Week 7 Biology Lab & Inquiry: The Floating Leaf Disk

Part 1 — The Big Picture

This week you learned that photosynthesis runs in two stages, in order: the light-dependent reactions in the thylakoid membrane split water and release oxygen, and the Calvin cycle in the stroma spends ATP and NADPH to build sugar from carbon dioxide. In this lab you watch the first stage at work on a kitchen counter. Spinach disks with the air drawn out sink in a baking-soda solution, which supplies dissolved carbon dioxide. Under a lamp, the leaves make oxygen, the oxygen collects inside the disks, and they float up one by one; a cup kept in the dark is your control. You write a hypothesis before you start, count the floating disks over 12 minutes, work out two averages, and explain where the oxygen came from. Then you check a chatbot's account of your lab and catch where it scrambles an order, mislabels a structure, oversimplifies the mechanism, or invents a number.

Time budget: Plan for about 120 minutes.

Part 2 — The Task

Guiding question: Does light change how fast spinach leaf disks make oxygen, and what does a dark control show about where that oxygen comes from?

Before you begin — hypothesis: Before the cups go under the lamp and into the dark, write one hypothesis in the if… then… because… form: if leaf disks sit under a lamp, then they will float sooner or later than disks kept in the dark, because of a reason about the light-dependent reactions that you can state. Then write a prediction for how many disks will be floating in each cup at 12 minutes.

Workshop data: Your own leaf disk counts, or the illustrative fallback counts

Source: Floating-disk counts you record yourself at a kitchen counter with the procedure in Part 3, five readings in the light and five in the dark; if you cannot run the protocol, use the illustrative fallback counts printed in the fallback, which were constructed for this lab and are not real data.

• Rows: 5 readings per cup (at 0, 3, 6, 9, and 12 minutes), shown as 5 rows of the count table with one column for each cup; the first scaffold table has 6 rows that name the parts of the test rather than data, and the reasoning table has 3 rows
• Columns: minute, disks floating in the light cup, disks floating in the dark cup, and the increment in the light cup from the reading before
• Unit: a count of disks floating out of 10 per cup

Part 3 — Materials & Procedure

Tools and materials

• Free virtual labs and simulations (PhET, HHMI BioInteractive, Learn.Genetics), a virtual microscope, and a spreadsheet (Google Sheets or any free spreadsheet)
• A handful of fresh spinach leaves, a drinking straw for punching disks, a clean plastic syringe with no needle, baking soda, a drop of dish soap, two clear cups, water, a desk lamp, a dark cupboard or a box, and a clock or phone timer
• A document or notebook for your written answers

Ways of working

• virtual-simulation-inquiry
• at-home-protocol-and-data-table
• figure-and-diagram-interpretation

Procedure

1. Stir a small pinch of baking soda and one drop of dish soap into a cup of water until it dissolves, then split it evenly between the two clear cups. Punch 20 disks from the spinach leaves by pressing the straw through the leaf, avoiding the thick veins, so you have 10 disks for each cup. Tool: Free virtual labs and simulations (PhET, HHMI BioInteractive, Learn.Genetics), a virtual microscope, and a spreadsheet (Google Sheets or any free spreadsheet). Check: Two cups holding the same solution, and 20 small, similar spinach disks.
2. Before the light goes on, write your hypothesis, your predictions, and the parts of the test in the first table in Part 4: what you change, what you count, what you keep the same, and which cup is the control. Tool: Free virtual labs and simulations (PhET, HHMI BioInteractive, Learn.Genetics), a virtual microscope, and a spreadsheet (Google Sheets or any free spreadsheet). Check: The first table has an if… then… because… hypothesis, a prediction for each cup, and the dark cup named as the control.
3. Pull the syringe plunger out, drop 10 disks into the barrel, push the plunger back in, and draw up some of the solution. Cover the tip with a finger and pull the plunger back gently a few times to draw the air out of the disks, then tap the syringe until the disks sink. Empty the sunken disks into one cup, and repeat with the other 10 disks for the second cup. Tool: Free virtual labs and simulations (PhET, HHMI BioInteractive, Learn.Genetics), a virtual microscope, and a spreadsheet (Google Sheets or any free spreadsheet). Check: All 10 disks rest on the bottom of each cup; any disk that still floats is pushed down or replaced.
4. Set one cup under the lamp and put the other in a dark cupboard or under a box. Start the timer. At 0, 3, 6, 9, and 12 minutes, count the floating disks in each cup, opening the dark cupboard only long enough to count, and record both counts in the count table. Tool: Free virtual labs and simulations (PhET, HHMI BioInteractive, Learn.Genetics), a virtual microscope, and a spreadsheet (Google Sheets or any free spreadsheet). Check: Five counts for each cup, every one between 0 and 10.
5. Open a blank sheet. Type the five minutes into cells A2 to A6, the light counts into B2 to B6, and the dark counts into C2 to C6. In E2 type =AVERAGE(B2:B6), in E3 type =AVERAGE(C2:C6), and in E4 type =B6-C6. In D3 type =B3-B2 and fill it down to D6 for the light increments. Tool: Free virtual labs and simulations (PhET, HHMI BioInteractive, Learn.Genetics), a virtual microscope, and a spreadsheet (Google Sheets or any free spreadsheet). Check: E2 and E3 match the averages you work out by hand, and the light increments in D3 to D6 add up to the last light count minus the first.
6. Complete the reasoning table, answer the analysis questions in your document, then paste your completed tables into an approved chatbot with the prompt in Part 6 and check its answer against the scripted checks. Tool: Free virtual labs and simulations (PhET, HHMI BioInteractive, Learn.Genetics), a virtual microscope, and a spreadsheet (Google Sheets or any free spreadsheet). Check: Completed tables, written answers, and a list of the chatbot's errors with corrections.

