Author Archives: paulweeks2014

“So, who wants to have xxx…?”

Year 11 Genetics blog entry the 2nd.

A quick recap…

The students have looked at chromosomes, figured out the principles of sex determination, and been introduced to the idea that the information carried by the chromosomes is in the form of discreet instructions on how to make specific proteins.

My rationale for all this is based on my observation that learning is so much easier and so much more enjoyable and oh so much more effective when it is built upon understanding. I am, for example, currently giving private tuition to a Year 11 student who was completely stumped by a question that began, “Plants need nitrates to make protein.”

She didn’t know where to start. She thought that proteins were made at ribosomes, had never heard of nitrates, and had never been taught about the atoms required to make amino acids. Instead, she was expected to learn, by rote, with no context or understanding, that plants need mineral ions, and then, somehow, had to unpick a difficult question on the effect of water logged soil on nitrate absorption by root hair cells. Good luck with that!

There is a similar lack of scaffolding in much genetics teaching. I remember a GCSE textbook where the inheritance of eye colour was explained by parents holding cards which they gave to their children, and that some cards took preference over other cards and that you could represent the cards by using different letters….

This kind of approach gives a workable tool for solving genetic cross problems, but it leaves horrible gaps and gives rise to all manner of confusion. How can one version of a gene be “dominant” to another? How can letters determine characteristics? How does any of it link to DNA and what genes actually do?

You end up having to do an awful lot of heavy lifting to translate the simplified stuff into a form that makes any kind of sense, and very often the misconceptions are the things that stick. Even now, I will get A-level students trying to pass off the nucleus as “the brain of the cell” because it was an accessible, if meaningless, metaphor taught to them in Year 7.

So try this.

Before the lesson, I put out two pieces of paper and a pair of scissors for each student.

They will either have this…

Cystic Fibrosis inheritance guide 2020 Female

or this

Cystic Fibrosis inheritance guide 2020 Male

For reasons that will become obvious, all the students on one side of the class are “females” together and the other side are all “males”.

They will also have a sheet with instructions and questions, but let’s start with the picture.

Notice that it has a karyotype at the top – either male or female – and with a gene marked towards the bottom of chromosome 7, labelled with a C on one and a c on the other.

Below this, they have two haploid versions of the same karyotype, so that the chromosome 7 either has a C or a c.

The questions on the other sheet Questions to go with Cystic Fibrosis inheritance guide 2020 start with a review…

  • Look at your Karyotype – are you male or female?

Now look at the smaller karyotypes below…

  • How are these different to the main karyotype?
  • What kind of cells would have this kind of karyotype?
  • What do we call cells that have this number of chromosomes?
  • Why is this important?

A section of chromosome 7 that carries a specific bit of information has been highlighted and represented with the letter C.

  • What do we call small sections of chromosomes that carry specific information?
  • What do these bits of information have in common?
  • What do you notice about how the letter C has been written?


They get through all of this very quickly, confidently and accurately. The previous lessons have obviously worked! Then the fun starts.

They use the scissors to cut out the two smaller karyotypes. Once they’ve done this, they have to find someone of the opposite sex. They both place their smaller karyotypes face down on the desk and select one at random from each person.

Then they turn them over and record the following information in a table

  • the sex of the baby
  • the combination of Cs that the baby has (i.e. CC, Cc or cc).

They retrieve their bits of paper and repeat with 3 more people of the opposite sex, so that everyone ends up with 4 babies.

This activity was greeted with great enthusiasm, hence the title of this piece, a quote from one girl leaping to her feet and announcing to the class, “So, who wants to have sex…?”

Once they’ve completed this, I ask the “mothers” to put the data on their babies on the board… Just check this out…

CF class cross

They can already explain the ratio of baby boys and girls – though they are very impressed with how this has somehow just happened! – but the next challenge is to explain whether the distribution of C genotypes is as expected. The brighter ones actually figure out the Punnett Square for themselves – but as understanding sinks in, the class is again amazed that the numbers match the predicted ratio so closely. Not having done it before myself, I was both delighted and relieved!

