In the parts of the investigation that follow, we conduct experiments using leaf disk samples to test whether they are undergoing photosynthesis based on whether they float in solution. What gives the leaf disks their buoyancy?

Explanation:

Leaf disks float normally,when the air spaces are replaced with water the overall density of the  leaf disk increases and the disk sinks

The saturating solution should include a small amount of  sodium bicarbonate

Bicarbonate ion serves as the carbon source for photosynthesis

As photosynthesis proceeds, oxygen accumulates in the air spaces of the spongy mesophyll, the leaf becomes buoyant and floats

While this is going on, the leaf is also carrying out cellular respiration; this respiration will consume the oxygen that has accumulated and possibly cause the plant disks to sink

The rate that the disks rise is an indirect measurement of the net rate of  photosynthesis