### Introduction

Temperature changes have profound effects upon living things. Enzyme-catalyzed reactions are especially sensitive to small changes in temperature. Because of this, the metabolism of poikilotherms, organisms whose internal body temperature is determined by their surroundings, are often determined by the surrounding temperature. Bakers who use yeast in their bread making are very aware of this. Yeast is used to leaven bread (make it rise). Yeast leavens bread by fermenting sugar, producing carbon dioxide, CO2, as a waste product. Some of the carbon dioxide is trapped by the dough and forms small “air” pockets that make the bread light. If the yeast is not warmed properly, it will not be of much use as a leavening agent; the yeast cells will burn sugar much too slowly. In this experiment, you will watch yeast cells ferment (burn sugar in the absence of oxygen) at different temperatures and measure their rates of fermentation. Each team will be assigned one temperature and will share their results with other class members.

You will observe the yeast under anaerobic conditions and monitor the change in air pressure due to CO2 released by the yeast. When yeast burn sugar under anaerobic conditions, ethanol (ethyl alcohol) and CO2 are released as shown by the following equation:

${{\text{C}}_{\text{6}}}{{\text{H}}_{{\text{12}}}}{{\text{O}}_{\text{6}}} \to {\text{2 C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{OH + 2 C}}{{\text{O}}_{\text{2}}}{\text{ + energy}}$ ${\text{glucose }} \to {\text{ ethanol + carbon dioxide + energy}}$

Thus, the metabolic activity of yeast may be measured by monitoring the pressure of gas in the test tube. If the yeast were to respire aerobically, there would be no change in the pressure of gas in the test tube, because oxygen gas would be consumed at the same rate as CO2 is produced.

### Objectives

In this experiment, you will

• Use a Gas Pressure Sensor to measure the change in pressure due to carbon dioxide released during respiration.
• Determine the rate of fermentation of yeast at different temperatures.