Biology Reference
In-Depth Information
the carbon-carbon skeleton as well as the high energy bonds of ATP and NADPH
which are formed during the process of photosynthesis.
The carbon cycle essentially describes how photosynthesis supports organic life
in the biosphere. The carbon skeleton formed via the process of photosynthesis is
converted into the simple and complex food consumed by organic life. The needed
energy for enzymatic inter conversions and biosynthetic processes is provided by
ATP and NADPH. The organic matter of dead biota is converted in turn into CO
2
,
H
2
O, and inorganic elements by bacterial activity. Then the carbon cycle repeats
itself all over again.
At issue then, is whether agricultural productivity at today's levels of photosyn-
thetic efficiency is efficient enough to feed a growing world population. This issue
will be explored below.
16.2 Relationship of Agricultural Productivity
to Photosynthetic Efficiency
Since plants form food by conversion of solar energy, CO
2
, and H
2
O into chemical
energy via the process of photosynthesis, it ensues that agricultural productivity
depends in turn upon photosynthetic efficiency. Let us therefore briefly describe the
components of photosynthetic efficiency.
Photosynthetic efficiency is controlled by intrinsic and extrinsic factors (Lien and
San Pietro
1975
). Extrinsic factors include the availability of water, CO
2
, inorganic
nutrients, ambient temperature, and the metabolic and developmental state of the
plant. The most important intrinsic factor is the efficiency of the photosynthetic
electron transport system (PETS). The latter is driven by two photochemical
reactions that take place in membrane-bound photosystem I (PSI), and PSII chloro-
phyll (Chl)-protein complexes.
16.2.1 The Primary Photochemical Acts
of Photosystem I (PSI) and PSII
Conversion of solar energy into chemical energy is the results of two photochemical
acts that take place in PSI and PSII. The primary photochemical act of PSII is
initiated by the absorption of light by antenna Chl
a
and
b
. The absorbed photons
are conveyed to special Chls in the PSII reaction center. There, the light energy is
used to generate a strong oxidant Z
+
that liberates oxygen from water. It also
generates a weak reductant Q
-
that together with plastoquinone electron acceptor
pools serve as temporary storage of the electrons extracted from water. The primary
photochemical act of PSI is also initiated by the absorption of light by antenna Chl
a
and
b
, and in this case too, the absorbed photons are conveyed to special Chls in
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