Light-harvesting provides a means of capturing photonic energy and transferring
ID: 174470 • Letter: L
Question
Light-harvesting provides a means of capturing photonic energy and transferring it to a Reaction Center where photochemistry takes place. Describe energy flow in terms of relative energies of donor and acceptor molecules in performing energy transfer. Calculate the photon flux (I) that strikes a surface per unit area per second under a sunlight intensity of 2000 mu E m^-2 s^-1. Provide answer in Angstromsign^-2 s^-1 I = EN_A = # of photons that strike a surface per unit area per second N_A= Avogadro's Number = 6.02 times 10^23 mol^-1 E=Einstein = 1 mole of photons (6.02 times 10^23) of photons A Light-harvesting complex contains 10 chlorophylls per complex, each capable of absorbing light to transfer to the Reaction Center. Target size is proportional to extinction coefficient (epsilon)= absorptivity of a molecule at a given wavelength Target size (sigma)= 2.303 epsilon/N_A Assume the Extinction coefficient for each chlorophyll is 25,000 M^-1cm^-1 Calculate the target size (sigma) of each chlorophyll and each light-harvesting complex Calculate how many photon hits/LH complex = 1 sigma If a Reaction Center turns over 3000 times per second, how many light-harvesting complexes are needed to allow for such turnover, not considering any absorption by the RC directly?Explanation / Answer
Energy flows in terms of relative energies of donor and acceptor:
In light harvesting process energy is transferred from P680 by accepting photon to NADP+. The overall reaction involves 2 H2O + 2 NADP+ + 8 photons--- 2 NADPH + 2 H+ + O2
The gobs free energy is 102 Kcals for this reaction. The reaction needs 8 photons and 40 kilocalories energy is given by one photon. Total 320 Kcals energy comes from photons. The donor molecule is P680. On absorbing photon it exits and release electron. The 1/3 energy from photons is used to move to NADP+, the acceptor. There is two molecules of H2O and they capture 2/3 of total energy for their splitting process i.e. 212 Kcal which is 106 Kcals per H2O.
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