The Chlorophyll Biosynthetic Heterogeneity and Chloroplast Bioengineering - Chlorophyll Biosynthesis and Technological Applications - page 333

16.3 Molecular Basis of the Discrepancy Between
the Theoretical Maximal Efficiency of the PETS
and the Actual Solar Conversion Efficiency
of Photosynthesis Under Field Conditions
The discrepancy between the 12 % theoretical maximal efficiency of the PETS and
the actual 0.1-0.4 % solar conversion efficiency of photosynthesis observed under
field conditions can be attributed to (a) factors extrinsic to the PETS, and (b) to
intrinsic rate limitations of the PETS (Lien and San Pietro 1975 ).
These factors will be examined below.
16.3.1 Contribution of Extrinsic PETS Parameters
to the Discrepancy Between the Theoretical
Photosynthetic Efficiency of 12 % and the Actual
Photosynthetic Field Efficiency of 0.1-0.4 %
Extrinsic factors such as ambient weather conditions, availability of water, CO 2 ,
and inorganic nutrients, as well as the metabolic and developmental state of the
plant directly affect photosynthetic efficiency under field conditions. Some of those
factors are under human control while others are not. They do contribute neverthe-
less, to variations in photosynthetic efficiency under field conditions.
The rest of this discussion will focus however upon the impact of intrinsic
factors that affect the PETS and photosynthetic efficiency.
16.3.2 Contribution of Intrinsic PETS Parameters
to the Discrepancy Between the Theoretical
Photosynthetic Efficiency of 12 % and the Actual
Photosynthetic Field Efficiency of 0.1-0.4 %
The 12 % theoretical efficiency of the PETS assumes that under natural conditions,
PSI and PSII operate at a maximal quantum efficiency of ONE. In other words, it is
assumed that every absorbed photon is completely converted into energy without
losses (Lien and San Pietro 1975 ).
For a photosynthetic unit (PSU) size of 200 i.e. for 200 light harvesting Chl
molecules per reaction center (RC), under the moderate light intensities of a shady
sky (about 1/10 of full sunlight), each RC would receive about 200 photons per
second (s) (Lien and San Pietro 1975 ). In other words, each RC would receive about
200 hits or excitons per s. In order to maintain a quantum efficiency of ONE, the