Page 32 - Biofuels Refining and Performance

Energy and Its Biological Resources 15

cited earlier. An oxidative pathway is considered to be more effective
from a biochemical energetic viewpoint.
One anabolic example of photosynthesis is briefly discussed.
Theoretically, reversal of this known reaction should fit well for photo-
synthesis:

C H O 6O → 6(CO H O) 686,000 cal
6
12
6
2
2
2
But in fact, we find a slightly different figure. The entire reaction may
be symbolically represented as
hv
2H O 2NADP ⎯⎯⎯→ 2NADPH (H ) O
2
Chloroplast 2
3CO 9ATP 5H O Triosephosphate 9ADP
2
2
⎯⎯→
6NADPH (H ) 8H PO 6NADP
3
4
But the actual stoichiometric presentation shows
n(CO H O) → ( CH O) nO n(113,000 cal)
2
2
2
n
2
almost 22,000 cal higher than expected; fortunately, however, the ender-
gonic reaction derives its energy from light energy. These figures are jus-
tified because the part of the reaction occurring in the absence of light
needs a large excess of energy-rich compounds (ATP). The deficiency of
ATP is, however, taken care of by two linked reactions:
Cyclic photophosphorylation:
hv
nADP nH PO ⎯⎯→ nATP nH O
3
2
4
Noncyclic photophosphorylation:
hv
4Fe 2ADP 2H PO 4H O ⎯→ 4Fe red 2ATP O 2H O 4H
2
ox
3
2
2
4
or 2Co II red 2ATP O 2

2H O 2H
2
The deficiency of 1 mole of ATP per mole of CO fixed is provided by cyclic
2
photophosphorylation. The other anabolic process is the nitrogen fixa-
tion, which is also highly energy consuming.
The heat of formation of NH by a chemical pathway can only be
3
determined indirectly. By the Haber process, high pressure and tem-
perature is needed and the yield remains very low. So the input in
energy in the technological process remains in large excess than the the-
oretical heat of formation of NH .
3
Nitrogen fixation can take place in nature in two major ways.
Molecular nitrogen is converted to oxides of nitrogen in the atmosphere