Measuring a Potto

Before I launch into this week’s burble, here are a few pictures of some of the things we’ve been up to.

Blood typing cards for the Year 10 6^th form taster day.

Year 7s with hamsters and mazes – quote, “we didn’t think it was possible, but this is even more fun than the flambe banana lesson!”

But for the main event, it’s Year 9, plant transport. There’s a combination of words to strike dread into a hardened biology teacher. Not just plants, but plant transport. And not just plant transport. Plant transport with Year 9. Just shoot me now…

I like to kick off with this totally flippant, irrelevant, basically silly game of Hangman.

Potto Meter

What’s the animal?

Click on the numbers as the wrong letters are suggested to build the scaffold and beyond. No-one has ever heard of a Potto, but they generally get there in the end.

Huh?

So, I beam brightly, what might you reasonably expect to do with a Potto-meter?

Measure a Potto?

Indeed. But, alas, a potometer is used for something else entirely.

And this is where the power of “flipping” exerts itself. I demonstrate how to set up a potometer but I don’t tell them what it’s for or what they’re going to do with it. See the accompanying sheet.A Potto Meter introduction It’s all about observation and interpretation.

What can they see? What does it mean? What could they measure with it?

If they’ve set it up correctly, they should be able to see a bubble travelling up the capillary tube. Ah, water must be moving up the stem. What could they measure? They eventually figure out the idea of rate. Excellent! Go on, then. Measure it.

The main difficulty for this bit is getting enough working potometers to gain critical mass. You rarely have a class where every single group gets them going, but you want the majority to do so.  I think it’s important to set it as a challenge for them – say something like, “you have to be really skilful and careful and persistent, but you can do it….”

There’s nothing terribly surprising at this point. OK, so plants take up water. They knew that. OK, so we could measure how quickly it happens. Big deal.

But once they’ve done this and got some number – 5mm a minute, something like that – note the next step. They strip off all the leaves and see what effect it has. This is usually quite a dramatic moment. They love the active defoliation of the plant, but aren’t sure what to expect. When the bubble stops moving, as it will, they’re surprised, even confused. But, hang on, does that mean the leaves are pulling up the water? But that can’t be right…

I leave (geddit?) that thought hanging and move them on to something a bit more tangible. Water is clearly moving up the stem, so we need to look at a stem. Some TS microscopy + detailed drawing and interpretation follows. If you have one of these – Celestron Digital Imager http://www.celestron.com/browse-shop/microscopes/microscope-accessories/imagers/digital-microscope-imager– you can project/capture their microscope images (this is a fantastic bit of kit –hugely recommended – best thing I’ve bought this year – I plan to eventually have one in every lab).

They do their usual careful observations and beautifully observed drawings.

Which bits do they think the water travels along?

Interesting, they all correctly point out the xylem, even if they don’t know the name yet.

We finish with the time honoured Pull The Red Xylem Out of the Celery activity and mush it up to see the spiral lignin patterns. This is much easier to see with tinned rhubarb, but they don’t get the reinforcement of seeing the water tubes inside the stem visualised by the red dye. Why not do both?

Observing, interpreting, measuring, drawing, thinking… but no notes, no information, no facts… oh, look, that’s the end of the lesson.

In the following lesson, I will expect them to remember that:

1. Water travels up a stem
2. In some kind of pipe
3. You can measure how quickly this happens with a potometer
4. It seems to have something to do with the leaves

At this point I will start to introduce a few technical terms, look at the structure of a leaf to try and figure out what’s going on, and invite them to try and suck water up 10m of hosing, a bit like this:

This gets very competitive and hugely entertaining. A lot more entertaining than writing Year 12 UCAS subject references, which is what I’m off to do now.

Next week, protein forensics…