Week 7 — Lab / Scientific Inquiry · "The Floating Leaf Disk"

Course: Introduction to Biology — General Biology I (BIOL 101) · Silver Oak University (fictional sample) · Prof. Castellano
Objective: Objective 4 — observe photosynthesis producing O₂; design a light-vs-dark comparison; collect and interpret data · SLO A (scientific reasoning)
Worth 50 points · Labs group = 15% of the grade · Lab 7
Format: a hands-on leaf-disk assay (a verified free protocol; common kitchen items) — you'll 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 uses a simple at-home/lab-bench protocol; the procedure links to a verified free resource (the Exploratorium's "Photosynthetic Floatation" Science Snack, with the Science Buddies version as an alternative). All lab resources are links to external sites — nothing to buy or download beyond a spinach leaf, baking soda, dish soap, and a syringe.

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Part 1 — The Big Picture

This week you learned the machinery of photosynthesis: light-dependent reactions in the thylakoid that split water and release O₂, and a Calvin cycle in the stroma that fixes CO₂ into sugar. Now you'll see the oxygen for yourself. Punch small disks out of a spinach leaf, sink them in a baking-soda solution, and shine a light on them. As the disks photosynthesize, they make O₂ gas, which collects inside them and makes them float. The more photosynthesis, the more disks float — so floating disks are a stand-in for the rate of oxygen production. Put them in the dark, and they stop.

The phenomenon: spinach disks sink at first (the air in their leaf spaces is replaced by solution). Under light, photosynthesis makes O₂ that re-fills those spaces, and the disks rise. The baking soda supplies dissolved CO₂ (a reactant); the light supplies the energy; the water is the medium. In the dark, no light reactions → no O₂ → the disks stay down.

Procedure resource (verified live): Exploratorium — "Photosynthetic Floatation" Science Snack (full materials + steps + the dark-control idea): 🔗 https://www.exploratorium.edu/snacks/photosynthetic-floatation
Alternative version (verified live): Science Buddies — "Use Floating Leaf Disks to Study Photosynthesis": 🔗 https://www.sciencebuddies.org/science-fair-projects/project-ideas/PlantBio_p053/plant-biology/photosynthesis-leaf-disk-assay
Optional background (~5 min): Amoeba Sisters — "Photosynthesis (UPDATED)": 🔗 https://www.youtube.com/watch?v=CMiPYHNNg28

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Part 2 — Your Scientific Question & Hypothesis

The question: Does light drive oxygen production in spinach leaf disks — i.e., do more disks float in the light than in the dark over the same time?

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

If I give one set of leaf disks light and keep another set in the dark, then __ will have more disks floating over time, 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.)

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Part 3 — Materials & Procedure

You need (common items; see the Exploratorium link for photos):
- Fresh spinach leaves · baking soda (sodium bicarbonate) · liquid dish soap (a drop or two) · water · a 10-mL syringe (no needle) · a straw or hole punch (to cut disks) · two clear cups · a bright lamp · a timer · something to make the dark condition (a cabinet, a box, or foil).

Procedure (summary — follow the linked resource for the detailed version):
1. Make the solution: stir a small amount of baking soda into water (about 0.5 g per 2 cups / ~0.1%), plus a drop of dish soap (the soap lets the solution wet the leaf). Mix gently to avoid suds.
2. Cut 20 disks from spinach with the straw/punch (avoid big veins). You'll use 10 for "light" and 10 for "dark."
3. Sink the disks: put 10 disks in the syringe with some solution, expel the air, then plug the tip and pull the plunger to create a vacuum; release. Repeat until all 10 disks sink. Pour them into a cup of the solution. Do the same for the second set of 10.
4. Light vs. dark: place one cup under a bright lamp; place the other in the dark (cabinet/box/foil). Start the timer.
5. Record the number of disks floating in each cup at 0, 3, 6, 9, and 12 minutes.
6. Hold these the same in both cups (your controlled variables): same solution, same number of disks, same cup, same starting time, same room temperature — only the light differs.

No spinach or syringe handy? You can run the linked Exploratorium demo conceptually (it includes a video), or use the model data table in Part 8 to practice the analysis — but the hands-on version takes about 20 minutes and is genuinely fun to watch.

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Part 4 — Data Table (fill this in)

Record the number of disks floating (out of 10) in each condition at each time.

Time (min)	Light — disks floating	Dark — disks floating
0	______	______
3	______	______
6	______	______
9	______	______
12	______	______

Tip: also note the time the first disk floats in the light, and the time half (5 of 10) are floating — that midpoint is a common way to compare conditions.

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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): ___

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Part 6 — Analysis Questions

1. In which condition did more disks float over time — light or dark? At 12 minutes, how many were floating in each?
2. Why do the disks float? Explain using photosynthesis — what gas is being produced, by which stage, and why does it lift the disks?
3. Why do the dark disks barely float? Tie it to the light-dependent reactions (no light → no ATP/NADPH → the Calvin cycle stalls too).
4. The solution contained baking soda (a CO₂ source). Which reactant of photosynthesis does that supply, and why is it needed for the disks to float?
5. Connect it: the oxygen filling these disks came from splitting water. In one sentence, explain how this little experiment demonstrates where the O₂ you breathe ultimately comes from.

