Hydrophobia rules

I structure the Year 12 SoW to build a foundation of important fundamental principles as quickly as possible. So we start with water, making interesting and topical links to the possibility of life on exo-planets, or Europa, and establishing all those vital properties that make it essential for Life As We Know It.

I then focus in on its role as a solvent. How can we tell if something is going to be soluble in water? We build the important vocabulary and concepts and I get them to work out whether any given molecule is likely to be water-soluble.

But is it soluble…

I get them to think of oxygen as the child at a party who won’t share its electrons nicely, so that not only is water polar, but so are -OH groups. And we explore why lipids cannot dissolve – they bring nothing to the water party, and quickly get jostled out of the way so that the water molecules can resume their endless round of speed dating, meeting new and exciting molecules every nano-second.

We then have an imaginative role play where I get them to imagine the very first origins of life on earth, a little corner at the bottom of the ocean where some organic molecules spontaneously form and can start reacting becaue they are soluble, and are therefore free to move and collide. The students are the organic  molecules, standing in the corner of the lab (the deep sea vent), but what will happen?

This takes them a while to figure out. Go on, I urge, what will you do? In solution, in the deep sea? Sure enough, they start to drift apart, diffusing to the far corners of the lab. Why is this a problem for kick-starting life? They get it – they’re too far apart to ever collide. Life will never get beyond the odd nucleotide.

So what do we need? We discuss the concept of a barrier, something that hold these molecules in one place, to prevent them diffusing into oblivion, and allow the very first reactions of life to take hold. And I ask them, what property must this barrier NOT HAVE??? This takes them a little longer as they’re not quite sure what I’m asking. But then the light dawns – any barrier intended to prevent water-soluble molecules diffusing away into the big blue sea MUST NOT BE SOLUBLE ITSELF. In other words, it must be hydrophobic. Can they think, off hand, of any potential hydrophobic molecule that might serve as the basis for this barrier, this, for want of a better word, MEMBRANE?

We look at lipids, figure out why a simple lipid can’t work, introduce the idea of a phospho-lipid, and try drawing diagrams of how they would interact with water, and how you could arrange them so that could associate happily with water while yet remaining insoluble. There’s a lovely lightbulb moment when they figure out the phospho-lipid bi-layer.

Still with me?

OK, so now they can start to work out some of the properties of this MEMBRANE thing. It’s a really nice example of how even a largely theoretical topic can be turned into a journey of discovery, where students figure things out for themselves, rather than just telling them what’s going on.

I set them this exercise.

Movement across cell membranes

A whole load of molecules/ions that must cross membranes if cells are to survive – but can they cross a phospholipid bi-layer? It’s back to the very first principles of water and solubility. If it’s soluble in water it must be hydrophilic, but the membrane itself is hydrophobic. So can water cross it? No. Can ions cross it? No! Can glucose cross it? No, no and thrice over no!!!!

I start introducing bigger ideas. Using this animated PP as an illustration.

channel proteins

Na+ cannot cross the membrane, it will be repelled by the hydrophobic core, and yet it must cross. Your entire nervous system depends on it. Every thought you have, every thing you feel and see and hear and believe is based on Na+ crossing cell membranes. Cl- cannot cross membranes. But it must. If Cl- doesn’t cross membranes you have cystic fibrosis and your lungs fill up with mucus and you are very ill. Even water itself, by definition, cannot cross a hydrophobic barrier!!! And yet clearly it must, not just for simple rehydration purposes, but the whole basis of osmo-regulation in the kidney is built on the water permeability of cells in the collecting duct.

So it must be more complicated than this…. Can they suggest anything?

The suggestions come in, slowly, tentatively, until someone eventually (and they always do) wonders whether there could be another pathway through the membrane?

What, like this? A click on the PP and a channel protein inserts itself. And slide by slide, we build up the properties of these rather wonderful structures. Specific, gated, facilitative of diffusion… It’s a wonderful moment as they start to understand the first basic principles of selective permeability. And it’s why Membranes are one of my favourite topics at A-level.

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Bloody Year 10s…

Here’s how I introduced a new topic at the start of the autumn term for my bright and eager Year 10 classes this year.

