Investigation 5: Photosynthesis and the Floating Leaf Disk

A fresh leaf disk is full of air in the spaces between its cells, so it floats. Pull that air out with a vacuum, usually while the disk sits in a solution of bicarbonate, and the spaces fill with solution instead. The disk sinks. Now put it under light. If the cells are photosynthesizing, they consume CO2 (supplied by the bicarbonate) and release oxygen gas. That oxygen collects in the intercellular spaces, and once there's enough of it, the disk becomes buoyant again and rises. The time it takes to float is a window onto the rate of net photosynthesis: faster photosynthesis means faster oxygen buildup means a quicker float.

To turn this into data, you count how many disks (out of some total, say 6 or 10) are floating at each timepoint and plot percent floating against time. The curve climbs from 0 toward 100 percent as photosynthesis proceeds. ET50 is the time at which half the disks are floating, read off that curve, and it acts like a half-life: a lower ET50 means photosynthesis is running faster under those conditions. Comparing ET50 across different light intensities, CO2 concentrations, or wavelengths is how this investigation turns a floating-disk trick into a quantitative comparison of photosynthetic rate.

This sits alongside the other rate-based investigations in the unit: wherever you can't watch a reaction directly, you find a proxy that changes in step with it, graph that proxy over time, and extract a rate or a characteristic time like ET50 from the curve. The same logic of linear interpolation between bracketing data points shows up anytime you need a value that falls between two measured points.