Week 3 A&P Lab & Inquiry: Which Way Does the Water Go?

Part 1 — The Big Picture

This week you learned that the plasma membrane is a selectively permeable phospholipid bilayer, that passive transport runs downhill for free, that osmosis moves water toward the side with more solute, and that tonicity is read from the cell's side. In this lab you watch particles cross a membrane in a free simulation, then you use the same rule to predict, for six baths, which way the water goes and what happens to a model cell. You define the variable so it can be measured, write a hypothesis before you start, and finish by checking a chatbot's account of your lab and catching its slips.

Time budget: Plan for about 120 minutes.

Part 2 — The Task

Guiding question: For a model cell at 320 mOsm/L, which way does water move in each of six baths, and how does the membrane decide what crosses it?

Before you begin — hypothesis: Before you open the simulation or fill the table, write one hypothesis in the if… then… because… form about a bath with more solute than the cell: if the model cell sits in that bath, then water will move in or out and the cell will swell or shrink, because of a reason about solute that you can state. Then define your variable so it can be measured: name what changes from bath to bath and the unit it is measured in.

Workshop data: Six illustrative baths for a 320 mOsm/L model cell, and your own membrane-simulation observations

Source: Six bath concentrations constructed for this lab (illustrative, not real measurements), plus the observations you record yourself in the free membrane-channels simulation; if the simulation will not run, the printed fallback observations stand in for yours.

• Rows: 6 baths, shown as 6 rows of the tonicity prediction table; the simulation table has 4 rows, one per setup you try
• Columns: bath, bath concentration in mOsm/L, comparison with the cell, tonicity, which way water moves, and the cell's fate
• Model cell: an illustrative cell interior of 320 mOsm/L

Part 3 — Materials & Procedure

Tools and materials

• A free virtual anatomy atlas or 3D anatomy viewer (Innerbody, GetBodySmart), a virtual microscope for histology (Histology Guide), free physiology simulations (PhET pH Scale, Membrane Channels, Neuron, Gene Expression Essentials), and a spreadsheet (Google Sheets or any free spreadsheet)

Ways of working

• structure-identification-on-a-virtual-atlas
• histology-on-a-virtual-microscope
• at-home-physiology-measurement-and-data-table

Procedure

1. Open the free Membrane Channels simulation at https://phet.colorado.edu/en/simulations/membrane-channels in a web browser and start it; no account is needed. Find the membrane across the middle of the screen, the controls that add particles to either side, and the channels you can drag into the membrane. Tool: A free virtual anatomy atlas or 3D anatomy viewer (Innerbody, GetBodySmart), a virtual microscope for histology (Histology Guide), free physiology simulations (PhET pH Scale, Membrane Channels, Neuron, Gene Expression Essentials), and a spreadsheet (Google Sheets or any free spreadsheet). Check: The simulation is running and shows a membrane with an empty space above and below it.
2. Add a large number of one type of particle to the top side only, with no channel in the membrane. Watch for about one minute and record in the simulation table whether any particles cross. Tool: A free virtual anatomy atlas or 3D anatomy viewer (Innerbody, GetBodySmart), a virtual microscope for histology (Histology Guide), free physiology simulations (PhET pH Scale, Membrane Channels, Neuron, Gene Expression Essentials), and a spreadsheet (Google Sheets or any free spreadsheet). Check: The particles stay on the top side, because nothing gives them a way through the membrane.
3. Drag one channel that matches that particle type into the membrane. Watch for about two minutes and record which way the particles move and whether the two sides become more even. Tool: A free virtual anatomy atlas or 3D anatomy viewer (Innerbody, GetBodySmart), a virtual microscope for histology (Histology Guide), free physiology simulations (PhET pH Scale, Membrane Channels, Neuron, Gene Expression Essentials), and a spreadsheet (Google Sheets or any free spreadsheet). Check: Particles drift through the channel from the crowded top side to the sparse bottom side, until the two sides are closer to even.
4. Add two or three more matching channels and watch again; then try a channel that matches the other particle type only. Record what changes in each setup, and note that you never supplied any energy. Tool: A free virtual anatomy atlas or 3D anatomy viewer (Innerbody, GetBodySmart), a virtual microscope for histology (Histology Guide), free physiology simulations (PhET pH Scale, Membrane Channels, Neuron, Gene Expression Essentials), and a spreadsheet (Google Sheets or any free spreadsheet). Check: More matching channels let the sides even out sooner; a non-matching channel lets the particles stay put.
5. Open a blank spreadsheet and type the six bath concentrations, 160, 240, 320, 440, 560, and 640, into column A. For bath F, type =640/2 into a cell to find its concentration after the stock is diluted into twice its volume. Tool: A free virtual anatomy atlas or 3D anatomy viewer (Innerbody, GetBodySmart), a virtual microscope for histology (Histology Guide), free physiology simulations (PhET pH Scale, Membrane Channels, Neuron, Gene Expression Essentials), and a spreadsheet (Google Sheets or any free spreadsheet). Check: The bath F cell returns a single concentration in mOsm/L that you copy into the prediction table.
6. For each bath, make the three moves in order in the prediction table: compare the bath with the cell's 320 mOsm/L using <, =, or >, name the bath hypotonic, isotonic, or hypertonic, then state which way the water moves and the cell's fate. Tool: A free virtual anatomy atlas or 3D anatomy viewer (Innerbody, GetBodySmart), a virtual microscope for histology (Histology Guide), free physiology simulations (PhET pH Scale, Membrane Channels, Neuron, Gene Expression Essentials), and a spreadsheet (Google Sheets or any free spreadsheet). Check: Every row of the prediction table has a comparison, a tonicity name, a water direction, and a cell fate.