Fallback route

When to use it: Use this route if you cannot get spinach, a syringe, baking soda, or a lamp, or if you cannot run the cups at home.

Use the illustrative fallback counts below, constructed for this lab and not real data, as your count table, then complete every scaffold table, analysis question, and the critique step from them and submit the same items for equal credit. Fill the empty column yourself.

This route earns the same credit as the primary route.

Illustrative fallback counts of disks floating out of 10, constructed for this lab, that stand in for your own cups.

Fallback model data

Minute	Disks floating in the light cup	Disks floating in the dark cup	Increment in the light cup
0	0	0
3	1	0
6	4	1
9	7	1
12	9	1

Part 4 — The Scaffold

Name every part of the test before the light goes on, so the comparison is fair from the start.

Student scaffold table 1

Part of the test	Your answer
Hypothesis (if… then… because…)
Prediction for each cup at 12 minutes
What you change between the cups (independent variable)
What you count (dependent variable)
Two things you keep the same for both cups
Which cup is the control, and why

Record both cups at each reading, then work out each average and the light increments.

Student scaffold table 2

Minute	Disks floating in the light cup	Disks floating in the dark cup	Increment in the light cup
0
3
6
9
12
Average across the five readings

Explain the two results that matter most, from the chemistry of the two stages.

Student scaffold table 3

Question	Your reasoning
Which gas lifted the disks, and which molecule did its atoms come from?
Why does the dark cup stay nearly flat?
What job does the baking soda do in the cup?

Part 5 — Analysis Questions

1. Show the sum and the division for each cup's average, find how many more disks float in the light than in the dark at 12 minutes, and check that the light increments add up to the last light count minus the first.
2. Does your evidence support or fail to support your hypothesis? Write one conclusion sentence that uses supports or fails to support rather than proves, and name the stage of photosynthesis, and its place in the chloroplast, that made the oxygen.
3. Explain why the dark cup stays nearly flat, and say whether the Calvin cycle in those dark disks could keep building sugar for long, naming what it would run out of.
4. The baking soda supplies carbon dioxide. Use atom tracking to say where the carbon from that carbon dioxide ends up and where the oxygen in the bubbles came from, and connect both to the puzzle of where a tree gets its mass.
5. Choose one and answer it in 3–4 sentences. (a) Your lab counts floating disks, not oxygen itself: name one thing besides oxygen production that could change how fast a disk floats, and say how you would keep it the same next time. (b) Name one difference between your two cups other than light, such as warmth from the lamp, and say how it could change your conclusion and how you would remove it.

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 Week 7 Biology Lab & Inquiry tables above. I punched spinach leaf disks, drew the air out of them so they sank in a baking-soda solution, and set one cup under a lamp and one cup in a dark cupboard, counting the floating disks every 3 minutes. Please do seven things. First, name the two stages of photosynthesis in order and the place in the chloroplast where each one runs. Second, name the gas that made my disks float and say which molecule its atoms came from. Third, check the two averages in my table, showing each sum and division. Fourth, explain whether the Calvin cycle can keep building sugar in the dark cupboard, and why. Fifth, tell me the exact rate of oxygen production, in milliliters per minute, that a spinach disk should reach under a desk lamp, with its source. Sixth, as a stretch question for later in the course, say how many cells a body cell makes when it divides for growth. Seventh, as another stretch question, say what phenotype ratio a cross of two Tt parents gives.

