Week 3 — Which Way Does the Water Go? A Membrane and Transport Report

Assignment 3 of the term — dilute a solution, read a bath from the cell's side, count the pump, and put the numbers into plain words for someone new to anatomy and physiology.

This adaptive assignment uses a coaching dialogue to help you turn a first attempt into a clear, evidence-based report. You stay the author: the coach asks questions, checks the reasoning you show, and waits for you to revise before it writes its report. Week 1 gave you homeostasis, the body holding its inside steady, and Week 2 gave you ions, water as the solvent, and ATP; this week you watch all three at work at the border of one cell. This week's lecture tutorial, quiz, discussion, and A&P Lab & Inquiry work the same ideas from other angles; this assignment is where you put them together.

The task

This week's shared dataset is Dataset C, the tonicity baths (illustrative): a model cell at 300 mOsm/L inside, in baths of 100, 300, and 500 mOsm/L. Read every bath from the cell's side in three moves: compare the bath to the cell, name the bath, then point the water. The sodium-potassium pump moves 3 Na⁺ out and 2 K⁺ in per ATP.

Work these four problems, showing your arithmetic:

1. The model solution from Dataset C, 300 mOsm/L, is diluted into twice its volume, and doubling the volume divides the concentration by 2. What is the new concentration in mOsm/L?
2. In Dataset C, the model cell is 300 mOsm/L inside and the third bath is 500 mOsm/L. By how many mOsm/L does the bath's solute concentration exceed the cell's?
3. The sodium-potassium pump moves 3 sodium ions out of the cell for each ATP it spends. How many sodium ions does it move out for 2 ATP?
4. A human body cell carries 46 chromosomes in its nucleus, and they come in pairs of 2. How many pairs is that?

Beside each answer, add one brief interpretation:

• Problem 1: name the tonicity of the diluted bath for the 300 mOsm/L model cell and the direction the water moves, and explain why an IV fluid is mixed to be isotonic instead.
• Problem 2: name the 500 mOsm/L bath's tonicity, the direction the water moves, and what happens to the cell, and explain why osmosis moves water rather than the salt.
• Problem 3: explain why the pump counts as active transport, and why writing it as 2 sodium out and 3 potassium in is a swap.
• Problem 4: name the organelle that holds the DNA, and match the ribosome, the Golgi apparatus, and the mitochondrion each to its one-verb job.

Explain it to a non-expert

A roommate watching a hospital drama asks why the fluid in an IV bag has to match the inside of cells. Write a 3–5 sentence message that explains what tonicity means, why it is read from the cell's side, and what a hypotonic or a hypertonic fluid would do to a cell. Name one limit, for example that Dataset C's 300 mOsm/L cell is a classroom model and that this message is not about anyone's care. Put any technical term in everyday words. More than one good message is possible.

For a different, ungraded example, imagine a friend who asks why sliced eggplant sprinkled with salt beads with water after a while. Model answer: "Each slice is made of tiny cells wrapped in thin borders that water can cross. The salt on the surface leaves more dissolved stuff outside the cells than inside. Water drifts toward the side with more dissolved stuff, so it seeps out of the cells and beads on the slice. That drift of water is called osmosis. It explains the beads, but it does not tell you how much water leaves or how long it takes."

Use that pattern for the IV message, with your own wording and evidence.

Part 1 — Student instructions

1. Draft all four answers, the four brief interpretations, and your message to your roommate before you open a chatbot.
2. Open any course-approved chatbot. Copy the entire Coach Prompt box, including its first and last marker lines, and paste it as one message. The coach asks for your draft and then works one question at a time.
3. Answer in your own words. Show calculations when asked. Use the coach's feedback to revise at least one calculation or explanation, even if your first attempt was correct.
4. Keep going until the prompt's exit condition is met.
5. Save the chat's share link and copy the final report exactly as the coach gives it.

The chatbot is a coach, not a ghostwriter. It draws out your reasoning; it does not write the report for you.

Coach Prompt

COPY EVERYTHING BELOW THIS LINE
ROLE: You are a careful learning coach for a first-semester anatomy and physiology course. Draw out and challenge MY thinking about the cell membrane and membrane transport. Do not write my assignment for me and do not invent work I never did. Never invent a measurement, a study, a concentration, or a percentage, and never offer medical advice or comment on anyone's health or care.

ASSIGNMENT: Work through these four problems in order, one at a time. Do not invent, replace, or reorder a problem. The data: Dataset C, the tonicity baths (illustrative): a model cell at 300 mOsm/L inside, baths of 100, 300, and 500 mOsm/L. Read every bath from the cell's side: compare it to the cell, name it, then point the water. Water follows solute. The sodium-potassium pump moves 3 Na+ out and 2 K+ in per ATP.
PROBLEM P1: The model solution from Dataset C, 300 mOsm/L, is diluted into twice its volume, and doubling the volume divides the concentration by 2. What is the new concentration in mOsm/L?
PROBLEM P2: In Dataset C, the model cell is 300 mOsm/L inside and the third bath is 500 mOsm/L. By how many mOsm/L does the bath's solute concentration exceed the cell's?
PROBLEM P3: The sodium-potassium pump moves 3 sodium ions out of the cell for each ATP it spends. How many sodium ions does it move out for 2 ATP?
PROBLEM P4: A human body cell carries 46 chromosomes in its nucleus, and they come in pairs of 2. How many pairs is that?
After the four answers, I must briefly interpret each one: for P1, name the diluted bath's tonicity for the 300 mOsm/L cell and the water's direction, and explain why an IV fluid is mixed to be isotonic; for P2, name the 500 mOsm/L bath's tonicity, the water's direction, and the cell's fate, and explain why osmosis moves water rather than the salt; for P3, explain why the pump is active transport and why 2 sodium out and 3 potassium in is a swap; for P4, name the organelle that holds the DNA and match the ribosome, the Golgi apparatus, and the mitochondrion to their one-verb jobs. I must then write a 3-5 sentence message to a roommate explaining what tonicity means, why it is read from the cell's side, what a hypotonic or hypertonic fluid would do to a cell, and one limit. I must explain technical terms in everyday words.

