Light Harvesting Complexes
Sun is the source of an incessant flow of energy which forms a range of electromagnetic spectrum, of a portion of which is perceived by us as a light. A small portion of this light between the wavelength 39-700 nm is absorbed by the chlorophylls and other photosynthetic pigments and becomes available for photosynthesis, as evidenced by their absorption spectra.
Light may be thought as made up of waves of particles, called photons. Wavelength and energy are inversely proportional; light of shorter wavelength carries more energy and of longer wavelength carries less energy. Usually in any calculation regarding energy carried by light, it is done for an Einstein, an Einstein being 6.02 X 1023 photons and the energy which carries has been termed as a quantum.
When light is absorbed by chlorophyll, it gets excited which means that an electron is shifted to an outer orbital, making the chlorophyll molecule charged. The electron that moved to higher orbital tends to regain its original level to bring the chlorophyll molecule to its ground state. As it does so, energy in the form of heat is produced and the chlorophyll fluoresces. Since some energy was lost as heat, the fluorescing chlorophyll molecule emits light lesser energy and so of longer wavelength.
The frequency of light of a given wavelength is defined as the number of waves that hits the surface each second. At the speed of light travels, the frequency of any light wave is bound to be very high. If the light of too short wavelength and therefore of very high frequency and high energy hits the surface, the dislodgement of the electron is bound to be permanent and the damage is irreparable. If on other hand, as red and blue light waves hit chlorophyll , they are absorbed (as evidenced by the absorption spectrum of chlorophyll) and energy so become available causes an electron to move into an outer (higher) orbital, without any danger of its being lost altogether. As the dislodged electron is getting back taking the chlorophyll molecule to its ground state, energy is released and this is put to use in photosynthesis. Incidentally in the latter event the chlorophyll‘s electron-hunger is satisfied from another source, allowing it to get back to its ground state. When we say that energy released in the process of dislodgement of electron from chlorophyll and return of the chlorophyll to its ground state is used in photosynthesis, we mean that the light reactions of photosynthesis are on.
We know that photosynthesis in its essence, consists in the reduction of CO2 to CH2O and the source of hydrogen is water. Otto Warburg, wanted to determine at the figure of 4 quanta per molecule of oxygen released, which means that 30 kcals of energy per Einstein is spent. This means that photosynthesis has about 75% efficiency, for the quantum energy carried by an Einstein of red light is about 40 kcals.
Emerson and his co-workers did not get the same results when they repeated the experiments of Warburg. They found that an average, 10 quanta are required for every molecule of CO2 reduced or O2 evolved. This value certainly does not lend to photosynthesis the same degree of energy efficiency of Warburg’s value suggests. But this is nearer the calculated value of 8, since each O2 released or CO2 reduced required for 4electron transfers, each of which requires 2 quanta of energy.
We may arrive at the same conclusion from other angle. For the reduction of CO2 to CH2O about 118 Kcals, are required. One Einstein of red light of 660 nm, calculation shows, carries 43 kcals of energy. It means that at least three photons are required for the purpose. Even if the blue light of 420 nm carrying 70 kcals is considered, it will be still low for one single photon to effect the reduction.
Furthermore calculations showed that under the most intensive light conditions when the highest rate of photosynthesis was recorded the alga would absorb enough number of photons before the first molecule of oxygen was released, but in reality, the evolution of oxygen begins almost immediately upon illumination. Against this background, the idea of a photosynthetic unit (PSU) has been suggested. It is not one molecule of chlorophyll, in its individual entity that subserves the purposes of photosynthesis but rather a collection of them.
The experiments of Emerson and Arnold (1932) on Chlorella led them to think that about 2500 molecules of chlorophyll constituted a PSU. They envisaged special reaction sites among the 2500 chlorophylls to which the light harvested was transferred, allowing a photoact, i.e. absorption and transfer of a light quantum to a trapping centre where it promotes release of an electron.
Some experimental evidence was obtained towards the confirmation of the existence of PSU. Broken chloroplasts with more than a thousand and more chlorophylls alone were able to show Hill reaction activity. Photosynthesis inhibitors such as DCMU (Dichloromethyl urea) were effective only when applied in concentrations of one molecule or more per 2000 chlorophylls, in lesser concentrations, they were ineffective.
Later studies showed that a PSU needs to consist of 250 chlorophyll molecules, a figure obtained by dividing the number of 2500 chlorophylls by quantum requirement, i.e., 10. This appears to be more realistic as the size of the 2500 chlorophylls would be too unwieldy for effective physiological activity. Other components of light reaction like cytochrome-f, ferredoxin and P-700 occur in the ration of 1 molecule each for 300 chlorophylls. All this points to the possibility of a PSU, made up of 250-300 chlorophylls and their accessory pigments and electron carriers.
The occurrences of a PSU as a distinct morphological entity were obtained by Park and his co-workers and they named it quantasome. Now, it is believed that the quantasome and PSU are respectively biophysical and biochemical aspects of the same entity.