Biochemistry

Biochemistry

Lecture 15

Photosynthesis

Chemiosmotic Gradient

Endosymbiotic Theory

Photosynthesis

Assimilation of CO2 by Plants

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

• 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

Photosynthesis

Review: Light Reactions Generate ATP and NADPH

The Calvin Cycle

Rubisco (ribulose 1,5 bisphosphate carboxylase /

oxygenase )

The Calvin Cycle

Rubisco is Activated via Covalent Modification of the

Active Site Lysine

Fates of G3P

Remaking R1,5BP

Photorespiration• 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

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

• The active site meant for CO2 also

accommodates O2

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

Salvage of 2-Phosphoglycerate

• 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 C3 Plants

Cannot Avoid Oxygen

• Plants that assimilate dissolved CO2 in the

mesophyll 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 CO2 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