Yeast Metabolism: Measuring Cell Growth, Glucose Utilization, and Ethanol Formation

Assessment Overview

In this laboratory, you will investigate the outcome of two strains of yeast grown under aerobic and anaerobic conditions by:

• Using absorbance to estimate cell concentration after the yeasts have grown.
• Detecting ethanol produced by metabolism.
• Measuring glucose concentration after the cells cease growing on glucose.

Theory

Introduction to eukaryotic metabolism:
The yield of energy from anaerobic metabolism is far lower than the yield of energy from aerobic metabolism. ATP, the currency of energy in the cell, is produced by the pathways of metabolism and is then used to produce more cells.

The brewer’s yeast, Saccharomyces cerevisiae, is a suitable organism used to compare the yield of energy via anaerobic or aerobic metabolism.

Yeasts are eukaryotic organisms with the ability to metabolise carbon sources aerobically to CO₂ and H₂O or anaerobically to produce alcohol (ethanol).

Assignments 1 - 3 Guideline

1a. Describing values and trends (Presents sufficient descriptions of the trends in the text summary in a logical manner).

• Accurately describes all trends and values. Provides detailed yet concise descriptions which makes text summary easy to read and visualise all aspects of the graph/table without having to refer to graph/table.

1b. Graphical representation (Presents analysed data accurately and correctly)

• Accurately calculates all data. Uses all the correct figure types and/or table formats. Figures and tables are accurately and appropriately formatted with informative captions.

2a. Discussing and interpreting results (Establishes logical and appropriate reasonings as to why results were obtained, relating to the purpose/s of the experiment).

• Provides a clear, logical, in-depth and scientifically sound explanation and interpretation of all data, by linking all discussion points to findings and clearly linking them to the experiment’s objectives. Draws insightful comparisons and contrasts among data trends.

2b. Comparing data to findings from peer-reviewed journal articles (Compares and contrasts findings to appropriate and relevant peer-reviewed journal articles).

• Uses appropriate and relevant literature to clearly compare own findings with literature and explains similarities or discrepancies, showing a thorough understanding of all concepts.

3a. Layout (Presents information in accordance with the layout conventions of a scientific report taught in BIO152).

• Adheres to all layout and structural conventions (e.g., tenses) of a scientific report.

3b. Scientific writing (Presents information in accordance with the written conventions of a scientific report taught in BIO152) 

• Written expression is clear, concise, scientific, objective, coherent and cohesive, with appropriate paragraphing throughout. Demonstrates a high level of readability and professionalism.

3c. Referencing (Presents at least three references and in-text citations in APA 7th Edition)

• All in-text citations and references are present, correctly and appropriately formatted, and are valid. Demonstrates a thorough understanding of citation conventions and uses credible, peer-reviewed and relevant sources.

Activity 1: Production of Ethanol

Materials:

• 12 samples of yeast cultures grown in flasks
• Acidified Potassium Permanganate (APP) solution.

Method:
Click here to see how the experiment is performed. Note: There may be slight variations in the protocol refer to the lab manual for the most up-to-date instructions.

1. At your bench, set up and label twelve Eppendorf tubes reflecting the different cultures on page 36.
   Note which pipette should you use in steps 2 and 3? p200
2. Add 0.1 mL of each of the yeast culture samples to the respective Eppendorf tube.
3. Head towards the fume hood at the ends of the lab and add 1.0 mL of the APP solution to each of the tubes and mix by inversion.
   Make sure that the lid is capped properly before inverting the tubes.
4. If ethanol is present in the sample, a brown colour should develop within 15 minutes at room temperature. If no ethanol is present, the sample will remain purple.
5. Note the result (+/-) in the table below.

Activity 2: Growth of yeast

The amount of growth in each culture can be measured using the spectrophotometer to measure the absorbance at 600 nm. The absorbance value is an estimate of the number of cells in suspension since cells in suspension scatter light.

Materials

• 12 samples of yeast cultures.

Method
Click here to see how the experiment is performed. Note: There may be slight variations in the protocol refer to the lab manual for the most up-to-date instructions.

