Photon absorption photosynthesis

In contrast to a conventional p-n-junction-type solar cell, the mechanism of the DSSC does not involve a charge-recombination process between electrons and holes because electrons are injected from the dye photosensitizers into the semiconductor, and holes are not formed in the valence band of the semiconductor. In addition, electron transport takes place in the Ti02 film, which is separated from the photon absorption sites (i.e., the photosensitizers) thus, effective charge separation is expected. This photon-to-current conversion mechanism of the DSSC is similar to that for photosynthesis in nature, where chlorophyll functions as the photosensitizer and electron transport occurs in the membrane. 

Conventional solar cells are built from inorganic silicon-like materials. Efficiency of such solar cells is high, but they originate from expensive materials and special techniques are required for their processing. Recently hybrid and photoelectrochemical solar cells [54] have been cost effective alternatives for conventional silicon solar cells. The correspondence between the photon absorption and charge separation events is the point of differentiation between the photovoltaic effect in a semiconductor junction, and that in a photon-induced generation of a chemical potential in natural systems, i.e. photosynthesis. In the latter, and this is very simple but highly relevant in the context of artificial photosynthetic systems, the point in space at which the... 

The first situation that can be envisaged concerns functioning close to the compensation point for photosynthesis (very low specific photon absorption rates 0) corresponding to the strongest physical limitation by radiant light energy transfer. In this condition, the specific rate is the... 

Photosynthesis is initiated by the absorption of a photon, which produces a charge-separated P680+Pheo state between P680, a special arrangement of four chlorophyll a molecules, and pheophytin... 

Generating one NADPH from NADP requires the transfer of two electrons through PS I and PS II. Transfer of one electron requires that a photon be absorbed by both PSI and PSII, so that the generation of 12 ATPs requires the absorption of 12x2x2 = 48 photons. These 48 photons allow the synthesis of (just) 18 ATPs by coupling factor. Thus, the equation for photosynthesis can be written in two steps. 

For many purposes in plant studies, it is important to know the photon flux density. For instance, the rate of photosynthesis depends on the rate of absorption of photons, not on the rate of absorption of energy. Some instruments are sensitive only to photosynthetically useful photons (e.g., wavelengths basically from 400 to 700 nm), the so-called photosynthetic photon flux (PPF) or photosynthetic photon flux density (PPFD), both of which are expressed in jimol m-2 s-1 (Fig. 4-4b). 

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