When a tree loves a tree…

Plant sex with the Year 9s and a veritable plague of misconceptions to overcome.

I’m always fascinated by why students persistently get the wrong idea about something and enjoy the challenge of trying to find alternative approaches to these topics.

With plant sex, quite apart from the usual “It’s Plants So It Must Be Boring” reaction, there’s confusion about what’s actually going on – they don’t see any parallels with animal reproduction – and confusion over what all the different structures and processes actually are.

So I start by getting them to dissect and draw a seed – some kind of bean is ideal. A drop of iodine emphasizes the starchiness. They’re quite happy to do this, though slightly mystified.

Time for discussion.

What is a seed?

A baby plant!

OK, where’s the baby?

Back to their drawings/dissections. Look! Most of them haven’t seen it. When they finally do, they realise that the “baby” actually takes up a tiny proportion of the overall seed.

So what’s the rest of it for?

We discuss the parallels with the English aristocracy, packing your kids off to boarding school at the age of 2 – plants just take it one stage further, thought they do, at least, provide a little picnic to sustain their offspring before they can fend for themselves.

We set up some germination experiments with sweetcorn and our little propagator trays.

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As you can see, these work beautifully (takes a week, for anyone hoping to cram this into a single lesson!), and we will have a busy lesson calculating class means (n=18 for each temperature!) and plotting graphs with range bars, and discussing what we didn’t control (the 4’C ones were in the fridge – and therefore in the dark – not controlling light with a plant??? tut tut…), and saying, hmmm, look, this is rather similar to your enzyme graphs….

Back to the story of seeds…

So, that thing inside the seed,  it’s not really a baby, is it? We need a different name. And, in fact, it’s an embryo.

So what needs to happen in order to make an embryo?

Pause. Giggles.

To much amusement, I stress that I’m not interested in the mechanics, I just want to know at the cellular level.

Yes, that’s right, a sperm needs to fertilise an egg.

At this point I say that I want them to look at an answer written under exam conditions. The test in question is on human reproduction, and the student is a Geranium. What, I ask, is biologically wrong with the following statements? (extracts from the following Powerpoint).

Understanding Plant Sex

“Human males produce millions of sperm to increase the chance of fertilisation. The babies have legs and arms that help them crawl to find the egg to fertilise it…”

They think this is hilarious. I push them for a precise explanation. What mistake has this student made? It’s the same mistake in the next answer:

“Male and female frogs mate in ponds. The female can lay up to 2000 embryos a week. The tadpoles have tails that help them swim to the egg to fertilise it…”

Isn’t that silly? This poor student has confused babies with sperm, and eggs with embryos, and then tadpoles with sperm. True, the student IS a Geranium – but even so…

But what about this? Here is a human student writing about plant reproduction.

“Dandelion seeds are wind dispersed. The seeds have little parachutes that are caught by the wind to carry them a long way to increase the chance of fertilisation.”

This stops them in their tracks. They have to stop, and think, and some will say, “there’s nothing wrong with this…” But after a bit of thought, they realise that the human has made the same mistake as the Geranium – conflating embryos with gametes.

We then walk through the rest of the Powerpoint, followed by some questions, to emphasize the parallels between human and plant reproduction, as well as some of the key differences.

Understanding Plant Sex

In the next lesson, they’ll have a think about why land animals have to mate, why that’s not an option for plants, and how they ingeniously get around the problem of dessication for the precious male sex cells…

Have a great Easter holiday.

Questions of bacteria

So what did my Year 9s decide to investigate with their new-found micro-biological skills?

In the end, the classes split about 50/50 into groups that had a very specific question – comparing the efficacy of different soaps, testing the 5 second rule, looking at the anti-bacterial properties of saliva (more on this later!) – and groups who just wanted to see what happened if they put stuff on the agar.

