Transduction (biophysics)
In biophysics, transduction is the conveyance of energy from one electron (a donor) to another (a receptor), at the same time that the class of energy changes.
Photonic energy, the kinetic energy of a photon, may follow the following paths:
- be released again as a photon of less energy;
- be transferred to a recipient with no change in class;
- be dissipated as heat; or
- be transduced
In photosynthesis, when the electrons of the "chlorophyll pair" receive the photon energy from the "collecting" associated pigments, the photonic energy is "destined" to link one molecule of phosphate to one of NAD. The resulting NADP in turn will use the stored energy in the generation of ATP, which is the end point of the light-induced photosynthetic process. [This is quite wrong: NADP is reduced (electrons are added) to NADPH in the light reactions of linear electron transport. No ATP involved directly. Phosphate is "linked" to ADP, forming ATP, but this does not involve NADP(H), it involves the proton gradient (and ATP synthase) generated by photosynthetic electron transport processes.] This means that the photon's energy ends up its circuit by being transduced to an electron that takes part in the formation of a molecular link of energy-rich phosphate.
In the pathway of this end-point transduction, the energy is transferred along a number of molecules (cytochromes), in a downward way so that energy is partially dissipated at each step. The liberated heat energy serves the homeostasis of the plant, and at the end of the chain the remaining energy is perhaps exactly the one that is needed to build NADP.
This process is committed; i.e. there is no return path. Homeostasis, theoretically, might save the day only at the beginning: before the luminic energy transferred to the "chlorophyl pair" is conveyed to the first element of the cytochrome chain, there is a gap in the process when the energy is carried as a series of excitons. These are now called resonant-energy-transferring molecules of the chlorophyll class, which transfer what is considered electromagnetic energy, from one to its neighbor with no participation of electrons nor enzymes. At this stage, if the first pigment has received an excess of light, the "exciton" perhaps might dissipate the energy as heat.