Monday, June 16, 2008


I've been subbing in the public schools the last couple months (quite an experience after 10 years of homeschooling, but that's another story). Last week I had to go over plants with a class of third graders and then give them a test.

In the review materials, their teacher had given them the following formula:

CO2 + sunshine + water = food

This really made me nuts. When my kids were a little younger, I made it a point to find out just how plants turned sunshine into food. It took some doing, but I finally found a DK book that spelled out the relevant chemical formula. Which is this:

6CO2 + 12H2O + sunlight ---> 6O 2 + C6 H12O 6 + 6H2 O
carbon dioxide + water + sunlight --->
oxygen + carbohydrate + water

Now, she's already given them a chemical name (CO2 -- I asked and one child identified it as carbon dioxide). She could very easily have then given them H2O, water, and then done the math. The carbohydrate, glucose, is a form of sugar, which they would have readily understood -- especially here in upstate NY, where maple sugaring is common!

Actually, I was surprised to see "sunlight" in the actual formula; it provides the energy via chlorophyll, a green pigment that absorbs energy from sunlight. But most amazing of all --


(And photosynthesis was hand-written in on the test as an afterthought; I had to help the students out by letting them know that photo means light and synthesis is making something.)

Just to understand, as with most public-school science in my experience, the information the kids had to know was basically all vocabulary. For instance, they had to correctly label the cotyledon of a seed. Now, I doubt there are many adults who can identify cotyledon but not chlorophyll. Really.

Anyway, here, for the record, is my third-grader-friendly, chemistry-literate explanation of how plants make food. I am looking forward to exploring biology again (my plan for next year) in light of my ever-growing comfort with chemistry.


Anonymous said...

From what I can see this is all true but from what scientific people have told me is that the formula you wrote was wrong. It should be

6CO2+6H2O - C6H12O6 + 6O2
Carbon Dioxide+water-glucose+oxygen

Love my life said...

I just stumbled across your site today. I am a high school chemistry teacher and my kids are in grade school. The content of their science classes are frustratingly too high in vocab and low in questioning and searching. I try to make up for that at home.
I do feel I need to help you with your frustrations with the science class you were subbing in. Many elementary teachers are not trained in science. School emphasis is on reading and math (since most state testing scores are in those content areas). It is only up to the individual teacher to expand on science and 99% of the elementary teachers I've met or worked with are not strong in the sciences nor work to understand it more.
By the way, your formula for photosynthesis is correct - plants require water and respire water so it is good that you have water on both the reactants and products side of the balanced equation.