiration through the measurement of oxygen consumption. Respiration is common to all plants, animals and other organisms which live in an aerobic environment. When plants are grown in the light they usually produce more oxygen through photosynthesis than they consume through respiration. However, when plants are grown in the dark, the trapping of light by photosynthesis can no longer occur, and more oxygen is produced by photosynthesis. Thus, when the syringe in this experiment is put in a dark place, or covered by a black film can, it is possible to investigate plant respiration. White light is composed of all of the colors of the spectrum. You can investigate which of these colors are necessary for photosynthesis by covering the syringe with cylinders of different colors of plastic film. We suggest you try at least the three primary colors: red, yellow and blue.

Follow up questions

  • What did you notice about the amount of time it took for each disk to float or sink?
  • How did the color of the disks associate with those times? What could the color of the leaves tell us about the rate?
  • What are the disks doing when they are exposed to the light? Dark? This would be a great time to introduce photosynthesis and the processes of photosynthesis.