Answer:
Through this experiment we study the process of photosynthesis.
• In the begining the leaf disks's tissue has oxygen, this allows them to float.
• Then, in the experiment we infiltrate the leaf disks with a solution that has bicarbonate, and as a result their density changes (it increases) and so the leaf disks sink.
• Once the disks are on the bottom and we direct the light to them the process of photosynthesis starts, being the bicarbonate the source of carbon for the process.
• As we know a product of the photosynthesis proces is oxygen, so when it is released into the leaf disks the overall density changes again and the disks rise.
So as we can see, in this experiment the purpose of the bicarbonate is being the source of carbon for the process of photosynthesis that occurs in the leaf disks.