Week 3 — Which Way Does the Water Go? A Membrane and Transport Report — Traditional alternative (no AI)

Instructors: publish this instead of the adaptive version if you prefer, and unpublish or delete the adaptive one.

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 week you learned how the plasma membrane is built, what each organelle does, how substances cross the membrane downhill for free or uphill at a cost in ATP, and how to read a bath's tonicity from the cell's side. 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. In this assignment you work four short problems, interpret each one, and explain why an IV fluid is isotonic to someone who has never studied anatomy and physiology. This week's lecture tutorial, quiz, discussion, and A&P Lab & Inquiry practise the same ideas from other angles.

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.

Show your arithmetic. A reader should be able to rebuild every number you report from the values given, without guessing which values you used or which operation you chose.

Deliverables

Submit a single report containing:

• the four answers, each with the arithmetic shown and the brief interpretation the task names for that problem;
• your message to your roommate, explaining what tonicity means, why it is read from the cell's side, what a hypotonic or a hypertonic fluid would do to a cell, and one limit; and
• a one-line restatement of the values you used, so your results are reproducible.

Keep it to roughly one page.

Choose how you show it. Present your report as written text, or as a recorded audio or video explanation of about three minutes 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. 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.

How you'll be graded

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

Academic integrity and AI use

This version is designed to be completed without AI: do the computations, the interpretations, and the writing yourself, without a chatbot or other AI tool. You may use a calculator or a spreadsheet for the arithmetic. Submitting AI-generated work as your own, or copying another student's work, is an integrity violation under the course policy.

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