My colleague finds a mistake in Feynman
Since I first saw it on Minds of Our Own, I loved the seed and a log question, where Harvard and MIT graduates are given a seed and a log, and asked how the seed becomes a log. Students wax scientific about photosynthesis, but then when pushed to explain where all that mass comes from, is it the air, the ground, or water, almost all of them default to thinking it’s the dirt that makes up the mass of the tree. Minds of our own goes on to show how this shows students are missing both a very basic scientific idea—that most of the mass of the tree comes from Carbon Dioxide, a gas, and this is because they don’t have a real understanding of the idea that gases has mass.
Recently, I stumbled upon a video of Richard Feynman giving his own answer to this question, via the excellent blog, It’s Okay to be Smart.
I forwarded this video to my colleague, who shares a similar love for this question (we were both indoctrinated in its value at the Klingenstein Summer Institute (Have you been teaching in an independent school for less that 5 years? If so, apply now).
Not to long after I sent the email, he wrote me back.
Sorry to say this about such a great speaker and great scientist, but Feynman gets things badly wrong here.
He suggests that the oxygen generated during photosynthesis is the product of a splitting apart of carbon and oxygen in carbon dioxide. (See min. 3:39)
In fact, all the oxygen produced by photosynthesis is generated by the splitting of water to make H+ e- and . The role of carbon dioxide in photosynthesis is to get reduced, to have hydrogen atoms added to it, not to have oxygen removed. This is easy to demonstrate experimentally by using radioisotopically labeled and . Only when the label is in the is the produced radioactive.
I’d like to think I would have caught this error if I’d paid closer attention to the video as I was watching it and trying to decide whether to forward it, but honestly, my recollection of the AP chemistry class I took 20 years ago is now rather faded, and redox reactions are definitely one of the gaping holes in my understanding.
But I see this as a really exciting interdisciplinary moment—here’s a great physicist, stepping pretty far outside his area of expertise to talk about a topic in biology, photosynthesis, only to be called out on incorrect chemistry by a biologist. It’s all the more exciting that Feynman made this mistake, since it shows we all make mistakes, which is a great lesson for my students to see.
This is exactly the type of interdisciplinary learning I’d like to be setting my students up to do. However, I’m not sure I am setting them up to understand ideas like this, since they see most topics in year long courses completely isolated from one another. Could a biology student hear an explanation from a physicist about photosynthesis and bring up his understanding of redox reactions from the previous year’s study of chemistry to check Feynman’s work? I’m skeptical. And I’m sure than none of them would be able to push to the extra layer of thinking about how we might be able to know this experimentally by using radioisotopes. So how do we teach science students to think in this interdisciplinary manner?
If you want even more information about this, my colleague wrote this excellent article in The American Biology Teacher: Dust Thou Art Not & unto Dust Thouh Shan’t Return: Common Mistakes in Teaching Biogeochemical Cycles.