Fallback route

When to use it: Use this route if the membrane simulation will not load on your device, or if you cannot use a computer for the simulation part of the lab.

Use the four illustrative fallback observations below, constructed for this lab and not recorded from the simulation, to complete the simulation table, then complete the prediction table, every analysis question, and the critique step, and submit the same items for equal credit. Fill the passive-or-active column yourself.

This route earns the same credit as the primary route.

Four illustrative fallback observations, constructed for this lab, that stand in for your own simulation runs.

Fallback model data

Setup	What you saw cross, and which way	Did the sides even out?	Passive or active, and why
No channel in the membrane	Nothing crossed; every particle stayed on the top side	No
One matching channel	Particles drifted from the crowded top side to the sparse bottom side	Slowly, over a few minutes
Several matching channels	The same drift from top to bottom, through every channel	Yes, sooner than with one channel
A channel for the other particle type only	Nothing crossed	No

Part 4 — The Scaffold

Name the parts of your inquiry before you start, so the comparison is fair and the variable can be measured.

Student scaffold table 1

Part of the inquiry	Your answer
Hypothesis (if… then… because…)
The variable that changes from bath to bath, and its unit
What you observe and record for each bath
One thing you keep the same for every bath

Record each simulation setup as you try it, so you can show what a channel does and does not do.

Student scaffold table 2

Setup	What you saw cross, and which way	Did the sides even out?	Passive or active, and why
No channel in the membrane
One matching channel
Several matching channels
A channel for the other particle type only

Make the three moves for every bath, so each tonicity name is read from the cell's side.

Student scaffold table 3

Bath	Bath concentration (mOsm/L)	Compare with the cell's 320 mOsm/L	Tonicity	Water moves	Cell fate
A	160
B	240
C	320
D	440
E	560
F (640 diluted into twice the volume)

Part 5 — Analysis Questions

1. Show the dilution for bath F with C₁V₁ = C₂V₂, writing the four values on one line, and name its tonicity. Then count how many of the six baths you called hypotonic, isotonic, and hypertonic, and check that the three counts add up to 6.
2. Describe what happened in the simulation with no channel, with one matching channel, and with several matching channels. Which way did the particles move each time, and did you ever have to supply energy?
3. Write the rule for predicting water movement in your own words, using the phrase water follows solute and reading each bath from the cell's side. Then use it to explain why bath C and bath F give the same result even though they were made differently.
4. The channels in your simulation only let particles even out. The sodium-potassium pump moves 3 Na⁺ out and 2 K⁺ in per ATP. Explain why the pump can build and hold a difference across the membrane that the channels never could, using the words passive, active, and gradient.
5. Answer one option. (a) The 320 mOsm/L cell and the six baths are an illustrative model: name one way a real cell in a real fluid could differ from this model, and say why the model is still useful. (b) The simulation shows particles moving through channels but never shows water itself: name one thing the simulation cannot show you about osmosis, and say how your prediction table fills that gap.

Part 6 — AI-Critique Moment

Choose one equal-credit critique route. Route A: ask an approved AI tool to critique your completed work, then check its response against your own evidence. Route B is the no-AI equivalent printed below.

Approved tools: Claude, Gemini, ChatGPT, Grok, Microsoft Copilot.

Prompt to paste:

I've pasted my completed Which Way Does the Water Go? tables above. In my lab, an illustrative model cell is 320 mOsm/L inside, and it sits in six baths: bath A is 160 mOsm/L, bath B is 240, bath C is 320, bath D is 440, bath E is 560, and bath F is a 640 mOsm/L stock diluted into twice its volume. Please do six things. First, for each bath, name its tonicity from the cell's side, say which way water moves, and say whether the cell swells, shrinks, or stays the same. Second, work out the concentration of bath F, showing the arithmetic. Third, say which part of a phospholipid faces the water in the plasma membrane and which part is tucked inside. Fourth, say what the channel proteins did in my membrane simulation and whether that cost the cell ATP. Fifth, list in order the steps a cell uses to build a protein and send it outside. Sixth, tell me the exact sodium concentration inside a real human cell, with a source.

