This may be defined in various ways. The minimum photon energy input at the outside antenna pigment molecules (i.e. not at the reaction centre) may be given as four photons of 1.77 eV (PS2 absorption for D2 at 700 nm) and four of 1.82 eV (PS1 absorption for D1 at 680 nm), totalling 14.4 eV. The actual excitations D2 to A2, and D1 to A1, are about 1.1 eV each. So four operations of each require 8.8 eV. The outputs may be considered to be four electrons lifted from H2O to NADP over redox potential 1.15 eV (4.60 eV) plus three ATP molecules at 0.34 eV each (1.02 eV) to give a total output of 5.6 eV. The output may also be considered as one O2 molecule, and one C atom fixed in carbohydrate, requiring 4.8 eV.
A reasonable maximum efficiency from light absorption to final product can be taken as 4.8/14.4 = 33%. The larger proportions of 5.6/14.4, 5.6/8.8 and 4.8/8.8 may sometimes be considered.
In discussing photon interactions, the unit of the einstein is often used. One einstein is Avogadro's number of photons of the same frequency, i.e. one mole of identical or similar photons.
The solar spectrum consists of many photons with quantised energy too small to be photosynthetically active (X > 700nm,hv< 1.8eV), and photons of greater energy than the minimum necessary (hv > 1.8eV). The situation is very similar to that with photovoltaic cells (see Figure 7.12 and Section 7.4), so that only about 50% of the energy of the photons photosynthetically absorbed is used to operate PS2 and PS1. This effect would reduce the maximum efficiency to about 16%. However, leaves are not black and there is considerable reflection and transmission, reducing the maximum efficiency to about 12%, considering only the solar radiation incident at the photosystems. Efficiencies near to this have been obtained in controlled laboratory conditions. Considering solar irradiation on land generally, which will include parts other than leaves, such large efficiencies are not reached in even the best agriculture, or in natural conditions (Table 10.1).
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.