Because notice how well this works and how many different concepts it illustrates. Random fertilisation, large sample size, probability, inheritance of different alleles – and how in particular the letters they’ve been sharing are directly linked to a specific instruction on a specific chromosome. They can see it and they have done it and they therefore understand it.

But we’re not finished yet!

They already know that the letter C represents a gene, a set of instructions coding for a particular protein. Their worksheet continues…

The letter C represents a gene coding for a protein that is made in cells lining the bronchi and bronchioles. The protein transports salt from the cells into the mucus lining of the airways. This process keeps the mucus wet and slippery so that it can be easily moved out of the lungs.

  • What is the function of mucus in the lungs?
  • Why is it important that it can be moved?
  • Why will putting salt in the mucus keep it wet?

The lowercase c represents a version of the gene that doesn’t work – i.e. the instructions for making the protein are broken.

  • What effect do you think this will have on children with a cc genotype? Explain your answer.


Cystic fibrosis is a horrible disease, but it is brilliant context for so many biological topics. This part of the exercise has them reviewing gas exchange and immune system and infection and osmosis. They work out and understand the key symptoms and side effects of cystic fibrosis, even if they’ve never heard of it (many of them have, someone usually knows someone who has it). And, critically, they now have an understanding of dominant/recessive alleles at the functioning level of the gene….

As I said right at the start of this two part blog, I’ve tried this twice now on groups of varying ability, and it works really well – and by that I mean that the lesson is fun, interactive, interesting, problem-solving and effective. Admittedly, I work in a girls’ school, so you might be more tentative if working in a mixed school – I guess it would depend very much on the class. But let me know what you think!

Long time no write

Ah, my blog, how have I neglected thee?

And my fan! So sorry to have been absent.

But things have changed since the long time ago when I last put fingers to keyboard. As from September 2019, I have stepped down as Head of Biology and gone part time – 4 days a week. The change in just about everything has been a revelation. And as far as work is concerned, I can now concentrate almost entirely on what I enjoy – teaching and running Go Apiary – and have cast aside the other stuff…. HoDs will, I’m sure, recognise what I’m not describing…

And this has allowed my to open up the canula of creative juices once more and, with a few new Twitter followers claiming to be eager for new teaching ideas, I feel inspired to share some recent new resources.

Starting with Year 11 Genetics. I really think that this year I’ve absolutely nailed it. But see what you think!

I start with this projected on to the board…

….female karyotype

What’s this?

And we start to unpick it. They can usually name them as chromosomes. And most will spot that these are the chromosomes from a woman. And then gentle prompting along the lines of, “what else do you notice?” picks out the other key features. These chromosomes come in a variety of sizes. There’s 2 of each, making 23 pairs, or 46 in total. You would find them in the nucleus. Why are they in pairs? Yes, of course. Because you get one from each parent.

OK, very good. What about this?

male chromosomes

They’re right on it now! Yes, here we have a set of male chromosomes. They’re normally intrigued that the Y chromosomes is so diddy – and they (being girls) enjoy hearing my rant about this runty bit of rubbish DNA that is responsible for more wars, violence and aggression than anything else in human history.

I now put the two karyotypes side by side and we work through sex determination together, talking about the importance of haploid gametes, and showing that while every egg from the mother will have an X chromosomes, it’s the father’s sperm, being 50% X and 50% Y that will decide the sex of the zygote. Without telling them what it is, they draw a Punnett Square of possible fertilisations, showing the 50/50 outcome.

All nice and easy with lots of potential for throwing interesting stuff in on Henry VIII or how the X chromosome is winning its evolutionary battle against the Y.

