ScienceFreePrintable

Yeast Respiration Lab

A free science worksheet ready for your classroom. Open in Kuraplan to grab the print-ready PDF, customize it for your students, or generate a fresh version in seconds.

Download now

Name:Date:

Yeast Respiration Lab

A bakery wants to know what makes bread dough rise. Use the model results and science clues below to investigate how yeast gets energy from sugar. Do not taste or smell any lab mixture directly.

A Model Test

In a classroom model, equal amounts of yeast and warm water were placed in three bottles. One bottle also received sugar; one received no sugar; one received sugar but very cold water. Balloons were placed over the openings. After 20 minutes, the balloon on the warm-water-and-sugar bottle was largest, the no-sugar balloon stayed nearly flat, and the cold mixture’s balloon was small. The gas in the balloons was tested and identified as carbon dioxide (CO₂).

Bottle setup

Balloon result

Warm water + yeast + sugar

Largest

Warm water + yeast, no sugar

Nearly flat

Cold water + yeast + sugar

Small

Investigate the Evidence

Use the model results and science ideas in each question. Yeast is a living, single-celled fungus. It can release energy from sugar. When oxygen is scarce, yeast ferments sugar, producing carbon dioxide and ethanol; cells also capture some released energy in ATP. In bread dough, carbon dioxide bubbles become trapped and help the dough expand.

1.Which bottle result is strongest evidence that yeast needs sugar to make much carbon dioxide?

The warm-water-and-sugar balloon was largest.
The no-sugar balloon stayed nearly flat.
The cold mixture’s balloon was small.
All three balloons were the same size.

2.What gas inflated the balloon, and what observation supports this conclusion?

3.The cold mixture produced less gas than the warm mixture. Give one reasonable explanation for this result.

4.Which variable was changed in the comparison between the warm-water-and-sugar bottle and the warm-water-without-sugar bottle?

Amount of sugar
Water temperature
Amount of yeast
Time allowed

5.Complete the idea: During fermentation, yeast uses and releases gas. Some of the released energy is captured in .

6.True or false? The balloon getting larger means new matter appeared from nowhere. Explain your choice.

7.A bakery doubles the sugar in a recipe but keeps everything else the same. Can you be certain the dough will make twice as much carbon dioxide? Explain why or why not.

8.A student says, “The cold bottle proves cold water kills yeast.” Is that conclusion supported by these results? Explain.

9.Bread dough rises as yeast ferments sugar. Explain how the gas produced in the bottle model helps explain rising dough.

10.Plan one fair test to find out whether warmer water increases yeast’s carbon dioxide production. State what you would change and what you would measure, keeping other conditions the same.