Tag Archives: plant transport

Year 9 feel the pressure

I’ve got lots of stuff I want to talk about this week. I know I promised Year 12 Protein Forensics, but the gels are still de-staining so I haven’t any results to pin on the report – though the comparison of Lamb Kebab protein with Lamb Meat protein is looking pretty definitive.  So maybe next week. And then I had a lesson yesterday with Year 7 that was just fabulous fun, getting them to be rats and putting them in a model Skinner Box. Trust me, you’ll want to do this one – watch this space!

But this week, for the sake of completeness, I need to follow on from the Year 9 introduction to water transport in plants. If you recall, they had been introduced to a potometer and figured out, from their own observations, that:

  1. water gets sucked up a stem
  2. in special pipes called xylem
  3. you can measure the rate of this uptake with a photometer
  4. the leaves seem to have something to do with it

Happily, they could remember all of this from the week before, so it was time to think about How.

At the start of the lesson, I had provided every girl had a beaker with water/Ribena and a straw. I asked them to measure the straw (20cm) and then put it in the beaker.

What happens?

Nothing! OK, water does not spontaneously move up a tube.

OK, so we need to change something. Make the water go up the straw!

They dutifully suck the water up the straw and enjoy a refreshing drink on a hot Friday afternoon in sunny Oxford.

Excellent! Everyone manage that? Well done. Very good. So, how did you make the water go up the straw?

I know what their answer will be, and that’s the whole point, as it’s the springboard for the ideas that follow.

We sucked on the straw.

No, that’s just a word to describe what you did. It doesn’t explain why the water moved up the straw. What did you actually do? What did you actually change?

This takes a while. Pushing past the easy, superficial answer that doesn’t actually explain anything, to an understanding of what needs to happen for a liquid to defy gravity and move up a tube needs to be taken slowly and thoroughly or nothing that follows will make any sense. I ask lots of questions of lots of girls to get them thinking.

For water to move up, how must the top of the straw differ from the bottom of the straw? In other words, what conditions do you create at the top of the straw by sucking?

Bit by bit we get there. There must be a pressure difference. Sucking lowers the pressure at the top of the straw, so the relatively higher pressure at the bottom of the straw pushes the water up.

We spend a bit of time drawing and annotating a beaker with an upwardly flowing straw.

Right! How far can you suck water through a straw? Is 20cm your limit?

Now it gets fun. I produce a 3 meter length of plastic tubing. Any volunteers…?

I pick a couple of girls from the forest of hands – important to do repeats! We need rules – no blocking the end of the tube with tongue/finger in between sucks – it’s got to all come up in one single action. And disinfect the end of the tube in the handy beaker of TCP between girls.

Who thinks they can do it?

They’re not sure. Yes, no, maybe? It’s an experiment to which they don’t know the answer – so they want to find out.

By standing the sucker on a table with another girl holding the beaker of Ribena (must be Ribena, or something coloured, to follow the progress up the tube) on the floor, we can all see what happens. And, as it turns out, sucking Ribena up 3 meters of tube is easy.

What about 4 meters?

At this point we have to go outside and use the fire escape staircase, which is conveniently located next to a fairly broad, empty walkway.

4 meters is also easy.

6 meters?

They have to go higher up the staircase and now it’s significantly more difficult. They report feeling as if their tongue is being sucked back into the tube. Several girls can’t do it. This is quite a challenge.

8 meters?

The only girl who succeeds here admits later that she cheated by blocking the end of the tube so she could grab another breath.

We head back inside – but not before we’ve attracted a large crowd of fascinated girls and staff. “Can I be in your class?” says the Head of Russian as she passes.

Now the theoretical maximum height for sucking water up a vertical tube is a little over 10m, and even this is only possible if you can generate a perfect vacuum at the top. No wonder we were struggling at 8m.

But let’s put it back into context. What’s the topic here?

They have to blink a bit before recalling, oh, yes, plants.

Is 10m the limit for plant height?

Clearly not. The school’s Whomping Willow alone is an impressive 40m.

And what obvious advantage do you have over plants in terms of moving stuff?

We have muscles. Plants are imprisoned by their cell walls.

So how on earth do they do it?

Well, we’ve seen that it’s somehow controlled by the leaves. We therefore need to take a closer look at leaves.

Back to the microscopes and a TS of a ligustrum leaf. Beautifully observed, carefully drawn leaf structure…. The various layers of the leaf are all identified and drawn. We talk about which side must be the top of the leaf and why. We talk about equivalents – all these air spaces – what are they for? Where do you have air spaces in your body? So if there’s gas exchange, what else must there be? Look closely…. They find the stomata, the equivalent of our noses.  We dutifully label and annotate… all ready for the next lesson when we’ll look at stomatal distribution, vaseline some leaves and talk about Joe Simpson in Touching the Void – why he nearly died because of his need to breathe.

Homework is to watch these two video clips from YouTube. Isn’t this guy great?

Nb on tube sucking activity

I’m not wholly satisfied with how I structured this. With only one tube per length, and with nearly every student wanting to try it, too many students are standing around watching. Plus despite disinfecting the ends of the tubes, I still worry slightly about dribble in the tubes themselves. I think next year I’ll order in much more tubing (it’s cheap as chips) and have maybe 4 of each length, which get assigned at random. Then everybody gets a go, it can be a race between girls, and we get through the whole thing much quicker. Plus there’s absolutely no risk of cross infection!

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 6th 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.


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…