Active Learning Exercises for Teaching: Photosynthesis

I teach a 3rd year Plant Physiology course and a 4th year Environmental Stress Biology of Plants course. In both of these courses I go into quite a bit of detail about photosynthesis. This is material that my students have been exposed to in previous undergraduate courses, but in the context of these two courses my aim is to show students why the process of photosynthesis can be a double edged sword. I think that most people assume that plants love any level of light and that the more light that plants have access to the better. We often talk about photosynthesis as a steady-state pathway when in reality plants are constantly acclimating to their light environment likely on the timescale of milliseconds. I use an active learning exercise in my classroom in order to teach my students that photosynthesis is a dynamic process fraught with dangers for plants.
For this exercise I bring in a bag of soft plastic balls. These were left over from a ball play-set that my son had when he was younger (think the ball pits that you can find at IKEA or that used to be present at indoor play areas). I also bring in a large metal bucket. I ask for 6 volunteers from the audience to participate in the activity. Each student represents a complex/mobile carrier involved in the photosynthetic electron transport chain (e.g. photosystem II, plastoquinol, etc.) and the last student in the row is the enzyme ferredoxin-NADP+-reductase. The balls are used to represent electrons. The first student’s job is to accept the balls that I pass to them and then pass them to the next student. The other students in the chain in turn accept the balls and pass them along the chain. The last student in the chain aims to deposit the balls in the metal bucket. The bucket represents the ability of the plant to use the electrons to produce NADPH and ATP and to fix carbon.
The first time through the exercise I put balls into the chain at a very low rate. This shows the students that sometimes plants can have difficulty generating energy and fixing carbon if light is limiting. This would be similar to severe shading effects for example. I then put the balls into the chain at a reasonable rate. This represents a “steady-state” for photosynthesis where the process is running efficiently. For the last part of the activity I put the balls into the chain at an extremely high rate; as quickly as I can pass the balls to the first student in the chain. Inevitably the balls get dropped frequently at various parts of the chain and many of the balls do not make it into the bucket, or the bucket overflows. I use this to demonstrate over-reduction of the electron transport chain and to explain the generation of reactive oxygen species during photosynthesis.
Based on previous course evaluations, the students enjoy this exercise and claim that it helps them to remember key aspects of the photosynthetic electron transport chain. I’ve found it to be an engaging and effect way to teach about photosynthesis in my classroom.

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