I wondered whether I should be frustrated by this lack of focus from the latter group, but in the end decided that they were simply displaying the genuinely curious approach of naieve scientists. They might not have formulated a hypothesis to test, but they were still exploring the natural world, poking it to see what happens, going “wow, that’s amazing!” when their plates disappeared under a luxuriant fungal foliage speckled with Staphylococci colonies…

But whatever shape their project took, the motivation and excitement of having complete ownership of their experiment was a joy to see. They could not wait to inspect their plates to see what had grown, to see what they had found out. Without exception, their write-ups reflected this, discussing their results thoughtfully and thoroughly.

So what did they find out?

Several discovered the importance of aseptic technique and the problems that can occur if you don’t employ it.

One very thorough and professional pair discovered that the fruit in the canteen did, indeed, accumulate more bacteria through the course of lunch time, as it was handled by more and more people.

I liked the project that sampled the locker handles from all the different year groups, suggesting that Year 7 are significantly less hygienic than any other.

There was a pretty graphic demonstration of the efficacy of Dettol – and several rather well planned projects that suggested the uselessness of washing hands at all.

Two groups showed very clearly that the 5 second rule has some validity – and I was amused by their comparison of apple, bread and jelly (!?!?). Splat.

And then there was dog saliva. A student managed to bring in a little vial of doggy drool from her canine pet. She and her friend set up plates seeded with E.coli and with paper discs soaked in the saliva. There were control discs, and comparison discs with human saliva and (wait for it) hamster saliva. Really? I asked, when they were preparing it. Yes, really. They were adamant they wanted salvia from the hamster.

So we awoke Herbie from his slumber and they persuaded him to chew on a cotton bud for a few minutes, and then rolled the cotton bud on the agar…

The hamster results were inconclusive, but there was no mistaking the huge halo of inhibition around the dog saliva. Awesome demonstration of lysozymic power!

Great fun, the whole thing, from beginning to end. And highly recommended.

Cell Cycle Snippets

I’m just starting the Cell Cycle/Cell Divison topic with Year 12.

I usually kick off with a question along the lines of what they were like when they first started life’s journey. Yes, that’s right, a zygote. A fertilised egg.

I draw a little cell on the board.

So what did that cell have to do in order to become you?

Again, correct; it had to divide.

I draw two little cells on the board.

And divide again.

I draw four little cells on the board.

What’s it become now?

No longer a zygote, but not yet a fetus, you’re an embryo. And so on. Until there’s roughly 50 trillion of them. Depending on the group, I might get them to calculate how many division this wold take.

Pause.

OK, if that’s all that happened, what would you look like now?

A nice comedy moment this as they visualise the ever increasing 50 trillion cell blob that cell division alone would make them.

So what do cells need to know???

This is a really good discussion point as they figure out the necessary skill set of any cell. We eventually agree on the following:

  • it must know when to start dividing
  • it must know when to stop dividing
  • it must know where it is
  • it must know what’s it’s going to be

Right! Now we’ve got something to work with. Let’s start with the division process.

I’m sure you already have an impressive armoury of amazing replication factoids to impress your students with.

Here are some of my favourites.

  • The DNA in your cells could stretch from here to the sun and back 600 times. That’s 68,000,000 x 2 x 600 miles.
  • From zygote to fully formed, differentiated, multi-tissued, multi-organed functioning organism, takes a mouse 19 days. 19 days! That’s just incredible.
  • Copying takes place in a cell at 2000 base pairs per second.

So how long would it take to replicate the entire genome???

If they work their calculators correctly, they should figure that the 3,000,000,000 base pairs are copied in about 8 hours.

  • Polymerase has an error rate of between 1 in 1000 to 1 in 100,000.
  • Yet the overall mutation rate is 1 in 100,000,000 per cell cycle.

These mutations are the basis of ageing. But how to explain this discrepancy between error rate and mutation rate?

It’s a good way of introducing the idea of check points, controlled by an army of checking and correcting enzymes that oversee the process. You might even want to flirt with the concept of nucleotide excision repair.

But the key point is that these check points are REALLY important because you want your genome to look like this…

karyogram.png

not like this…

karyogram 2.pngwhich is what the karyotype of a human breast cancer cell line, MDA 231, looks like.

