Evolution of Photoautotrophy Ecol 182 452005

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Evolution of PhotoautotrophyEcol 182 4-5-2005
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PLANT ECOLOGY UNDERGRAD RESEARCH POSITIONS

• Mix of lab and field work
• in labs of Dr. Travis Huxman Dr. Larry Venable
• 15-20 hrs/week during semester
• Up to 40 hrs/week in summer
• Contact 621-8220 or
• gregbg_at_email.arizona.edu

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Ground rules

• Lecture notes will be posted the night before
  each lecture (182 portal link to my website)
• Figures and tables from the text MAY NOT ALWAYS
  be posted online
• Additional figures or pictures will ALWAYS be
  available
• Several questions (2-5) will be posted after each
  lecture (within 1-2 days) and study guides
  after a series of connected lectures is finished
• On email please put ECOL 182 in the subject
• I hold my office hours M 200-300, T 345-445

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Big Questions

• What have been the important constraints and / or
  principles that have shaped the evolution of
  plants.
• Diversification
• Form and function
• How do organisms interact with their environment
• Community dynamics
• Ecosystem structure and function

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Major Points for Today

• The nature of the physical environment
• Evolutionary history of photoautotrophy
• (structure and function of the photosynthetic
  apparatus)
• Modern view of photosynthesis in plants

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What is the ultimate constraint facing most
plants?

• Salient qualities of the environment
• Temperature - range, extremes
• Humidity - evaporation, precipitation
• Wind
• Soils
• Biotic influences
• Radiation - quality and quantity

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What is your favorite equation?
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What is your favorite equation?

• Interconversion of mass and energy
• E mc2
• Hydrogen - Helium
• Maintains the surface of the sun at 5800K!
• Extremely high temperature results in radiation
  of energy (as light) into space
• 1360 W m-2 (solar constant) hits the outer
  atmosphere.
• Scattering in the atmosphere
• interception (Rayleigh) and diffusion (Mie)
  results in 420 Wm-2 global average (or up to
  840 Wm-2 at equator)

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• Newton (1666) - light is made up of many things
  (prisim)
• Foucault (1850) - verification of wave theory
• Hertz (1887) - photoelectric effect
• wavelength dependent
• independent of total beam energy
• Planck (1901) - light can be particle-like
  (quanta)
• Einstein (1905) - explained photoelectric effect
• relative amount of energy in short - vs - long
  wave lengths

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The Interactions of Light and Pigments

• Discrete packets of visible light called photons.
• Photons can be absorbed by receptive molecules.
• Photons have energy which can be converted to
  perform work

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What is your favorite constant?
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My favorite constant

• Plancks constant - h - conversion of a photon to
  energy
• El h v
• El h c / lvaccum
• El - energy of a particular wavelength
• v - frequency of oscillation
• l - wavelength
• c - speed of light

How much energy is in sunlight? 260 kJ
mol-1 Average daytime photosynthetic photon flux
density 1000 mmol m-2 s-1 100 seconds result
in a mole of light compare to ATP hydrolysis
yielding 40 to 50 kJ mol-1
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• How do organisms take advantage of this free
  energy?
• Consider the evolutionary history of
  photoautotrophy
• Initial events NOT well understood
• Glycolysis had already evolved
• Photosynthetic apparatus co-opted from some other
  function (more specific on this later)

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Evolution of Photoautotrophy

• Likely evolved from chemoautotrophs
• Fossils of photosynthetic Archean bacteria ( 3.6
  billion yrs old)
• Photosynthesis is found in both prokaryotes and
  eukaryotes
• Eukaryote distribution includes algae and
  embryophytes (for our purposes this is the
  definition of a plant note this is different
  than your text!)
• Prokaryotes distribution is throughout Bacteria
  and Archea

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Phylogenetic distribution of photosynthesis

• Prokaryotes (5 of 10 clades)
• One of the most interesting - proteobacteria
• A range of other clades, including, greensulfur
  bacteria, gram positive bacteria (recall
  peptidoglycan cell walls), and filamentous green
  non-sulfur bacteria
• Cyanobacteria
• only clade with oxygenation abilities (what are
  those?)

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Figure 27.20 Extreme Halophiles - Euryarchaeota
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Figure 27.11 Cyanobacteria (Part 2)
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Biological soil crusts
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Universal Photosynthetic Structure?

• Similar form in both prokaryotes and eukaryotes
• A simple dogma of photoautotrophic organisms -
  energy acquisition, a common physiological
  paradigm for a diverse set of organisms
• 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

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Antenna / Reaction Center Design

• One exception from this general design -
  Halobacteria (Euryarchaeota - extreme saline
  environments)
• Contain retinal - protein system (as a complex
  molecular structure)
• Recall that retinal is found in the vertebrate
  eye
• Consequences?
• Photosynthesis has evolved at least TWICE!

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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

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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

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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)

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Light harvesting structures

• Photosystem I uses 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

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Stealing electrons capturing light energy
producing high energy compounds
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Endosymbiotic origins of eukaryote photosynthesis

• Coexistence of multiple photosystems when both
  can be found in isolation in nature
• Similarities between cyanobacteria and
  chloroplasts
• Multiple endosymbiotic events (not just one)

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BACTERIA
Remaining EUKARYA
Cyanobacteria
PLANTAE
Chloroplasts
Mitochondria
Proteobacteria
Chlamydiales
ARCHAEA
Spirochaeles
If mit. or chl. DNA were derived from nuclear
DNA, we would expect there would be braches here
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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!

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The CalvinBenson Cycle

• Ribulose 1,5-bisphosphate carboxylase / oxygenase
  (rubisco) catalyzes the fixation of CO2 into a
  5-carbon compound, ribulose 1,5-bisphosphate
  (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.

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The CalvinBenson 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!

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Sink regulation of photosynthesis different
concept of metabolic regulation in photosynthetic
organisms
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Figure 8.13 The Calvin-Benson Cycle
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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.

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Controls over photosynthesis

• Spatial heirarchy is important for understanding
  photosynthetic regulation
• Physicochemical constraints
• Biochemcial constraints
• Diffusive constraints
• Whole-organism constraints

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Figure 8.3 An Overview of Photosynthesis
Chloroplast
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Figure 8.1 The Ingredients for Photosynthesis
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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.

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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.

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Figure 8.15 Organelles of Photorespiration
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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.

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