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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. In which condition did more disks float, and what does this say about photosynthesis? Which stage produces the oxygen, and where does that oxygen come from? Also name the independent and dependent variables."
2. Check everything it says against your own work and this week's lecture:
   - Did it correctly say the oxygen comes from splitting water — or did it claim it comes from CO₂? (This is the #1 slip.)
   - Did it credit the O₂ to the light-dependent reactions — or did it confuse the stages, or say the Calvin cycle makes the oxygen?
   - Did it correctly call the light the independent variable and the number of floating disks the dependent variable — or did it swap them?
   - Did it get the dark result right (no light → no O₂ production → disks don't float), or did it over-claim?
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 against the lecture — that's the skill.)

The habit all term: the tool drafts, you judge. A chatbot will confidently say the oxygen comes from CO₂, or flip the variables — catching it is the point.

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Part 8 — What to Submit

Submit a single document (or text entry) with: your hypothesis, your completed data table (both conditions, all five times), your Part 5 variable labels, your Part 6 answers, and your Part 7 AI-critique paragraph. Due Sunday, Oct 18, 11:59 p.m. (50 points).

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

Students collect their own numbers, so exact counts vary with light intensity, leaf freshness, and disk size. The model dataset below is for grading the analysis and reasoning; all numbers are pre-computed and independently re-verified (a Python check re-derives the trend; quantitative gate: PASS).

Model data table (illustrative, clean values — disks floating out of 10):

Time (min)	Light	Dark
0	0	0
3	2	0
6	5	0
9	8	1
12	10	1

• Light rises steadily: 0 → 2 → 5 → 8 → 10 (all disks floating by 12 min). ✓
• Dark stays essentially flat: 0 → 0 → 0 → 1 → 1 (almost nothing floats). ✓
• Endpoint difference at 12 min = 10 − 1 = 9 more disks floating in the light. ✓
• Half the disks (5 of 10) float at ~6 min in the light (a clean midpoint for comparison). ✓
• Mean disks floating across the five readings: light = 5.0 (0+2+5+8+10 = 25; 25 ÷ 5 = 5.0); dark = 0.4 (0+0+0+1+1 = 2; 2 ÷ 5 = 0.4). ✓
• Trend, not exact counts, is the gradable fact: light → steadily increasing floats; dark → little or none.

Expected answers:
- Part 5: (1) IV = light (light vs. dark); (2) DV = number of disks floating (over time); (3) two of: same solution/baking-soda concentration, same number of disks, same cup, same start time, same temperature; (4) control group = the dark cup (the no-light baseline).
- Part 6: (1) Light had more; at 12 min ≈ 10 light vs. 1 dark (model). (2) The disks float because photosynthesis produces O₂ gas (from the light-dependent reactions splitting water); the O₂ collects in the leaf's air spaces and makes the disks buoyant. (3) In the dark, the light reactions can't run, so no ATP/NADPH and no O₂ is produced (and the Calvin cycle stalls too) — nothing lifts the disks. (4) Baking soda supplies dissolved CO₂, the reactant the Calvin cycle fixes into sugar; without a CO₂ source, photosynthesis (and O₂ output) would be limited. (5) The O₂ filling the disks came from splitting water — the same source as the oxygen in every breath you take.
- Part 7 (AI-critique): full credit for a specific catch — most commonly the AI saying the O₂ comes from CO₂ (it's from water), confusing the two stages (the Calvin cycle does not produce O₂), or swapping the IV/DV. Full credit also if the student verified each AI claim against the lecture and their data.

Grading rubric — 50 points

Criterion	Full	Partial	None
Hypothesis — a clear, testable "if…then…because…" prediction comparing light vs. dark (8)	8	4–6	0–2
Data table — both conditions, all five time points, with the light-vs-dark trend evident (15)	15	8–12	0–6
Variables (Part 5) — IV (light), DV (floating disks), two constants, and the dark control group all correct (12)	12	6–10	0–4
Analysis (Part 6) — O₂ from the light reactions (splitting water) explains floating; dark explained by no light reactions; CO₂/baking-soda role correct (10)	10	5–8	0–4
AI-critique (Part 7) — names a specific thing checked/corrected in the AI's interpretation (5)	5	3	0–2

Quality gate (self-checked): every number in the model dataset is pre-computed and independently re-verified by a Python check — the light trend (0/2/5/8/10) is monotonically increasing, the dark trend (0/0/0/1/1) stays ≤ 1, the endpoint difference is 9, the means are 5.0 (light) and 0.4 (dark), and half the disks float at ~6 min in light (quantitative gate: PASS). The biology is correct: O₂ is produced by the light-dependent reactions (from splitting water), light drives it, the dark control yields essentially none, and the baking soda supplies CO₂. Variables map correctly (IV = light, DV = floating disks, control = dark). No student-collected number is asserted as "the" answer — the key grades the trend and the reasoning, not a specific count.