
Interpretation:
ΔG0 value should be determined for the excitation of P700 and its comparison with the energy in an Einstein of 700nm photons should be explained.
Concept introduction:
P700 is primary donor for photosystem I where P denotes the pigment and since it absorbs at 700nm hence named as P700.
In the process of photo excitation, when light is absorbed by the photosystem, one electron of chlorophyll P700 gets excited to the higher energy level.
An einstein energy is defined as the energy associated with one mole of photons of certain wavelength.

Answer to Problem 1P
Solution:
Total ΔG0=164.0245kJ/mol
ΔG0 obtained is almost 95.92% of the Einstein energy of 700nm photons.
Given:
The raise in potential for the P700 reaction center is +0.4 to −1.3 volts.
Explanation of Solution
Step1:
Let us find change in potential first as:
ΔE0=E0final−E0Initial=−1.3V−(+0.4V)=−1.7V
Now, the free energy change is calculated as follows:
ΔG0=−nFΔE0
Where, n is number of electrons involved, F is Faraday's constant (96485C/mol) and E0 is cell potential.
Here,
ΔE0=−1.7V=−1.7J/C(1V=1J/C)
n=1
Since, the P700 gets excited through only one electron, so value of "n" will be 1.
Putting all the values we get:
ΔG0=−nFΔE0=−1×96485C/mol×−1.7J/C=164024.5J/mol=164.0245kJ/mol
Step2:
Einstein is energy of one mole of photons.
Let us find the energy of one photon as:
E=h×cλ
Where, h is Planck's constant, c is speed of light and λ is wavelength.
Putting the values:
h=6.626×10−34J.sc=3×108m/sλ=700nm=7×10−7m
Hence,
E=6.626×10−34J.s×3×108m/s7×10−7m=2.84×10−19J
Here, energy of one photon is 2.84×10−19J.
Hence, an einstein energy will be energy of one mole of photon, which can be calculated as:
Einstein=E×NA=2.84×10−19(J/photon)×6.022×1023(photons/mol)=1.71×105J/mol=171kJ/mol
Step3:
Comparison of the two energies (ΔG0 and Einstein) is done as follows:
In the given photoexcitation, the percentage of energy conserved by the P700 is calculated as:
Percent of energy conserved in P700= Energy conserved by P700Einstein energy of photon×100= 164.0245kJ/mol171kJ/mol×100=95.92%
Hence, ΔG0 obtained is almost 95.92% of the Einstein energy of 700nm photons.
The total obtained, ΔG0=164.0245kJ/mol.
It is almost 95.92% of the Einstein energy of 700nm photons.
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Chapter 15 Solutions
Biochemistry: Concepts and Connections (2nd Edition)
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