Sugar and carbohydrates, protein, fat and water - that’s what little plants are made of!

Modified by Rachel Hughes and Kirstin Bittel
Portions modified from Teaching Science for Understanding: Plant Nutrition Activity 5.2 and 5.3
Andy Hind, Jenny Lewis, and Phil Scott, Center for Studies in Science and Mathematics Education, the University of Leeds

Time: 2-3 class periods
Materials: Overhead of basic plant cell
Fact sheet
Activity sheet
Lab Instruction sheet
Lab equipment
Approximately 100 large test tubes
Light source
Test tube racks
Graduated cyclinders
Safety goggles
Aluminium foil
Wax pencils
Elodea plants
NB: Sodium hydroxide is caustic, BTB is toxic if ingested
Add Bromthymol Blue Stock Solution (0.5g of BTB to 500ml distilled water. Then add 9 drops of 1MNaOH)Bromthymol Blue Working Solution. Finally, dilute 100ml of concentrated stock solution with 400ml of distilled water.


Earlier lessons have established that photosynthesis is the source of energy for the plant, but how does that energy becomes part of the plant’s growth? Students draw upon their experiences growing plants to establish that the plant needs energy all the time to grow. This lesson starts with a discussion and pencil and pen exercise and then moves into a student designed experiment. Students see that some of the sugar made in photosynthesis is used to assimilate the matter that plants are made of; some of which is used to provide a constant source of energy.

Students will be able to:-
1. identify photosynthesis as the mechanism by which plants convert sunlight energy into a useable energy source for plant processes
2. identify photosynthesis as the mechanism by which plants create a molecule that can be used to build or grow.
3. recognize that the plant needs a mechanism by which to access energy all the time, otherwise known as respiration
4. design an experiment which tests variables one by one

Teacher Background

Resource Websites




Remind students to check on their cotton plants.

1. Ask students “Now that we know that photosynthesis is the energy source for a plant, how do you think that the energy is used to build the stuff plants are made of?” “What are the molecules that make up plants?” “Where do they come from?”
This can be a pre-assessment in students’ notebooks.

2. Using the overhead of the basic plant cell, review with students their knowledge of basic cell chemistry. Identify the types of molecules that make up the cell parts. Using the student sheet, review what these molecules are made of. Students should recognize that photosynthesis produces one huge component of all these molecules, glucose. Ask students where the additional minerals might come from. Referring back to van Helmont’s experiment, ask the students “Where might these minerals have come from?” Students should recognize that the soil did provide some nutrients. Class discussion reviews students’ ideas about basic cell chemistry.

3. Using the activity sheet 1, have students identify whether the plant derives everything from the process of photosynthesis or whether there needs to be some incorporation of another chemical into the substance on the activity sheet.

4. Remind students that chemical reactions might give off energy or need it. Ask students how plants might get energy to build the parts of a cell it needs. While most students will likely respond with photosynthesis at this point, remind them that the plant needs energy all the time. What happens in the dark? How does a seed get energy to build cells to become a seedling? The seedling is in the dark, but it still grows. Hopefully, students will remember the storage of energy in the insoluble form - starch. Remind them of the geranium exercise if they don’t remember. Ask them again why the glucose can’t be stored just as it is.

5. Introduce the idea of respiration as a way of freeing energy from starch. Students should be familiar with the idea of cellular respiration from the second quarter. However, reviewing cellular respiration is important as students often connect the term respiration strictly with the use of lungs to breath. Aerobic cellular respiration is the oxidation of glucose producing energy, carbon dioxide and water. So plants breathe too!

6. Challenge students to support the idea that plants respire as well as photosynthesize with evidence that they really do give off carbon dioxide. Students are to design an experiment using the materials provided. Before they design the experiment provide them with some tools to design their experiment.

  • Show students that Bromthymol blue (BTB) can be used as an indicator for carbon dioxide. BTB will turn light green or yellow in the presence of an acid formed by carbon dioxide and water. Demonstrate how BTB acts as an indicator by blowing through a straw into a solution of water and BTB in a beaker. The BTB solution will turn from blue to yellow or light green with the addition of carbon dioxide. Keep this beaker out on display; it is important that students see that given time, the yellow color will return to blue once the dissolved carbon dioxide reaches equilibrium with that in the air.
  • Introduce students to the water plant Elodea, Canadian Pond Weed. Explain that the cut sprigs of Elodea will continue to photosynthesize and respire if placed in test tubes with water and BTB. Students are to make the assumption that the BTB in the plant’s environment will not affect photosynthesis or respiration.
  • Remind students that they should focus on differing one variable at a time within their experimental design.
  • Encourage students to incorporate the 24 hour time period into their experimental design.

7. Students are provided with an information sheet stating their challenge, the materials available and safety cautions and the points that they should include in their experimental design. They should be given time as a group to design the experiment. Before starting the experiment students must first get approval for their experimental design.

8. After students have performed their experiments and analyzed their data have them present their findings to the class. Once everyone has presented to the class, establish what elements might be essential to an experiment that tests the idea that plants also respire.

For homework: Ask students to draw a diagram which demonstrates the relationship between photosynthesis, respiration and the nutrition and growth of a plant.

Embedded Assessment

Drawing the diagram at the end of this three-day exercise pushes students to incorporate ideas from the mini-unit as a whole. The lab is an opportunity to assess students’ ability to identify variables and create controls in an experiment. The earlier activity sheet encourages students to articulate in written form the class discussion.



PULSE is a project of the Community Outreach and Education Program of the Southwest Environmental Health Sciences Center and is funded by:

NIH/NCRR award #16260-01A1
The Community Outreach and Education Program is part of the Southwest Environmental Health Sciences Center: an NIEHS Award


Supported by NIEHS grant # ES06694

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Last update: November 10, 2009
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