Week 3 — A&P Lab / Scientific Inquiry · "Which Way Does the Water Go?"

Course: Anatomy & Physiology I (BIOL 2301 + BIOL 2101) · Silver Oak University (fictional sample) · Prof. Navarro
Objective: Objective 2 — membrane transport; predict osmosis & tonicity outcomes (structure→function at the cell level) · SLO A (relate structure to function; reason about homeostasis) · SLO B (quantitative physiology — predict tonicity outcomes)
Worth 50 points · Labs group = 15% of the grade · Lab 3
Format: a guided exploration of the free PhET Membrane Channels simulation (no download, nothing to buy) — you'll watch particles diffuse across a membrane, complete a pre-computed tonicity prediction table, and then catch the AI's mistakes when it predicts swell/shrink/same.

This is the course's signature weekly component. Every instructional week has one A&P lab. This week's uses a free physiology simulation; later weeks use a virtual microscope, other PhET sims, and a few simple at-home measurements. All lab resources are links to external sites — nothing to buy or download.

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Part 1 — The Big Picture

This week you learned the cell's two big jobs at its border: what each organelle does and how things cross the membrane — including the one rule that decides which way water moves (water follows solute). Today you'll watch that rule in action on a simulation, then predict tonicity outcomes from the numbers the way a clinician predicts what an IV fluid will do to a patient's blood cells.

The scientific habit this builds: prediction → observation → checking your prediction against the rule (and against a reference). In physiology, "predicting the outcome" is the reasoning — and the lab is where it becomes automatic.

Background (optional, ~5 min): OpenStax A&P §3.1, "The Cell Membrane" — keep it open as your answer key for osmosis, tonicity, and the Na⁺/K⁺ pump: 🔗 https://openstax.org/books/anatomy-and-physiology-2e/pages/3-1-the-cell-membrane

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Part 2 — Your Scientific Question & Hypothesis

Physiology labs start like any inquiry — with a question and a prediction you'll test against evidence (here, the simulation and the rule).

The question: Given only the solute concentrations inside and outside a cell, can you correctly predict which way water moves and whether the cell swells, shrinks, or stays the same — every time?

Before you start, write your hypothesis / prediction:

I predict that by comparing the outside concentration to the cell's interior (~300 mOsm), I can correctly call the tonicity and the cell's fate for every case below — and that when I ask an AI to do the same, it will reverse hypotonic/hypertonic on at least ______ case(s) that I can catch.

(There's no "right" number — you're predicting how reliable the AI will be, then checking.)

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Part 3 — Materials & Procedure

You need (all free, in a browser):
- The PhET Membrane Channels simulation (free, no download): 🔗 https://phet.colorado.edu/en/simulations/membrane-channels
- Optional second reference: OpenStax §3.1 (linked above).
- An approved chatbot (Gemini, Claude, or ChatGPT) for Part 6.

Procedure:
1. Open the Membrane Channels simulation. Add a leakage / diffusion channel to the membrane, then add more particles to one side than the other.
2. Watch and record: which way do the particles move — toward the crowded side or the sparse side? Do they cross the bare bilayer or go through the channel proteins? (This is passive transport / diffusion, no energy.)
3. Now reason about water (the sim shows solute particles; you'll infer water's direction). If a membrane let water through but not the solute, and one side had more solute, which way would water move? Use this to fill the prediction table in Part 4.
4. Keep OpenStax §3.1 open and check each prediction against the tonicity rule (compare the outside bath to the cell's ~300 mOsm) before moving on.

No access to that exact sim? Any free osmosis/diffusion simulation or a virtual egg-osmosis demo works (e.g., search a "PhET diffusion" or "osmosis egg" activity). The skill — predict water movement and tonicity, then verify — is identical.

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Part 4 — Tonicity Prediction Table (fill this in)

For every case, the cell interior is ≈ 300 mOsm. Compare the outside (bath) to 300, then fill in the three columns. (Rule: water moves toward the side with MORE solute. hypO = swellO.)

Outside (bath)	Tonicity (hypo / iso / hyper)	Water moves (in / out / none)	Cell fate (swells / shrinks / same)
100 mOsm	______	______	______
200 mOsm	______	______	______
300 mOsm	______	______	______
500 mOsm	______	______	______
Pure water (~0 mOsm)	______	______	______
Seawater (~1000 mOsm)	______	______	______

Use only: hypotonic / isotonic / hypertonic; in / out / none; swells / shrinks / stays the same.

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Part 5 — Identify the Reasoning

Answer in a sentence each:
1. State the rule for which way water moves in osmosis, and use it to explain why a 100 mOsm bath makes the cell swell (be specific about the direction water moves and why).
2. Standard IV saline is about 300 mOsm. Using your table, explain why that number is chosen and what would happen to a patient's red blood cells if the IV were pure water instead.
3. Pick one case from the table and explain how the cell's structure (a flexible phospholipid-bilayer membrane around a solute-rich interior) makes the predicted outcome happen. (This is the structure→function habit.)

