Download Carbon Fixation www.AssignmentPoint.com Carbon fixation or

Carbon Fixation
www.AssignmentPoint.com
www.AssignmentPoint.com
Carbon fixation or сarbon assimilation refers to the conversion process of
inorganic carbon (carbon dioxide) to organic compounds by living organisms.
The most prominent example is photosynthesis, although chemosynthesis is
another form of carbon fixation that can take place in the absence of sunlight.
Organisms that grow by fixing carbon are called autotrophs. Autotrophs include
photoautotrophs, which synthesize organic compounds using the energy of
sunlight, and lithoautotrophs, which synthesize organic compounds using the
energy of inorganic oxidation. Heterotrophs are organisms that grow using the
carbon fixed by autotrophs. The organic compounds are used by heterotrophs to
produce energy and to build body structures. "Fixed carbon", "reduced carbon",
and "organic carbon" are equivalent terms for various organic compounds.
Net vs gross CO2 fixation
It is estimated that approximately 258 billion tons of carbon dioxide are
converted by photosynthesis annually. The majority of the fixation occurs in
marine environments, especially areas of high nutrients. The gross amount of
carbon dioxide fixed is much larger since approximately 40% is consumed by
respiration following photosynthesis. Given the scale of this process, it is
understandable that RuBisCO is the most abundant protein on earth.
www.AssignmentPoint.com
Overview of pathways
Six autotrophic carbon fixation pathways are known as of 2011. The Calvin
cycle fixes carbon in the chloroplasts of plants and algae, and in the
cyanobacteria. It also fixes carbon in the anoxygenic photosynthetic
proteobacteria called purple bacteria, and in some non-phototrophic
proteobacteria.
Oxygenic photosynthesis
In photosynthesis, energy from sunlight drives the carbon fixation pathway.
Oxygenic photosynthesis is used by the primary producers—plants, algae, and
cyanobacteria. They contain the pigment chlorophyll, and use the Calvin cycle
to fix carbon autotrophically. The process works like this:
2H2O → 4e− + 4H+ + O2
CO2 + 4e− + 4H+ → CH2O + H2O
In the first step, water is dissociated into electrons, protons, and free oxygen.
This allows the use of water, one of the most abundant substances on Earth, as
an electron donor—as a source of reducing power. The release of free oxygen is
a side-effect of enormous consequence. The first step uses the energy of
sunlight to oxidize water to O2, and, ultimately, to produce ATP
ADP + Pi is in equilibrium with ATP + H2O
and the reductant, NADPH
www.AssignmentPoint.com
NADP+ + 2e− + 2H+ is in equilibrium with NADPH + H+
In the second step, called the Calvin cycle, the actual fixation of carbon dioxide
is carried out. This process consumes ATP and NADPH. The Calvin cycle in
plants accounts for the preponderance of carbon fixation on land. In algae and
cyanobacteria, it accounts for the preponderance of carbon fixation in the
oceans. The Calvin cycle converts carbon dioxide into sugar, as triose
phosphate (TP), which is glyceraldehyde 3-phosphate (GAP) together with
dihydroxyacetone phosphate (DHAP):
3 CO2 + 12 e− + 12 H+ + Pi → TP + 4 H2O
An alternative perspective accounts for NADPH (source of e−) and ATP:
3 CO2 + 6 NADPH + 6 H+ + 9 ATP + 5 H2O → TP + 6 NADP+ + 9 ADP + 8
Pi
The formula for inorganic phosphate (Pi) is HOPO32− + 2H+. Formulas for
triose and TP are C2H3O2-CH2OH and C2H3O2-CH2OPO32− + 2H+
Evolutionary considerations
Somewhere between 3.5 and 2.3 billion years ago, the ancestors of
cyanobacteria evolved oxygenic photosynthesis, enabling the use of the
abundant yet relatively oxidized molecule H2O as an electron donor to the
electron tranport chain of light-catalyzed proton-pumping responsible for
efficient ATP synthesis. When this evolutionary breakthrough occurred,
www.AssignmentPoint.com
autotrophy (growth using inorganic carbon as the sole carbon source) is
believed to have already been developed. However, the proliferation of
cyanobacteria, due to their novel ability to exploit water as a source of electrons,
radically altered the global environment by oxygenating the atmosphere and by
achieving large fluxes of CO2 consumption.
Carbon concentrating mechanisms
Many photosynthetic organisms have acquired inorganic carbon concentrating
mechanisms (CCM), which increase the concentration of carbon dioxide
available to the initial carboxylase of the Calvin cycle, the enzyme RuBisCO.
The benefits of CCM include increased tolerance to low external concentrations
of inorganic carbon, and reduced loses to photorespiration. CCM can make
plants more tolerant of heat and water stress.
Carbon concentrating mechanisms use the enzyme carbonic anhydrase (CA),
which catalyze both the dehydration of bicarbonate to carbon dioxide and the
hydration of carbon dioxide to bicarbonate
HCO3− + H+ is in equilibrium with CO2 + H2O
Lipid membranes are much less permeable to bicarbonate than to carbon
dioxide. To capture inorganic carbon more effectively, some plants have
adapted the anaplerotic reactions
HCO3− + H+ + PEP → OAA + Pi
www.AssignmentPoint.com
catalyzed by PEP carboxylase (PEPC), to carboxylate phosphoenolpyruvate
(PEP) to oxaloacetate (OAA) which is a C4 dicarboxylic acid.
CAM plants
CAM plants that use Crassulacean acid metabolism as an adaptation for arid
conditions. CO2 enters through the stomata during the night and is converted
into the 4-carbon compound, malic acid, which releases CO2 for use in the
Calvin cycle during the day, when the stomata are closed. The jade plant
(Crassula ovata) and cacti are typical of CAM plants. Sixteen thousand species
of plants use CAM. These plants have a carbon isotope signature of -20 to -10
‰.
C4 plants
C4 plants preface the Calvin cycle with reactions that incorporate CO2 into one
of the 4-carbon compounds, malic acid or aspartic acid. C4 plants have a
distinctive internal leaf anatomy. Tropical grasses, such as sugar cane and maize
are C4 plants, but there are many broadleaf plants that are C4. Overall, 7600
species of terrestrial plants use C4 carbon fixation, representing around 3% of
all species. These plants have a carbon isotope signature of -16 to -10 ‰.
C3 plants
The large majority of plants are C3 plants. They are so-called to distinguish
them from the CAM and C4 plants, and because the carboxylation products of
the Calvin cycle are 3-carbon compounds. They lack C4 dicarboxylic acid
www.AssignmentPoint.com
cycles, and therefore have higher carbon dioxide compensation points than
CAM or C4 plants. C3 plants have a carbon isotope signature of -24 to -33‰.
www.AssignmentPoint.com