It’s an interactive Powerpoint – no, wait! – nothing complicated, just a click and reveal quiz as they choose the blocks based on number or colour.

Blood eating animals introduction

Simple question to start with, can they name any of the organisms?

In fact, why don’t you try it? See how many you get…

(brief pause while eager reader attempts quiz)

How did you get on? Here’s what Year 10s generally make of it.

They all get mosquito and vampire bat. Most of them will get flea and remember it as the Robert Hooke drawing from their introduction to microscopes in Year 7. The leech is sometimes identified as a slug, or a flatworm, but a little prompting gets them there. They often call the tick a spider – for valid reasons! – but usually get Tick, VG! (joke). Bed bugs is a lottery. I point them to the clues if they get stuck. Very few get beyond “bird” for the red-billed oxpecker, though students I have taught in previous years sometimes remember that this is my PhD species… and hardly any ever get the Masai warrior.

Next question – what do all these organisms have in common? The Masai and the oxpecker throw them, because they were thinking, “they all eat blood!” but this is new.

Yet they’re absolutely right. These are all organisms that subsist entirely or largely on a diet of blood.

I take time out to explain the Masai traditional lifestyle and how they use their cattle as a source of blood and milk. This year, I made up some stage blood and asked if anyone was brave enough to try the Masai diet. One brave girl tasted the blood/milk cocktail – and exclaimed, “that’s not blood!!!” (stage blood is syrup, corn starch and food colouring).

But, key question: what does this tell you about blood?

They instantly see that blood provides a perfect balanced diet. So it must contain what?

We discuss the types of carbohydrate, protein and lipids that appear in blood and their function. So many proteins! So many functions! This takes us neatly into the four main functions of blood as a whole (thanks to Bill Burnett for the following Powerpoint review)…

Year 10 Circulation intro

… and its overall structure….

…setting them up nicely for a look at their own blood in the next lesson.

Loadsa dough…

First week of the summer holidays, and I’ve been spending my mornings in the village primary school, offering some sciencey type lessons. I must admit, I was a bit wary – I was told that as part of the end of term arrangements, the children were being divided into four teams of 25 of mixed age. Please could I do four mornings with each group.

Gulp.

I find mixed ability hard enough – I don’t believe that it’s possible to consistently and successfully differentiate in a class any bigger than, say, 2, – but mixed ability AND mixed age? From 6 to 11? From some fairly severe SEN to precoscious semi-genius? Deliver a hands-on practical science lesson?

Yeah right.

So, what would you do?

Tricky, isn’t it?

I wanted it to be fun, I wanted it to be messy, I wanted it to be different to what they would normally do, I wanted it to be science. It also had to be safe and it had to somehow involve all the children in some way.

In the end, I opted for the dough rising at different temperatures investigation. I raided the lab for water baths, measuring cylinders, spatulas, beakers, glass rods, timers and balances. My technician had the inspired idea of throwing in 25 junior lab coats. I added yeast, flour and glucose.

I asked their regular teacher to divide them into mixed age teams of 4, each at a table with the relevant apparatus. I reasoned that if nothing else, they would enjoy handling real science apparatus, learn to take some measurements, and make a lot of mess. If they learned anything else along the way, that would be a bonus.

Setting up, the water baths, set to 45′, 35′ and 20′, were around the edge of the room. Each table had the following:

  • 3 x 100ml measuring cylinders
  • 3 x empty plastic beakers
  • 1 plastic beaker containing strong white flour
  • a glass rod
  • a tea spoon
  • a small saucer of dried yeast
  • a small  beaker of glucose
  • a stop clock

One of the most useful aspects, from my perspective, was teaching the same lesson 4 days in a row. The first day was OK, but I made lots of mistakes and had to completely rethink the lesson plan – there were too many blank faced children who clearly either weren’t engaged or just couldn’t follow what was going on. In short, I tried to explain everything in one go, and then let them get on with it. As you’ll see, this was way too big an ask for primary school children! They managed, but needed lots of support…

Incidentally, I think this is a key skill to being a successful teacher. Teachers stress over lesson observation, thinking that their performance is being assessed, but actually a good observer is just looking at the students. Are they interested? Are they involved? Are they doing lots of different things? Are they asking good questions?  Are they actually learning something, or are they just writing stuff down? There are teachers who seem blissfully ignorant that their pupils are bored out of their skulls. You have to be aware of the success or otherwise of your lesson if you want to improve. That, of course, makes an assumption. Again, I am amazed by the number of teachers who show no interest whatsoever in becoming better at their job…

I digress..

