Tag Archives: water balloons

Kirsty tries not to pop

Year 9. A new idea for introducing Active Transport.

I started by showing them one of my favourite props – a small water balloon.

Why is this quite a good model of a cell, I ask.

  • Because cell’s are mainly full of water.
  • And have a squishy, flexible membrane.
  • And are 3D.
  • And are surprisingly tough (bounce them on the floor – they won’t burst!).

What kind of cell would a water balloon be?

An animal cell (you can always turn it into a plant cell by inserting it into a large beaker).

Can anyone remember what happens if too much water goes into an animal cell?

Indeed they can. It highlights the importance of memorable activities and demonstrations. Not only can they recall the investigation where they exploded red blood cells, they can vividly recollect the demo of the balloon attached to the tap, getting bigger and bigger and bigger, impossibly bigger, until it finally burst with a satisfyingly spectacular volume of water, most of it going over me.

Ah, yes, Osmosis.

So what’s the danger for a single celled organism, say, an Amoeba, living in a pond?

Oh no, Osmosis! The poor exploding Amoeba!

At this point I pull out the props and ask Kirsty to volunteer.

The props are pretty simple.

  • A large container of water (I used the paper recycling bin – paper carefully removed).
  • A large empty tub.
  • A large beaker.
  • A small beaker.

They’re intrigued. It catches their attention. What’s he up to now, they wonder.

So, I say, let’s imagine that Kirsty is an Amoeba. And she’s living in a fresh water pond. And she is much afflicted by Osmosis.

How is she going to survive? How is she going to avoid… exploding…?

I explain that the large container of water represents the pond, the external environment with the high water potential. Kirsty is the empty tub.

I hand Kirsty the small beaker. Ready?

She nods, uncertain. What’s going to happen?

I suddenly start transferring water from the bin to the tub with the large beaker. Look ! Water is moving in to Kirsty by Osmosis! Help! Quick! Kirsty, what are you going to do???

Everyone shouts suggestions and after half a second’s hesitation, Kirsty springs into action, bailing the water out of the tub and back into the bin. But my large beaker is out-competing her small beaker – the water level is inexorably rising! If it reaches the top, she’s a goner!

Kirsty bails quicker and quicker, more and more frantically, desperately shovelling the water out of her tub and back into the “pond”…. What a star!

We finally reach a kind of equilibrium and I call a halt. A truce. Amoeba vs Osmosis = a score draw. And the Amoeba lives!


So, how did Kirsty survive?

She had to move the water out as fast as it moved in.

Is it Osmosis?


Why not? It’s water moving across a selectively permeable membrane. That’s Osmosis, isn’t it?

No! It’s going against the gradient.

Ah, right. Pushing it up hill. What does that require? What was Kirsty using lots of?


Exactly. It’s called Active Transport.

Time to draw a cell with some little pumps in the membrane and show them some video clips of contractile vacuoles.

And to tantalise them with the idea that, at rest, 30% of their energy is powering active transport of sodium ions to keep their nervous system running. Seeing, feeling, hearing, imagining, falling in love… all dependent on active transport.

How can anyone not love Biology?