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Biochemistry. Lecture 15. Photosynthesis. Chemiosmotic Gradient. Endosymbiotic Theory. Photosynthesis. Assimilation of CO 2 by Plants. CO 2 Assimilation Occurs in Plastids. Organelles found in plants and algae Enclosed by a double membrane Have their own small genome

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Biochemistry

Biochemistry

Lecture 15






Assimilation of CO2 by Plants


Co 2 assimilation occurs in plastids
CO2 Assimilation Occurs in Plastids

Organelles found in plants and algae

Enclosed by a double membrane

Have their own small genome

The inner membrane is impermeable to ions such as H+, and to polar and charged molecules


Origin and differentiation of plastids
Origin and Differentiation of Plastids

Plastids were acquired during evolution by early eukaryotes via endosymbiosis of photosynthetic cyanobacteria

Plastids reproduce asexually via binary fission

The undifferentiated protoplastids in plants can differentiate into several types, each with a distinct function

Chloroplasts for photosynthesis

Amyloplasts for starch storage

Chromoplasts for pigment storage

Elaioplasts for lipid storage

Proteinoplasts for protein storage








Fates of g3p
Fates of G3P Site Lysine


Remaking r1 5bp
Remaking R1,5BP Site Lysine


Photorespiration
Photorespiration Site Lysine

So far, we saw that plants oxidize water to O2 and reduce CO2 to carbohydrates during the photosynthesis

Plants also have mitochondria where usual respiration with consumption of O2 occurs in the dark

In addition, a wasteful side reaction catalyzed by Rubisco occurs in mitochondria

This reaction consumes oxygen and is called photorespiration; unlike mitochondrial respiration, this process does not yield energy


Oxygenase activity of rubisco
Oxygenase Activity of Rubisco Site Lysine

The reactive nucleophile in the Rubisco reaction is the electron-rich enediol form of ribulose 1,5-bisphosphate

The active site meant for CO2also accommodates O2

Mg++ also stabilizes the hydroperoxy anion that forms by electron transfer from the enediol to oxygen


Salvage of 2 phosphoglycerate
Salvage of 2-Phosphoglycerate Site Lysine

Complex ATP-consuming process for the recovery of C2 fragments from the photorespiration

Requires oxidation of glycolate with molecular oxygen in peroxisomes, and formation of H2O2

Involves a loss of a carbon as CO2 by mitochondrial decarboxylation of glycine


Rubisco in c 3 plants cannot avoid oxygen
Rubisco in C Site Lysine3 Plants Cannot Avoid Oxygen

Plants that assimilate dissolved CO2 in themesophyll of the leaf into three-carbon 3-phosphoglycerate are called the C3 plants

Our atmosphere contains about 21% of oxygen and 0.038% of carbon dioxide

The dissolved concentrations in pure water are about 260 M O2 and 11 M CO2(at the equilibrium and room temperature)

The Km of Rubisco for oxygen is about 350 M


Separation of co 2 capture and the rubisco reaction in c 4 plants
Separation of CO Site Lysine2 capture and the Rubisco Reaction in C4 Plants

Many tropical plants avoid wasteful photorespiration by a physical separation of CO2 capture and Rubisco activity

CO2 is captured into oxaloacetate (C4) in mesophyll cells

CO2 is transported to bundle-sheath cells where Rubisco is located

The local concentration of CO2 in bundle-sheath cells is much higher than the concentration of O2


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