So, on the second day, I started by getting each team to decide on an identifying picture/cartoon/initials that they can draw on the top of their 3 measuring cylinders so they can tell which ones are theirs.

And then I ask one child in each team to pick up the balance.

And turn it on.

They then passed it on to the child on their left. That child had to…

…find an empty plastic beaker and put it on the balance.

I ask each table to report the weight of the beaker. The beaker/balance combo is then passed to the next child who…

presses the on-button again.

This, of course, tares the balance. I check that all the balances now read 0.0g, before the two items are passed on again. The next child has to…

add exactly 20g of flour to the empty beaker.

This is more challenging, and if this has landed on one of the younger students, the older ones pitch in and help. I have told them that the best scientists work as part of a team. But I’m pleased with how this approach is guaranteeing that all the children get to do something. Plus it’s training them how to carry out a procedure they’ll have to do twice more.

The beaker with 20g of flour is then passed on to the next child (who in a team of 4 would be the first one again). This one has to…

add 3 spatulas of yeast to the 20g of flour.

It’s fascinating watching how so many of them struggle with this. It re-emphasizes the point that practical work requires and develops all kinds of skills, which we can’t take for granted.

The beaker of flour and yeast is passed on and….

… one level teaspoon of sugar is added.

I now ask them to choose one member of their team who can both walk in a straight line whilst at the same time carrying a beaker of water. This lucky student gets to

collect half a beaker of water from the 35′ water bath

and take it back to their table. Now it’s time to introduce the measuring cylinders. Taking nothing for granted, I demonstrate the lines, the numbers, the intervals. They seem quite happy with this, so I ask them to…

measure out exactly 25ml of water from their beaker into the measuring cylinder.

Nice and easy, though never underestimate the ability of your students to devise ways of ballsing up a simple procedure. Because although they now have to…

pour the 25ml of warm water into the beaker with the 20g of flour, 3 spatulas of yeast and one teaspoon of sugar….

One cheerful group of boys pours it into the original stock beaker of flour. This only becomes apparent when they try to

use the glass rod to stir it all to a thick, runny paste

and merely succeed in creating a slightly damp floury lump.

And now, I announce, just in case they were finding this all a bit too easy, comes the really challenging bit! They have to…

pour their thick runny paste from the beaker into one of the measuring cylinders without any of it touching the sides….

Children of all ages love a challenge, and these are no different. The screwed up concentration on their faces is just a joy to behold. Of course, they all fail – though one girl was doing very well until her friend made her laugh and some dough smeared the side. But overall, it’s fine – they have enough paste in the bottom of the cylinder  for them to…

…measure the volume of dough in the measuring cylinder, write it down, and then transfer the cylinder to the 45′ water bath.

Phew! Now they just have to do it all again, twice, except that their second two cylinders will go into the 35′ and 20′ water baths respectively. And only then does the real fun start. Starting their stop clock, each team has to…

every 2 minutes, record the volume of dough in each measuring cylinder at the 3 different temperatures.

Initially, they are puzzled. What’s the point in this? But they quickly realise that the dough is rising! This has them scurrying back and forth in earnest. The primary school concept of a table is an interesting one, and we have to police the water baths as the measuring cylinders have an annoying habit of turning turtle, filling up with water, and losing all the crucial dough mix. But they take it very seriously, reading the volume carefully, and writing down the value with great aplomb, one girl writing “55ml EXACTLY”, just to emphasize the point. The excitement builds as dough in the 45’C measuring cylinders threatens to rise above the top and spill everywhere. Whose will rise the highest?

Eventually, it’s time to call a halt. I get them to all sit down with their measuring cylinders in front of them. So, they’ve done lots of measuring and used lots of different apparatus and developed all kinds of skills from team-work to spatula-work. But can they see the wood for the trees? What exactly have they shown?