Once they’re secure on this, we then take a tour of unusual karyotypes and the knock-on effects. Trisomy 21 they’ve heard of, but look at Klinefelters with the XXY, or Turners with just X, and check out the knock-on effects. These chromosome things clearly have a very profound effect on who you are!

human karyotypes (full introductory powerpoint here…)

This all takes a double lesson so we then spend the next double lesson very happily breeding and making ReeBops…



…which reinforces the idea of parents contributing one of two chromosomes at random – modelling meiosis and fertilisation – and how this throws up enormous variation. (nb if you don’t do this, you absolutely must – just ask for resources and instructions – it is one of the absolute highlights of the year).

For the next double lesson, it’s back to a karyotype for a quick review, and especially back to the idea that the chromosomes carry information for making a person. But what kind of information would that be? This powerpoint which genes are on which chromosomes gives a hint. Click on the letters and an arrow appears, indicating the location on a chromosome where you could find the instructions on how to make amylase… insulin… muscle…. haemoglobin…. antibodies….pepsin….collagen…..keratin…. melanin…. lipase… mucus….

These are all things they have studied, or heard of, as part of their GCSE course. I stress the point – if you want to make a human, you have to know how to make, for example, muscle. But if you want to make muscle, you need instructions. And there they are! On a tiny bit of chromosome 19! But look at all of them, the muscle and antibodies and haemoglobin and the rest. What do they have in common? This question can take them a little time, but they get there eventually.

They’re all proteins!

And at this point I introduce the word “gene”, which of course they know, but which we can now put an accurate definition to….

OK – confession time – none of this is new. This has been my introduction to genetics for several years now. What is new is what follows, an idea I’m really pleased with and which worked very well with 2 different classes. But to find out what I did, and why I think it works so well, you’ll need to tune in next week…

Beanz meanz tweetz

Here, as requested (many times! thank you!) are the details of how to set up the Bean Mineral Ion Deprivation Experiment. It does need a bit of planning….

Start by ordering the relevant kit from Philip Harris or Timstar (just shy of £30 – does enough for about 40 groups). It’s the Sach’s Water Culture set or Plant Water Culture set. You actually get 9 different minerals in the set – so there’s lots of scope for investigations…

Sach's kit.JPG

You will also need beans. We use Sutton’s Dwarf Broan Beans (£2.99 for pack of 45) as they sit better on top of the boiling tubes. You’ll need 5 beans per student/group so do a few more to allow for wastage. A week before the lesson,  soak the beans in water for 24 hours and then germinate them; a layer of damp vermiculite in a tupperware box works well…

beans 1

Each group needs:

  • 5 boiling tubes
  • boiling tube rack
  • something to label the tubes with
  • parafilm/clingfilm
  • mounted needle
  • access to the following solutions…
    • distilled water
    • normal
    • –  magnesium
    • – nitrates
    • – phosphates
  • 5 germinated beans at same stage of development – must have a prominent radical…beans 3

The radical doesn’t need to be this long, but it must be there. Warn your students to handle them carefully – they snap quite easily!

Students then label the test tubes accordingly and add the relevant solution. The tubes must be filled to the brim as the radical must make contact with the liquid.

Then they put a piece of parafilm or clingfilm over the top of the tube and make a small hole in it with the mounted needle. Make sure the film has a slight depression for the bean to sit in.

beans 2

Then they carefully insert the radical through the hole…

beans 4.JPG

Repeat for all solutions and leave for a week. Keep an eye on the water levels in the tubes – top up with a pipette if it drops below the level of the radical…

There are really lovely clear results in a week. Beware that as the beans grow, the extra rootage and leafage drains the water in the boiling tubes very fast, so if you want to keep them going, you’ll need to transfer them to a beaker or plant them out.

Enjoy! Let me know how it goes…






A root through plants 2

Lessons 2 and 3…

Having tried to establish that plants are, indeed, living organisms, it’s time to launch into experimental investigation time.

I display a range of plant material – garlic, mint, ginger, chilli, basil, rosemary… what have they got in common?

All nice in the kitchen for adding flavour to food. OK, fine, but why do these plants have this ability? They’re not making these tasty compounds for us, but for themselves. Why?

