Environmental Engineering Reference
In-Depth Information
shift between C
3
and CAM in response to environmental
stresses. Pineapple, aloe, and opuntia are the only notable
CAM crops.
In energy terms the simplest equation describes photo-
synthesis as an endothermic reaction that requires 2.8
MJ of radiant energy to synthesize one molecule of
glucose from six molecules of CO
2
and H
2
O. A
more realistic black box description is as follows:
106CO
2
þ
90H
2
O
þ
16NO
3
þ
PO
4
þ
mineral
nutrients
þ
5.4 MJ of radiant energy
¼
3258 g of new
protoplasm (106C, 180H, 46O, 16N, 1P, 815 g of min-
eral ash)
þ
154O
2
þ
5.35 MJ of dispersed heat. Where
light is a limiting factor of photosynthesis, individual
plants follow two basic strategies: shade-intolerant spe-
cies redirect their development into internodal extension
or stem growth at the expense of leaf development;
shade-tolerant species cope by increasing photosynthetic
efficiency and cutting down respiration losses (H. Smith
1982). Solar tracking by plants is also widespread, either
to boost the photosynthetic rates (with leaves perpendic-
ular to the sun's rays) or, with parallel leaves, to reduce
leaf temperature and transpiration water losses (Ehler-
inger and Forseth 1980). And although plants reject
near IR wavelengths and thus avoid overheating, the
heat absorbed by plants in the far IR is essential for the
initiation and progression of thermochemical reactions
of the RPP cycle.
The energy efficiency of actual carbon assimilation is
very high. To reduce one molecule of CO
2
requires three
molecules of ATP and two molecules of NADPH in the
RPP pathway. Free energies of the two compounds are,
respectively, about
29 and
216 kJ/mol. The reacting
compounds contribute 519 kJ, and the difference be-
tween the broken (in H
2
O and CO
2
) and newly formed
(in sugars) bonds during carbohydrate formation is about
465 kJ/mol. Theoretical efficiency of the process is
almost exactly 90%. Actually measured performance is
80%-85%, a great contrast with the much lower effi-
ciency of the whole photosynthetic sequence. The mini-
mum quantum requirement for the synthesis of the three
ATP molecules needed for the reduction of one molecule
of CO
2
depends on the H
þ
/ATP ratio. Theoretically,
there should be an efflux of 2H
þ
/ATP; the actual ratio
is up to 4. With 2H
þ
/ATP the minimum quantum
requirements would be 6, but synthesis of two molecules
of NADPH would raise this to 8 quanta for each mole-
cule of CO
2
assimilated, and 10 may be a more realistic
total.
Energy content of a quantum depends on the light fre-
quency, varying inversely with the wavelength. Assuming
the mean PAR wavelength at 550 nm, the energy con-
tent of an average-sized quantum is 3
:
61
10
19
J
(Planck's constant, 6
:
62
10
34
J, multiplied by the
light frequency, a quotient of the light speed and the
mean wavelength). One einstein (1 mol, or Avogadro's
number, 6
10
23
) of green photons would have energy
of 217 kJ and 8 einsteins would supply 1.736 MJ of radi-
ant energy. The overall maximum theoretical efficiency of
photosynthesis would be almost 27% (465 kJ/1.736
MJ). Alternatively, using 680 nm (the peak for chloro-
phyll), 1 einstein of red photons carries 176 kJ, and with
the more realistic requirement of 10 quanta per fixed
CO
2
molecule, the total radiant energy input would be
1.756 MJ, virtually identical with the result of the first
calculation. A sequence of adjustments brings this theo-
retical maximum to realistic levels.
Adjustment for PAR (43% of total insolation) reduces
the maximum theoretical efficiency to about 12%. Some
of the incident light is reflected by plants, and some of it
is transmitted through canopy leaves. These losses of
