Week 3 — Reading Molecules by Their Parts: The Macromolecule 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 — read the four classes of large molecules by their parts and explain a starch test to someone new to biology.

This week you learned how cells build large molecules by dehydration synthesis and break them by hydrolysis, met the four classes of macromolecules, and saw that structure determines function. In this assignment you work four short counting problems, interpret each one, and explain an iodine starch test to someone who has never studied biology. This week's lecture tutorial, quiz, discussion, and Biology Lab & Inquiry practise the same ideas from other angles.

The task

The iodine starch test is an illustrative, qualitative set of results built for this course (not real data). Iodine starts amber, an orange-brown color, and turns blue-black when it meets starch. Plain water, the negative control, stays amber. Bread and potato turn blue-black. Sugar water and cooking oil stay amber. Fruit stays amber or changes faintly. Starch is a polysaccharide built from glucose by dehydration synthesis, and the worked example joins 3 glucose units in a row.

Work these four problems, showing your arithmetic:

1. In the worked example, 3 glucose units are joined in a row by dehydration synthesis. How many new bonds form, which is also the number of water molecules removed?
2. In the iodine starch test (illustrative results), 2 foods turned blue-black, 2 stayed amber, and 1 changed faintly. How many foods were tested in all?
3. An illustrative protein chain of 8 amino acids is built by dehydration synthesis. How many peptide bonds join it, which is also the number of water molecules removed?
4. An illustrative paper model shows 3 triglycerides, and each triglyceride is one glycerol and three fatty acids. How many fatty acids are there in all?

Beside each answer, add one brief interpretation:

• Problem 1: name the reaction that builds the chain and the reaction that breaks it, saying which way water moves in each, and explain why a chewed cracker starts to taste sweet.
• Problem 2: name the negative control and explain, from the shape of the molecules, why sugar water stays amber even though sugar is a carbohydrate.
• Problem 3: put the four levels of protein structure in order, and say whether these peptide bonds survive when heat denatures the protein, and why.
• Problem 4: explain why a triglyceride is not a polymer, and describe how straight saturated tails and kinked unsaturated tails decide whether a fat is solid or liquid.

Explain it to a non-expert

Your younger cousin watches you test foods with iodine and asks, "Why did the bread turn dark but the sugar water didn't? Isn't sugar a carb too?" Write a 3–5 sentence message that explains what iodine detects, reports the result in everyday words, and names one limit, including that these results are illustrative and that an amber color means no starch rather than no carbohydrate. Explain any technical term in everyday words. More than one good answer is possible.

For a different, ungraded example, imagine a friend asking why a jacket zipper stopped closing after one tooth got bent. Model answer: "A zipper works because every tooth has the same shape, so each one hooks neatly into the teeth on the other side. When one tooth is bent, it no longer fits, and the slider jams at that spot. Nothing is missing; the parts are all still there, but one part has the wrong shape. A zipper is only a rough comparison for a molecule, so it can show the idea that shape decides the job without showing how a real molecule folds."

Use that pattern for the starch test, with your own wording and evidence.

Show your arithmetic. A reader should be able to reproduce every number you report from the 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 cousin, explaining what iodine detects, the result in everyday words, and one limit; and
• a one-line restatement of the iodine colors 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 molecules 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 counting, 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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