This is really interesting. They don’t have the vocabulary or framework to make the link between the experimental design and what has happened in their measuring cylinders. But frame the question in a different way, and most of them seem to get the point:

What would be the worst place in your kitchen to leave dough to rise?

And this triggers some excellent questions from the older, brighter ones – but why? why do things happen faster at higher temperatures?

I’ve had positive feedback from parents, teachers and students, but I’ve got to be honest, though. I reckon that most of the students would say that the best thing about their science lesson was getting to wear the white lab coat…

That’s me done for the summer. Hope to be back in September.

 

Kill my babies, but let me eat meat!

Just a visual PS to the last post on energy flow through food chains.

Desk 1

Here’s the start of the game: there’s the pristine tropical island, with just a little corner of cultivation dedicated to growing enough plant food to sustain two people.

Desk 2

BUT…. they’ve decided they want to eat meat every day. Oh dear. The pristine wilderness is under threat!

Meanwhile, on vegetarian island…

Desk 3

…the population has doubled, but most of the island is still untouched.

Back on the isle of meat eaters, a doubling of the population has a more profound effect…

Desk 4

Animals at the top of the food chain are running out of space. The monkey eating eagles are endangered….

Veg-ville continues to cope reasonably well with population growth….

Desk 5

The population doubles again yet we’re still using less land than two people eating meat.

Compare this to the other island…

Desk 6

And here’s the really important question. Never mind the eagles, never mind the orang utans, never mind the vanished rainforest – what exactly are they going to do when the population increases again…?

As a sobering way to wrap up the topic, you could do worse than this…

Decisions, decisions…

Year 10 and Energy Flow Through Eco-systems. Not an obviously exciting lesson, I grant, but one with scope for some entertainingly confrontational role play. After all, it’s a tricky decision, give up eating meat, or kill your babies….

We spend a couple of lessons establishing the basics. The Leaf Litter practical – identifying, counting and weighing the crawly denizens of the detrivore community – after which I collect and collate all their data for them to draw accurate pyramids of numbers and biomass.

I also show them the Flamingo/Fish Eagle/Lake Bogoria clip from Life of Birds, to emphasize the spectacular fall in numbers as you move up the food chain, from algae (gazillions) to flamingoes (one million) to fish eagles ( a few hundred). David (Attenborough) provides the necessary descriptors.

Because this is a really interesting question – what’s going on here?

The following PowerPoint (@Burnett 2005) gives a simplified breakdown of how this all might be quantified as well as a beginner’s guide to why the energy stored in, say, grass, does not all get converted into rabbit.

Energy loss in food chains

In the past, I’ve tried to make this more rigorous using owl pellet dissection – can they use the contents of an owl pellet to calculate the area that a pair of barn owls would require?

Energy Flow from Voles to Barn Owls

But it’s over elaborate and I don’t think it worked very well – they miss the wood for the trees.

In any case, having provided the necessary background, ideas and theory, I’m more interested some of the broader implications of this inexorable law of energy transfers. In particular, how should it inform our choice of food and what we do with available land.

I’ve been playing around with ideas for how to explore this through role play for quite a few years. Again, none of my previous efforts have quite worked. So, for example, I came up with a kind of Monopoly version, where they had a laminated version of a map

island map for role play lesson

that they had to divide up into vegetable and meat cultivation, and then collect food calories (based on their allocation) from another student playing the part of Gaia.

Instructions for desert island survivor game

Having established this, I added more and more students to the island, playing the part of shipwreck survivors/African refugees/rivals from a local school, forcing them to reallocate land use if everyone was going to get sufficient calories per day. It was quite good fun (important note: even if a lesson fails, your students will forgive you if they can see that you were making an effort), but, again, it was over-elaborate and they spent so much time trying to do the calculations that they missed the key point of the exercise.

This year, however, I think I finally cracked it. See what you think.

I send all but six of the students out of the lab. These six form three pairs, each of which has a 70cm x 140cm desk. Each desk represents the island on which that pair have been ship-wrecked. There are no other survivors. The island is a lush, tropical paradise, awash with orang utans and parrots and butterflies and orchids. But they’ve got to eat, so they need to clear land for cultivation.

