Experiment: Test the Effect of Changing the Concentration of Yeast on the Rate of Fermentation of Glucose

SACE Number: ______________________________

Instructions for school supervisor:

Please note – this practical report must be submitted to the OAC supervisor within 5 days of completing the practical.

The practical skills section below must be assessed on the continuum during the practical by the supervising Biology/Science teacher. Please use a at the level of achievement for each skill.

Experiment Supervisor’s Details:

• (name, address, telephone no.
• Experiment Supervisor’s DECLARATION
• I declare that the practical work was conducted under the conditions specified by the following guidelines:
  • All work included is only that of the student whose name appears on this form;
  • No overt assistance was received by the student in the completion of the practical work and the report write up;
  • The practical was completed within a suitable time (~2 hours) and the report was handed in within 5 days.
  • The practical skills above were assessed by me during the practical.
• Experiment Supervisor’s Signature: ______________________________
• Date experiment was completed: ______________________________
• Date report received by supervising teacher: ____________________________
• *R = Required for Moderation (you must return this at the end of the year with your other marked work ie tests, prac reports & issues investigation)

Introduction

Yeast cells can carry out both aerobic and anaerobic respiration, depending on the availability of oxygen in their environment.  Anaerobic respiration by yeast is called fermentation.

To ensure that fermentation takes place, it is essential to exclude as much oxygen as possible from the solution in which the yeast cells are growing.

Fermentation can be summarised by the following equation. The enzymes are found in the yeast cells.

• C6H12O6 2C2H5OH + 2CO2
• glucose ethanol + carbon dioxide

The rate of fermentation can be found by measuring the volume of carbon dioxide produced in a given time.

Aim: To test the effect of changing the concentration of yeast on the rate of fermentation of glucose.

Complete the following before you start the experiment.

Hypothesis:

The hypothesis can be stated as follows: Increasing the concentration of yeast cells increases the rate of fermentation.

(a) Predict the results expected if this hypothesis were supported.

If the independent variable is increased, the dependable variable will increase.

(b) Explain why these results would be expected:

Each yeast cell is known to metabolize glucose via anaerobic respiration to produce a certain volume of carbon dioxide within a certain period of time. If the number of respective yeast cells is increased, the volume of carbon dioxide produced will increase in proportions that directly reflect the number of yeast cells that have been added.

• State the independent variable: The amount of yeast used.
• State the dependent variable: The volume of carbon dioxide gas produced.

Materials

• dried yeast (solid) ~ 20 g
• glucose (solid) ~ 12 g
• patty pans ~ 12
• 1 x 50 mL measuring cylinder
• Stopwatch
• 1 x large test tube with stopper and tubing (as per photo)
• 1 x 250 mL beaker to hold reaction mixture in place in a water bath
• 1 x thermometer ( -10C to 110C)
• apparatus as shown
• water bath at approx. 40 C (alternatively use a large beaker of hot water at approx. 40 C).
• 1 stoppered 250 mL container of distilled water in a water bath – this is for making the glucose solution.

Method

1. An ice cream container was half-filled with cold tap water. The measuring cylinder was filled with water, inverted, and then immersed in the reservoir. Carefully, the measuring cylinder was lifted up a little from the bottom of the dish and supported loosely with the clamp as shown in the photo.
2. The temperature of the water bath was checked to ensure that it was about 40oC. 
3. 1g of glucose was weighed in a patty pan, using the electronic balance.
4. 1g of dried yeast was weighed in a patty pan, using the electronic balance.
5. The 1g of glucose was placed in a test tube.
6. 10 mL of hot water (from the container in the water bath) was measured to the glucose in the test tube. The glucose was dissolved by swirling gently.
7. The yeast was then added to the glucose solution and mixed thoroughly by shaking.
8. Immediately the delivery tube was attached to the test tube and the test tube was placed in the beaker in the hot water bath at the same time, the end of the plastic tubing was placed under the measuring cylinder in the reservoir, making sure that the measuring cylinder was firmly resting on the tubing and then the clamp tightened. This was to collect the gas made by the yeast, so it was important not to let any escape.
9. The volume of gas collected in 10 minutes was recorded. NB: Any froth was not allowed to enter the delivery tube. The stopper and tubing were removed from the test tube immediately if the froth looked like it will do this.
10. Steps 3 – 8 were repeated 5 times but the masses of yeast were altered so that the following are used: 0.0 g, 0.5 g, 1.0 g,1.5 g, 2.0 g, and 2.5 g.
11. The whole experiment was repeated once.

Identify 3 major factors that must be deliberately held constant during this experiment.

