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Cells and Systems 6-8 / Week 02 / Wednesday
3/6
Week 02 · One Cell or Many

Wednesday

Windowsill Lab: The yeast balloon
// Living things, from one cell to trillions
⏱ about 20 min

Wednesday: Windowsill Lab: The Yeast Balloon

Rocket has two jars, two balloons and a spoon. "Yeast, sugar, warm water, balloon on top. Instant proof!"

"Two jars," Raven says. "One with sugar and one without. Otherwise we cannot tell what the yeast needed."

Nova hovers between them. "What do you expect each balloon to do?" she asks.

"The sugar one puffs up," Rocket says. "The plain one stays flat. Yeast needs food."

A grown-up pours warm water into both jars. Raven stirs in the yeast, then stretches a balloon over each mouth.

"Now we watch and measure," she says. Nova sets a timer. "Every ten minutes. Flat balloons are data too."

Your mission

Today you test whether yeast is alive by watching what it does with food. Living things use energy from food and release gas.

You will compare two jars. Jar A gets yeast, warm water and sugar. Jar B gets yeast and warm water only.

Predict first, then set up, then measure every ten minutes for forty minutes. Record everything in the data table.

  • Two clean glass jars or clear cups the same size, labelled A and B
  • One packet of baking yeast, split in half, and one spoonful of sugar
  • Warm tap water, poured by a grown-up, warm to the touch and never hot
  • Two balloons, a piece of string and a ruler
  • A kitchen timer or phone timer and your Windowsill Log
Safety first
A grown-up pours the warm water. It is warm to the touch, never hot.
Yeast and sugar are lab materials. Nothing is tasted, eaten or sniffed. Keep hands away from your face.
Jars stay on the sill or table. If a balloon pops off, step back and let the grown-up reset it.
When you finish, pour the jars down the sink with running water and wash your hands.
  1. Pour the same amount of warm water into jar A and jar B, about a quarter full. A grown-up does this.
  2. Add half the yeast packet to each jar and stir each gently with the spoon.
  3. Add one spoonful of sugar to jar A only. Stir again. Jar B gets no sugar.
  4. Stretch a balloon over the mouth of each jar so no air can escape.
  5. Wrap the string around the widest part of each balloon. Mark the length on the ruler. Record it as minute zero.
  6. Start the timer. Every ten minutes, measure each balloon with the string and record the number.
  7. After forty minutes, look at the liquid in each jar. Record any foam or bubbles.
  8. Pour both jars down the sink with running water, rinse the jars and wash your hands.
PREDICT BEFORE YOU POUR
  • Read the question.
  • Tap your answer.
Which balloon do you expect to inflate?
Why is jar B, with no sugar, part of the test?
TimeJar A balloon (string length in cm)Jar B balloon (string length in cm)Foam or bubbles?
0 minutes
10 minutes
20 minutes
30 minutes
40 minutes

Reading the result

Yeast takes in sugar as its source of energy. It grows best with oxygen but can also use fermentation when oxygen is gone.

Either way, the yeast releases carbon dioxide, the same gas that makes bread rise. In your jar, that gas has nowhere to go but the balloon.

A balloon that grows is evidence of a living thing using food. Water alone cannot do that.

If neither balloon moved, the water may have been too cool or the balloon not sealed. Scientists try again.

What the lab showsTrue or false?
Gas in the balloon is evidence that the yeast is using sugar.?
Jar B shows that warm water alone makes the gas.?
The gas yeast releases is carbon dioxide.?
A flat balloon on jar B is useless data.?
WHY THIS EXERCISEAn investigation needs a comparison. Jar B is what makes jar A count as evidence.
Yeast released a gas into the balloon. Which gas? Type two words or one.
WHY THIS EXERCISENaming the gas links your balloon to every loaf of bread that ever rose.
Sketch both jars at forty minutes. Show the balloon sizes and any foam. Label A and B.

Careful work. Tomorrow you read a real size table and work out how many cells fit on a pin head.

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