Test the response for these likely failures:

• Chatbots sometimes list the Calvin cycle first, as though sugar is built before any light is captured, or describe the two stages as running in either order.
• Chatbots often swap the two places, putting the Calvin cycle in the thylakoid membrane and the light-dependent reactions in the stroma, or call the stroma a stack of membranes.
• Chatbots often say the oxygen that lifts the disks comes from the carbon dioxide in the baking soda, or that the Calvin cycle needs no light and keeps building sugar all night.
• Asked for a rate, chatbots often state a precise milliliters-per-minute figure for a spinach disk and cite a study or source that was never given.
• Asked the stretch question, chatbots sometimes say a body cell dividing for growth makes four cells, which describes meiosis, not mitosis.
• Asked the stretch question, chatbots sometimes give the phenotype ratio of a Tt by Tt cross as 1:2:1, which is the genotype ratio.

Write 3–5 sentences naming one thing the chatbot got right and at least two places it went wrong or overreached, quoting its words and correcting each with evidence from your own tables or this week's chapter; mark the two stretch answers as ones to check in later weeks.

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: "My leaf disks floated because the carbon dioxide from the baking soda turned into oxygen bubbles. In photosynthesis the Calvin cycle runs first, inside the thylakoid membranes, and the light reactions finish the job afterward in the stroma. The dark cup stayed flat only because the Calvin cycle needs no light, so it went on building sugar all through the test. Under my lamp each disk made exactly 0.8 milliliters of oxygen per minute, as a study shows. As a preview of later weeks, a body cell dividing for growth makes four different cells, and a cross of two Tt parents gives a 1:2:1 phenotype ratio." Mark it up: quote each flawed sentence, name what is wrong (a stage out of order, a mislabeled structure, an over-simplified mechanism, an invented figure, or a preview fact stated wrong), cite the evidence from your tables or this week's chapter, and write a corrected sentence beneath it; for the two preview sentences, say which later week will check them. This route assesses the same objectives (Objective 1 and Objective 4) 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, filled from your own cups or the illustrative fallback counts
• Written answers to all five analysis questions
• The chatbot critique report with its share link, or the equivalent route's marked-up flawed analysis

Deliverable:

A short Biology Lab & Inquiry report on the floating leaf disk: three completed tables (the named parts of the test, the counts for both cups with the averages and light increments, and the reasoning about the oxygen, the dark cup, and the baking soda), 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, variables, and dark control	A falsifiable if… then… because… hypothesis and a prediction for each cup appear before the light goes on, the independent and dependent variables are named correctly, two held conditions are named, and the dark cup is named as the control.	A hypothesis appears but is not falsifiable or lacks predictions, or a variable, a held condition, or the control is missing or swapped.	No hypothesis, or the parts of the test are not named.
Completed count table with averages and increments	Both cups have five counts from the student's own cups or the fallback counts, each average shows its sum and division and is correct for those counts, and the light increments are subtracted correctly and check against the change from first to last reading.	The table is filled, but one or two averages or increments are wrong, or the sums and divisions are not shown.	No counts are recorded, or the counts are invented rather than observed or taken from the fallback.
Two-stage analysis and atom tracking	Questions 1 to 4 are answered accurately: the conclusion uses supports or fails to support, the oxygen is traced to water split in the light-dependent reactions in the thylakoid membrane, the dark cup is explained through the missing light and the Calvin cycle's need for ATP and NADPH, and the carbon is traced from carbon dioxide into sugar.	The questions are attempted, but one or two answers trace the oxygen to carbon dioxide, swap the thylakoid and the stroma, say the Calvin cycle needs no light, or claim the result proves the hypothesis.	The analysis questions are missing or do not address the results.
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, stretch answers are marked to check later, 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.
Reflection on the limits of the test	The chosen reflection option is answered in 3–4 sentences that tie a specific limit of counting floating disks, or a specific difference between the cups, to a concrete change in the test or in what can be claimed.	The reflection is answered but stays general or names no specific change.	No reflection is submitted.

Quality Gate

• Every count in the data table comes from your own cups or the printed fallback counts, never from memory or a chatbot.
• Each average shows its sum and its division, and each increment shows its subtraction.
• The hypothesis was written before the light went on and has the if… then… because… form.
• The oxygen is traced to water split in the light-dependent reactions, not to the carbon dioxide.
• No measurement, rate, 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.