Plants occupy a fundamental part of the food chain and the carbon cycle due to their ability to carry out photosynthesis, the biochemical process of capturing and storing energy from the sun and matter from the air. At any given point in this experiment, the number of floating leaf disks is an indirect measurement of the net rate of photosynthesis.
In photosynthesis, plants use energy from the sun, water, and carbon dioxide (CO2) from the air to store carbon and energy in the form of glucose molecules. Oxygen gas (O2) is a byproduct of this reaction. Oxygen production by photosynthetic organisms explains why earth has an oxygen-rich atmosphere.
The equation for photosynthesis can be written as follows:
6CO2 + 6H2O + light energy → C6H12O6 + 6O2
In the leaf-disk assay, all of the components necessary for photosynthesis are present. The light source provides light energy, the solution provides water, and sodium bicarbonate provides dissolved CO2.
Plant material will generally float in water. This is because leaves have air in the spaces between cells, which helps them collect CO2 gas from their environment to use in photosynthesis. When you apply a gentle vacuum to the leaf disks in solution, this air is forced out and replaced with solution, causing the leaves to sink.
When you see tiny bubbles forming on the leaf disks during this experiment, you’re actually observing the net production of O2 gas as a byproduct of photosynthesis. Accumulation of O2 on the disks causes them to float. The rate of production of O2 can be affected by the intensity of the light source, but there is a maximum rate after which more light energy will not increase photosynthesis.
To use the energy stored by photosynthesis, plants (like all other organisms with mitochondria) use the process of respiration, which is basically the reverse of photosynthesis. In respiration, glucose is broken down to produce energy that can be used by the cell, a reaction that uses O2 and produces CO2 as a byproduct. Because the leaf disks are living plant material that still require energy, they are simultaneously using O2 gas during respiration and producing O2 gas during photosynthesis. Therefore, the bubbles of O2 that you see represent the net products of photosynthesis, minus the O2 used by respiration.
When you put floating leaf disks in the dark, they will eventually sink. Without light energy, no photosynthesis will occur, so no more O2 gas will be produced. However, respiration continues in the dark, so the disks will use the accumulated O2 gas. They will also produce CO2 gas during respiration, but CO2 dissolves into the surrounding water much more easily than O2 gas does and isn’t trapped in the interstitial spaces.