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Lecture 8

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University of Toronto Scarborough
Biological Sciences
Stephen Reid

Lecture 8: Chloroplasts: •First organisms were heterotrophs –Needed nutrients from environment -Autotrophs: manufacture organic nutrients from inorganic molecules (CO2) •About 2.7 million years ago, cyanobacteria used electrons from water to produce oxygen as a waste product •Synthesis of complex molecules from CO2 requires large input of energy –Chemoautotrophs: use energy from inorganic molecules. -Photoautotrophs: use radiant energy to make organic compounds •Photosynthesis: converts energy from sunlight into chemical energy stored in carbohydrates. –Low energy electrons are removed from a donor molecule : CO2+ H2O – +light ->(CH2) + O2 Chloroplast Structure and Function •Photosynthesis in eukaryotes takes place in the chloroplast, a cytoplasmicorganelle •Arose from phagocytosis of photosynthetic cyanobacteria •Uses energy to convert carbon dioxide and water into glucose –Plants, eukaryotic algae, some protists and several prokaryotes •Chloroplasts have a double membrane. –The outer membrane contains porins and is permeable to large molecules. –The inner membrane contains light-absorbing pigment, electron carriers, and ATP-synthesizing enzymes. •Outer envelope membrane contains porin proteins •Inner envelope membrane is highly impermeable: requires transporters -Thylakoid: arranged into grana (stacks of thylakoids) -Stroma lamellae: attach grana -Stroma: inside fluid Thylakoids–Internal membrane system •The inner membrane of a chloroplast is folded into flattened sacs (thylakoids), arranged in stacks called grana •Thylakoid membranes contain a large percentage of glycolipids, which make the membrane highly fluid for diffusion of proteins complexes. Membrane has 75:25 protein : lipid ration •Thylakoid sacs have lumen inside •Stroma lamellae –flattened membrane structures that connect thylakoidsfrom different grana •Stroma –Contains DNA and ribosomes, tRNA –Chloroplast DNA encodes for 100 genes, but most chloroplast proteins encoded by nuclear DNA –Chloroplast proteins must be imported •Proteins on the thylakoid membrane: encoded by chloroplast genes. Ribosomes assemble on the thylakoid membrane like RER Chloroplasts -Function •Photosynthesis is a redox reaction; transferring an electron from water to carbon dioxide -6CO +212H O 2 C H O 6 12 O6+ 6O 2 2 •Experiments using radioactive O showed that O2molecules released during photosynthesis came from H2O not from CO2 •Photosynthesis oxidizes water to oxygen; respiration reduces oxygen to form water –Respiration removes electrons from reduced organic substrates to form ATP and NADH. –Photosynthesis uses electrons to form ATP and NADPH, which are then used to reduce CO2 to carbohydrate Photosynthesis •Photosynthesis occurs in two stages: –Light-dependent reactions (light reactions)in which sunlight is absorbed, converting it into ATP and NADPH.: Thylakoid membranes –Light-independent reactions (dark reactions) use the energy stored in ATP and NADPH to produce carbohydrate: stroma 1) Light-dependent reactions •Absorption of photons (light “particles”) by a molecule makes them go from ground to excited state –Energy in photon depends on wavelength –Energy required to shift electrons varies for different molecules •Photosynthetic pigments are molecules that absorb light of particular wavelengths •Chlorophyll –photosynthetic pigment in thylakoid membrane: harvests light •300 chlorophyll molecules are arranged into photosynthetic units (PSI and PSII) -also contains many proteins similar to proteins of the ETC •Chlorophyll absorbs light and electrons are excited to a higher orbital –If electron dropped down to original orbital we’d have fluorescence –This doesn’t happen, instead: electrons passed to another acceptor pigment molecule and or proteins in ETC Photosynthetic ETC 1) Light-dependent reactions PSI: •Chlorophyll captures a photon of light •Electron excited and passed to acceptor molecules –NADP+ and H+ make NADPH (becomes reduced) -a chlorophyll molecule is still missing an electron PSII •Chlorophyll in its reaction centre captures a second photon of light •Electrons passed to several acceptors –Similar to ETC of mitochondria –Electron energy is used to pump H+ into thylakoid lumen –Electron moves into PSI to replace missing electron -now chlorophyll in PSII is missing an electron •Electron in chlorophyll replaced by the photolysis of H2O •PSII has proteins that split water –Into protons, electrons and oxygen: electrons moves into PSII to replace missing e –Protons stay in thylakoid lumen: oxygen is liberated Flow of electrons from H2O to NADPH 2 H2O + 2 NADP+  O2+ 2 NADPH  Establishes a proton gradient (H+) •Protons accumulate in thylakoid lumen •Develops an electrochemical gradient: high H+=acidic pH 5 = Thylakoid lumen •Gradient energy will be used to make ATP: photophosphorylation Chloroplast ATP Synthase •Multi-protein complex (8 polypeptides) •2 major complexes a)CF0 -Embedded in thylakoid membrane, forms a channel b)CF1 -Spherical head group facing stroma, where ATP synthesis occurs •Like in mitochondria, H+ moves through channel down electrochemical gradient •Movement of H+ causes conformational change in chloroplast ATP synthase –Causes CF0and CF1to rotate relative to one another •Rotation drives reaction 1) Light-dependent reactions •Summary: Light energy is absorbed and converted to chemical energy –Makes ATP and NADPH -uses water and creates oxygen 2) Light-independent reactions •Chemical energy is used to convert CO2into carbohydrates •Melvin Calvin, Andrew Benson and James Bassham studied process of carbon dioxide fixation and synthesis
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