Demonstration of the Light Dependent Reactions of Photosynthesis Using Students as Molecules

Written by Rhoda Perozzi, Virginia Commonwealth University

Adapted by Kelly A. Hogan

Learning Outcomes:

– To overcome students’ difficulties with understanding how light energy excites electrons

– To enable students to visualize the way sequential events occur in membranes

– To enable students to understand that water is split in photosynthesis to supply electrons to chlorophyll

Activity Description: Students act out the light dependent reactions of photosynthesis by passing electrons to one another from taller to shorter students. The activity uses a water molecule student as well to demonstrate how water is the source of electrons for the process. This activity has been used in classes of 20 students as well as in classes of more than 300.

Time Needed: Approximately 20 minutes

Materials Needed:

– A long sturdy table for students to stand on in the front of room (not necessary but useful)

– Seven students representing: Chlorophyll of Photosystem I, Chlorophyll of Photosystem II, NADP+, electron transport system (2-3 students), water

– Crumpled paper wad representing an electron. Nametags (large if used in large lecture hall)

Activity Instructions:

1. Ask for a few students to help with a demonstration. From them, choose the second to the tallest and ask if he/she is willing to stand on the table. Explain that the table represents a thylakoid membrane in a chloroplast. Name the first person “Chlorophyll in Photosystem II.”

2. Next, ask the tallest person to stand on the table to the far left of “Chlorophyll in Photosystem II.” This person is named “Chlorophyll in Photosystem I.” (This is ideal if they happen to be wearing green.)

3. Depending on the size of the table, ask for either two or three shorter people to stand between them. These are arranged so that the tallest person is next to “Chlorophyll in Photosystem II” and the shortest next to “Chlorophyll in Photosystem I” (see figure). These two to three people are named “Electron Transport System.”

4. Ask for a relatively tall person to stand on the floor next to “Chlorophyll in Photosystem I.” This person is named NADP+.

5. Look around the room and ask the person with the brightest blue shirt to stand on the floor at the opposite end next to “Chlorophyll in Photosystem II.” Consider “forgetting” to give the person in a blue shirt a name (this will be the water).

6. Identify yourself as “Solar Energy.”  Hand “Chlorophyll in Photosystem II” a paper wad and explain that it is an electron. Explain that ideally everyone in class would be clustered around the two chlorophylls and would be handing electrons to them, but with space being limited, they will have to imagine what that would be like.

7. Hit the paper wad in the hand of “Chlorophyll in Photosystem II,” forcing the hand to go up above the person’s head (Gently of course!). Relate this motion to the action of photons of light. (They kick the electron to an excited state. Point out that if chlorophyll were all alone on the table, the electron would simply fall back down to ground state. Since, it is in a membrane, however, the electron transport chain grabs the electron in its excited state and passes it from one electron transport molecule to the next until it reaches the chlorophyll molecule in PS I.)

8. Have the students pass the electron (from PS II to the electron transport students to the Chlorophyll student of PS II). When the electron reaches PS I, hit that person’s hand, “exciting” the electron again. The electron is then passed to NADP+.

9. Throughout this time, note the person in blue (water has not been named and has been ignored).  Now, walk back to PS II and try to excite an electron again, but demonstrate that there is no electron to excite. Ask everyone what the problem is. (Students should have no problem seeing that chlorophyll has lost its electron.)

10. Ask students where another electron will come from. (Someone should point out that the person in blue has not been identified.) Consider acting surprised and say something like, “Oh, of course! This is ‘Water.’” Turn to the water student and say, “You really have a very important job here. I am going to need your full cooperation. You have to split into pieces and part of you has to leave as a gas.” (The class often finds this amusing.)

11. Slip “Water” another paper wad electron, which he or she then passes to “Chlorophyll in PS II.” Point out that there is always plenty of water in cells to constantly supply electrons to chlorophyll.

12. Have the students applause their classmates that have role played. Have the participating students state their names to help form a classroom community.

Photosynthesis and Respiration: Are They Similar?

Written by Kelly A. Hogan, University of North Carolina at Chapel Hill

Learning Outcomes:

– To compare and contrast two challenging topics: photosynthesis and cell respiration

– To appreciate that comparing and contrasting topics is an important study tool

– To promote discussion and questions from students about where they are confused

Activity Description: After students have already had lectures on cell respiration and photosynthesis, they are asked to compare and contrast these topics. This could be a very open-ended activity or can be done with a quick worksheet.

Time Needed: Activity should take about 15 minutes but could go on longer with much discussion and review

Materials Needed: Copies of the worksheet below

Activity Instructions: After students have learned about both processes, allow students time to compare and contrast photosynthesis and cell respiration. Students often get confused by the similarities of the topics but can’t always pinpoint where they are similar or different. This activity will open up discussion to find out where students are having trouble.

Worksheet: Photosynthesis and Cellular Respiration Worksheet