Evolution of Photoautotrophy Posted on web 2 27 08 at 5 30 pm Ecol 182 2 28 2008 Summary from last time We talked about Universal Photosynthetic Structure Similar form in prokaryotes and eukaryotes The dogma of photoautotrophic organisms energy acquisition a common physiological paradigm Structure antenna reaction center design chlorophyll based light harvesting pigments Chlorophylls can absorb visible light and delocalize energy across their molecular structure heterodimeric protein core of reaction center Two distinct yet related proteins Suggests origin as monomeric structure with gene duplication and neofunctionalization leading to novel function Antenna Reaction Center Design One exception from this general design Halobacteria Euryarchaeota from extreme saline environments Contain retinal protein system as a complex molecular structure Energy yield from this conversion is quite small no electron transport chain Recall that retinal is found in the vertebrate eye Consequences Photosynthesis has evolved at least TWICE Chlorophyll based pigments Harvest light by trans cis interconversion resulting in greater energy states all oxygen evolving photosynthetic groups use chl a all other bacteria use other chl bacteriochlorophylls Biosynthetic pathway Does this present an evolutionary problem Does biosynthesis recapitulate phylogeny Evolutionary solutions 5 aminolevulinic acid protochlorophyllidae chlorophyll c chlorophyllide a chlorophyll a chlorophyll b bacteria chlorophylls Dimeric protein complex reaction center Converts that energy to a usable form Types 1 iron sulfur clusters 2 pheophytin and quinones From a variety of groups but in cyanobacteria and eukaryotes they coexist Coexist as Photosystem I 1 above and Photosystem II 2 Light harvesting structures Photosystem I reduces NADP to NADPH H Photosystem II uses light energy to oxidize water molecules producing electrons protons and O2 Both of these are stand alone energy systems but combined they can maintain energy flow through a system Evidence for the endosymbiotic origin of eukaryote photosynthesis Coexistence of multiple photosystems when both can be found in isolation in nature Similarities between cyanobacteria and chloroplasts Suggests multiple endosymbiotic events not just one Remaining EUKARYA PLANTAE ARCHAEA Proteobacteria Mitochondria Chloroplasts Cyanobacteria Chlamydiales Spirochaeles BACTERIA If mit or chl DNA were derived from nuclear DNA we would expect there would be braches here Regulation of Photosynthesis where does the ATP and NADPH following light harvesting The Calvin cycle Carboxylation enzymatic Reducing energy dependent Regenerating energy dependent Turns out there is plenty of light energy most of the time what regulates photosynthetic rate is carboxylation The Calvin Benson Cycle Ribulose 1 5 bisphosphate carboxylase oxygenase rubisco catalyzes the fixation of CO2 into a 5 carbon compound ribulose 1 5bisphosphate RuBP An intermediate 6 carbon compound forms which is unstable and breaks down to form two 3 carbon molecules of 3PG see fig 8 14 Rubisco is the most abundant protein in the world The Calvin Benson Cycle Consists of three or four processes Fixation of CO2 to RuBP catalyzed by rubisco Reducing to G3P uses ATP and NADPH Regeneration RuBP uses ATP Transport by inorganic phosphate Sink regulation of photosynthesis different concept of metabolic regulation in photosynthetic organisms Figure 8 13 The Calvin Benson Cycle Making Carbohydrate from CO2 Products of photosynthesis are critical for energy on Earth Most photosynthetically acquired energy is released by glycolysis and cellular respiration of photoautotrophs Some of the carbon incorporates into amino acids lipids and nucleic acids Some of the stored energy is consumed by heterotrophs where glycolysis and respiration release the stored energy Controls over photosynthesis Spatial heirarchy is important for understanding photosynthetic regulation Physicochemical constraints Biochemcial constraints Diffusive constraints Whole organism constraints Figure 8 3 An Overview of Photosynthesis Stroma Thylakoids Light photon Chloroplast Chlorophyll Light reactions CO2 fixation reactions Thylakoid Figure 8 1 The Ingredients for Photosynthesis Factors shaping constraints over photosynthetic rates a series of heirarchical controls with multiple trade offs Relative allocation of nitrogen to carboxylation versus light harvesting Field and Mooney 1986 Farqhuar et al 1980 Trade off between exchanges of water and carbon Cowan 1977 Cowan and Farqhuar 1977 Leaf life span returns on C N investment reproductive timing and output Bloom 1986 Bazzaz 1996 Other issues Photorespiration Rubisco is a carboxylase adding CO2 to RuBP It can also be an oxygenase adding O2 to RuBP These two reactions compete with each other When RuBP reacts with O2 it cannot react with CO2 which reduces the rate of CO2 fixation Photorespiration and Its Consequences Photorespiration RuBP O2 phosphoglycolate 3PG Glycolate diffuses into organelles called peroxisomes Peroxisomes convert glycolate to glycine Glycine diffuses into mitochondria and is converted to glycerate and CO2 Figure 8 15 Organelles of Photorespiration Photorespiration and Its Consequences Photorespiration uses the ATP and NADPH produced in light reactions CO2 is released rather than fixed Rubisco acts as an oxygenase if CO2 is very low and O2 is high O2 becomes high when stomata close preventing plant water loss Big Questions What have been the important constraints and or principles that have shaped the evolution of plants Diversification Form and function Important particularities on evolution and speciation in plants R A Fisher 1958 Fundamental Theorem of Natural Selection Rate of increase in the mean fitness of a population is proportional to the genetic variance in fitness In order for there to be evolution there must be genetic variation Major ways genetic variation is introduced into populations 1 Mutation UV random error 2 Genetic recombination meiosis including crossing over 3 Immigration into population But plants do two additional tricks that enhance genetic variation 4 Polyploidy an organism that has more than one complete set of the normal chromosome compliment most animals are diploids many plants are polyploids occurs through processes such as chromosome duplication 5 Hybridization crossing of closely related taxa usually between species within a genus Multicellularity and plant evolution Multicellularity