Tag Archives: xylem

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!