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Investigation 4 – Concept Day








Photosynthesis: Investigation 4

Concept Day


In this relatively short Investigation, we will finish our study of photosynthesis. We will address the component of the photosynthesis reaction we have not yet discussed; the production of the other product (in addition to O2) of the photosynthesis reaction, glucose.

We will also provide you with an overall review of the photosynthesis reaction and how you studied it in the lab.

Finally, we will end with a slide that attempts to give you an appreciation of the flow of solar energy from the Sun through processes on Earth.



  • This is a review slide that asks the important question “How can we determine if the sugar glucose (C6H12O6) is formed by photosynthesis?” Glucose is the only component of the photosynthesis reaction that we have yet to consider.



  • This slide shows that the glucose molecules produced in plants by photosynthesis are stored in the leaf not as the sugar itself, but as a “polymer” of glucose molecules know as starch.
  • As depicted, individual glucose molecules bind to each other in a very specific way to form long chains. In the starch molecule shown here, there are three glucose subunits. However, notice the brackets around the central subunit and the subscript circled in red. This subscript means that the chain of glucose subunits ranges anywhere from about 300 to 600 glucose subunits! Thus, the starch molecule is rather large.
  • Cells cannot directly use starch for energy. Starch is therefore a stored form of chemical energy derived through photosynthesis from the energetic photons from the Sun. As stated in this slide, when additional free glucose is needed by the plant cell, enzymes break down the starch molecule into its constituent glucose molecules for immediate utilization by the cell.
  • Finally, the last bullet on this slide states that starch molecules can be detected by reaction with iodine. In Investigation 4 lab, you will use iodine staining to localize the distribution of stored starch in the leaf of the Coleus plant.



  • This slide depicts Coleus leaves at the upper right. Coleus leaves are used in the experiment in Investigation 4 because the chloroplasts (and therefore chlorophyll) are not evenly distributed. The lack of green chlorophyll in the central regions of Coleus leaves tells us that photosynthesis does not take place in these regions.

Note: In Investigation 4 lab, you will follow an interesting multistep procedure to first extract the pigments from a Coleus leaf and then stain the resulting leaf with an iodine solution to determine the distribution of starch molecules in it. This experiment helps demonstrate that starch, and therefore glucose, is a product of photosynthesis.



  • This slide provides an overview of the entire photosynthesis reaction once again. At the bottom of the slide, the question is asked, “How did we assess the involvement of the various components of the photosynthesis reaction in the lab?

Note: It is intended that this slide will not only serve as a good review of the photosynthesis reaction itself but that it will also stimulate discussion that correlates the lab procedures and experiments performed during the course of this CELL with the basic chemistry of the photosynthesis reaction.



Note: This slide may be used in combination with the previous slide in the discussion of laboratory procedures and findings that you used to explore photosynthesis.


  • Let’s identify the lab procedures on this slide by referring to the components of the photosynthesis reaction in order from left to right.
  • Carbon dioxide formation: You used the pH indicator phenol red to detect CO2 depletion caused by Elodea plants in Investigation 2.
  • Chloroplasts: The phenol red Elodea experiment mentioned above suggested that plant tissue was required for photosynthesis. However, the wet mount of an Elodea leaf in Investigation 3 localized the site of photosynthesis to the chloroplast.
  • Chlorophyll: In Investigation 1, you performed paper chromatography on a bright green extract from spinach leaves. This green pigment was, of course, chlorophyll.
  • Photons (light): In Investigations 2 and 3, you followed the photosynthesis reaction in the presence and absence of light and found that light (photons) are required for photosynthesis to occur.
  • Glucose: In Investigation 4, you localized the glucose polymer, starch, to the precise areas of the Coleus leaf where photosynthesis occurs, namely, the green areas containing chloroplasts and chlorophyll.
  • Oxygen: Finally, the oxygen meter and probe were used in Investigations 2 and 3 to directly measure O2 production by photosynthesis in the presence and absence of light in the aquatic plant Elodea.



  • This final slide shows the overall flow of solar energy from the Sun through various processes on Earth.
  • Photon production is achieved in the center of the Sun where conditions permit nuclear fusion. In the fusion process, the nuclei of 4 hydrogen atoms are fused to form one helium atom. In the fusion process, a photon of energy is released.
  • Once a photon leaves the surface of the Sun, it takes about 8 minutes to arrive at Earth. Some photons strike the leaves of plants and cause photosynthesis to occur. In the process of photosynthesis, the energy of the photon is captured and converted to potential chemical energy in the glucose molecule.
  • The potential chemical energy of glucose and starch is stored and utilized by plants to increase their tissue biomass. Wood harvested from trees contains large amounts of stored potential chemical energy.
  • Finally, at the lower left, when wood is burned in the “combustion” reaction of fire, the bonds storing potential chemical energy from the tree are broken and the potential chemical energy is converted and released as the kinetic energy of light (photons) as well as kinetic radiant energy (heat).
  • Obviously, the scenario shown in this slide represents only one of many energy pathways. For example, potential chemical energy stored by trees through photosynthesis that occurred millions of years ago can be converted into coal and oil over time. When these fuels are burned, this ancient stored and highly concentrated potential chemical energy is released in various kinetic energy forms and can be used for heat and to do work.
  • Alternatively, the stored potential chemical energy of plants may be consumed by animals and transferred to kinetic energy of heat, growth, and movement. In a later CELL, Ecosystems, we will return to the concept of energy flow from the Sun through ecosystems on Earth.