Of the several photosynthetic pigments, only chlorophyll-a can convert light into chemical energy. However this does not mean that light absorbed by the other pigments is waste. On the other hand, the light absorbed by the other pigments is transferred to chlorophyll-a . This transfer may be direct or as it happens more usually, some other mediating agencies might affect it. Chlorophyll-a thus utilizes the light that it absorbs on its own and also the light transferred o it by other pigments to chlorophyll-a has been called light trap or light sink. The light trap makes for a much better light-harvesting efficiency, for it ensures funneling of light quanta towards one acceptor molecule of chlorophyll. Were it not for this, there would be random migration of the quanta, leading to dissipation of energy.
We might say that even chlorophyll-a molecule is not the seat for the conversion of light into chemical energy. It passes it on to a very special category o chlorophyll-a molecule, called P-700 of photosystem I or P-690 of photosystem II. The P-700 & P-690 molecules are so called because of their absorption peaks, 700nm and 690nm respectively. The P-700 and P-690 are protein-bound and are at the helms of light traps of the respective photosystems. The light trap mechanism functions as follows:
One of the pigments(other than P-700 and P-690) gets excited on absorption of light energy. But the excited pigment quickly loses the energy so gained by transferring it to another unexcited pigment and in the process, goes back to the ground state. This entire process of absorption, retention and transfer of energy is completed in 5X10-9 sec.
The transference of energy from an excited pigment molecule to an unexcited pigment molecule takes place in either of the two ways: electron transport or resonance. In the electron transfer, the ejected electron gets back to its original site after hopping from one electron acceptor to another and in the process, energy becomes available. On the other hand, in resonance, energy is directly transferred, the tight packing of pigment molecules and the overlapping of the fluorescence frequencies of the pigments in question, proving helpful.
Chlorophyll-b absorbs light at about 644nm and gives off light energy by fluorescence at longer wavelengths . But this fluoresced light is good enough to excite chlorophyll-a because this one absorbs light maximally at about 660 nm. If chlorophyll-a would not use this light for any physiological purposes, it can only radiate that energy by further fluorescence. There is no question of chlorophyll-b absorbing that light, for now it is of a longer wavelength and therefore it is not useful by that pigment. This wavelength barrier creates a one-way track o energy flow and this is called light trap.
The light traps mechanism is inherent in the organization of photosynthetic unit. Though anticipatory in nature, we might state here that the light reaction actually consists of two photochemical reactions which are separated both in time and space. They are respectively designated as Photoact I and II, and the two reactions are mediated by two different systems, whose compositions differ in terms of pigments, electron carriers and light trap mechanisms. The mediating agencies of the two photoacts respectively called Photosystem I and Photosystem II. The products of light reaction as a whole ae reduced NADP(NADPH2), ATP & O2 , Photosystem I yielding NADPH2 and photosystem II yielding ATP and molecular oxygen.