We circle back to the idea of how plants defend themselves, not just against predators, but also against infection.

I then ask them to design an investigation to test the anti-bacterial properties of various plant extracts. I used to provide this method…

Investigating the Antibacterial properties of plants UPDATED December 2017

…but I now much prefer, where possible, to let them design the experiment for themselves. Learn by doing. Learn by making mistakes. So I demonstrate the technique – extracting the compounds, soaking the discs, being aseptic – but let them decide on the approach. Most of them will forget a control disc, many won’t do repeats, they won’t all think of all the necessary control variables – but my feedback will highlight these basic omissions and they’ll do better next time….

This is motivating and interesting and different and they clearly enjoy it….

… and it’s a practical that really works. Garlic – particularly if you put it through a garlic crusher – inhibits bacteria to a spectacular extent, as they’ll discover in the next lesson, creating vast halos of inhibition around the paper discs. Homework is to finish their experimental write up and then design an advert for a garlic flavoured toothpaste.

Collecting the results doesn’t take much time, so it’s time to move things on.

So I ask them if they remember their babies, the little brassica seeds that they planted a week ago. We take a look. These Rapid Cycling Brassicas are not misnamed – one week is quite enough to see dramatic results. Oh look, I say disingenuously, one lot of plants went under the light bank, but this lot got left at the back of the lab. Tut tut, careless me, hem hem….

The difference is dramatic and the impact immediate. There are – I kid you not – audible coos and gasps when I do my big reveal.

I get them to make careful observational drawings of the difference between this “inadvertent” experiment. This is always a good exercise in forcing them to look closely and carefully at something. I also get them to list the controls and to think about the measurable differences between the two treatments.

They produce beautiful drawings and we discuss the differences. Plants under light bank have….

  • larger leaves
  • more leaves
  • thicker stems
  • purple stems
  • much taller stems
  • hairier leaves

We’ve controlled temperature and air and number of seeds per module and type of soil and volume of soil and availability of water and type and volume of fertiliser and species of plant.

Yep, the only difference is the light. Simple conclusion: light = growth. More light = more growth.

I’ve yet to come across any other demonstration or practical that so quickly and effectively gets this vital learning outcome across. I would choose this over and above all my other photosynthetic teaching resources.

But where to go next….? Find out next week!


A root through plants 1

Year 10 and plants is a graveyard slot tough gig at the Glasgow Empire at the best of times. Even bright, engaged, enthusiastic students will fail to engage with the topic and I have sometimes despaired at what can often seem like a pre-conceived rejection of everything to do with plants. Plants = Boring and there’s nothing you can do to change my mind!

As always, I keep refining, keep trying new ideas, keep looking for ways to excite their imagination and thence their interest. This might be an individual lesson, a new practical, a slightly different angle, and so on. But this year it has embraced the entire scheme of work, where I have gone back to what I consider to be the absolute basics.

  1. It has to tell a story
  2. It has to be an interesting story
  3. It has to be a story that makes sense, where the ideas and the concepts fall comfortably into the (hopefully!) receptive brains

Part of the problem, I suspect, is that too much of GCSE plant biology is abstract – the balanced equation, the various experiments which should be really fun and interesting but fail to find a a satisfactory context, all of which can leave students feeling confused, bored, disengaged.

So for the next 6 blogs or so, I’m going to take you through my new approach to teaching plants to Year 10. Lots of it is stuff I’ve done before – with a few new ideas thrown in, but it’s the over-arching theme that interests me and how it all flows from lesson to lesson.

Lesson 1

My usual introduction. I gather the class around me, pick a student who I know will be happy with what follows, and ask her to stand in a large cardboard box. It’s an unusual start to a lesson and amongst the giggles, there’s clearly curiosity. Where’s this leading?

I then ask the class if they’ve heard of the Mafia.

