2013) show that the slowing of the forward reaction by the necessary uphill activation energy actually decreases the efficiency of energy

storage.] The assumption that the energy of the thermally equilibrated excited state is the free energy is reasonable if the entropy change is small, as is the case in chlorophyll. An efficiency of >98 % for the primary reaction of green plant photosynthesis when excited at the main absorption band is thus allowed. The energy of the first cation–anion pair in photosynthesis is not precisely known but the efficiency is ~95 %. Since the first step in photosynthesis is electron transfer, its yield depends on the rate of reverse electron transfer, assuming the other deactivation paths are slow as is required for maximum efficiency. As has been pointed out repeatedly, for a yield of X % one needs a reverse rate of (100 − X) % of the forward rate. This is usually written as the energy loss in the forward LY294002 chemical structure step to enable the minimum thermodynamically required slowing of the reverse step via a Boltzmann distribution. However, as I have pointed out, there is more than one way to skin a cat: at least a half-dozen, and these are unlikely to have exhausted the subject (Mauzerall 1988). Quantum mechanics in particular allows a variety of possibilities. Daporinad cost The simple Boltzmann-based argument of slowing the reverse rate

leads to the requirement of 0.6 eV decrease of free energy to ensure a 99 % yield of product on the 1 ms time scale required to form oxygen, given a forward reaction time of 3 ps. On the 10 s timescale of the most stable S-state of the oxygen forming cycle, which allows photosynthesis in very dim light, the required energy loss is 0.83 eV. Thus, a thermal efficiency of 54 % from a 680 nm (1.8 eV) photon is possible. The measured efficiency at

the trap energy is ~35 % (Mielke et al. 2011) so some gain is theoretically possible. However, this efficiency is very close to that delivered by the final products of photosynthesis, oxygen and glucose, if eight photons are required for the complete cycle. It may be difficult to outdo evolution. Exactly because it is a photochemical system, the thermal efficiency of photosynthesis is wavelength dependent: it Ketotifen decreases with decreasing wavelength. The energy of all photons greater than the equilibrated energy of the excited state is immediately degraded to heat. This is another reason why the thermal or Carnot cycle arguments are irrelevant. The efficiency then depends on the assumed “temperature” of the light source, which increases with decreasing wavelength. In fact the thermal efficiency depends in large part on the choice of the trap energy—i.e., the energy of the primary reaction—by evolution. This is clearly seen in the classic paper on the efficiency of photovoltaic devices by Shockley and Queisser (1961). They use only one temperature in their arguments, that of the sun, but stress the role of the energy gap in determining the efficiency of the device.