### Introduction

Yeast are able to metabolize some foods, but not others. In order for an organism to make use of a potential source of food, it must be capable of transporting the food into its cells. It must also have the proper enzymes capable of breaking the food’s chemical bonds in a useful way. Sugars are vital to all living organisms. Yeast can metabolize sugar in two ways, aerobically, with the aid of oxygen, or anaerobically, without oxygen. When yeast metabolizes a sugar under anaerobic conditions, ethanol (CH3CH2OH) and carbon dioxide (CO2) gas are produced. An equation for the fermentation of the simple sugar glucose (C6H12O6) is:

${\text{C}}_{\text{6}}{\text{H}}_{\text{12}}{\text{O}}_{\text{6}} \to {\text{2}}\ {\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{OH}} + {\text{2}}\ {\text{CO}}_{\text{2}} + energy$

If sugars are readily available, baker’s yeast (Saccharomyces cerevisiae) prefers to metabolize glucose and other sugars anaerobically, through fermentation. This is also known as the Crabtree effect. The metabolic activity of yeast can be determined by the measuring the rate of ethanol production using an Ethanol Sensor inside a fermentation vessel. The rate of ethanol production is a direct indicator of the rate of fermentation.

In the Preliminary Activity, you will use an Ethanol Sensor to determine the rate of fermentation of glucose by yeast.

After completing the Preliminary Activity, you will first use reference sources to find out more about sugars, respiration, and yeast. You will then choose and investigate a researchable question dealing with the respiration of sugars by yeast. Some topics to consider in your reference search include the following:

• sugars
• carbohydrates
• monosaccharides
• disaccharides
• polysaccharides
• yeast
• enzymes
• metabolism
• aerobic respiration
• anaerobic respiration

### Objectives

• Identify variables, design and perform the experiment, collect data, analyze data, draw a conclusion, and formulate a knowledge claim based on evidence from the experiment.
• Obtain graphic representations of fermentation rate.
• Determine fermentation rate by yeast while using different sugars.
• Determine which sugars can be used as a food source by yeast.