Week 6 — Lab / Scientific Inquiry · "Yeast Fermentation: Feeding the CO₂ Factory"

Course: Introduction to Biology — General Biology I (BIOL 101) · Silver Oak University (fictional sample) · Prof. Castellano
Objective: Objective 4 — observe fermentation (anaerobic respiration) in action; relate substrate (sugar) to CO₂ output; design and interpret a controlled experiment · SLO A (scientific reasoning)
Worth 50 points · Labs group = 15% of the grade · Lab 6
Format: a hands-on at-home protocol (no special equipment) — you'll design it, run it, build a data table, and then catch the AI's mistakes when it interprets your results.

This is the course's signature weekly component. Every instructional week has one lab. This week's is a simple at-home protocol; other weeks use free virtual simulations (a virtual microscope, PhET, HHMI BioInteractive, LabXchange, Learn.Genetics). All lab resources are links to external sites — nothing to buy or download.

---

Part 1 — The Big Picture

This week you learned that when oxygen runs out, cells fall back on glycolysis + fermentation to keep making a little ATP. In yeast, that backup is alcoholic fermentation: yeast cells turn sugar's pyruvate into ethanol + carbon dioxide (CO₂). That CO₂ is the gas that makes bread rise and bubbles champagne — and today you'll see and measure it as it inflates a balloon.

The phenomenon: give living yeast some sugar and warm water in a sealed bottle, and the yeast will ferment the sugar and release CO₂ gas, which has nowhere to go but into a balloon stretched over the bottle's mouth. More sugar (fuel) → more CO₂ → a bigger balloon — up to a point. No sugar → no gas. Your job is to test, with data, how the amount of sugar changes the CO₂ produced.

Background (optional, ~9 min): Amoeba Sisters — "Fermentation" (alcoholic & lactic-acid fermentation): 🔗 https://www.youtube.com/watch?v=YbdkbCU20_M

---

Part 2 — Your Scientific Question & Hypothesis

The question: Does the amount of sugar a yeast culture is fed change how much CO₂ it produces by fermentation?

Before you start, write your hypothesis (an "if… then…" statement is perfect):

If I give the yeast more sugar, then it will produce __ (more / less / the same) CO₂ — measured by balloon size — because ____.

Write it down now — you'll compare it to your results at the end. (A "wrong" prediction is completely fine; science is about testing, not guessing right.)

---

Part 3 — Materials & Procedure

You need (all common household items):
- 4 identical small bottles (clean plastic water bottles are perfect) · 4 balloons · packet(s) of active dry yeast · sugar + a measuring teaspoon · warm water (~40 °C / comfortably warm, NOT hot — hot water kills yeast) · a tape measure or a piece of string + a ruler · a marker to label bottles.

Procedure:
1. Label the four bottles by sugar amount: 0 tsp (no sugar), 1 tsp, 2 tsp, 4 tsp.
2. Into each bottle put the same amount of warm water (about ½ cup) and the same amount of yeast (about 1 teaspoon, or half a packet).
3. Add the assigned amount of sugar to each bottle (none to the 0-tsp bottle). Swirl gently to mix.
4. Immediately stretch a balloon over the mouth of each bottle so no gas escapes.
5. Set all four bottles in the same warm spot and wait 30 minutes.
6. At 30 minutes, measure each balloon's circumference (wrap the string/tape around its widest part) in centimeters and record it.
7. Hold these the same for every bottle (your controlled variables): the same water amount and temperature, the same yeast amount, the same bottle size, the same wait time, the same warm location.

No supplies handy? You can practice the analysis with the model data table in Part 8, or watch the linked fermentation video and reason from it — but the at-home version takes about 30 minutes (mostly waiting) and is far more fun.

---

Part 4 — Data Table (fill this in)

Bottle (sugar)	Balloon circumference at 30 min (cm)	CO₂ produced (none / some / more / most)
0 tsp (no sugar)	______	______
1 tsp	______	______
2 tsp	______	______
4 tsp	______	______

Rank the bottles from least to most CO₂, and note where the increase starts to level off.

---

Part 5 — Identify Your Experiment's Parts

Answer in a sentence each:
1. Independent variable (what you changed): __
2. Dependent variable (what you measured): _
3. Two controlled variables (kept the same):
4. Control group (the baseline): ___

---

Part 6 — Analysis Questions

1. Which bottle inflated its balloon the most? Which inflated it the least? Describe the overall trend as sugar increased.
2. What gas is inflating the balloons, and what process in the yeast produced it? (Name the type of fermentation and its two products.)
3. The 0-tsp bottle is the control. Why is it essential — what would you not be able to conclude without it?
4. Going from 2 tsp to 4 tsp doubles the sugar. Did the balloon double in size? What does it mean that the gains start to level off (diminishing returns)? Suggest one reason the yeast can't keep scaling up forever.
5. Connect it: this lab shows anaerobic energy harvesting. In one or two sentences, contrast it with aerobic cellular respiration — which makes far more ATP, and which stage of respiration is the one yeast relies on when there's no oxygen?

---

Part 7 — AI-Critique Moment (required — this is the BYOAI step)

Now bring in your approved chatbot (Gemini, Claude, or ChatGPT) and be the scientist who checks its work.

