Oxidized (blue) DPIP becomes colorless when it is reduced – that is, when it receives electrons. A diagram of the photosynthetic light reaction shows that water molecules are the source of these electrons. How is it that the DPIP was only able to receive reducing electrons from the photosystems (and ultimately from the water) when the lights were on, but not when the lights were off. Since the cuvette is mostly full of water, it is not the case that water is the limiting factor. Explain: what is the role of light energy in this system, and why does it make a difference in the reduction of the dye?
In the photosynthetic light reaction, light energy is crucial for the reduction of DPIP and the overall process of photosynthesis. The role of light energy is to excite electrons in the chlorophyll molecules of the photosystems, leading to a series of electron transfers.
When light is present, photons are absorbed by the chlorophyll molecules in the photosystems. This energy is transferred along a series of electron carriers in a process called the electron transport chain. Ultimately, the energy is used to pump protons (H+) across a membrane, creating an electrochemical gradient. This energy-rich environment drives the synthesis of ATP, the energy currency of the cell.
During this electron transport chain, DPIP acts as an electron acceptor. It accepts the electrons that are transferred from the excited chlorophyll molecules. In its oxidized (blue) state, DPIP can receive these electrons and becomes reduced (colorless).
Now, to address the specific question of why DPIP can only receive reducing electrons from the photosystems when the lights are on, and not when the lights are off - it is because the light energy is required for the initial excitation of electrons in the chlorophyll molecules. In the absence of light, there is no excitation of electrons, and therefore no transfer of electrons to DPIP.
Even though the cuvette is mostly full of water, it is not the limiting factor because water molecules serve as the source of these reducing electrons. They are continuously being split in a process called photolysis, which occurs in the presence of light. During photolysis, water molecules are broken down into oxygen (released to the atmosphere) and hydrogen ions (H+). The electrons from the photolysis of water are the ones that are ultimately transferred to DPIP.
To summarize, light energy plays a critical role in the reduction of DPIP by exciting electrons in chlorophyll molecules. When light is present, the excited electrons are transferred to DPIP, causing it to become reduced. In the absence of light, there is no excitation, and therefore no transfer of electrons, resulting in DPIP remaining in its oxidized state.