Yes, of course they have. We chat about Sicily, organised crime, gang loyalty, the rule of Omerta, and the punishments for anyone who breaks that rule. We agree that they are not nice people. For example, let us imagine that Izzi (in the cardboard box) has broken the rule of silence and an example needs to be set. We stand her in this box, fill the box with quick drying concrete, and then drop her off a remote bridge into very deep water.

This goes down very well and they are ghoulishly absorbed by the idea of a “cement shoe”. Except, I say, we’re feeling like a change, a more prolonged punishment. We’re not going to drop them into a lake. We are, instead, going to abandon them in the Serengeti. I let this sink in, and then ask, what sort of problems is Izzi going to face?

Predation is always the first suggestion. Yes, of course, all those lions and leopards and hyenas and vultures and so on. Why are they are a problem? Of course – she can’t run away and she has no protection.

OK, let’s give her a sub-machine gun to keep off all the things that want to eat her. What other problems will she face?

Food is generally next up. Yes, indeed, she can’t move around to find food. Still, never mind, let’s arrange for a sandwich delivery man to pass by once a day and give her a sandwich. And a bottle of water.

The point of the solutions is to a)keep the ideas flowing but also b)to show that she could survive with a few resources and a bit of ingenuity.

Once the immediate problems are suggested and overcome, I encourage them to start looking longer term. They realise, with much giggling, that if an attractive young man is deposited in the vicinity, also with a concrete block around his feet, friendship, romance, startingafamilyifitallgoeswellandtheydecideit’stherightthingtodohemhem is going to present another set of difficulties.

The brighter ones have already twigged where this may be heading, but I still ask the key question: what have I effectively turned this student into? They get it. A plant. The idea, of course, is that I’ve invited them to feel some empathy with a plant as a living organism faced with the same sort of problems any other organism faces.

Infuriatingly, despite the original and entertaining start, there is always some student who groans in despair and wails, “plants are so boring!!!” I control my temper and ask them sweetly to just give it a chance.

Anyway, boring or not, we’re off. We return to the problem of predation and discussing the kind of animals that predate plants – so not just cows and their ilk, but caterpillars, aphids, slugs, etc- and cover suitable defences – thorns, stings, spikes and chemicals. I show them these pictures of familiar plants up close and stress the biochemical ingenuity of plants, starting with the mechanism of sulphuric acid release from onion cells…

Trichomes and onion mechanism

I then put them into pairs and give them the name of a plant defence molecule. They have to find out the structure of the molecule, build it with molymods, and give a very brief presentation to the rest of the class about their molecule, which plant it comes from, and why it’s interesting. They really enjoy the challenge of building the molecules – some of which are fiendishly complicated – and the models give us a lovely lab display AND an important reference point for a future lesson. Stay tuned!

Plant secondary metabolite

I wrap up the lesson by getting them to plant lots of Rapid Cycling Brassica seeds, for reasons that will also unfold over the next few lessons….




So hormonal…

It’s time to introduce Year 10s to hormones.

But before I run you through my tried and trusted lesson for this, I need to issue a Health and Safety warning – namely, that virtually none of the following detail is on the iGCSE specification. What? Teach something that’s not on the specification???? Outrageous! This is the kind of dangerous bolshevik thinking that gets teachers a bad name… Shocking stuff.

Bear with me. I hope you’ll see why and how it works….

I start with this picture….

tanningWhat What are these two people trying to achieve?

OK, so that’s an easy one. They’re trying to get a tan.

By doing what?

By lying, scantily clad, in the sun.

So what’s this an example of? This interaction between sun and skin, what’s going on?

Straight off the back of their work on the Nervous System, they’re generally pretty quick to realise that we’re looking at a stimulus (UV rays) and a response (skin turns brown).

Can they suggest why is it unlikely to be a nervous response?

This can take a little longer, but the idea that skin goes brown slowly and gradually is obviously different to the rapid reflex actions they’ve been studying. Much comedy can be derived from imagining tanning as a reflex response. I’ll just go out in the sun – SPLAT – tan!