There’s enough questions and disuccsion and intriuge here to last us another half hour or more. But when we’re done, we’ll launch into the cell cycle, followed by Mitosis, or possibly watch the FOP video by way of looking at the catastrophic consequences of cells not knowing what they’re meant to be…

An update on Year 9 microbiology projects to come soon!

the power of curiosity

Never underestimate the power of giving students autonomy.

As regular readers (both of you :-)) will know, I have no time for practical work where students already know the expected result.

The obvious and easiest improvement is to “flip” the practical, so that the students are puzzled/intrigued/surprised by the result and have to interpret the results for themselves, or at the very least are forced to ask the question “why?” so that the theory you then teach is in response to their curiosity.

But far better still is giving students the opportunity to ask their own questions, so that they have complete control, complete autonomy.

Take my Year 9s.

We had zoomed through Enzymes and Digestion so quickly that I was suddenly faced with the prospect of starting Plant Transport in February. I don’t know if you’ve ever tried to get potometers working in February, but it’s a pretty joyless and pointless exercise.

So I thought I’d use the time to introduce some Micro-biology and let them carry out some investigations. But it’s not on the spec.! bleat the bucket filling information shovellers.

Perhaps not. But leaving aside other considerations – such as motivation, excitement, interest – it’s a great way of putting their experimental design skills into another context.

So, lesson 1, a quick intro to aseptic technique and agar plates, followed by a little mini-project – how does bacterial load on finger tips vary with stage of washing? This takes a double lesson, as they need to be clear about various points, and I want them to do it properly, all ready to look at for the following lesson. I also want them to think about Controls and Measurements. Not only are there lots of good ideas and intelligent suggestions here, but they are also already wanting to introduce their own variations (can they use hand gel? can they compare air dried with towel dried? and so on). It’s a great illustration of what can happen when you encourage curiosity and independence.

Once all the plates are in the incubator, each one carefully divided up into Unwashed (Control), Wet, Soapy, Rinsed, Dried sections, I explain that they now have the necessary skills/techniques to ask their own question. What could they use these techniques to find out?

Next lesson, they get to look at their incubated plates, and, as generally happens, they see that washing your hands appears to have very little, if any, effect on the number of bacteria they transfer to the plate! The results actually weren’t all that spectacular, with very little bacterial variation, but they’re still fascinated by what they can see, and keen to plan their own projects.

Here’s some examples of what they went for:

  • testing the 5 second rule
  • comparing the hygiene of different year groups by sampling the surfaces of locker handles for bacterial load
  • investigating bacterial biodiversity in different habitats
  • investigating the efficacy of different anti-bacterial agents
  • investigating the bacterial load of different food preparation surfaces

and a few others that I can’t immediately recall.

Actually, the questions are irrelevant – what was so wonderful was how motivated and interested and excited they were at being allowed to do whatever they liked. They will have complete ownership, from beginning to end, and their results will be their results – a real taste of the thrill of doing original research.

A very happy end to the week. I’ll let you know how they get on…

Mapping Retinas

You can’t really go wrong with eyes. Oh, unless a student starts feeling queasy with the eyeball dissection. But it’s one of those Slam Dunk Open Goal Shoo In topics that you would have to work really hard at to make dull. So the following is just throwing some random ideas out there which you might consider (if you don’t already do them).

I start by making the entire class stare at a huge red heart projected on to the board for two minutes.

red-heart-vision-trick

Then immediately switch to a white screen. They love this – it’s weird, it’s unexpected, it’s slightly unsettling, and it stimulates the obvious question – what the…?

So then we map their retinas.

mapping-the-visual-field

This is a resource I found in an age old file 15 years ago or more, so it’s not original, but I love it. Keeps them usefully occupied for 45 minutes or so and all you need is some A3 paper, sellotape, meter rules and two pen lids, one red, one green. I’ve adapted it a bit and added a homework exercise with interpretive questions to help them understand the wonderful maps that they produce…

retina-map

Once they’ve figured that out, you can go back to the red heart trick and ask them to explain it.

And if you are dissecting eyeballs, do make sure you see how high the lenses will bounce…

 

Protein Synthesis Race

And it’s back to my old favourite, Lego.

