Photosynthesis Lesson Plans

Looking outside at the amazingly green world, I can’t help but think about one thing – photosynthesis.  All the green leaves are working overtime, converting solar energy from sunlight into the chemical energy of glucose and other organic molecules.  If it wasn’t for those green leaves, life on Earth as we know it would not exist.

Photosynthesis is an essential part of any biology curriculum.  The chemical process can be difficult for students to understand, but the subject links cellular biochemistry and ecology.  I teach my photosynthesis unit after students learn about the parts of the cell and before we learn about ecology.  This helps my students to really understand the importance of photosynthesis and its role, which is my main goal.

I start my photosynthesis unit with a science fiction story.  I give students a scenario in which it is twenty years in the future, and they have children of their own.  A news story comes on the television saying that the chloroplasts of plants all around the world are mysteriously breaking down and photosynthesis is coming to a halt. Their children ask them what that means, and they have to tell me what they would reply.  This is also a good way to incorporate writing by having students write the end of the story. 

Once I have reminded students of the importance of photosynthesis, we begin to discuss the process itself.  I usually link this to prior learning by discussing the visible spectrum of light and the wavelengths of light that chlorophyll absorbs and reflects. Next, we look at the overall chemical equation for photosynthesis and break it down into the light dependent and light independent stages.  Having students label diagrams is a traditional and effective way to help them learn the parts of the process.

Labs are always an important way to deepen students’ understanding of a topic, and there are many great ones for photosynthesis. Gas exchange is an important part of photosynthesis, and stomata are the pores on the underside of leaves that allow oxygen to leave the leaf and carbon dioxide to enter.  Stomata and their accompanying guard cells are easily visible by painting the underside of a leaf with clear nail polish, then once the polish is dry, peeling it off with a piece of transparent tape.  If you tape the impression to a microscope slide, you will see the coffee bean shaped guard cells and the stomata between them.  Students can design an experiment to either compare stomata size or quantity in various leaf types, or to see the differences in stomata size in plants exposed to light or dark.

Another way to experiment with photosynthesis is to use elodea in bromothymol blue.  Bromothymol blue is an acid indicator that turns yellow in the presence of carbon dioxide and blue in its absence.  Put the elodea in a test tube of dilute bromothymol blue solution and have students conduct their own experiments to determine how various conditions can change the rate of photosynthesis.