1. Temperature of the water bath, which is the temperature at which the fermentation reaction proceeds
2. Time of fermentation reaction
3. Concentration of glucose utilized by yeast cells

Table of Results

• Yeast used(g)
• Volume of carbon dioxide gas produced (mL)
• Average volume of carbon dioxide gas produced (mL)
• Initial volume of water (mL)
• Final volume of water (mL)

Graph a line/curve of best fit. (Hand-draw this graph)

Discussion and Conclusion:

According to the results obtained in the experiment (see table 1), an increase in the number of yeast cells mixed with the same concentration of glucose resulted in an increase in the volume of carbon dioxide produced (see graph 1). As per the experiment, the increase in the number of cells was not continuous. Every batch of cells was allowed to metabolize the same concentration of glucose separately and the volume of carbon dioxide produced was recorded after each allowed time of fermentation reaction. With every increase in mass of yeast cells used, the rate of production of carbon dioxide was increased such that more volume of carbon dioxide gas was produced within the experimental set time.

According to observations, the increase was not directly proportional to the mass of yeast cells. Additionally, an increase in the volume of carbon dioxide produced was not recorded with every increase in the mass of yeast cells. For instance, when 2g of yeast cells were used in the first trial, the volume of carbon dioxide produced dropped from 34mL to 27mL.

An increase in the volume of carbon dioxide produced corresponds to an increase in the rate of fermentation. In order to ensure that the increase in the volume of carbon dioxide was only linked to the increase in the concentration of yeast cells, several factors, which could otherwise contribute to increasing in the volume of carbon dioxide produced were kept constant during the experiment. These included; temperature, the time allowed for the reaction to proceed, and lastly concentration of glucose. These factors were kept constant because an increase in temperature would have caused the rate of reaction to increase, thus increasing the volume of carbon dioxide produced within the set time.

An increase in time will also result in recording more volume of carbon dioxide produced. Lastly, an increase in the concentration of glucose would also have resulted in more volume of carbon dioxide produced within the experimentally allowed time. The importance of keeping these factors constant was to ensure that the experimental setup yields reliable results (Chen, 2013). To ensure that the factors were kept constant, several approaches were followed in the experiment. The temperature was checked at all times to ensure that it was about 40oC. 

The initial concentration of glucose was used in the entire experiment. All reactions were allowed to proceed for 10 minutes after which, the volume of carbon dioxide produced was recorded. Random and systematic errors may have occurred in the experiment (Stanbury et al., 1995). Random errors may have been caused by incorrectly measuring both yeast and glucose, and the volume of water. Systematic errors may have been caused by obtaining wrong readings of the final and initial volume of carbon dioxide produced. The precision and accuracy of the results obtained in the experiment were poor. The second trial gave completely different volumes of carbon dioxide produced. For instance, in the first trial using 1.5g of yeast cells gave 34 mL of carbon dioxide while the second trial gave 27 mL of carbon dioxide gas produced.

The methods used to set up this experiment were appropriate. However, they did not give accurate results. Possibly, the inaccuracy of the results obtained resulted from the random and systematic errors committed while conducting the experiment. The experimental setup is appropriate but the use of exact measurements of the mass of yeast cells and glucose can be obtained by measuring the respective quantities two times for confirmation.

As stated before, the volume of carbon dioxide produced increased when the number of yeast cells was increased by allowing the increased mass size of cells to react with the glucose in solution form. Each yeast cell is known to metabolize glucose under anaerobic conditions to produce a certain volume of carbon dioxide after the reaction has been allowed to proceed for a particular experimental time. Yeast cells require energy for survival. The energy is known to be stored in glucose, but yeast cells can not utilize the energy directly. Therefore, yeast cells metabolize glucose during respiration to obtain their energy in form of adenosine triphosphate (ATP).

Usually, two molecules of ATP are produced. Carbon dioxide gas is usually produced alongside ethanol as bi-products of the respiration process occurring in the absence of energy (anaerobic respiration). The respective process by which yeast cells act on glucose molecules to derive their energy and produce carbon dioxide and ethanol as bi-products can be summarized in the equation below:

• C6H12O6 Enzymes in yeast cells 2C2H5OH + 2CO2 + 2ATP;
• Glucose ethanol + carbon dioxide + Energy.

When the number of cells becomes more, the volume of carbon dioxide produced within the set experimental time proportionally relates to the number of cells in the respective reaction (Boulton and Quain, 2008). Therefore, an increase in the number of yeast cells results in an increase in the volume of carbon dioxide produced if temperature, the concentration of glucose, and reaction time are kept constant. In this experiment, an increase in the volume of carbon dioxide was noted with an increase in the mass of yeast cells allowed to metabolize 1g of glucose. Hence it can be deduced that the outcome of the experiment supports the hypothesis, which was stated as follows: If the independent variable is increased, the dependable variable will increase, i.e increasing the concentration of yeast cells increases the rate of fermentation.

Every member of my group had a role to play while setting up the experiment. One or two members of the group executed each protocol step. We discussed as a team how best to perform all the steps starting from measuring water, glucose, and yeast cells until the final step of reading the results prior to disassembling the apparatus used in the experiment, cleaning them, and storing them in their appropriate places.

Reference list:

• Boulton, C., & Quain, D. (2008). Brewing Yeast and Fermentation. Chicester, Wiley.
• Chen, H. (2013). Modern Solid State fermentation theory and practice. Dordrecht, Springer.
• Stanbury, P. F., Whitaker, A., & Hall, S. J. (1995). Principles of fermentation technology.

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