Environmental Engineering Reference
In-Depth Information
3.1.5 Plant Respiration and Glycolysis
Carbon also can be fixed during the crassulacean acid
metabolism (CAM) or Hack-Slack cycle. This type of reduc-
tion can be envisioned as an aerobic reduction of CO
2
, unlike
the anaerobic reduction of CO
2
that produces CO
2
and CH
4
(methanogenesis). Shallow-rooted plants in the desert under
conditions of essentially permanent water limitation have to
be more efficient in taking up CO
2
than plants with less
water stress. Desert plants accomplish this by fixing CO
2
into a C
4
carbohydrate, such as oxaloacetic acid, malic, and
aspartic acids, during the day before converting it to the
glucose typical of the C
3
and C
4
plants at night when water
stress is lower.
Unlike C
3
,C
4
, or CAM plants, plants that reside in aquatic
environments fix CO
2
similar to single cell blue-green algae
with respect to photosynthesis and gas exchange. Terrestrial
plants fix CO
2
after it passively dissolves in the water of the
outer layer of mesophyll cells; the products are then
translocated throughout the plant. Conversely, aquatic plants
passively absorb CO
2
dissolved in the water column, either as
a dissolved gas (CO
2(g)
) or as an ion such as carbonic acid
(H
2
CO
3
), bicarbonate (HCO
3
), or carbonate (CO
3
2
); this
occurs directly as no or minimal stomata are present in aquatic
plants. If air is in equilibrium with water, the amount of
dissolved CO
2
is near 0.5 mg/L, compared to about 8 mg/L
for oxygen. It is this minimum concentration of CO
2
that must
then diffuse passively through the water before entering cells;
in fact, concentrations need to be at least 10 times higher to
overcome aqueous boundary layers of still water near leaf
surfaces, as exist in air near terrestrial leaf boundaries.
Aquatic plants differ from terrestrial plants in other ways.
Aquatic plants do not have a strict xylem in the manner that
terrestrial plants have. As such, the presence of roots in most
aquatic plants is for anchorage and food storage rather than
for water or solute uptake, as evidenced by the lack of root
hairs. Aquatic plants have very thin leaves with thin epider-
mal cuticles, and many are dissected. Such plants also have a
more extensive network of pore spaces interconnected
throughout the plant, similar to terrestrial plants that grow
in waterlogged soils, such as phreatophytes that can be used
for phytoremediation. These pore spaces also conduct CO
2
formed by methanogenesis in the sediments near the roots to
the leaves for use in photosynthesis.
Aquatic plants also have adapted to their watery environ-
ment in terms of oxygen fate. Oxygen is a byproduct of
photosynthesis and needed for respiration. Whereas terres-
trial plants use stomata to emit oxygen produced by photo-
synthesis and to take up oxygen for passive transport to
roots, aquatic plants use aerenchyma tissues for gas trans-
port. Oxygen produced by photosynthesis is stored in these
tissues after collection by diffusion. Once significant internal
pressures are produced during gas collection, oxygen is
transported to roots that often grow in anaerobic sediments
where oxygen is limiting to root growth and survival.
Photosynthesis results in the formation of food for plants,
but this trapped light energy cannot be used directly by the
plant. The trapped energy first needs to be unlocked and the
key is the equally important process of respiration. Respira-
tion in plants is similar to that in animals and is essentially
the reverse of photosynthesis shown in Eq.
3.1
. Energy is
required to drive respiration much like energy is required to
drive photosynthesis. The energy for respiration is derived
from (1) the excitation of electrons in the chlorophyll mole-
cule, and (2) the hydrolysis of water. In general, during
respiration the trapped energy is transformed into the
organic compound glucose, which can then be used by the
plant. In doing so, glucose is converted back to CO
2
and is
released, ready to be reduced again by plants. These sugars
also can be used to synthesize the necessary chemicals of the
plant structure. Individual glucose molecules, for example,
can be polymerized to form cellulose that is used in the cell
wall, as previously mentioned. Most of the carbohydrate
products in the natural world are built from glucose
molecules arranged in various manners to give rise to vari-
ous compounds.
Most of the enzymes that participate in plant aerobic
respiration come from the mitochondria located in the cyto-
plasm. Similar to respiration in mammals, plant respiration
permits the oxidation of photosynthetically produced
reduced organic matter, or photosynthate to produce energy,
water, and CO
2
, according to
C
6
H
12
O
6
þ
6O
2
!
6CO
2
þ
6H
2
O
þ
energy
:
(3.3)
This is an essential part of the global carbon cycle and is
discussed in Chap. 11.
As is shown in Eq.
3.3
, respiration produces CO
2
and the
rate of production is variable. Plants cannot afford to resub-
mit the very CO
2
they fixed right back into the atmosphere
during respiration, so respiration proceeds slowly, often
resulting in the production of other compounds that will
store this energy for later use. In part, slower reaction rates
occur because diffusion is the process by which CO
2
is lost
and O
2
is gained. This is perhaps best demonstrated by the
extensive network of air spaces between cells of aquatic
macrophytes. In these cases, the majority of the bulk plant
consists of nothing but air space, surrounded by a 1-cell-
thick wall that permits maximum oxygen and CO
2
diffusion.
In plants, as in animals, the conversion of stored chemical
energy in glucose to useful energy is through glycolysis
where photosynthate is broken down in a series of steps.
Glycolysis leads to the conversion of sugars that contain six
carbon atoms into two molecules of pyruvic acid, or pyru-
vate, that each contains three carbon atoms. This step occurs
within the cell cytoplasm and does not require oxygen.
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