Biochemistry Lecture 15 Photosynthesis Chemiosmotic Gradient Endosymbiotic Theory

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 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 G 3 P

Remaking R 1, 5 BP

Photorespiration • So far, we saw that plants oxidize water to O 2 and reduce CO 2 to carbohydrates during the photosynthesis • Plants also have mitochondria where usual respiration with consumption of O 2 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 CO 2 also accommodates O 2 • 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 C 2 fragments from the photorespiration • Requires oxidation of glycolate with molecular oxygen in peroxisomes, and formation of H 2 O 2 • Involves a loss of a carbon as CO 2 by mitochondrial decarboxylation of glycine

Rubisco in C 3 Plants Cannot Avoid Oxygen • Plants that assimilate dissolved CO 2 in the mesophyll of the leaf into three-carbon 3 phosphoglycerate are called the C 3 plants • Our atmosphere contains about 21% of oxygen and 0. 038% of carbon dioxide • The dissolved concentrations in pure water are about 260 M O 2 and 11 M CO 2 (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 • Many tropical plants avoid wasteful photorespiration by a physical separation of CO 2 capture and Rubisco activity • CO 2 is captured into oxaloacetate (C 4) in mesophyll cells • CO 2 is transported to bundle-sheath cells where Rubisco is located • The local concentration of CO 2 in bundle-sheath cells is much higher than the concentration of O 2