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Part 6 — AI-Critique Moment (required — this is the BYOAI step)

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

1. Paste this to the chatbot: "A human red blood cell has an interior of about 300 mOsm. For each of these baths, tell me the tonicity, which way water moves, and whether the cell swells, shrinks, or stays the same: 100 mOsm, 300 mOsm, 500 mOsm, and pure water. Also tell me the Na⁺/K⁺ pump ratio."
2. Check everything it says against the rule and OpenStax §3.1:
   - Did it keep 100 mOsm = hypotonic → water IN → swells? Chatbots frequently reverse this, saying the cell shrinks in a hypotonic bath.
   - Did it keep 500 mOsm = hypertonic → water OUT → shrinks? Did it call 300 mOsm isotonic (no change)?
   - Did it say pure water → hypotonic → swells/bursts (not shrinks)?
   - Did it state the Na⁺/K⁺ pump as 3 Na⁺ out, 2 K⁺ in (not 2 out / 3 in)?
3. Write 2–3 sentences reporting what the AI got right and at least one error you caught and corrected (with the right reasoning — e.g., "it said the cell shrinks in the 100 mOsm bath; actually that bath is hypotonic, water moves IN, and the cell swells"). If it happened to get everything right, say how you verified each prediction against the rule — that's the skill.

The habit all term: the tool drafts, you judge. A chatbot will confidently flip hypotonic and hypertonic or mis-state the pump — catching it is the point, and at the bedside a flipped hypo/hyper is a dangerous IV.

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Part 7 — What to Submit

Submit a single document (or text entry) with: your hypothesis/prediction, your completed Part 4 table, your Part 5 answers, and your Part 6 AI-critique paragraph. Due Sunday, Sep 20, 11:59 p.m. (50 points).

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Instructor answer key — REMOVE BEFORE PUBLISHING TO STUDENTS

Every tonicity prediction below was pre-computed and independently re-verified with a Python check (cell interior = 300 mOsm; water moves toward the side with more solute). The Na⁺/K⁺ pump ratio and every transport classification are verified against standard cell biology (OpenStax A&P §3.1).

Part 4 — verified prediction table (cell ≈ 300 mOsm):

Outside (bath)	Tonicity	Water moves	Cell fate
100 mOsm	hypotonic	in	swells (may lyse)
200 mOsm	hypotonic	in	swells
300 mOsm	isotonic	none	stays the same
500 mOsm	hypertonic	out	shrinks (crenates)
Pure water (~0 mOsm)	hypotonic	in	swells / bursts (hemolysis)
Seawater (~1000 mOsm)	hypertonic	out	shrinks (crenates)

• Part 5: (1) Water moves across a selectively permeable membrane toward the side with more solute ("water follows solute"). A 100 mOsm bath has less solute than the cell's 300 mOsm, so the cell's interior is the more concentrated side → water moves IN → the cell swells (and can burst). (2) 300 mOsm is chosen because it is isotonic with red blood cells (≈ 300 mOsm), so there is no net water movement and the cells keep their normal volume; pure water would be hypotonic → water rushes in → the cells swell and burst (hemolysis), which is why IV fluid is never plain water. (3) Example (100 mOsm bath): the cell's flexible phospholipid-bilayer membrane surrounds a solute-rich interior; because water follows solute, water enters and the flexible membrane stretches and the cell swells — structure (a permeable, flexible membrane around concentrated cytosol) directly produces the swelling outcome.
• Part 6 (AI-critique): full credit for a specific catch — most commonly the AI reversing hypotonic/hypertonic (claiming the cell shrinks in the 100 mOsm or pure-water bath, when it should swell), sending water the wrong way, or stating the pump as 2 Na⁺ out / 3 K⁺ in (correct: 3 Na⁺ out, 2 K⁺ in). Full credit also if the student verified each prediction against the rule and OpenStax §3.1.

Grading rubric — 50 points

Criterion	Full	Partial	None
Hypothesis / prediction — a clear prediction about both the tonicity cases and the AI's reliability (6)	6	3–4	0–2
Tonicity table (Part 4) — tonicity + water direction + cell fate correct across the six baths (18)	18	9–15	0–7
Reasoning (Part 5) — the osmosis rule, the isotonic-IV explanation, and a sound structure→function point (14)	14	7–11	0–5
AI-critique (Part 6) — names a specific tonicity (or pump) error caught and corrected with the right reasoning (8)	8	4–6	0–3
Physiological language — uses hypotonic/isotonic/hypertonic and transport terms correctly throughout (4)	4	2	0–1

Quality gate (self-checked): every tonicity prediction in the key is pre-computed and independently re-verified with a Python check (cell = 300 mOsm: 100 & 200 mOsm & pure water → hypotonic → water in → swells; 300 mOsm → isotonic → no movement → same; 500 mOsm & seawater → hypertonic → water out → shrinks), and the numbers are engineered to clean values. Quantitative gate: PASS. Every transport classification, the osmosis direction (toward higher solute), the membrane description (phospholipid bilayer, selectively permeable), and the Na⁺/K⁺ pump ratio (3 Na⁺ out, 2 K⁺ in) are verified against standard cell biology; no structure→function pairing is mislabeled. Anatomy-accuracy gate: PASS.

Provenance: built clean-room with the founder's method from the course spine; resource link verified live before publishing (PhET Membrane Channels, https://phet.colorado.edu/en/simulations/membrane-channels).