Chlorophyll a in the leaves has an absorption band at the wavelength of 680 nm.

Question

Chlorophyll a in the leaves has an absorption band at the wavelength of 680 nm. As part of the photosynthesis in the plant, chlorophyll a absorbs photon energy to drive the CO2 fixation process. Calculate the energy in J per photon and in the electron volts per photon which absorbs by chlorophyll a at the wavelength of 680 nm CO2 fixation in photosynthesis can be represented by the reaction CO2 + H2O rightarrow (CH_2O) + O2 where (CH_2O) represents one-sixth of a carbohydrate molecule such as glucose. The enthalpy change for this endothermic reaction is delta r Hdegree= +485 kJ mol^-1 of CO2 fixed. Calculate the minimum number of photons (in mol) that need to be absorbed to provide the energy to fix mol CO2 via photosynthesis. Experimentally many values have been determined for the number of photons required to fix one molecule, the presently accepted "best value" is 9 photons per CO2 molecule. Calculate the photochemical quantum yield, based on your answer to part (b) for this process

Explanation / Answer

Energy = h c/ l; where h is Planck's constant = 6.64*10^-34 J-sec, c = speed of light, l = lambda= wavelength

Energy (J/photon) = (6.64*10^-34 J-sec * 3*10^8m/s) / 680 * 10^-9m = 2.92 * 10^-19 J/photon

Energy (eV/photon) = 4.13 * 10^-15 eV-sec * 3*10^8m/s) / 680 * 10^-9m = 1.82 eV/photon

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