Adapted from “Is Yeast Alive?” by Penny Bernstein at Kent State University, Stark Campus
copyright 2011 by Dr. Ingrid Waldron and Jennifer Doherty, University of Pennsylvania Biology Department[1]
Humans use yeast every day. What is yeast, and what are some common uses of yeast?
You can buy yeast to make bread in the grocery store.This yeast consists of little brown grains.Do you think that these little brown grains of yeast are alive?Why or why not?
To find out whether yeast is alive, we first need to think about what makes something alive.What are some characteristics of living organisms?
To begin to answer the question, "Is yeast alive?”, you will test whether the grains of yeast have two characteristics of living things -- the ability to grow and the ability to use energy (referred to as metabolism).
We will carry out an indirect test for metabolism.In other words, we will be indirectly testing whether yeast can use energy, which is one of the characteristics of living organisms.
When yeast, humans, and other living organisms use energy, they break down high-energy molecules like sugar to get the energy they need and give off a gas called carbon dioxide as a by-product of this reaction.
We will test whether yeast can metabolize sugar and produce a gas which we will presume is carbon dioxide.Specifically, we will test whether yeast produces a gas when it has sugar available as a food vs. when no sugar is available.
Research Question:
Does yeast metabolize sugar and produce a gas?
Predictions:
Do you expect yeast to produce a gas when sugar is available?______
Do you expect yeast to produce a gas when no sugar or other food is available?_____
Explain the reasons for your predictions.
Procedure to Test Your Predictions
If the yeast grains are capable of metabolism, it will take some time to produce enough carbon dioxide to see the change in the balloons.While you are waiting for this change, set up the experiment to test growth (see page 4).
Observations of Yeast Test Tubes
0 minutes | 5 minutes | 10 minutes | 15 minutes | 20 minutes | 25 minutes | |
Test tube 1 (with sugar) | ||||||
Test tube 2 (with sugar) | ||||||
Test tube 3 (water) | ||||||
Test tube 4 (water) |
Procedure to Test for Growth
Research Question: Can the little brown grains of yeast grow?
Instructions
37° C is equivalent to _______° F.
Observations
Conclusions
Based on your findings, do you think the little brown grains of yeast are alive? Explain why or why not.
[1] Teachers are encouraged to copy this student handout for classroom use.A Word file (which can be used to prepare a modified version if desired), Teacher Preparation Notes, comments, and other hands-on activities are available at http://serendip.brynmawr.edu/sci_edu/waldron/.Additional biology activities are available at http://serendip.brynmawr.edu/exchange/bioactivities.