SCORING CRITERIA: Score only evidence I supply: correct computations (40 points), membrane and transport reasoning (32 points), clarity for a non-expert reader (16 points), and showing work so it can be checked (12 points). For each of P1-P4, give 2, 1, or 0 evidence units per criterion; one unit is worth 5 computation points, 4 reasoning points, 2 clarity points, or 1.5 showing-work points. Sum the four problems within each criterion. Count my roommate message only with P1, and only for clarity.
COMPUTATION UNITS: A correct result in any equivalent notation = 2; the right values and operation with one arithmetic slip = 1; a missing result or a wrong operation, such as adding where the dilution divides = 0.
REASONING UNITS: For each problem, an interpretation that covers both named parts = 2; one accurate part = 1; none = 0. Calling a weaker bath hypertonic, saying osmosis moves the salt, defining active transport by speed or by a protein alone, or swapping the pump's ions is not accurate. An arithmetic slip alone is not a reasoning error.
CLARITY UNITS: For each problem, labeled evidence a newcomer can follow, with terms in everyday words = 2; evidence that needs a reread because of an unclear label or unexplained term = 1; no readable evidence = 0. Include my roommate message with P1. Missing reasoning alone does not lower clarity for otherwise clear work.
SHOWING-WORK UNITS: For each problem, values and steps that let a reader rebuild the result = 2; one relevant but incomplete value or step = 1; a bare result or no method = 0. A visible method stays checkable despite an arithmetic slip.

Do NOT invent grading rules.

DIALOGUE RULES: Ask exactly one question per message and wait for my answer. Start by asking for my complete first attempt. Build each next question from my exact words. Probe an unsupported claim and ask me what supports it. Keep the conversation on questions, not on finished work.

ENGAGEMENT GUARDS: A greeting, a copied prompt, a yes/no reply, or a one-word answer is not a substantive exchange. Do not flatter or treat agreement as evidence. Praise only a concrete move I made. If evidence is missing, say what is missing and ask for it. Require a real revision plus a short reflection on why the revision is better. Never score evidence I did not supply, and do not issue the report early.

EXIT CONDITION: Continue until at least five substantive exchanges have occurred AND I have supplied all four answers and their brief interpretations AND written my message to my roommate AND revised at least one calculation or explanation AND reflected on the revision. Stop only after whichever requirement happens last.

REPORT: After the exit condition is satisfied, output exactly these labelled lines in this order. Replace X with the exact earned score; it may end in .5 and must not be rounded. Base every line only on evidence in this chat; write "Not supplied" for missing evidence and never raise the score for polished chatbot prose.
STUDENT'S SCORE: X/100
EVIDENCE FOR THE SCORE: [specific evidence from my calculations and reasoning]
REVISION MADE: [what I changed and why]
STRENGTH: [one earned strength]
NEXT STEP: [one concrete improvement]
Then tell me to submit this report, my revised membrane and transport report, and the share link.
COPY EVERYTHING ABOVE THIS LINE

Deliverables

Submit these three items in the assignment entry:

• your revised membrane and transport report with all four answers, their brief interpretations, and your message to your roommate — written, or recorded as audio or video with a transcript, or as a labeled sketch of the three baths with a short note; every form is graded by the same scoring criteria;
• the coach's five-line report; and
• the conversation share link.

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.

Equivalent route (no AI required)

This route assesses the same Week 3 skills — diluting a solution and reading the new bath from the cell's side, naming a bath's tonicity and the direction the water moves, explaining why the sodium-potassium pump is active transport, and matching organelles to their jobs — for comparable time and effort, about 60 to 90 minutes, the same as the coached route. Complete the same four problems, the four brief interpretations, and the message to your roommate. Then audit your own first draft against the attached scoring criteria, one criterion at a time, and write a second draft. Add a 150–200 word revision memo that names one calculation or explanation you changed, the evidence that prompted the change, and one question you still have. Submit the revised report, your completed criteria checklist, and the revision memo in the assignment entry; together they replace the share link and the coach's report. This route uses the same scoring criteria and the same points, with no penalty for choosing it.

How you'll be graded

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

Academic integrity and AI use

The calculations, reasoning, revision, and final report must be your own. Do not ask the chatbot to create the assignment or to add evidence you did not produce. Submit an honest share link from your own dialogue. The equivalent route keeps the same workload and evidence and needs no AI account.

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