Investigating Photosynthesis: Discovering what plants need for photosynthesis
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.
Context for Use
Resource Type: Activities:Lab Activity
Grade Level: Middle (6-8)
Description and Teaching Materials
- 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
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.
18.104.22.168.1 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.