Week 7 — Tracking the Atoms: The Leaf Disk and Calvin Cycle Report — Traditional alternative (no AI)

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

Assignment 7 of the term — read a photosynthesis experiment and count the atoms, then explain to a houseplant owner where a plant's mass comes from.

This week you learned that photosynthesis runs in two stages, in order and in place — the light-dependent reactions in the thylakoid membrane, then the Calvin cycle in the stroma — and that atom tracking shows where the oxygen and the carbon come from. In this assignment you work four short problems, interpret each one, and explain to someone who has never studied biology where a plant's mass comes from. This week's lecture tutorial, quiz, discussion, and Biology Lab & Inquiry practise the same ideas from other angles.

The task

The floating leaf disk example is an illustrative dataset constructed for this course (not real data). Two cups each hold 10 spinach disks with the air drawn out, sunk in the same baking-soda solution. One cup sits under a lamp; the other sits in the dark. The floating disks are counted at 0, 3, 6, 9, and 12 minutes:

• light cup: 0, 2, 5, 8, and 10 disks floating out of 10;
• dark cup: 0, 0, 0, 1, and 1 disks floating out of 10.

The photosynthesis equation is 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂, and three turns of the Calvin cycle export one G3P.

Work these four problems, showing your arithmetic:

1. In the floating leaf disk example (illustrative), 10 disks float in the light cup and 1 disk floats in the dark cup at 12 minutes. How many more disks float in the light than in the dark at 12 minutes?
2. In the floating leaf disk example (illustrative), the dark cup reads 0, 0, 0, 1, and 1 disks floating at 0, 3, 6, 9, and 12 minutes. What is the dark cup's average across the five readings?
3. Three turns of the Calvin cycle export one G3P. An illustrative leaf exports 5 G3P. How many turns of the Calvin cycle did that take?
4. In the photosynthesis equation, 6 H₂O molecules go in, and each water molecule carries 2 hydrogen atoms. How many hydrogen atoms enter on the water side?

Beside each answer, add one brief interpretation:

• Problem 1: name the independent variable, the dependent variable, and the control group, and say which gas lifted the disks and which molecule its atoms came from.
• Problem 2: explain why the dark cup stays nearly flat, naming the stage of photosynthesis that needs light, and state what the two cups together show using the honest verb (supports, not proves).
• Problem 3: name the stage that does the turning and where in the chloroplast it runs, say what it spends and which stage makes those molecules, and explain why it stops soon after the light goes out.
• Problem 4: use atom tracking to say where the hydrogen and the carbon in glucose come from and where the released oxygen comes from, and connect that to where a tree's mass comes from.

Explain it to a non-expert

Your neighbor, who keeps a shelf of houseplants, asks, "My plants keep growing, so they must be eating the potting soil, right?" Write a 3–5 sentence message that explains where most of a plant's new mass really comes from, reports it in everyday words, and names one limit, including what the soil does supply and that the leaf disk numbers in your report are illustrative rather than measured. Explain any technical term in everyday words. More than one good answer is possible.

For a different, ungraded example, imagine a friend asking why ice cubes float in a glass of water instead of sinking. Model answer: "Water molecules cling to each other with weak attractions called hydrogen bonds. When water freezes, those bonds lock the molecules into a pattern that holds them a little farther apart than in the liquid. The same amount of water takes up more room as ice, so the ice is lighter for its size and floats. This explains floating ice, but it does not tell you how fast a pond freezes, which depends on other things."

Use that pattern for your neighbor's houseplants, with your own wording and evidence.

Show your arithmetic. A reader should be able to reproduce every number you report from the readings and counts in the problems without guessing which values or steps you used.

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 neighbor, explaining where most of a plant's new mass comes from in everyday words, with one limit; and
• a one-line restatement of the leaf disk readings you used, so your results are reproducible.

Keep it to roughly one page. Clear and correct beats long.

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 labelled sketch of the chloroplast and the two stages 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 arithmetic, 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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