Retrieving the Krebs’ Disaster

Last year (October 16th: Chemiosmosis and DNP) I came clean about an idea that simply hadn’t worked. In my endless search for ways of making students do figure things out for themselves – I had designed an activity whereby they would work out Krebs’ Cycle for themselves.

I wasn’t expecting them to do this completely from scratch! They had already covered ATP, Chemiosmosis, and proton pumping by electrons fed into the ETC by reduced NAD. They knew that the NAD was reduced with hydrogens originally stripped from glucose by dehydrogenase enzymes. They had carried out an investigation with yeast and TTC and interpreted the results (INSERT). And they had met Glycolysis and the Link Reaction, so had met metabolic pathways and the slow tweaking and adjusting of the molecules as each enzyme makes its own small contribution. So the foundation was there. Could they complete the jigsaw?

It looked good on paper. First, a bit of human interest, the story of Krebs fleeing Nazi Germany and finally settling in Sheffield for his entire career because the people were so friendly. Then a list of the various molecules that you find in actively respiring cells (liquidised liver, I believe) – the citrate and the oxaloacetate and so on, along with their basic chemical formula. Next, a brief account of Krebs’ approach – enzyme inhibition followed by measuring the relative concentrations of these molecules. Finally, an outline of what some of the results of the enzyme inhibition.

From that, I reckoned they could figure it out. But when I trialled it on a class of intimidatingly bright students, it failed. They got frustrated and annoyed and ultimately confused. Not great.

At this point, I had two choices. Throw the whole thing in the bin and abandon it as a basically bad idea – and revert to just chalking and talking through the process. Or I could try and figure out why it didn’t work, adjust the exercise accordingly, and try again. Given the inestimable value of giving students ownership of their learning, of the understanding and recall that result from them finding things out for themselves, I opted for the latter.

The trial class had struggled, I realised, because they were trying to account for all of the atoms in the molecules, and couldn’t make any sense of the oxygens. But oxygen is not an important atom to follow in the Krebs’ Cycle – they need to keep track of the carbons and the process of decarboxylation producing CO2 – and they need to appreciate the over-arching importance of dehydrogenation and the production of NADH to power the Electron Transport Chain and thus maintain the proton gradient for chemiosmosis. But oxygen can be safely ignored.

So I went back to the drawing board and re-worked the exercise, summarising the oxygen content of each molecule with (n). Unravelling metabolic pathways a la Hans Krebs

And last week I ran it again. It worked beautifully. They quickly figured out the sequence of the metabolic pathway from the enzyme inhibition results. From there, they deduced that carbon dioxide was being produced as the number of carbon atoms went down AND they identified when dehydrogenation was taking place. To put the icing on the cake, they then figured out that by attaching Acetyl(2C) to Oxaloacetate (4C) it would take them back to Citrate (6C) and the whole thing could start again. It was bloody wonderful.

So I would urge you to try new things. That’s the only way you can find out if they work! Your students will appreciate the effort, even if it’s not 100% successful, and even an apparent disaster will teach you something about your students, your approach, your idea.

Finally, a couple of pictures taken with our Celestron Digital Imager, a bit of kit that is proving so popular with the department that I’ve had to order two more.

cheek cells by Mariablood

These are student slides of blood cells and cheek cells respectively (cheek cells extracted from the micro-centrifuge approach – look how plump, juicy and numerous they are!). With the imager, you can project live microscope work on to your white board, capture images (as here) and even take video footage.

Have a good week!

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2 thoughts on “Retrieving the Krebs’ Disaster

  1. Perks, Fiona (SUT)

    Hi Paul
    Was about to try your squiggle sheet out with my year 13 tomorrow.
    Will try your new one instead thanks
    How much was your digital imager?

    Fiona Perks
    Head of Biology
    Sutton High School
    55 Cheam Road, Sutton, Surrey SM1 2AX
    Switchboard: 020 8642 0594|Direct: 020 8225 3022 |Internal: 33022
    F.Perks@sut.gdst.net |www.suttonhigh.gdst.net

    Sutton High School is part of the GDST network of schools

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    Reply
    1. paulweeks2014 Post author

      Hi Fiona,
      Good luck with Squiggling! Let me know how it goes. Digital Imager was around £100, I think. My only gripe with what is otherwise a wonderful bit of kit is that it’s not intuitively obvious how what to do with images once you’ve captured them. I’ve prepared a User’s Guide with screenshots and helpful arrows which I’d be happy to send over if you’re interested. Best wishes Paul

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