Welcome. Today you will use a clever, low-tech assay to measure how fast a leaf can photosynthesize — by watching disks rise from the bottom of a beaker. You'll manipulate variables, gather data, and reason about why one disk takes longer than another.
⏱ Estimated time: 30 minutes. Your work auto-saves silently to this device.
A leaf is a chemical factory. Inside every green leaf, microscopic organelles called chloroplasts capture light and use that energy to split water and stitch carbon dioxide into sugar. The pigment that absorbs the light is called chlorophyll, and it absorbs red and blue wavelengths much more than green — which is exactly why a leaf looks green to your eye. The leftover green light is the light the leaf didn't use.
The reaction we call photosynthesis takes in CO₂ and water and outputs glucose plus oxygen gas. A leaf normally exchanges these gases through tiny pores on the underside of the leaf called stomata. The spongy interior — the mesophyll — is full of air spaces. That trapped air is the reason a leaf disk floats: it's not denser than water, it's just full of bubbles.
If we replace that internal air with liquid (using a syringe and a little suction — vacuum infiltration), the disk sinks. Now we have a trick: as the leaf photosynthesizes, it releases oxygen. The O₂ fills the air spaces back up, and the disk rises again. Because the leaf is also doing cellular respiration at the same time (consuming O₂), what we actually measure is net photosynthesis — the photosynthesis happening on top of the respiration. Gross photosynthesis is the total rate before respiration is subtracted off.
To make sure the leaf has enough CO₂ to work with, we infiltrate with a bicarbonate solution (NaHCO₃), which acts as a CO₂ source dissolved in the water. The faster the leaf can build sugar, the faster O₂ accumulates, the sooner the disk's buoyancy wins out and it rises. We summarize a whole trial with one number: ET50, the time when half of the disks have floated. A shorter ET50 means a faster rate.
Several factors can change the rate: how bright the light is (light intensity), what color it is (wavelength), how much CO₂ is dissolved, and the temperature of the water. Today you'll change one factor at a time and ask, "Did the disks rise faster, slower, or the same?"
Tap a card to flip it. Only one card opens at a time. Each open card stays revealed for 8 seconds before re-closing. You can open the same card more than once.
Click a term on the left, then click its definition on the right. Correct matches dim out. Each correct match = 1 pt.
Short responses. Each question = 1 pt for an answer that addresses the question.
What you're looking at. Below is a virtual beaker holding 10 leaf disks that have already been vacuum-infiltrated with bicarbonate solution. They sit at the bottom. When you press START, the simulated lamp turns on and the disks rise at a rate determined by your settings. The clock measures the time, in seconds, until each disk floats. The simulation reports the ET50 — the time when 5 of the 10 disks have surfaced. Use this number as your rate proxy.
Directions. Run at least 4 trials: one trial at each light intensity (Low, Medium, High) with the white filter, then one extra trial of your choice. Record results in the data table on the next page. The simulator stops automatically at 240 seconds if not all disks have surfaced — record "NF" (did not float) if so.
Fill in all 8 trials below. A complete table is worth 4 pts (full credit if every cell has a value).
| Trial | Light Intensity | Filter / Color | Temp (°C) | ET50 (s) | Rate (1/ET50) |
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The graph below shows class-averaged rates (1/ET50) versus light intensity at 22 °C. Use it to answer the questions.
Dr. Mireille Okafor studies salt-tolerant coastal plants at a research station in northern California. In summer 2025, she noticed that the ivy growing closest to a shallow tide pool — which warms to around 32 °C on still afternoons — appeared paler and grew more slowly than ivy 10 m inland. She wondered whether photosynthesis itself was being affected by the warmer microclimate, or whether the salt spray was the real culprit.
To test this, she ran a floating-leaf-disk assay using ivy from three locations: (A) next to the warm tide pool, (B) 10 m inland in moderate shade at ~22 °C, and (C) 10 m inland in full sun at ~28 °C. All trials used 0.2% sodium bicarbonate solution, identical LED lighting at 800 μmol m⁻² s⁻¹, and the assay was run at 22 °C laboratory temperature for fairness. Results from 30 disks per group:
| Group | Source | ET50 (s) | Notes |
|---|---|---|---|
| A | Tide pool (32 °C native) | 178 | Pale, smaller disks |
| B | Inland shade (22 °C native) | 96 | Dark green, supple |
| C | Inland sun (28 °C native) | 71 | Thicker, waxy disks |
Dr. Okafor also recorded that the ivy at the tide pool was not visibly salt-burned. A quick chlorophyll extraction showed Group A had roughly 40% less chlorophyll per gram than Group B.
Answer all parts. Each lettered part = 1 pt.
A biology student is investigating how the wavelength of light affects the rate of photosynthesis in spinach leaves using the floating-leaf-disk assay. They hold light intensity at 800 μmol m⁻² s⁻¹, temperature at 22 °C, and use 0.2% sodium bicarbonate. They run trials under four colored filters: red, blue, green, and a no-filter (white-light) control. Each trial uses 10 fresh disks.
Questions are drawn at random from a large bank, so your set is different from your neighbor's. Each correct answer = 1 pt. You get 2 attempts; your higher score counts.
Mr. Brown · Biology · Floating Leaf Disk Photosynthesis Assay · v1.0