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lecture-6%2017March - Plastids: 17th March 2011 Aerobic...

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Unformatted text preview: Plastids: 17th March 2011 Aerobic mitochondria dependent respiration is the reverse of Photosynthesis - - Aerobic respiration in the mitochondrion results in energy generation - requires material for breaking down Mitochondrial Respiration reduces O2 to water • releases energy Photosynthesis oxidizes water to O2 • requires energy Plants generate the energy rich compounds that can be broken down for releasing energy !"#$%&'() CO2 + H2O CO2 + H2S light (CH2O) + O2 light (CH O) + 2 2S A variety of plastids exist Early chemical equation for photosynthesis Chemical equation for light based carbohydrate production in sulfur bacteria CB van Niel, 1931 postulated similar chemical mechanism in photosynthesis CO2 + H2A 6 CO2 + 12 H2A light light (CH2O) + H 2O + 2A C6H12O6 + 6 H2O + 12 A glucose Photosynthesis is basically a process of oxidation - reduction. H2A / H2O / H2S are electron donor (reducing agent) h CO2 is an oxidizing agent: gets reduced to form hexose-sugar 6 CO2 + 12 H2O light C6H12O6 + 6 H2O + 6 O2 glucose Protist, Fungal, Animal Plant lineage Plastids exhibit range of shapes and numbers in a cell © Martin Schattat Euglena -unicellular protist: Mixotroph : heterotroph in dark; autotroph in light Spiral-ribbon like plastid in Spirogyra Star-shaped plastids in Zygnema http://www.biologie.uni-hamburg.de •Chloroplasts are motile •Amoeboid motility •Produce dynamic extensions •contains circular, double stranded DNA •prokaryotic ribosomes •Outer membrane contains porins •Inner membrane -impermeable - specific transporters required http://marineodyssey.co.uk Chloroplasts are directly involved in photosynthesis •are sites of carbon dioxide reduction and assimilation into carbohydrates, amino acids, fatty acids, and terpenoid compounds. •sites for nitrite and sulfate reduction and their assimilation into amino acids. •Biogenesis of chloroplast membranes requires a variety of lipids including polar glycerolipids. photosynthetic machinery on a system of flattened membrane sacs - thylakoids stacks of thylakoids -grana unstacked thylakoids -stroma thylakoids connect grana •Lipid biosynthetic enzymes from different subcellular compartments participate in the biogenesis of the thylakoid membrane system. This process requires the extensive exchange of lipid precursors between the chloroplast and the ER (endoplasmic reticulum). • space inside thylakoid - lumen • outside thylakoid and inner envelope is stroma • stroma contains proteins responsible for carb synthesis lipid granules Harvesting light: Thylakoid membranes have high galactose-containing glycolipids- very fluid - allows lateral diffusion of proteins •Several hundred Chl molecules act together as one PHOTOSYNTHETIC UNIT •one member (reaction centre) actually transfers electrons to acceptor molecules Photosynthesis consists of light dependent & light independent (dark) reactions Light dependent : Solar energy is absorbed & stored in ATP (cell’s primary source of chemical energy) and NADPH (main source of reducing power) The Antenna system harvests light of various wavelengths & transfers excitation energy to the reaction center Dark reactions: Carbohydrates synthesized from CO2 using energy from ATP/NADPH Chlorophyll - most important light absorbing pigment - Mg2+ containing porphyrin ring - phytol chain embeds pigment in bilayer •Chlorophylls absorb strongly in violet-blue & red •Carotenoids in green region •Chlorophyll-a is most efficient in promoting photosynthesis Granum Granum The Apicoplast •Parasites like Plasmodium and Toxoplasma (Apicomplexan genera) possess a vestigial plastid homologous to the chloroplasts. •The apicoplast is nonphotosynthetic •retains many hallmarks of its ancestry including a circular genome. •four bounding membranes •Secondary endosymbiogenesis McFadden.Protoplasma.2010 Outer envelope Outer & inner chloroplast envelope membranes contain distinct translocon complexes TOC - TIC, respectively. Inter-plastid connections Expression of an outer envelope protein (OEP7)::GFP fusion protein ? Mathur Lab 2011 Chloroplasts have 6 sub-compartments into which proteins can be delivered Protein import into Chloroplasts •outer chloroplast membrane •inner chloroplast membrane •inter-membrane space •stroma •thylakoid membrane •thylakoid lumen Nuclear encoded genes synthesized in cytosol & imported through pores in Chl envelope •Chaperones (eg. Hsp70) aid in STD-containing proteins remain in stroma after removal of STD STD-containing proteins (1a) and TTD-containing (1b) proteins unfolding polypeptides in cytosol and folding proteins inside chloroplast TTD-containing proteins (many encoded by Chl DNA and synthesized on Chl ribosomes) move into thylakoid membrane or through it into the thylakoid lumen •Proteins targeted to chloroplasts / subdomains have a removable Nterminal transit peptide sequence. - stroma targeting domain (STD) - thylakoid transfer domain (TTD) Fig.8.48 pg.320 Stromules Plastid - ER relationship Plastids extend & retract stroma-filled tubules Diameter 0.4 to 0.8 !m Extend-retract, branch, create polygonal shapes Plastids are enmeshed within a loose, dynamic arrangement of the ER * plastids are appressed against the cortical ER Mathur Lab 2011 ER-cup ER-channel • Plastids surrounded by a loose ER envelope. • Stromules appear to extend between ER-lined channels. • Plastid-ER connectivity responsible for bi-directional flow of metabolites, lipids The ER surrounding a plastid modulates stromule morphology, extension and retraction Bulked ER causes stromule retraction Peroxisomes producers as well as scavengers of toxic radicals mitochondria chloroplasts Mathur 2010 • microbodies; seedlings glyoxysomes • membrane bound ca. 1 !m vesicles crystalline core; catalase • major role in H2O2 based redox • • oxidation of very-long-chain-fatty acids peroxisomal membrane matrix • do not have own DNA / protein machinery • majority of peroxisomal proteins are imported directly from the cytosol • a few proteins brought in via the ER • biogenesis now accepted from the ER • movement along cytoskeletal filaments •involved in ROS scavenging & signalling * crystalline core peroxisomes peroxules mitochondria Peroxisomes extend tubules when cells sense ROS ‘stress’ matrixules stromules chloroplasts ʻperoxulesʼ Mathur 2010 Organelle Interactions • Peroxule extension is rapid (within seconds) and transient • Peroxule extension can be elicited by intense light & H2O2 • Peroxule extension is a normal phenomenon in plant cells Sinclair et al. 2009 ; Mathur 2009 Peroxules • extensions of peroxisomes • often directed towards chloroplasts • formed rapidly in response to oxidative stress © Mathur Lab 2010 elongated mitochondria (red) & peroxisomes (green) • Mitochondrial extensions - matrixules • induced rapidly • involve dynamin related proteins that tubulate & constrict •created transiently - extend and retract rapidly membranes ...
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This note was uploaded on 12/09/2011 for the course MCB 2210 taught by Professor R.mullen,g.vandermerwe during the Winter '09 term at University of Guelph.

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