They have a collection of green and red bits of square card – 10cmx10cm, 20cmx20cm, and 40cm x 40cm.

The small green card represents the area of land that would provide enough plant based food for one person to survive. So with two of them on the island, they need a minimum of two small green cards. The rest of the island is left untouched. It’s an immediately visual impression of land use.

But they may not want to be entirely vegetarian. Most of them will want to include meat in their diet. Thing is, though, if they want to eat meat, one small red card represents the area of land that would support sufficient animals to eat meat only once every 10 days. So if they want to eat meat every day, they need to clear ten times as much land. Suddenly the island is looking rather different, the orang utans are feeling a bit constricted.

At this point I go and collect six more students and allocate two to each “island”. Look! Two more shipwreck survivors! And by an amazing coincidence, they’re old school friends! But what are they going to do about food?

The original students have to explain the game to the new students (peer to peer learning!) and then they all need to discuss and decide how they’re going to carve up the island. It’s an interesting contrast. One table is made up of vegetarians and their island is still largely pristine. Another is occupied by unapologetic carnivores and is covered in large red cards. But everyone is still able to enjoy their preferred diet, even if the orang utans are only just hanging on.

And then they have children. The remaining students are allocated, 3 per table, as the offspring of the original settlers. Obviously their babies are beautiful and adorable but, ahem, how are you going to feed them?

At this point, things get heated. Because based on the premise of the game, the island is not big enough to support seven people eating meat every day…. And so, with the brutal and decisive certainty of teenagers, faced with a choice of cutting back on their meat intake, or discarding their babies, the babies are thrown into the sea…

At this point, the interesting discussion can start. Why does it take so much more land to sustain a meat based diet? No, it’s not because the animals need space to move around – as one student suggests, it’s because you’re putting the energy in the vegetation through another organism before eating it. As 90% is going to be lost, you need 10x as much land.

This also explains why meat is so much more expensive than bread.

And it means how we decide to utilise farmland has profound ethical implications. Should we, for example, be clearing rainforest to grow cows/grow food for cows, just so rich people can eat beef every day?

And would any of them be willing to make any compromises to their diet (such as eating meat a few times a week, rather than every day) if it could contribute to conserving biodiversity, or using the same land to feed more people? Isn’t it immoral to eat meat in a world where so many people are starving?

If you like a feisty argument in your classroom, it works a treat!

Outline and questions here: Ship wrecked on a desert island energy flow June 2017 (1)

 

 

Tell them stories

Still on plant transport, I thought I’d backtrack a little and explain how I try to make the topic more memorable, accessible and comprehensible. One way is to look at parallels between plants and themselves and this next idea was prompted by the frequent misunderstanding that the function of stomata is to allow water to evaporate and pull water/minerals up to the leaves.

So I tell them the story of Joe Simpson and Touching The Void.

If you haven’t read this book, I’d really recommend it, though – spoiler alert! – I’m going to retell it now, as I tell it to my Year 9s/10s. But this is obviously a very abridged version, probably full of errors, and for the full effect you really need to go to the original source.

So, in 1985, a young British climber called Joe Simpson went to Peru with his friend Simon Yates to climb the west face of Siula Grande, an unclimbed route in the Peruvian Andes. It was a very remote mountain and 6356m high. They had to hike, with donkeys, for a week just to reach the place where they would camp. They then spent a week acclimatising and practising, before setting off for the climb.

They took lots of equipment – ice axes, crampons, sleeping bags, ropes, other climbing gear, instant food, a stove and some gas cannisters.

I ask the students why the importance of a stove and gas, and then discuss the need to melt ice both for cooking and drinking. Climbing at altitude, using lots of energy, breathing thin air, you dehydrate very quickly, as water evaporates from your lungs. I get them to breathe on a mirror to remind them of condensation. The whole disaster that followed, I say, was because they didn’t take enough fuel…

This intrigues them, captures their imagination and their curiosity.

I sketch Siula Grande on the board. I describe the challenges of climbing up a 1500m wall of near vertical ice. I tell them how the first day of climbing, after trekking over 6 miles of rocky ground and glacier to get to the base of the mountain, went well, and how they found a sheltered snow hole to spend the night, melting ice to make tea and a supper.

