Photosynthesis leaf disk lab online: controls, floating counts, and ET50
Use a virtual leaf disk investigation to interpret infiltration, control cups, ET50, light saturation, and the difference between net and gross photosynthesis.
A floating leaf disk can provide indirect evidence about oxygen accumulation in leaf tissue. It is not a direct count of sugar molecules or a universal measure of plant growth. In Scivra's Photosynthesis Leaf Disks, the investigation connects vacuum infiltration, light conditions, floating counts, and a net-photosynthesis model.
The catalog lists AP Biology Unit 3 and Investigation 5 alongside the course's NGSS connection. Use its modeled procedure and measurements to explore the evidence chain; that listed alignment does not establish hands-on lab credit or complete coverage of every official investigation step.
Why do infiltrated disks sink first?
The course models gas in the leaf's spongy tissue being replaced by solution during vacuum infiltration. That change lets the disks sink. Under light, photosynthetic oxygen can accumulate in the spaces again and support later floating.
This starting preparation matters. A disk that never sank is not evidence of a newly measured photosynthetic response after the same starting condition. Retain the infiltration procedure and initial state when comparing cups. The represented detergent and vacuum steps belong to sample preparation, while the later counts describe the response under the selected light and solution conditions.
Which conditions should stay the same?
Choose one independent variable, such as lamp distance, bicarbonate condition, or temperature. Keep the remaining sample and apparatus conditions matched. Record the disks counted, solution, temperature, and light setting beside the time series.
The water-only control helps interpret the bicarbonate comparison. A control does not merely supply another curve; it tests whether the chosen treatment provides evidence beyond the matched reference condition. If the cups differ in both solution and lamp position, their floating-time difference no longer isolates the solution treatment.
Write a prediction before collecting the comparison and retain the original run after changing settings.
What does ET50 mean?
ET50 is the elapsed time at which half of the selected disks have floated. The course obtains it from the count record, including interpolation when the midpoint falls between observations. Keep the counted total attached to the threshold: half refers to the sample in that cup, not a fixed universal number of disks.
Compare time and its reciprocal carefully. A shorter ET50 corresponds to a larger reciprocal-time rate indicator. The indicator is a proxy under the course model, not the same quantity as a direct gas-volume measurement. A curve that never reaches the threshold within the observation interval does not justify inventing a precise ET50 beyond the collected evidence.
Why does brighter light not always give a proportional response?
In the specified standard reference, a lamp distance of 20 cm gives model midpoint time 690.0 s, or 11.50 min. At 10 cm, the listed midpoint is 624.1 s, or 10.40 min. The shorter distance substantially raises incident light in this setup, but the midpoint does not become proportionally smaller.
The comparison illustrates saturation and other limiting factors under the selected conditions. Retain the lamp geometry and remaining inputs instead of assuming every halved distance creates the same photosynthetic improvement. At a lower light setting, the response can change much more strongly because the system is operating in another part of the modeled light-response relationship.
The reference values describe the course model. They are not a promise that a physical leaf sample will reproduce identical times.
Is floating a measure of net or gross photosynthesis?
The course's floating response reflects net oxygen accumulation while respiration continues. Plants respire in light as well as darkness. A later dark comparison can help inspect the respiratory contribution, but the floating result alone should not be labeled gross production.
Follow Cellular Respiration to distinguish oxygen-linked metabolism from the gas or displacement signal used by another apparatus. In Elodea, Snails & CO₂, net carbon change also depends on competing biological transfers.
How should you compare repeated cups?
Keep each cup's conditions and threshold estimate visible. Use the repeated values and the course's uncertainty representation instead of choosing only the fastest run. If you suspect a preparation difference, state how it changes the initial buoyancy or later observation and test that idea in a matched comparison.
A useful conclusion names the changed factor, ET50 evidence, and the limiting-factor explanation. Start the leaf disk investigation, then retain both the count curve and the conditions so another student can reconstruct what the comparison supports.