Is It Alive? Cells as the Unit of Life

We are learning to describe the cell as the smallest unit that carries out all the processes of life, and to tell single-celled organisms from many-celled ones using evidence.

Before you startAbout 65 minutes · what success looks like · notes for the helper

About 65 minutes. It's fine to stop after the Quick Check and come back another day: the lesson opens where you left it, at the Quick Check.

You'll know you've got it when

I can say what a cell is, and why a part of a cell cannot carry out all the processes of life on its own
I can sort organisms as single-celled or many-celled, and name life processes a single cell carries out
I can write a reasoned prediction, and name what I change, what I measure and what I keep the same
I can explain what a gas given off by yeast shows, and what it does not show on its own

For the helperhow to sit beside this lesson, and what this step is for

This step: Spark

Start with the question at the top and chat about it. There is no wrong answer here: it gets your learner thinking about what they already know.

Getting ready: Three small plastic drink bottles, three balloons, a packet of dried baker's yeast, sugar, a teaspoon and a tablespoon, a jug, a tape measure or a length of string and a ruler, and a clock. You might test the warm water yourself on the inside of your wrist before it goes near a bottle, so that it is comfortably warm and never hot. The controls choice needs three more bottles and balloons; the evidence file needs six kitchen or garden things. The readings run for an hour alongside the lesson, which is about 65 minutes in all; many families stop after the check and finish the task the next day.
What understanding sounds like: Listen for your learner arguing from evidence rather than from looks: 'the gas only came when there was sugar and warmth, so something in there is using the sugar'. Listen for a cell described as the smallest thing that does every life process, for single-celled and many-celled organisms told apart with a reason, for what was changed, measured and kept the same, and for 'gas alone isn't proof, because bicarb and vinegar make gas too'. A prediction that gives a reason, even if it turns out wrong, is worth more than a lucky one.
A likely mix-up, and something that helps: The common belief is that something that does not move, like dry yeast grains, cannot be alive, or that a single cell is somehow only part of an organism. You might ask, 'What would we expect to see if it were alive and had been given sugar and warmth?', and, 'Does a bacterium have a heart? Is it alive anyway?'
If it comes easily: A question to leave hanging, if the lesson is going well: how could they be sure the gas came from the yeast and not from the sugar and water alone, and what extra bottle would answer that?
If it's a struggle: If it is a struggle, one way is to work on one bottle at a time: write one sentence of prediction, take the readings, and only then compare it with the next bottle. A table drawn on paper before the first reading often steadies the whole task. Words such as unicellular, dormant and respiration carry this lesson; saying each with the thing in front of you, and in the family's home language if that helps, makes them easier to hold. If your learner has had enough, the balloons can be read by whoever is around and the writing left for another day.

Step 1 of 6 · Spark

Lesson 1 of 5 · Small on Purpose: Cells and What Their Parts Do

A packet of grains that might be alive

For the helper

Start with the question at the top and chat about it. There is no wrong answer here: it gets your learner thinking about what they already know.

Open a packet of dried baker's yeast and tip a little onto a plate. It is pale, dry and grainy, about as lively as sand, and it has sat in a cupboard for months without moving, growing or changing. Your task for this lesson is to find out whether those grains are alive, using nothing but a bottle, a balloon, sugar, water and a clock. Before you read on, write down what you think: alive, not alive, or something in between, and what evidence would convince you?

Dry grains in a packet, and bread that rises in an hour. Something changed, and it was not the flour.

Set up the investigation now (about 10 minutes)

The bottles need time to start working, so set them up first and read while they run. For each of three clean plastic bottles: pour in 200 mL of water, add one tablespoon of sugar and one level teaspoon of dried yeast, and swirl for ten seconds. Stretch each balloon by blowing it up and letting it go flat, then fit it over the neck. Bottle A uses cold tap water, bottle B lukewarm water (only just warm to the touch, roughly 25 to 30 °C) and bottle C warm water (about 40 °C, comfortably warm on the inside of your wrist like a warm bath, never hot). Stand them side by side out of direct sun. Write a prediction before you start: which balloon will inflate most, and why? Then, every 10 minutes for an hour, wrap a piece of string around the widest part of each balloon, hold the string against a ruler, and write the length down in a table. Write the time 0 reading as soon as the balloons are on.