The next day was harder as they ran into the meringue like flutings and cornices that form on Andean peaks. Fragile, unstable, nerve wracking to climb on. They floundered for hours, trying to force a way to the peak that was only just above them, but ended up having to spend another night on the mountain. They were behind schedule, and running out of fuel.

The next day, exhausted and scared, they summited, but now the weather, which had been glorious, was closing in. A storm on top of a mountain was not a great prospect, so they floundered along the ridge, not sure if they were on the mountain or a fragile overhanging meringue of snow and ice. The storm forced them to spend a 3rd night on the mountain, and, making breakfast the next morning, the gas ran out. They would have no more water until they got to the lakes back at their campsite…

The next day, in terrible weather, no visibility, struggling along the ridge, Joe fell and shattered his leg. The only pain killer they had was paracetemol. He should have died. Nobody breaks a leg on top of a remote mountain and survives. But, instead, Simon lowers him, on a rope, down the side of the mountain, 30 meters at a time. While he is lowering Joe, Simon makes a little snow seat to take the weight. When the rope runs out, Joe does the same while Simon climbs down. Over and over again, down this massive face.

They’re nearly there, they nearly make it…

But they can’t communicate. The storm means they can’t see each other or hear each other. And near the bottom of the mountain, Simon gets to the end of the rope and Joe has not taken his weight. Reason being, Joe has been lowered off the edge of a cliff and is danging in mid-air. I sketch this on the board for effect. And then explain how Joe’s weight is slowly pulling Simon out of his snow seat… At the last second, Simon remembers his Swiss army knife, grabs it out of his pack and… cuts the rope.

Pause for effect. There is always a gasp, a moment of shock.

Simon, battered and exhausted, builds a snow hole and collapses into it to shelter from the storm. The next day, he wakes to glorious weather, perfect visibility, and he is able to safely climb down and walk back to camp. On the way down he sees the gaping crevasse in the glacier that Joe has fallen into and is torn apart by the thought that he has killed his friend.

Except that Joe is not dead. He’s landed on a little ledge. He pulls the rope down and see the cut end. He knows what Simon has done. Looking around, he knows that he can’t possibly climb out of the crevasse. He also slowly realises that he doesn’t want to just sit there and wait to die, so he fixes an ice screw into the wall and slowly lowers himself into the void…. he has decided that if he reaches nothing by the end of the rope, he will just let go.

So he lowers himself. At the end of the rope he’s still in hanging free, so he lets go… and falls a few centimeters onto a flat surface. A flat surface that slopes away and up to the top of the crevasse. He has an escape route!

And so Joe slowly crawls out of the crevasse. And with a broken leg, crawls out of the glacier and along rocky ground, the 6 miles back to camp. He does, at one point, find some water, and drinks and drinks and drinks, feeling his strength returning (though he has lost many kilos in mass, he is so dehydrated). Even so, he only realises he is back in the camp when a horrible stench alerts him to the fact that he’s just crawled into their latrine….

Simon after several days of mourning his friend, had decided to leave the next morning. He had burned all Joe’s clothes. They have to get back to Lima – which takes several days – and then he has to lie in a hospital, untreated, until the insurance confirms it will pay.

It’s an amazing story of courage and endurance and survival. But it almost certainly wouldn’t have happened if they had just packed another couple of gas cartridges…

…for if they’d taken enough fuel, they could have sheltered on top of the mountain until the storm had passed, and made a much more controlled, safer descent, rested and hydrated. But with no water, they had to get off the mountain as quickly as possible….

Still with me?

If nothing else, it reveals the power of stories. In the next lesson they can recall every detail – the names, the sequence of events, the country, the meringue cornices, even the mountain. And they remember why it all went horribly wrong. Which is when we finally get round to talking about stomata…

Because stomata are the plant equivalent of your nose and mouth, they allow their owner to breathe, so they can carry out vital gas exchange. Trouble is, gas exchange surfaces like alveoli or spongy mesophyll are moist, and water will evaporate into the air spaces and escape. You don’t have a nose in order to exhale water and dehydrate, it’s just an inevitable consequence of needing oxygen. Same with a leaf – the water loss is a necessary evil. Sure, you can stop it – but is it a good idea to stop breathing?

