I tried something new this week with Year 13 Photosynthesis. Instead of giving them a brief introductory overview of the process, I challenged them to figure it out for themselves, using the observation that it’s Respiration in reverse. Simple idea, but it worked really well. They started by going for the trees rather than the wood, trying to remember all the intermediate compounds and exactly what happens in the Krebs’ Cycle. But after a hint to ignore the specifics and focus instead on the general principles, they put it all together. Since Respiration finishes with oxygen accepting hydrogen to form water, Photosynthesis must start with water being split to form oxygen and hydrogen. Oh, says one of my brightest students, is that what they need the sunlight for?
From there, they decided there must be a hydrogen carrier… but to do what? What do hydrogen carriers do? They end up reducing other things. But what? Well, what’s the other waste product in respiration? Oh, is the hydrogen added to the carbon dioxide? Bingo! So what happens to the carbon dioxide? Oh, it’s being reduced. To form? Carbohydrate.
They felt very pleased with themselves so had some fun deriving the absorbance spectrum for geranium leaf photopigments.
Nice, quick, easy practical – yet another use for colorimeters – with some gentle questions for homework. Just left time to read the Carbon chapter from Primo Levi’s Periodic Table – which makes for a glorious introduction to this topic. I always feel that students’ negative reaction to plants is just a failure of imagination – Levi’s carbon atom fantasy tries to counter this.
Meanwhile, the introduction to Cells with Year 7 is going down well. I really like recreating scientific breakthroughs of the past – apart from anything else, it allows you to tell your students that they’ve just emulated a genius – so getting them to slice cork and make a detailed drawing of what they observe sets things up nicely. I accompany this with a reading of Cops and Robbers (Janet and Allan Ahlberg) for this priceless page
“Here are the robbers of London town
In cells all gloomy and grim.
“Let us out! Let us out!
Not guilty!” they shout,
And, “It wasn’t me – it was him!”
which absolutely nails the concept of why he called them Cells. And then they build little models of the cells and use their homework to write a letter from Hooke to a friend about his exciting new toy. They love ageing the letters with tea, burning or tearing the edges, using a wax seal, and so on. This year, I combined their letters with their cell wall models to make this display.
Note the Year 12 protein model dangling in the foreground….