Test the response for these likely failures:

• Chatbots sometimes call the phosphate heads hydrophobic or draw the tails facing the watery fluid, swapping the two parts of the phospholipid.
• Chatbots often say the simulation's channels pumped particles uphill, pairing a channel with the job of an ATP-spending pump.
• Chatbots sometimes multiply instead of divide for bath F, reporting 1,280 mOsm/L and calling it hypertonic.
• Chatbots sometimes send the protein to the Golgi apparatus before the ribosome has built it, or place exocytosis at the start.
• Chatbots often flip hypotonic and hypertonic, calling bath A hypertonic or saying the cell shrinks in it.
• Chatbots readily supply an exact sodium concentration inside a real cell, with a confident-sounding citation that this lab never gave.

In 3–4 sentences, name what the chatbot got right, quote at least two of its errors, and correct each one with evidence from your tables or the chapter.

Equivalent route (no AI required)

No AI account is required for this lab. Instead of the chatbot, critique this flawed analysis, printed here in full: "In my lab the model cell was 320 mOsm/L. Bath A, at 160 mOsm/L, is hypertonic, so water leaves the cell and it shrinks. Bath F is 640 × 2 = 1,280 mOsm/L, so it is hypertonic too. In the plasma membrane the phosphate heads are hydrophobic, so they hide in the middle while the tails face the water. The channels in the simulation pumped the particles uphill using ATP. To export a protein, the Golgi apparatus packages it first, then the ribosome builds it, then exocytosis releases it. A real human cell holds exactly 17 mmol/L of sodium inside, as studies show." Mark it up: quote each flawed sentence, name what is wrong (a mislabeled structure, a wrong structure-to-function pairing, a miscomputed value, an order scrambled, a confused tonicity term, or an invented figure), cite the evidence from your tables or this week's chapter, and write a corrected sentence beneath it. This route assesses the same objectives (Objective 2 and Objective 1) as the chatbot critique, takes comparable effort (about 30 minutes and 200–300 words), is scored by the same scoring criteria for the same credit, and its own submission artifact is the marked-up flawed analysis, posted in place of the chatbot share link and report.

Part 7 — What to Submit

• All three completed scaffold tables, with the simulation table filled from your own runs or the illustrative fallback observations
• Written answers to all five analysis questions, including the option you chose for question 5
• The chatbot critique report with its share link, or the equivalent route's marked-up flawed analysis

Deliverable:

A short A&P Lab & Inquiry report on which way water goes: three completed tables (the parts of your inquiry, the four simulation setups, and the six-bath prediction table), written answers to five analysis questions, and a critique of a chatbot's account of your lab, or, on the equivalent route, a marked-up correction of the printed flawed analysis. 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. Choose a written, recorded, or visual form for your answers; every form is graded by the same scoring criteria.

Rubric

The rubric below shows how each submitted part is evaluated; bands are full, partial, or none.

Workshop evaluation criteria

Criterion	Full	Partial	None
Testable hypothesis and a measurable variable	A falsifiable if… then… because… hypothesis appears before the simulation, the changing variable is named with its unit, and one condition held the same is named.	A hypothesis appears but is not falsifiable, or the variable or its unit is missing.	No hypothesis, and the parts of the inquiry are not named.
Simulation observations	All four setups are recorded from the student's own runs or the fallback observations, each with the direction of movement, whether the sides evened out, and a passive-or-active call with a reason.	Some setups are recorded, or the direction or the passive-or-active call is missing for some rows.	The simulation table is blank or not based on the simulation or the fallback.
Tonicity prediction table	All six baths carry a comparison with 320 mOsm/L, a tonicity name read from the cell's side, the water's direction, and the cell's fate, and bath F's concentration is worked from the dilution rule.	Most baths are complete, but a tonicity is flipped, a step is skipped, or bath F is missing or miscomputed.	The prediction table is blank, or the tonicity names describe the cell instead of the bath throughout.
Analysis and the rule in your own words	Answers all five questions with evidence from the tables: the bath F work and the counts, the simulation pattern, the water-follows-solute rule applied to baths C and F, the passive-versus-active contrast with the pump, and one chosen limitation option.	Answers most questions but leaves one unsupported, or the rule is stated without applying it.	Analysis questions are unanswered or unsupported by the tables.
Critique of the chatbot or of the printed flawed analysis	On either route, at least two errors are quoted and corrected with evidence from the student's own tables or the chapter, and one correct point is acknowledged; both routes earn this band equally.	On either route, errors are named but corrected without evidence, or only one error is caught.	No critique appears, or pasted output is submitted without any checking.

Quality Gate

• Every row of the prediction table makes the three moves in order: compare, name the bath, then state the water's direction and the cell's fate.
• Each tonicity name describes the bath, read from the cell's side, never the cell.
• The hypothesis was written before the simulation and has the if… then… because… form, and the variable is defined so it can be measured.
• The simulation table comes from your own runs or the printed fallback observations, never from memory or a chatbot.
• No measurement, concentration, percentage, study, or source is invented anywhere in the submission.
• The critique quotes the chatbot or the flawed analysis and corrects at least two errors with evidence from your tables or the chapter.