Looking at rates of transpiration under different conditions, explaining guard cell movement, now has an immediate, relevant and identifiable context.

Yes, my little piranha fish?

Basil!!!!!!

That gets their attention. Slight pause, adjust voice for ultra shrillness, and…

BASIL!!!!!!!!!

To my surprise, many of my Year 9s spot the allusion – Fawlty Towers – though some think I’m referring to Basil Brush, and most look mystified.

We fondly recap on the health inspector episode -“it’s a rat, Manuel. Hamsters are small and cuddly. Cuddle that, you’ll never play the guitar again…”

…before I introduce a third Basil. Here he is:IMG_2038

Isn’t he lovely? All bright and green and perky? Well, what do you expect from M&S???

And, actually, there are four of them, but I start with one, placed on a balance.

We review possible changes to Basil, and they recall that he will probably lose mass because of water loss from his leaves. They can remember stomata and evaporation and gas exchange and so on. And then we record the starting mass: 374g.

Basil 2 weighs in at a similar 385g. But after putting him on the balance, I position a fan to blow air directly at him.

Basil 3 is a rather weedy 260g. He gets covered with a clear plastic bag.

And Basil 4 is a chunker, a bruising 451g. He also gets covered with a plastic bag, but this one is black.

So first important lesson of the day, M&S Basil plants all cost £1.50, so heft them carefully before you buy – the variation in mass is considerable!

Having carefully recorded all the starting masses, I ask them to write down predictions for how each Basils will have changed after 24 hours, when I see the class next. I also ask them to justify their predictions.

Moving round the lab, some need prompting to think about evaporation, some are wondering about growth, but I redirect them to transpiration – how will these treatments affect water loss from the leaves? I don’t mind if their predictions are right or not – I want them to think – so if, as some do, they argue that the black bag will absorb heat and thus increase evaporation, that’s fine by me.

And note that they don’t know what’s going to happen – this is not an experiment/demonstration to confirm something they’ve been taught, it’s a genuine investigation (though note the ease with which mass can be adjusted by a surrepstitious leaf removal or a sneaky out of lesson watering, if the results aren’t what you want…).

All of this takes 30 minutes, setting things up nicely for the hour long lesson the next day.

24 hours later, there is, happily, no need for such dishonesty. The Basils have behaved beautifully. We record the new mass for each plant and then I get them to answer the following questions.

  • Work out the mass change for each Basil.
  • Calculate the rate of mass change in g/hr.
  • Explain why this is not a fair comparison.
  • Now calculate the %age change in mass for each Basil and present this data in a suitable graph.
  • Do these results match your predictions? Try to explain the differences between the plants.
  • Which of the treatments is not properly controlled? What should we have done instead?

I was delighted with how well this worked. Firstly, the %age mass loss was exactly what you would expect – 25% for the windy Basil, 15% for the standard Basil, 8% for the Basil in the clear plastic bag, 5% for the Basil in the black plastic bag.

But the questions really got them thinking. They had to evaluate the experiment, process data, decide on the best way to present it, use SKU to explain the differences, and think carefully about controls. And it illustrated how even really bright students don’t immediately see what seems obvious to us. It took them a suprisingly long time to realise that the different starting masses made any simple comparison of mass loss invalid. Similarly, they needed a lot of prompting to realised that the black plastic bag altered both the humidity AND the light, though they were quick to suggest a better way of doing this when they did get there.

They all did super graphs – correctly choosing a bar chart – and they all talked intelligently about humidity and the effect of wind and, again with a little prompting, what might happen in the dark to reduce water loss still further.

We finish the lesson with my adaptation of the song from the musical Oliver, Fagin’s I am reviewing the situation… which I’ve turned into I am revising the transpiration.

Lyrics on request…

And next week, after further exposure to the fan, Basil 2 will look like this…

IMG_2041

…and we can discuss wilting… and how to recover from it (wilted Basil responds brilliantly to watering).

And at the end of all this, 4 members of the department can go home with a Basil plant.