1. Set the spectrophotometer to read absorbance at 600 nm and rinse with water.

2. Zero the spectrophotometer with water.

3. Swirl each sample (12 in total) before measuring in the spectrophotometer. Note that you do not need to blank the spectrophotometer between the samples.

4. Record the absorbance in the table below.

5. To obtain the net change in absorbance, subtract the absorbance at T₀ from the corresponding culture at Tₘ.

Assessment Requirements

Assessment Type: Laboratory Report (Yeast Metabolism Study)

Purpose:

• Investigate the growth and metabolic outcomes of two strains of Saccharomyces cerevisiae under aerobic and anaerobic conditions.
• Develop scientific reasoning, analytical, and laboratory reporting skills.
• Apply concepts of eukaryotic metabolism to practical experiments and data interpretation.

Key Pointers to be Covered:

1. Describing Values and Trends (1a):
   • Present clear, concise descriptions of experimental data trends.

   • Enable readers to visualise the data without referring constantly to graphs or tables.

2. Graphical Representation (1b):
   • Accurate calculation and presentation of data in appropriate figures or tables.

   • Proper formatting and captions for clarity.

3. Discussion and Interpretation (2a & 2b):
   • Logical, scientifically sound explanations linking results to experimental aims.

   • Compare and contrast findings with peer-reviewed literature to validate results.

4. Report Structure and Scientific Writing (3a & 3b):
   • Follow standard scientific report conventions taught in BIO152.

   • Maintain clarity, conciseness, and professional scientific writing style.

5. Referencing (3c):
   • Include at least three credible references with proper APA 7th Edition formatting.

Experimental Activities:

• Activity 1: Production of Ethanol (detect ethanol via Acidified Potassium Permanganate reaction).
• Activity 2: Growth of Yeast (measure absorbance at 600 nm to estimate cell concentration).

Assessment Approach Step-by-Step Mentor Guidance

1. Understanding the Experiment:

   • Mentor explained the purpose: comparing aerobic vs anaerobic metabolism in yeast.

   • Reviewed theory behind energy yield differences and ethanol production.

2. Planning and Setup:

   • Guided student on experimental setup: labelling tubes, pipette selection, and following safety procedures.

   • Emphasized careful handling of reagents and proper fume hood use.

3. Data Collection:

   • Ethanol Production: Step-by-step instruction for adding APP solution, mixing, observing colour change, and recording results.

   • Yeast Growth: Training on using the spectrophotometer, zeroing, measuring absorbance, and calculating net changes.

4. Data Analysis and Representation:

   • Mentor assisted in calculating averages, net absorbance changes, and summarising trends.

   • Guided student on creating clear tables and graphs with appropriate captions.

5. Discussion and Interpretation:

   • Explained linking experimental results to metabolic theory.
   • Encouraged comparing data with literature to highlight consistencies or discrepancies.

   • Advised on drawing conclusions about aerobic vs anaerobic metabolism efficiency and ethanol production.

6. Report Writing and Referencing:

   • Checked adherence to scientific report structure: introduction, methods, results, discussion, conclusion.
   • Ensured clarity, scientific tone, and proper APA 7th Edition referencing.

Outcome Achieved

• Student produced a well-structured laboratory report summarizing growth and ethanol production trends.
• Demonstrated clear understanding of yeast metabolism under different conditions.
• Presented data effectively through tables and graphs.
• Showed ability to critically interpret results and compare them with relevant literature.
• Developed professional scientific writing and reporting skills, including accurate referencing.

Learning Objectives Covered

1. Understanding Eukaryotic Metabolism:

   • Compare energy yield from aerobic vs anaerobic pathways in yeast.

2. Data Collection and Analysis:

   • Measure absorbance, detect ethanol, calculate glucose consumption, and analyse trends.

3. Scientific Reporting:

   • Apply professional conventions, clear writing, logical structure, and referencing.

4. Critical Thinking and Interpretation:

   • Draw conclusions, link results to theoretical knowledge, and validate findings with literature.

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