Swammerdami
Squadron Leader
The "efficiency" of photosynthesis is an ambiguous term: losses occur at various stages of the process, beginning with the incoming photons: Many photons are of the wrong wavelength, or not aimed at a chlorophyll. However one key step in photosynthesis is said to be almost 100% efficient.
The incoming photon strikes the magnesium atom in a chlorophyll molecule, creating an "exciton" (electron-hole pair). This exciton bounces around, transferring via other magnesium atoms (the chlorophyll molecules acting like ping-pong paddles!) and eventually finds its way to the photosynthetic reaction center (featuring manganese ions) where the harvesting of the electron's energy begins. Apparently almost 100% of these excitons do make their way to the reaction center. They have to get there quickly: the exciton's energy would otherwise dissipate as waste heat in less than a nanosecond IIUC.
In classical physics, the exciton would travel a "Drunkard's Walk" and very seldom make its way to the reaction center. But in quantum physics the exciton's path is a superposition of hundreds of random paths, some of which will get lucky and find the reaction center in time. It is one of those "lucky" paths that is instantiated (when the wave function collapses?); this is an example of "quantum tunneling."
My question is . . . WHY? Why does the wave function collapse on that successful path, rather than on some other random path? Articles that write about this, or quantum tunneling more generally, seem to ignore this question. I'm afraid that I am missing something very basic. (It is said that this aspect of photosynthesis is similar to quantum computing algorithms. Is that the connection I should ponder? Or does the arrival at the reaction center just minimize some thermodynamic measure?)
I've had this question for ten years! ... ever since I read Life on the Edge, by Al-Khalili & McFadden. Here's a more recent article on the same topic.
The incoming photon strikes the magnesium atom in a chlorophyll molecule, creating an "exciton" (electron-hole pair). This exciton bounces around, transferring via other magnesium atoms (the chlorophyll molecules acting like ping-pong paddles!) and eventually finds its way to the photosynthetic reaction center (featuring manganese ions) where the harvesting of the electron's energy begins. Apparently almost 100% of these excitons do make their way to the reaction center. They have to get there quickly: the exciton's energy would otherwise dissipate as waste heat in less than a nanosecond IIUC.
In classical physics, the exciton would travel a "Drunkard's Walk" and very seldom make its way to the reaction center. But in quantum physics the exciton's path is a superposition of hundreds of random paths, some of which will get lucky and find the reaction center in time. It is one of those "lucky" paths that is instantiated (when the wave function collapses?); this is an example of "quantum tunneling."
My question is . . . WHY? Why does the wave function collapse on that successful path, rather than on some other random path? Articles that write about this, or quantum tunneling more generally, seem to ignore this question. I'm afraid that I am missing something very basic. (It is said that this aspect of photosynthesis is similar to quantum computing algorithms. Is that the connection I should ponder? Or does the arrival at the reaction center just minimize some thermodynamic measure?)
I've had this question for ten years! ... ever since I read Life on the Edge, by Al-Khalili & McFadden. Here's a more recent article on the same topic.