Investigating Photosynthesis: Discovering what plants need for photosynthesis

Cindy Boese, John Adams Middle School, Rochester, MN
This activity is an adaptation of the lesson "Autotroph Inquiries: Comings and Goings" from the book Biology Inquiries by Martin Shields (ISBN: 0-789-7652-0).


This activity will be used prior to any direct instruction for photosynthesis. In this inquiry lab, students design and conduct simple experiments using elodea and Bromthymol blue to determine whether plants consume or release carbon dioxide in the process of photosynthesis. Students will record their data which will be used to conclude whether carbon dioxide was consumed or released by the elodea. Through class discussion of student data, students will learn that carbon dioxide was consumed during photosynthesis. At the end of the class discussion students will be asked to answer 5 follow-up questions.

Learning Goals

This activity is designed for students to discover that plants consume carbon dioxide when undergoing photosynthesis. Students will use higher-order thinking skills as they design their experiment, analyze their data and draw conclusions. Skills developed in this inquiry lab are as follows: creating and conducting a controlled experiment, using Bromthymol blue as an indicator for carbon dioxide, creating a data table on which to record data and making observations. The key concept for this investigation is that plants use carbon dioxide, water and sunlight to make glucose, oxygen and water during the process of photosynthesis. Vocabulary words to be reviewed/learned during this lab are photosynthesis, carbon dioxide (CO2), autotroph, producer, elodea, flask, bromthymol blue and indicator solution.

Context for Use

Use this activity for 7th grade life science students prior to teaching about photosynthesis. This inquiry lab can be used for a class of 32 students with students working in pairs or groups of 4. Depending on the background of the students and the length of your class period, it will take 1-2 class periods. Prior experience in setting up a controlled experiment would be helpful for this lab. This fits into my curriculum during our study of cell processes following the study of the cell structure.

Description and Teaching Materials

This activity is used at the beginning of your study of photosynthesis. It allows students to discover for themselves that plants consume carbon dioxide and release oxygen when photosynthesizing through experiments designed by the students. The experiments are done using elodea in a Bromothymol blue indicator solution. The lesson begins with an assessment of student preconceptions and ends with students reflecting on their new understandings.

- One student handout per student
- Elodea – two to four sprigs per lab group (found at pet stores)
- Flasks – 125 ml, two to four per lab group
- Rubber stoppers (#5) or parafilm, two or four per lab
- Bromthymol blue (BTB) 0.1 percent solution, diluted by adding seven drops per 30 ml of water used
- Water, enough to fill all flasks
- Light source
- Drinking straws – one per lab group

Assess Prior Knowledge (15 minutes):
Partners will consider the following questions and then will out the chart on the student handout with their thoughts. What do plants need to live? What do they need to carry out photosynthesis (photosynthesize)?

Demonstrating the tools (10 minutes):
Show the class how they will be collecting evidence of CO2 consumption by doing the following:
1. Fill a beaker half-full with the diluted BTB.
2. Insert a straw into the liquid and gently blow until the color changes from blue to green to yellow. BE SURE TO WEAR GOGGLES WHEN DEMONSTRATING THIS!
3. Explain that BTB is an indirect indicator for the presence of CO2. Carbonic acid is formed from CO2 and water which causes BTB to change to green, and with increasing amounts of acid, to yellow. Conversely, when CO2 is taken out of a BTB solution, the color change reverses back toward blue.
4. To see if CO2 is consumed over time, you need to start with yellow BTB. If it changes back to green or blue, the CO2 was taken out of the solution.

The Investigation (15 minutes):
Students will be given 15 minutes to discuss/plan and write their procedure. The teacher may provide feedback during the planning time or papers may be collected and returned with feedback the next day. Experiments will need to be set up under lights and left overnight. The lights need to be on for all twenty-four hours. The next day students collect data and draw conclusions. The teacher may then lead a discussion on student results and their meaning. Students will then complete the analysis questions as homework.

This activity is an adaptation of the lesson "Autotroph Inquiries: Comings and Goings" from the book Biology Inquiries by Martin Shields (ISBN: 0-789-7652-0). Student Handout for Investigating Photosynthesis: Discovering what plants need for photosynthesis (Microsoft Word 35kB Aug25 09)

Teaching Notes and Tips

Safety precautions: Students should wear splash-proof goggles when working with BTB. Caution students to blow very gently into the BTB when they are using the straw to add CO2.

Term to review/reinforce: Controlled variables in an experiment

Some Possible Student Experiments:
1. To eliminate light some may put a flask in a cabinet or cover it with paper or aluminum foil.
2. Some may have some plants in yellow BTB and others in blue BTB (not helpful).
3. The best will involve a plant in yellow BTB in the light, a flask with only yellow BTB in the light, and a plant with yellow BTB in the dark.

Answer to question 5: The purpose of photosynthesis is to make organic molecules (sugars). It is VERY important to keep students from concluding that photosynthesis evolved to produce oxygen. Oxygen gas is a waste product of the process that organisms then adapted to using.

I have not done this activity in the past. I like the idea of students discovering what is used during photosynthesis. Students will remember this much more than simply being told what is used.


Students will be assessed both as a group during class discussion and as individuals based on their answers to the analysis questions.

Standards Recognize that a chemical equation describes a reaction where pure substances change to produce one or more pure substances whose properties are different from the original substance(s). Recognize that producers use the energy from sunlight to make sugars from carbon dioxide and water through a process called photosynthesis. This food can be used immediately, stored for later use, or used by other organisms.

References and Resources