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Photosynthesis and Carbon Fixation
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Photosynthesis was a process that took a very long time to occur
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In the visible range of light at 550nm, high energy photons range from 100
to 30 Kcal/mole
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[C-C
80Kcal/mole ; O=O 120Kcal/mole]
Pyrrolle structure- 2 variations- linear tetra pyrrolle [phycobilins] and cyclic
tetra pyrrolle [ present in pigments that is important for photochemistry
like Hemoglobin]
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Page 132- Positively charged chlorophyll can act as a strong oxidizing agent
and can reduce water to create oxygen
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ATP synthase used for
the production of ATP
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Carbon Fixation pathways
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2 different systems- Carboxylation and Cyclic Regeneration
Carboxylation is something thats relatively easy to do
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In carboxylation you reduce
the Carbon
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PGA is a
3C molecule
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This Complex step is required
because the regeneration of the substrate is required
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Potentiation process allows us to generate the Ribulose bisphosphate
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Then take a reducing equivalent NADPH to reduce the
second carbon to remove the phosphate and create an aldehyde
[phosphoglyceraldehyde]
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This aldehyde can be chemically modified easily without an energy input
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Can take the aldehyde
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There is an additional complexity associated with this entire process which
has to do with a serious mistake on the part of photosynthesizing organisms
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Oxygen, was an undesirable product
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Problem is that the co2
molecule is not all that different from an oxygen molecule
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The 21% Oxygen is a lot more than the
levels of CO2
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The rubisco enzyme does this
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Ribulose 1,5 bisphosphate carboxylase oxygenase
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Issue is that
if you take the ribulose 1,5 diphosphate and add an oxygen molecule instead
of a CO2; it creates 2x 3 PGA + a glycolate [2 C molecule] which causes no
net Carbohydrate formation by the photosynthetic organism
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Organisms over the past 500 years have developed ways to deal with the
oxygen problem by utilizing alternative ways of utilizing the oxygen
molecule
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Used a different carboxylation reaction which uses O2 in a different form
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If this CO2 is
dissolved in water, its present as a bicarbonate anion
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Organisms
have now taken Phosphoenol pyruvate using the enzyme [phosphoenol
pyruvate carboxylase - 3C] which can capture the carbonate anion to create
an oxaloacetate [4C]
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Mitochondrial respration- dark respiration and Photorespiration- light
respiration



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