The Chlorophyll Biosynthetic Heterogeneity and Chloroplast Bioengineering - Chlorophyll Biosynthesis and Technological Applications - page 332

At the maximal quantum efficiency of one, two photons are required to move
one electron across the potential difference of about 1.25 V between Z + and A 0 .
The maximal efficiency of the photochemical reactions leading to the formation of
Z + and A 0 is then given by

E ¼ 1 : 25 eV = 2h ν (16.1)

Where,
E ¼ efficiency of PETS
eV ¼ Energy units in electron volts
ν ¼ Energy of the absorbed photon in eV

Since the red 680 nm photons absorbed by PSI and PSII have an energy of
1.83 eV, it ensues from Eq. 1 that

E ¼ 1 : 25 eV = 2 1 : 83 eV ¼ 0 : 34 eV (16.2)

Therefore under red light, the absolute maximal efficiency of the PETS is

ð 0 : 34 eV = 1 : 25 eV Þ 100 ¼ 27 % (16.3)

Under natural white light, although the Chl concentration in photosynthetic
membranes is high enough to result in the near total absorption of all incident
photosynthetically active photons between 400 and 700 nm. These photons repre-
sent only about 44.5 % of the total incident solar radiation, under normal weather
conditions. Therefore under these conditions, the possible overall maximal energy
conversion efficiency amounts to:

ð % : %Þ= ¼ %

27 44 5 100 12 (16.4)

16.2.3 Actual Energy Conversion Efficiency of the PETS
of Green Plants Under Field Conditions

However, under field conditions, the average net photosynthetic efficiency results in a
net agricultural productivity in the range of 2-8 tons of dry organic matter per acre per
year (Lien and San Pietro 1975 ). This corresponds to a solar conversion efficiency of
0.1-0.4 % of the total average incident radiation. Therefore the discrepancy between
the 12 % maximal theoretical efficiency of the PETS, and the agricultural photosyn-
thetic efficiency observed under field conditions ranges from

ð 12 %= 0 : 4 %Þ 100 ¼ 3000 % (16.5)

to

ð 12 %= 0 : 1 %Þ 100 ¼ 12000 % (16.6)