Right! Now move to this image.

body + pituitary

A human body.

And this???? the yellow dot?

body +

I love it when someone suggests it’s a brain. More chance for top class banter. Well, maybe your brain is that small, but generally brains fill skulls. Try again!

They usually don’t immediately know, so it’s then time for a quick game of Hangman, where I inevitably fail to break my losing run of 1,200 straight defeats, despite outrageous cheating, rule bending and general mayhem from yours truly.

Right – the pituitary gland! Let’s take a closer look…


And watch what it does…

The animation on my Powerpoint shows little blobs appearing in the pituitary and then moving out into the bloodstream.

The little blobs, I tell them, are a chemical called MSH, manufactured by the cells that make up the pituitary, which are secreted into the blood. And once it’s in the bloodstream, where’s it going?

Yes, everywhere!!! It’s all good Heart and Circulation revision from earlier in the year… With a little prompting they might even remember the part of the blood it’s dissolved in (blood plasma) and the name for these chemicals that travel in the blood (hormones).

Zoom back out to the body. Watch where this MSH molecule is going! And watch what happens when it gets there!

body + MSH

The animation shows it emerging from the pituitary, whizzing round the body, and arriving in the foot. And the foot then turning brown… In other words, it tans.

Huh? How does that work???

We need to look at what’s happening in the skin!



Here it is. A skin cell. And right behind it, another type of cell. With some recently arrived MSH in the top left corner.

What do they notice?

It’s pretty obvious. The cell behind the skin cell has little shapes on its membrane that, following their work on synapse mechanism, they can confidently call receptors. Which, they instantly see, the MSH can fit into.

It’s like I keep telling them – if you can do jigsaws, you can do Biology.

Let’s see what happens when the MSH fits into the receptor…

MSH in receptor


What’s that?

That is a molecule of the tanning chemical, stuff they’ve all heard of, melanin. And the cell that makes it is called a Melanocyte. Hence MSH – Melanocyte Stimulating Hormone.

Now watch….

melanin round nucleus

The animation then shows the melanin migrating from the Melanocyte to the skin cell and accumulating around… where?

So why would it do that?

We discuss the importance of protecting a cell’s nucleus, and the DNA within, from the mutagenic effects of UV rays…

And what happens if you fill more receptors with hormone?

Exactly. More melanin production.

OK, so that’s the mechanism. How is it modified by the sun?

Back to the pituitary!

Look what happens when its stimulated by light from the sun!!!

pit sun

Wow! LOTS more MSH…

…. which is duly taken all round the body in the blood….

…. and arrives at the skin.

Hmmmm. But skin only tans if it, too, is exposed to the sun. It can’t just be changes in the pituitary. So something must happen here too. What might that be?

They get it. The melanocyte responds by making more receptors!

And with so many more receptors filled with so much more MSH, what happens?


Bingo. Lots more melanin – in other words, a tan.

There is usually an avalanche of questions in this lesson. Partly to clarify things, but mainly to push for further answers. So you need to know your stuff – or just be very good a blagging. Either way, tt’s a great way of engaging students’ imagination and curiosity.

Tried and tested, this lesson works. It works really well. The combination of a clear narrative, simple but memorable animations, and a link to something they’re familiar with, is a winning combination. How can I be so confident? Well, 48 hours later, in the follow up lesson, they can recall every single detail – despite having not written a single thing down.

The full powerpoint, with notes and animations, is here…

Melanin production and hormonal control

They are also then ready to have a go at the follow up questions….

Hormonal Control of Tanning questions

Give it a go! Let me know what you think….




Bubble, bubble, toil and trouble…

When I’m starting off with a new class, and encouraging them to “have a go, because, after all, what’s the worst thing that can happen?”, I like to tell them about when I myself was in Year 10. My best subject of that year was History, and we had been studying the Renaissance and the Voyages of Discovery. As the end of topic test approached, I was vying with my best friend, Chris Mortimore, for top spot in the class (this was back in the days when class results were pinned to the classroom door for all to see). Lots of the available marks were for key dates, who did what to whom, or when did how to what, and so on…. I learned them all. Or nearly all.