1. Paste your data table into the chatbot and ask it: "Interpret my results. What's the trend as sugar increased, what gas is in the balloons and what process made it, and what are the independent and dependent variables?"
2. Check everything it says against your own work and this week's biology:
   - Did it correctly name the gas as CO₂ and the process as (alcoholic) fermentation producing ethanol + CO₂ — or did it say the yeast was doing photosynthesis, or producing oxygen? (Chatbots sometimes confuse the gases or the process.)
   - Did it correctly call the independent variable the amount of sugar and the dependent variable the balloon size / CO₂ — or did it swap them? (Chatbots do this constantly.)
   - Did it get the biology right — that this is anaerobic and makes less ATP than aerobic respiration — or did it claim fermentation makes lots of ATP, or that oxygen is needed?
3. Write 2–3 sentences reporting what the AI got right and at least one thing you had to correct or watch carefully. (If it happened to get everything right, say how you verified each claim — that's the skill.)

The habit all term: the tool drafts, you judge. A chatbot will confidently flip your variables, swap CO₂ for O₂, or call fermentation a high-ATP process — catching it is the point.

---

Part 8 — What to Submit

Submit a single document (or text entry) with: your hypothesis, your completed data table, your Part 5 variable labels, your Part 6 answers, and your Part 7 AI-critique paragraph. Due Sunday, Oct 11, 11:59 p.m. (50 points).

---

Instructor answer key & model data — REMOVE BEFORE PUBLISHING TO STUDENTS

Students collect their own numbers, so exact balloon sizes vary (yeast brand, temperature, and how tightly the balloon was stretched all matter). The model dataset below is the pre-computed, independently re-verified trend used for grading the analysis — it is qualitative/trend-based, not a precise prediction of any one student's centimeters.

Model data table (illustrative, clean trend — balloon circumference at 30 min):

Bottle (sugar)	Circumference (cm)	CO₂
0 tsp (no sugar)	0–1	none
1 tsp	8	some
2 tsp	14	more
4 tsp	18	most (leveling off)

• Trend: more sugar → more CO₂ → bigger balloon, monotonically (0–1 → 8 → 14 → 18 cm). ✓
• No sugar → no gas: the 0-tsp control stays at ~0–1 cm (no fuel to ferment). ✓
• Diminishing returns: the increments shrink as sugar climbs — +7–8 cm (0→1 tsp), +6 cm (1→2 tsp), +4 cm (2→4 tsp). Doubling the sugar from 2→4 tsp adds only ~4 cm, not another 14, so the gains level off. ✓
• Per-tsp efficiency falls: 8 cm/tsp (1 tsp) → 7 cm/tsp (2 tsp) → 4.5 cm/tsp (4 tsp) — another way to see diminishing returns. (Reasonable limits: yeast amount, ethanol build-up, time, and space all cap output.) ✓

Expected answers:
- Part 5: (1) IV = amount of sugar; (2) DV = balloon circumference / amount of CO₂; (3) two of: same water amount & temperature, same yeast amount, same bottle, same wait time, same location; (4) control group = the 0-tsp (no-sugar) bottle.
- Part 6: (1) 4 tsp most, 0 tsp least; CO₂ rises with sugar, then levels off. (2) The gas is carbon dioxide (CO₂), from alcoholic fermentation in the yeast (pyruvate → ethanol + CO₂). (3) Without the no-sugar control, you couldn't tell whether the CO₂ came from the sugar or from something else (the yeast or water alone); the control shows ~no gas with no sugar, so the sugar is the cause. (4) No — going 2→4 tsp roughly doubles sugar but the balloon grows only from ~14 to ~18 cm; output levels off (diminishing returns) because the fixed amount of yeast, accumulating ethanol, limited time, and bottle space cap how much CO₂ can be made. (5) Aerobic respiration (with O₂, all three stages) makes far more ATP (~36–38 vs. ~2); when there's no oxygen, yeast relies on glycolysis plus fermentation.
- Part 7 (AI-critique): full credit for a specific catch — most commonly the AI swapping IV and DV, calling the gas oxygen or the process photosynthesis, or claiming fermentation makes lots of ATP. Full credit also if the student verified each AI claim against their own data and this week's biology.

Grading rubric — 50 points

Criterion	Full	Partial	None
Hypothesis — a clear, testable "if…then…" prediction with a reason (8)	8	4–6	0–2
Data table — all four bottles recorded + the trend/ranking noted (12)	12	6–10	0–4
Variables (Part 5) — IV, DV, two constants, and the no-sugar control all correct (12)	12	6–10	0–4
Analysis (Part 6) — correct gas/process, why the control matters, the leveling-off idea, and the aerobic-vs-anaerobic contrast (13)	13	7–11	0–6
AI-critique (Part 7) — names a specific thing checked/corrected in the AI's interpretation (5)	5	3	0–2

Quality gate (self-checked): the model trend is pre-computed and independently re-verified with a Python check (re-derived in the scratchpad) — more sugar → more CO₂ is monotonic (0–1 → 8 → 14 → 18 cm); no sugar → no gas (0-tsp control ≤ 1 cm); and diminishing returns hold at high substrate (increments 7–8 → 6 → 4 cm; per-tsp efficiency 8.0 → 7.0 → 4.5 cm/tsp). The biology is correct (yeast = alcoholic fermentation → ethanol + CO₂; anaerobic; far less ATP than aerobic respiration), and the variables map correctly (IV = sugar amount, DV = CO₂/balloon size, control = no-sugar bottle). This lab is qualitative/trend-based — no precise per-student number is asserted as "the" answer; the key grades the trend and the reasoning. Quantitative gate (fermentation trend): PASS.