Environmental Engineering Reference
In-Depth Information
than 550 nm. Even the vision of vertebrates, such as humans,
can be linked to this plant pigment, which has to be ingested
by eating plants since it cannot be synthesized by our
metabolism.
In 1926, the scientist Edgar Transeau made an interesting
observation that put the theoretical values of energy transfer
into the perspective of a real-world scenario (Transeau
1926). He was curious about the fraction of solar energy
input to a corn field that was actually captured in the form of
reduced carbon, meaning the corn produced. He assumed
that 10,000 corn plants per acre were harvested. Of the 100%
of solar energy input to the corn field, only 1.6% ended up as
primary production, or energy stored in the bonds of glucose.
Because very little solar energy, about 0.4%, was used by the
plant to support respiration, the process of making plants out
of solar energy was almost 77% efficient. Measurement of
CO
2
in the air over growing crops indicated that up to 5% of
the daily net
forest or food crop takes up 2,000 tons of water in one
growing season, only 3 tons (or roughly 1%) of the water
remains behind in the form of carbohydrate; the rest is
returned, or cycled, to the atmosphere.
The input of solar energy into the chlorophyll molecule
involves electrons. The incident solar energy in the form of
photons transfers their energy to the electrons of the chloro-
phyll molecule. The excited and energized chlorophyll mol-
ecule transfers this energy towards the production of ATP, a
more usable and transferable form of the photon energy. The
important component here is that unlike other forms of
heterotrophic or chemolithic life that requires energy to be
in the form of organic or inorganic species, the process of
photosynthesis relies on essentially ambient solar radiation.
The ATP so generated can then be used by a cell to synthe-
size the chemicals necessary for life.
The flow of energy is a one-way street. Much like the
energy that is transferred to a light bulb is less than 10%
efficient, with 90% of the input released as heat, the initial
solar energy captured by plants in the form of reduced
carbon compounds is lost as it transfers from producer
to consumer. Decomposers, however, can release these
elements, such as carbon, back to inorganic carbon that can
be used again by plants. The flow of energy is somewhat
cyclic as well, but a net loss also can occur, through burial by
sedimentation.
These fundamental concepts of the flow and cycling of
energy through plants provide a framework within which to
evaluate the phytoremediation of common groundwater
contaminants. Additional support for the use of phyto-
remediation of contaminated groundwater is derived from
the fundamental interaction of plants within the natural
biogeochemical cycles discussed below.
radiation is transferred to plants during
photosynthesis.
The amount of solar energy used to maintain transpira-
tion, however, accounted for almost 45% of the input energy
(Transeau 1926), and 54% was unabsorbed and lost. A
similar representation of inefficiency for water uptake and
that fixed into plants is shown in Fig.
11.1
. For example, if a
11.1.2 The Flow of Oxygen
Oxygen is a toxic gas. Oxygen has been present in the earth's
atmosphere for less than half the age of the planet, which
originally contained no oxygen and had an anoxic atmo-
sphere. As was discussed in Chap. 3, the production of
oxygen by plants as a waste product of photosynthesis grad-
ually reversed this scenario. As the amount of oxygen
increased in the atmosphere, the predominant anoxic life
forms died, escaped oxygen's toxicity through burial during
sedimentation, or adapted to oxygen by using it as part of
their metabolism—anoxic life adapted by developing
enzyme systems to deal with the oxidative effects of oxygen.
In fact, enzyme-based antioxidant properties eventually
evolved to be used as part of metabolism, which gave rise
to truly aerobic respiration and aerobic-based life forms
(Halliwell 2006). This change in atmospheric gas composi-
tion also produced vast amounts of oxidized minerals, such
Fig. 11.1
The efficient flow of energy from the sun to plant carbon and
the reverse inefficient flow of water through plants to the atmosphere.
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