Environmental Engineering Reference
In-Depth Information
3.1 ATP molecule is composed of adenine, ribose, and a
triphosphate tail.
I is excited by wavelengths of 700 nm, and photosystem
II works best at 680 nm. Photosynthesis is thus ener-
gized overwhelmingly by red light. In total, the photo-
synthetically active radiation (PAR), the wavelengths
between 400-700 nm, is less than half (43%) of total
insolation.
Light-driven enzymatic transfer of electrons from
water produces nicotinamide adenine dinucleotide phos-
phate (NADPH) and converts adenosine diphosphate
(ADP) and inorganic phosphate to ATP. The electron
transfer splits water in a single reaction step within a
MnCa
4
cluster and release of O
2
(Yachandra et al.
1993). ATP and NADPH are then used to energize the
reduction (fixation) of CO
2
to carbohydrate, which can
proceed without any light (dark reaction). ATP, the prin-
cipal energy carrier in the biosphere, powers a vast variety
of enzymatic reactions via the hydrolysis of its phosphate
bond (fig. 3.1). Evolution selected this phosphate be-
cause its enzymatic hydrolysis (loss of the endmost phos-
phate group) releases a large amount of energy, 31 kJ/
mol (Westheimer 1987). A reverse reaction, catalyzed
by ATP synthase, takes place when energy is not immedi-
ately needed. It reforms the compound by adding a third
phosphate molecule to ADP. ATP can thus be used again
and again.
The sequence of this multistep reductive carbon cycle
was first elucidated by Melvin Calvin and Andrew Ben-
son during the early 1950s (Bassham and Calvin 1957;
Calvin 1989). They introduced
14
C-labeled CO
2
(for
periods ranging from fractions of a second to minutes)
to photosynthesizing Chlorella, and after preselected
periods they stopped the enzymatic reactions by alcohol
and extracted materials for analysis. In the early 1950s,
once they began using newly invented paper chromato-
graphy, they could clearly demonstrate that the first
stable compound produced by photosynthesis was phos-
phoglyceric acid (PGA), a compound containing three
carbons (3-phosphoglycerate is a synonym). The RPP
