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Investigation 2 – Concept Day








Photosynthesis: Investigation 2

Concept Day


In this Investigation, we wish to focus on the various components of the photosynthesis reaction.

We will begin by describing one of the classic experiments of all times in plant science, an experiment performed some 400 years ago!

Most of our discussion will pertain directly to the Investigation 2 lab. You will see that you can determine if carbon dioxide is used in photosynthesis and if oxygen is produced. You will also determine the involvement of chlorophyll and photons in the photosynthetic reaction.



  • This is an introductory slide from Investigation 1. It is simply placed here to remind you of the abundance of photosynthesis activity on Earth.



  • This is the first of several slides describing a classic experiment in plant science.
  • Jean Baptiste Van Helmont, the great seventeenth-century chemist and physician, wanted to know where the mass comes from when a plant grows. Not knowing about the gases in the air, he supposed the mass must come either from the water that is supplied for growth or from the soil in which the plant grows.
  • Think about Van Helmont’s hypothesis, that the mass of a growing plant either comes from the water added during its growth or the soil in which it is planted. Without a knowledge of photosynthesis or even the knowledge of gases in the air, wouldn’t this be a reasonable hypothesis? After all, the increasing mass of a growing plant can’t come from nowhere, can it?



  • Van Helmont’s experiment was elegant given what was known at the time. He began by determining that the mass of a willow sapling to be 2.3kg. He also determined the mass of the dry soil he put in a pot. It was determined to be 90.9kg.
  • He planted the willow and allowed it to grow for a period of five years, only providing the plant with water during the course of the experiment.
  • After five years, Van Helmont once again determined the mass of the grown willow and the dried soil. These were found to be 77kg and 90.8kg, respectively. In other words, the amount of soil before and after the experiment were nearly identical, even though the mass of the willow increased by over 70kg! So clearly, the increased mass of the willow could not be accounted for by components found in the soil.



  • Van Helmont concluded that all of the mass gained over the five-year period of growth must have come strictly from the water that was added.
  • Even though he was incorrect in his conclusions, based on what was known at the time, his thinking and interpretation of his data was reasonable. He simply had no idea that a gas in the air, carbon dioxide, was incorporated into the growing willow’s tissues. Actually, CO2 was not discovered until shortly after Van Helmont’s death. It was discovered by a Scottish chemist and physician named Joseph Black. Oxygen gas was discovered shortly after, in the 1770s.
  • In fact, most of a plant’s mass comes from the CO2 in the air, certainly all of its carbon. In Investigation 2 lab, you will follow CO2 utilization and O2 production during photosynthesis. The data that you collect would have been very valuable to Van Helmont!  



Note: With this slide, we begin to discuss the experiments that you will perform in Investigation 2. Some of the questions posed at the bottom of this slide will be further addressed in Investigations 3 and 4.

  • On this slide, the photosynthesis reaction is presented and a number of questions about the reaction are posed.

Note: You might wish to consider what types of experiments might be done to answer these questions before moving on to the next slides.



  • This slide is included again here to remind you that, in nature, the Sun provides the Earth with a nearly endless supply of photons. Of course, the photons only directly strike the surface of the Earth during daylight hours. Therefore, at any given geographic location, photons for photosynthesis are available for only a portion of a twenty-four-hour day.




  • While photons are provided by the Sun in nature, in your experiments, you will provide photons for photosynthesis by using an incandescent light bulb.

Note: You may have first seen a form of this slide in the CELL Electricity and Magnetism and then again in the Light CELL.

  • Electrical energy, that is the kinetic energy of moving electrons, enters the light bulb and must pass through a tungsten filament of very high resistance to electron flow. This causes both heat and light to be given off by the filament. The light, of course, is given off in the form of photons. Such photons are the same as those provided by the Sun and can therefore drive photosynthesis. You will use these photons in the experiments you perform in Investigations 2 and 3.



  • In this slide, we present the basic experimental setup and background for the first experiment of Investigation 2.
  • The pH indicator, phenol red, indicates if a solution is neutral, basic, or acidic. It turns a different color in these pH ranges. As shown in the table, phenol red is yellow in an acidic solution and red in a basic solution.
  • Carbon dioxide interacts with water. When it is present, it causes a water solution to become acidic. Therefore, phenol red will be yellow in its presence. On the other hand, if the water is depleted of CO2, this will cause the solution to become more basic. You can therefore use phenol red to determine the relative amount of CO2 in water solutions.
  • As shown on the left, while both of the test tubes will contain water and phenol red, only one will contain the aquatic plant elodea. Since water has a certain amount of dissolved CO2 in it from the atmosphere, the starting conditions will likely show that both tubes are orange-reddish in color, indicating a somewhat basic pH.
  • If CO2 is used up during the photosynthesis reaction, this will cause the solution to become more basic and the phenol red will become a more reddish color and perhaps even turn purple if enough CO2 is depleted.

Note: From this experiment, you should conclude two things. First, CO2 is used in the photosynthesis reaction, and second, plant chloroplasts and chlorophyll are required for the reaction (because one test tube doesn’t have plant tissue and the other one does).



  • This slide shows the experiment setup for the second part of Investigation 2 lab. In this experiment, you will directly measure dissolved O2 with an oxygen meter and probe.
  • As the experiment is too long to perform all controls in the same Investigation, you will measure O2 concentration in the presence and absence of light over Investigations 2 and 3 as indicated in this slide.



  • This final slide revisits the photosynthesis reaction and asks how you will assess the involvement of the various components of the photosynthesis reaction in the lab.

Note: You should be able to discuss the depletion of CO2, the formation of O2, the requirement of light, and the requirement of plant tissue (chloroplasts and chlorophyll).

Note: You may wish to refer to this slide in PostLab for Investigation 2.