Week 6 — Feeding the CO₂ Factory: The Yeast and Respiration Report — Traditional alternative (no AI)

Instructors: publish this instead of the adaptive version if you prefer, and unpublish or delete the adaptive one.

Assignment 6 of the term — compute what yeast fermentation and cellular respiration produce, and explain to someone new to biology why more sugar does not mean twice the rise.

This week you followed glucose through four stages, placed each stage in its part of the cell, and saw what yeast and muscle do when oxygen runs short. In this assignment you work four short problems, interpret each one, and explain to someone who has never studied biology why more sugar does not mean twice the rise. This week's lecture tutorial, quiz, discussion, and Biology Lab & Inquiry practise the same ideas from other angles. It builds on Week 5, where you met ATP as the cell's energy currency.

The task

The yeast balloon example is an illustrative dataset constructed for this course (not real data). Four bottles hold the same amount of yeast in the same warm water with 0, 1, 2, and 4 teaspoons of sugar, and a balloon covers each opening. At 30 minutes the balloon circumferences are 0, 8, 14, and 18 cm. The gas is carbon dioxide from alcoholic fermentation, pyruvate → ethanol + CO₂, which yields only glycolysis's 2 ATP per glucose. In the full aerobic pathway, glycolysis splits each glucose into 2 pyruvate in the cytoplasm, and the complete breakdown releases 6 CO₂ per glucose: 2 in pyruvate oxidation and 4 in the citric acid cycle in the mitochondrial matrix.

Work these four problems, showing your arithmetic:

1. In the yeast balloon example, the 1-teaspoon balloon reads 8 cm and the 2-teaspoon balloon reads 14 cm at 30 minutes. How many centimeters of circumference does the second teaspoon add?
2. A model muscle cell runs glycolysis on 3 glucose molecules, and each glucose splits into 2 pyruvate. How many pyruvate molecules does the cell make?
3. Each glucose that is fully broken down releases 6 CO₂ molecules: 2 in pyruvate oxidation and 4 in the citric acid cycle. How many CO₂ molecules are released when 4 glucose molecules are fully broken down?
4. In the yeast balloon example, the first teaspoon of sugar adds 8 cm of circumference, and going from 2 to 4 teaspoons adds 4 cm. How many times larger is the first gain than the last?

Beside each answer, add one brief interpretation:

• Problem 1: explain what the 0-teaspoon control shows, and name the gas and the process that made it.
• Problem 2: say where in the cell glycolysis runs, and explain why it can keep running when oxygen runs short.
• Problem 3: say where in the cell that carbon dioxide is released, and where it goes after it leaves the cell.
• Problem 4: explain why the gains shrink as sugar is added, and use your neighbor message (below) to compare fermentation's payoff with the full pathway's.

Explain it to a non-expert

A neighbor who bakes bread most weekends asks, "If I double the sugar, will my dough rise twice as much?" Write a 3–5 sentence message that answers in everyday words using your results, compares fermentation's 2 ATP per glucose with the full pathway's about 30 to 32, and names one limit of the evidence, including that the yeast balloon numbers are an illustrative model and that real yeast and kitchens vary. Explain any technical term in everyday words. More than one good answer is possible.

For a different, ungraded example, imagine a café owner asking why hiring a third cashier helped less than hiring the second, using made-up numbers from one afternoon. Model answer: "A new cashier helps only when customers are waiting. The second cashier cleared a long line, so the gain was big; by the time the third started, the line was already short, so there was less left to gain. That is why each added cashier helped less than the one before. My numbers come from one made-up afternoon, so a busier day could look different."

Use that pattern for the bread question, with your own wording and evidence.

Show your arithmetic. A reader should be able to reproduce every number you report from the values in the problems without guessing which values or steps you used.

Deliverables

Submit a single report containing:

• the four answers, each with the arithmetic shown and the brief interpretation the task names for that problem;
• your message to your neighbor, answering the bread question in everyday words with one limit of the evidence; and
• a one-line list of the yeast balloon readings and per-glucose amounts you used, so your results are reproducible.

Keep it to roughly one page. Clear and correct beats long.

Choose how you show it. Present your report as written text, or as a recorded audio or video explanation of about three minutes with a transcript, or as a labelled sketch of the four bottles with a short note. Every form is graded by the same scoring criteria. Submit in any accessible file type or as a link; a recorded response may be submitted as audio or video with a transcript, or as text.

How you'll be graded

The full analytic rubric is attached to this assignment in Canvas.

Academic integrity and AI use

This version is designed to be completed without AI: do the arithmetic, the interpretations, and the writing yourself, without a chatbot or other AI tool. You may use a calculator or a spreadsheet for the arithmetic. Submitting AI-generated work as your own, or copying another student's work, is an integrity violation under the course policy.

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