When the final results were read out, Chris had beaten me to first place by one mark, because he had learned one date that I had not – 1517, the year when Martin Luther nailed his theses to the church door in Wittenberg and kicked off the Reformation. Nearly 40 years later, the only date I can remember from that year is the one I got wrong.

So my message is that getting things wrong is a really powerful way of learning. Don’t be scared of making a mistake – it will help you remember, it will help you learn.

It can also be a barrel load of fun, as my Year 10s found last week.

The context was Respiration, exploring this idea that living things need to release energy from food, and that while the reaction is chemically identical to combustion, the actual process must be slower and more controlled and take place at remarkably low temperatures, something made possible by the wonderful enzymes they learned all about in Year 9. I show them some yeast in a warm water bath, bubbling merrily away after the addition of some sugar. Look, I say happily, here’s an example of a living organism respiring sugar, using enzymes to do so, and producing carbon dioxide as a waste product.

They nod with varying degrees of enthusiasm and understanding.

But hang on, I say, how do you know that any of that is true?

They stop nodding and look puzzled. What do I mean?

Why take my word for it? I’ve just told you a whole load of stuff about what’s going on in that boiling tube – you’re just going to accept it on faith? What kind of scientists do you think you are?!?!?

They continue to look puzzled. Where is this going?

Go on, I say, design some experiments and test at least 3 of the statements I’ve made about respiration in yeast.

Both of my classes, Set 1 and Set 2, found this wonderfully difficult. Firstly, they struggled to grasp what I actually meant. I tell them to go back to the statement:

here’s an example of a living organism respiring sugar, using enzymes to do so, and producing carbon dioxide as a waste product

and think about how they could test the veracity of otherwise of what it says.

OK, they say, we could test the gas to see if it really is carbon dioxide….

Right! That’s the idea! Off you go!

Even on this easy starter for ten, there are some hilarious moments. The demo I showed them had a delivery tube passing bubbles through a test tube of tap water. Many of the groups let the gas bubble through the tap water and then attempted to collect the gas with a syringe, and then squirt the contents of the syringe through some lime water. When I suggest that they just move the delivery tube to a test tube full of lime water, they look at each other, and then laugh in mutual recognition of their own inability to see the simple and obvious. No matter – they won’t forget this experimental detail ever again.

I was also hugely amused by the group who tried to establish optimum temperatures of 37’C by heating the tube directly in the bunsen….

And then they really struggled. What else could they do? I tell them not to over-complicate. They have a very simple experimental set up – they only need to tweak the contents of the tube to test the other assumptions.

But what other assumptions????

And then, slowly, group by group, they started to see it.

What if we left out the sugar? What indeed! Try it!

They try it and, lo and behold, there is no bubbling. Seems that sugar is, indeed, vital for whatever is taking place in the boiling tube.

What next?

Well, there were, of course, some clues in the apparatus and reagents I had provided. Why the HCl and NaOH? Well, what effect would those chemicals have on conditions inside your boiling tube? Oh, pH change. Oh, enzymes. Oh…. oh!

Bingo, now they’re re-running the experiment but at high and/or low pH. And waddayaknow, the bubbling stops, or slows significantly. It seems reasonable to infer that some enzymes have been denatured.

The groups who had earlier tried to establish 37’C conditions in the blue flame of a roaring bunsen now realise that it gives another test – boiled yeast will be dead yeast. And dead yeast, it turns out, doesn’t bubble either.

What seems obvious to an experience scientist, or experienced teacher, was a really difficult thinking exercise for some very bright Year 10 students. But hard thinking is memorable thinking and without doing a single second of revision, or copied a single note from the board, the vast majority will have understood and remembered all the important points of the lesson.