My Year 11s meet Lego as a way of understanding the link between DNA and proteins, and how the same amino acids can make different proteins depending on what instructions are being followed. See this Burble from a long time ago.

At A-level, of course, this is all covered in much more detail, but you still need some kind of fun activity to break up the theoretical background and help them visualise/conceptualise the different ideas.

I start by showing them a simple Lego model.

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What might this represent?

DNA! announces someone, engaging mouth before brain.

OK, so right idea that it’s some kind of molecule, but why can’t it be DNA?

They see that it has more than 4 types of monomer.

Try again!

This time we arrive at protein. We revise amino acids, peptide bonds and so on.

OK, so it’s a protein, and you’ve just eaten it for lunch. What happens to it?

They rummage through their brains for some Year 9 digestion memories…. oh yes, it gets broken down…by proteases… so it can be absorbed… into the bloodstream…

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We add some A-level detail – the peptide bonds are hydrolysed – but then, key question:

Where do they go?

This takes them a little longer. Go? What do you mean? But eventually they work out that the amino acids are being delivered, by the bloodstream, to cells all over the body.

Right! And what do the cells use them for?

Again, they can be pleasingly perplexed by this. It usually needs a prompt or two.

What can cells make out of amino acids?

Once they’ve worked out the answer, there’s a bit more A-level revision on the kind of proteins that might be made – channel proteins, protein pumps, ATP synthase, hair (for those lucky enough to possess hair making cells), ENZYMES!, hormones, mucus, collagen, haemoglobin – hurrah, they’re on a roll!

Right. And where does this happen? And how will all these proteins be different? So how does the ribosome know which order to put the amino acids in?

By now, they’re happy and confident and make the link to the genetic information in the nucleus. We quickly revise the basic principles of transcription and translation and why they’re necessary…

So! One half of the lab becomes the nucleus, with some chromosomes… (there’s a set for each team).

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The other half of the lab becomes the cytoplasm, with some ribosomes… (again, a set for each team).

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And a great pile of communal amino acids…

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And in teams of two, they become RNA polymerase/mRNA, in a race to find the gene (they look for a Start codon on Chromosome 7), transcribe it (on a rough piece of paper) into mRNA until they hit a Stop codon, and then sprint to the ribosome to translate their copy into a colour-coded polypeptide…

It’s great. They have to think, they have to work together and communicate, they correct each other (if one forgets to turn T into U, for example), they have to apply their understanding, and it’s competitive!

One by one they bring me their completed polypeptides. Can you see which pair were given some mutated DNA….?

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Lots of discussion points here. mRNA on a bit of scrap paper? That’s right – it’s short lived and disposable. Trade off for cell? Need to do it quickly, but need to do it accurately. Fast and you out compete other cells. But incorrect, and mutations can be disastrous…

And so on.

Half term tomorrow. Can’t wait. Have a good one!

 

 

A little bit of TLC…

I encourage my classes to bring their mobile phones to lessons. And I encourage them to make full use of the available technology – taking photos of demonstrations or practical results, looking up answers to esoteric questions, using the timer or the calculator. They know that if they start texting each other then the arrangement comes to a quick and brutal end, but I always feel that if my lessons become so dull that they’re forced to text each other for entertainment, then I kind of deserve it.

Anyway, I got my Year 13s to try something else today – using the time lapse video function to film Thin Layer Chromatography of mint photopigments. Doesn’t need to be phones – the link here was filmed with my i-pad – so if you have a class set of i-pads, you can always use them instead.

(nb WordPress won’t let me add videos without a paid subscription, hence the use of YouTube…)

As you can see, the carotene races beautifully up the strip, the phaeophytin does a great supporting act, but then the chlorophylls and xanthophyll, after a promising start, get a bit bogged down. Though look at that beautiful blue-green of the chlorophyll A!

If anyone has a better recipe for a running solvent I’d love to hear it!

And the relevant practical protocol and questions are here.

chromotography-chlorophyll-2016

I like my last question: Why is butter yellow? It seems to random, after all the carefull rf value calculation and so on, but there are so many good biological synoptic ideas tied up in this.

Have a good weekend!