Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 50

whereas in non-photosynthetic animals, the input energy is
reduced organic matter such as plant matter.
An analogy can be made between ATP and a chemical
storage battery in that both store concentrated forms of
potential energy for later use. The use of ATP to drive life
processes releases maximum amounts of energy—up to

include the use of NADPH to reduce CO 2 to carbohydrates
and sugars. This is why dark reactions are sometimes
referred to as carbon-fixing reactions. Dark reactions take
place in the stroma of the chloroplast. The rate of dark
reactions is dependent upon temperature and will increase
with increasing temperature. Moreover, other forms of
hydrogen can act as a hydrogen donor during dark reactions,
such as hydrogen sulfide (H 2 S) by some photosynthetic
sulfur bacteria, or even molecular hydrogen (H 2 ). It is likely
that abundant quantities of H 2 S in ancient, anoxic
environments were used to reduce CO 2 and initially release
sulfur rather than oxygen, until supplies were depleted. As
concentrations of oxygen in the atmosphere increased, ozone
(O 3 ) formed prohibiting ultraviolet radiation from entering
the earth's surface and aiding in the invasion of land by
plants. Once an oxic atmosphere was established, aerobic
or oxidative metabolism was advantageous, whereas oxygen
previously had been (and still is) considered a toxin.
As a consequence, conditions existed such that captured
chemical-bond energy in photosynthates could be harvested
efficiently by organisms, including man, and oxidized back
to CO 2 . The abundant supply of water, however, for use not
only in cellular processes but also as a ubiquitous hydrogen
donor, makes it the preferred source of hydrogen for the
reduction of CO 2 in dark reactions.

8,000 cal—after hydrolysis. The high energy released is
a function of the bond energy in ATP, in which the last
2 phosphates are anhydride linkages that contain repulsion
energy between the closely spaced phosphate groups. This is
in contrast to the simple sugars first fixed during glycolysis,
such as glucose-6-phosphate, which yields less energy than
ATP upon hydrolysis—only 3,300 cal.

In other words, the storage of energy from the sun in
carbohydrates formed by plants is analogous to the energy
stored in water at elevation (Fig. 3.4 ). Both forms of energy
are considered potential energy. If water stored at elevation
flows downhill and turns a wheel the potential energy is
converted to kinetic energy that can be used to perform
work. This is the same with the sugar molecule. The stored
energy is not physical, like water elevation, but chemical,
which is converted to kinetic energy when it is used to drive
the flow of energy in a living organism.
From Eq. 3.2 it appears that the total reaction must pro-
ceed in the presence of light. This is in fact true; light is
needed to initiate photosynthesis. Additional reactions take
place in the absence of light, however. These dark reactions

3.1.4 Carbon Fixation: C 3 ,C 4 , CAM,
and Aquatic Plants

Light provides the energy to stimulate chlorophyll and split
water and release H + , and atmospheric CO 2 enters the cell
passively, but these processes together do not make CO 2
available for subsequent reduction. Most plants reduce, or
fix, CO 2 by using the enzyme ribulose biphosphate carbox-
ylase/oxygenase, called either Rubisco, or RuBP. This
enzyme is used by plants to take the carbon from CO 2 and
add it to pre-existing sugars to create two molecules of a
3-carbon (C 3 ) sugar called phosphoglyceric acid (PGA).
This process is referred to as the Calvin cycle, or the
Calvin-Benson cycle, after the work of Calvin's chemistry
group in California. Most C 3 plants tend to predominate
in moderate climates where water is readily available.
A 4-carbon (C 4 ) sugar also can result from carbon fixation
by certain plants. Most C 4 plants tend to predominate in hot,
dry climates or tropical ones with high light intensity. Pho-
tosynthesis is at a maximum around 30 C for C 3 plants, but
temperatures must be near 45 C for C 4 plants. C 4 plants still
use the Calvin-Benson cycle, but it occurs elsewhere in the
plant. C 4 plants have to be more efficient in the conversion of
light energy into reduced organic matter than C 3 plants that
grow in more humid areas.

Fig. 3.4 The potential energy stored in adenosine triphosphate (ATP)
is analogous to that of the potential energy associated with the elevation
of water, where both can be used to perform work for making proteins
or turning a water wheel, respectively.