Evolution of PhotoautotrophyEcol 182 – 4-5-2005

PLANT ECOLOGYUNDERGRAD RESEARCH POSITIONS

Mix of lab and field work in labs of Dr. Travis Huxman & Dr. Larry Venable

15-20 hrs/week during semesterUp to 40 hrs/week in summer

Contact: 621-8220 orgregbg@email.arizona.edu

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 2:00-3:00, T 3:45-4:45

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

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

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

What is your favorite equation?

What is your favorite equation?

• Interconversion of mass and energyE = 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)

• 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

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

What is your favorite constant?

My favorite constantPlanck’s constant - h -

conversion of a photon to energy

E = h v

E = h c / vaccum

E - 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 mol m-2 s-1

100 seconds result in a mole of light – compare to ATP hydrolysis yielding

~ 40 to 50 kJ mol-1

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)

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

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?)

Figure 27.20 Extreme Halophiles - Euryarchaeota

Figure 27.11 Cyanobacteria (Part 2)

Biological soil crusts

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

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!

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 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

Stealing electrons capturing light energy producing high energy compounds

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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If mit. or chl. DNA were derived from nuclear DNA, we would expect there would be braches here

BACTERIA

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 1,5-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.

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

Stroma

Thylakoids

Thylakoid

Chloroplast

Lightreactions

CO2 fixationreactions

Light(photon)

Chlorophyll

Figure 8.3 An Overview of Photosynthesis

Figure 8.1 The Ingredients for Photosynthesis

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.