Environmental Engineering Reference
In-Depth Information
ratio of net production (from photosynthesis) to the transpi-
ration of 1,000 g of water. As stated earlier, most of the
water taken up by plants is released as water vapor and is not
used in the photosynthetic equation; the
TE
of most plants,
therefore, is extremely low, about 2 g of water used in
photosynthesis per 1,000 g of water transpired.
Although not part of the structure of chlorophyll, one of
the elements essential for its synthesis is iron. In areas where
the earth's surface is exposed to oxygen, however, iron is in
an oxidized form and unavailable for direct passive uptake
by plant roots. Under anoxic conditions, however, iron is
reduced and can be released into solution. This process of
iron reduction under anoxic conditions primarily is mediated
by iron-reducing microorganisms. Plants that have the abil-
ity to survive periods of flooding or conditions of a high
water table and the attendant anoxic conditions have a higher
probability of accessing the reduced, dissolved iron with
little expenditure of energy. Too much dissolved iron, how-
ever, can be toxic to plants. The role of iron in plant growth
is discussed in Chap. 11.
During photosynthesis, an electron-transport system sim-
ilar to that used in mitochondrial cells also is present. The
energy necessary to start the system is absorbed in the form
of light radiation. Just as human skin is warmed by the
sun's light radiation, this energy also is absorbed by the
molecules in a plant. Energy absorption causes atoms to be
raised to an excited level, whereby electrons orbiting the
nucleus are pushed into a new, more distant orbit, causing
these atoms to have more potential energy. Thus, incoming
light energy is transformed into electrons and potential
energy. Specifically, chlorophyll is raised to an excited
state but also rapidly disposes of energy and returns to its
pre-excited state. The energy is released as heat or
light, called fluorescence, but the most important transfer
of energy is in the formation of high-energy molecules.
This light energy is absolutely necessary for the reaction
of CO
2
and water to occur, because these reactants have an
unfavorable Gibbs change in free energy in the absence of
light.
The solar energy captured by chlorophyll in the
chloroplasts in the leaves is used to split a water molecule
into hydrogen and oxygen, and oxygen is released as a
byproduct into the atmosphere. Four photons of light energy
are required to split hydrogen from oxygen—the hydrogen
then reacts with CO
2
. In general, CO
2
enters the leaf through
open stomata by passive diffusion along a concentration
gradient. It enters the mesophyll cells, also by diffusion,
along the watery boundary layer and then into the cytoplasm.
To produce 1 molecule of sugar, as CH
2n
O, requires 6
molecules of CO
2
to be reduced, which is the input of energy
in the form of 24 electrons; this states the balanced form of
Eq.
3.1
. This equates to an energy requirement of 28.8 kcal/
molecule, (kilocalories per molecule). The absorbed light,
at the blue and red wavelengths of 400 and 700 nm,
respectively, is about 40.5 kcal per 6
10
23
photons; this
is called photosynthetically active radiation (PAR). The
conversion of light energy into reducing power is about
71% efficient.
The path from light radiation energy to chemical bond
energy that occurs in the chloroplasts is complicated. There
are two different light-driven reactions. The overall reaction
is that water is split into hydrogen ions (H
+
) and oxygen,
which exits the plant through the stomata, and the evolved
H
+
is used to reduce the coenzyme nicotinamide adenine
dinucleotide phosphate (NADP) to NADPH, which is then
used by the cell to reduce gaseous CO
2
into carbohydrates.
The water molecule is split to remove 4 electrons by
oxidization. The water-splitting reaction occurs inside the
thylakoid membrane, and the reduction of NADP to NADPH
occurs on the outside of the membrane. Hence, there exists a
gradient in H
+
concentration from inside the membrane
where it is produced to the outside where reduction occurs.
The H
+
used to reduce NADP to NADPH also converts
adenosine diphosphate (ADP) and phosphate into ATP. The
synthesis of living cell matter takes energy, and the form of
energy used by all living organisms is ATP. The H
+
gradient
established by the splitting of water and its use in NADPH
also leads to the storage of ATP. The NADPH can reduce
CO
2
to sugars, and the ADP used to run energy requiring
reactions. Both are cycled back to the oxidized forms of
NADP and ADP, ready for use again.
As stated previously, the sugar synthesized is used by
plants not just for food but to construct other important
organic compounds, from the waxy cuticle that covers
many leaves to wood itself. As such, Eq.
3.1
also can be
written as
6CO
2
þ
!
;
;
;
6H
2
O
plant
light
enzymes
minerals
!
C
6
H
12
O
6
þ
6O
2
þ
H
2
O
ð
g
Þ
(3.2)
The formation of ATP from solar energy and then electron
energy can follow two paths; cyclic or noncyclic phosphory-
lation. In cyclic, ADP is linked to phosphate using the elec-
tron energy released from light sorption of chlorophyll as the
excited electrons drop back to ground state. These electrons
are cycled continually. In noncyclic phosphorylation, how-
ever, electrons are not cycled but are passed along a series of
electron carriers or transport systems. In doing so, ATP is
formed. In the noncyclic case, the electron energy to drive the
formation of ATP is derived from the splitting of water. This
electron transport system of noncyclic phosphorylation in
photosynthetic organisms is very similar to oxidative phos-
phorylation in the mitochondria of non-photosynthetic
organisms. During photosynthesis, the input energy is light;
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