A student uses the floating-disk assay to measure photosynthesis. Spinach leaf disks are punched out and vacuum-infiltrated with a 0.2% sodium bicarbonate solution containing a drop of detergent. The disks sink. Five beakers are placed under a lamp, each with a different bicarbonate concentration: 0, 0.05, 0.1, 0.2, and 0.5%. The student records the time for half of the disks (5 of 10) in each beaker to float (ET50). All beakers are kept at 25 C and equal light intensity.
Why do the leaf disks float as photosynthesis proceeds?
- A
Disks absorb light energy and become less dense
- B
CO2 from bicarbonate is released as a gas
- C
Detergent reduces water density
- Dcheck_circle
O2 produced in the chloroplasts accumulates in air spaces, increasing buoyancy
Explanation
Bicarbonate provides CO2 substrate for the Calvin cycle, but the gas that fills the leaf air spaces is O2 generated by the light reactions. As O2 accumulates, the disks become buoyant and rise.