Start by sinking
Leaf disks usually float because gas occupies spaces in the spongy mesophyll. Vacuum infiltration replaces this gas with solution, increasing density so the disks sink. Soap helps wet the leaf surface.
SMALL DISKS. BIG QUESTIONS.
What makes a
leaf disk rise?
Change the conditions. Track oxygen. Discover how photosynthesis brings spinach disks back to the surface.
01 / The lab bench
Ready to infiltrateEXPERIMENT CONDITIONS
DarkBright
Fixed: 10 fresh spinach disks per beaker, 70 mL solution, one drop of dilute soap, equal disk size and infiltration.
Vacuum infiltration replaces trapped gas with solution so the disks sink.
LIVE OBSERVATION
Watch oxygen accumulate
0.0minutes
☀ 70% light
70 mL
0.20% bicarbonate
10 / 10 disks floating
70 mL
Water control · 0% added
10 / 10 disks floating
NET O₂ RATE · A—relative units / min
NET O₂ ACCUMULATED · A0.00relative units
ET₅₀ · A—5 of 10 disks floating
Illustrative model, not experimental measurements. Oxygen values are relative units, not mL or dissolved oxygen concentrations. Inputs lock during a trial to preserve a fair comparison.
Oxygen over time
━ A · treatment┄ B · water control
Minute-by-minute observations
Both samples experience the same light and temperature.
| Time (min) | A floating | B floating | A net O₂ | B net O₂ |
|---|
02 / YOUR EVIDENCE
Trial notebook
Trials are saved on this browser when finished. Repeat each condition at least three times. ET₅₀ is the first observed minute with at least five disks floating; a smaller ET₅₀ indicates a faster response. 1/ET₅₀ is a relative rate index.
| Trial | Light | NaHCO₃ | °C | ET₅₀ A | ET₅₀ B | 1/ET₅₀ A (min⁻¹) |
|---|
Your writing saves on this browser. Download or print before changing devices.
03 / MAKE SENSE OF IT
Inside the leaf
Light powers the process
Chloroplasts capture light energy. Photosynthesis uses carbon dioxide and water to make carbohydrates and releases oxygen. Bicarbonate supplies inorganic carbon; the water control has no added bicarbonate, but is not completely free of CO₂.
Measure a net result
Respiration consumes oxygen in both light and darkness. Disks rise when retained oxygen makes them buoyant. Flotation is an indirect measure of net photosynthesis, not a direct measurement of all oxygen produced.
Student investigation prompts
- Write a null and experimental hypothesis for bicarbonate versus water.
- Run Part A at 70% light and 25°C. What does the control help you conclude?
- In Part B, test at least three light levels, holding bicarbonate and temperature constant. Repeat each condition three times.
- Graph mean ET₅₀ with sample standard deviation for each condition. How should a trial that never reaches ET₅₀ be reported?
- Why might additional light stop increasing the rate? Explain a limiting factor.
- Why can’t flotation alone tell you gross photosynthesis or sugar production?
Teacher notes & model limitations
Based on the uploaded floating disk student manual (adapted from the College Board AP Biology Student Lab Manual, 2012). Part A uses paired beakers, 10 spinach disks, 70 mL solution, 0.2% bicarbonate by default, equal light, and ET₅₀.
The model uses saturating light and carbon responses, a temperature optimum, respiration, and small randomized disk/trial differences. It illustrates relationships; it is not fitted to spinach measurements. Bicarbonate also changes pH in real experiments, which is not modeled. Very hot conditions have a reduced modeled rate without modeling irreversible damage. Water remains available in all beakers; changing water volume would also change assay geometry, so water availability is held constant.
Students adjust inputs and environmental conditions; oxygen and carbohydrate are outputs. This model tracks net oxygen and buoyancy, and does not independently manipulate products or estimate glucose from net oxygen. O₂ released in photosynthesis comes from water. No gas exchange, pH, wavelength, or acclimation mechanisms are resolved.
Start each trial with fresh disks. Not reaching five floating disks within 30 minutes is recorded as “not reached,” never as zero. Summary statistics exclude those censored trials and report their count. A single run does not support error bars. Local storage is device-specific; no work is submitted to the teacher automatically.