It is again the problem of how to simplify without lying too much.

Exactly. My solution is to avoid lying whenever possible. In this case, it might be appropriate to tell students that we are going to describe the traditional view of photosynthesis in flowering plants. You could mention that later on they will learn about different versions of photosynthesis in other species, such as bacteria and algae.

The problem with that version of teaching is that is misses several opportunities to really educate students. We all know that students enter the classroomâ??or open a textbookâ??with certain biases. One of the best ways to teach critical thinking is to confront those biases and challenge students to think of other possibilities. If we continue to cater our education goals to the pre-conceived notions that students already have then we aren’t really doing our job as teachers.

The fact that Earth’s atmosphere contains oxygen is due to bacteria, not plants. Even today, 30% of the oxygen is produced by bacteria. What’s wrong with telling students about this? And what’s wrong with explaining that the “synthesis” part of photosynthesis is production of ATP and NADPH then going on the explain that those “energy” molecules can be used to make proteins, nucleic acids, carbohydrates, and lipids? Wouldn’t than contribute to changing students’ perspectives about life? Isn’t that the goal of education?

As a curious question, how much of the history of photosynthesis research should be covered? Should students be told about the experiment which showed that the O2 comes from oxygen and not from CO2, for example?

This definition applies to all species, including those that don’t use water as a source of electrons and don’t release oxygen as a by-product. What it means is that my students will get a big picture view of photosynthesis covering bacteria, single-cell eukaryotes, and plants. One they’ve understood the basics, we can concentrate on describing more specialized versions of photosynthesis, including the one that occurs in plants.

I explain that many species can fix carbon. The process that often occurs is called the Calvin cycle and it’s the one that is most often mixed in with photosynthesis in introductory textbooks. That’s because we usually think of big plants when we think of photosynthesis and those big plants need lots of structural carbohydrate (e.g. cellulose). This old, out-of-date, view of photosynthesis comes from a time when bacteria and algae got short shrift in school.

In most other species, the ATP generated by photosynthesis is used for all kinds of things like protein synthesis, nucleic acid synthesis, and fatty acid synthesis. When you look at the big picture, you realize that the captured light energy isn’t just used to make carbohydrates. This is especially true in bacteria and algaeâ??species that don’t contain a lot of stored carbohydrates. There is no direct linkage between photosynthesis, as I define it, and the synthesis of carbohydrate.

And oxygen production is not a necessary part of photosynthesis.

That’s what I mean by the “big picture.”

In fairness, my (correct) viewpoint is a minority one. Almost all textbooks and websites describe photosynthesis as a process that converts light energy into sugars. If that’s the “big picture” then where do plants get the energy to make everything else? Is it by breaking down sugar as soon as it’s made?

jg