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3/5/03 Photosynthesis

animation of electron transport in thylakoid membranehttp://instruct1.cit.cornell.edu/Courses/biomi290/MOVIES/OXYGENIC.HTML

• The vast majority of the energy consumed by living organisms stems from solarenergy captured by the process of photosynthesis

• Only chemoautotrophs (aka chemolithotrophs) are independent of this energysource

Of the sunlight that reaches the earth each day:• 1% is absorbed by photosynthetic organisms and transduced into chemical

energy• of the remaining 99%: 2/3 is absorbed by the earth and oceans (heating the

planet) and 1/3 is lost as light reflected back into space

Often referred to as carbon dioxide fixation:

light energy6 CO2 + 6 H2O -------------> C6H12O6 + 6 O2

• The fixation of carbon is an endergonic (endothermic) reaction• Estimates indicate that 1011 tons of carbon dioxide are fixed globally per year• 1/3 is fixed in the oceans, primarily by photosynthetic marine microorganisms

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• cyanobacteria• other photosynthetic bacteria• algae• plants

Two stages of photosynthesis in a green plant or cyanobacteriaNADPH = NADH plus an extra phosphate

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Respiration as a series of energy transformations:• electrons released from carbohydrates(and other foodstuffs) are

transferred by a circuitous route to O2

• O2 is reduced to H2O• the free energy released as electrons flow from a high energy to a

low energy state is transformed into a proton gradient• the potential energy in this proton gradient is then converted into

chemical energy in the form of ATP

Aspects of photosynthesis can be described in similar terms:• Photosynthesis also involves a series of oxidation-reduction events• Flow of electrons is from water to CO2, reducing it to a carbohydrate• light provides the energy required to move electrons from a lower to

a higher free energy• energy interconversions carried out by chemiosmotic mechanisms

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Whether photosynthesis occurs in a cyanobacteria or a giant sequoia, somegeneralitites apply:• photosynthesis is associated with membranes• eukaryotic cells, photosynthesis occurs in the chloroplast• in prokaryotes, photosynthesis is associated with the plasma membrane

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Thylakoids• flattened membrane-bound sacs inside the chloroplast• location of chlorophyll

Chloroplasts possess three membrane bound aqueous compartments• the intermembrane space• the stroma -- inside the inner membrane and outside the thylakoid sacs (like the

mt matrix)• thylakoid space -- inside the thylkoid sacs

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Comparison of membrane components of the chloroplast and mt

Similar:• electron transport chains: entire set of proteins and small molecules involved in

the orderly sequence of electron transfers• ATP synthase• inner membrane very impermeable and narrow inter-membrane space

Different• photosystems: site where light energy is captured and harnessed to drive the

transfer of electrons• in chloroplasts the electron transport chains and ATP synthase are located in the

thylakoid membrane, not the inner membrane

The many reactions that occur during photosynthesis can begrouped into two broad categories:

Photosynthetic electron-transfer reactions• AKA light reactions• energy derived from sunlight “energizes” an electron in chlorophyll• chlorophyll obtains its electrons from water (generating O2)• the energized electrons move along an electron transport chain in the thylakoid

membrane (analogous to mt electron transport)• an H+ gradient is generated and drives the production of ATP• NADP+ is reduced to NADH

Carbon Fixation Reactions• also called Calvin cycle or dark reactions• the ATP and NADPH produced by the light reactions serve as the source of

energy and reducing power to drive the conversion of CO2 to carbohydrate• reactions take begin in the stroma of the chloroplast and end in the cytosol

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Dark Reactions:Oxidation of glucose to CO2:The formation of ATP from ADP and Pi is an endothermic process that is coupledto the exothermic oxidation of glucose

The reduction of NAD+ to NADH is an endothermic process that is coupled to theexothermic oxidation of glucose

Synthesis of glucose from CO2

The fixation and reduction of CO2 is endothermic and coupled to these exothermicreactions:

• Hydrolysis of ATP to ADP and Pi

• Oxidation of NADH to NAD+

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• Change in the redox potential during photosynthesis. The redox potentialfor each molecule is indicated by its position along the vertical axis.Transferring electron “up the scale” from H2O requires energy

• The net electron flow through the two photosystems is from water to NADP+

to form NADPH.• In the process of electron transfer a H+ gradient is generated across the

thylakoid membrane. ATP is synthesized by membrane bound ATPsynthase -- same process as in the mitochondria

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Maximal absorption by chlorophylls between 400-500 and 600-700 nm

atenna complex on leftPhotosystems consist of two closely linked components:

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Photochemical reaction center:• complex of proteins and chlorophyll (transmembrane protein-pigment complex)• converts light energy to chemical energy

Antenna complex:• pigment molecules that capture light energy and feed it to the reaction center• cluster of several hundred chlorophyll molecules and accessory pigments

(collect light of other wavelengths)

Reaction center:• acts as a trap for quanta of energy captured by the pigment molecules in the

antennae• excitation of the reaction center begins the actual light reactions

Reaction center chlorophylls are in a different environment from antennaechlorophylls and asborb energy at a slightly lower level

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Two different photosystems:

Photosystem I:

• chlorophyll molecules of the reaction center are a form of chlorophyll a, knownas P700

• the P stands for pigment and the 700 designates the optimal absorptionmaximum in nanometers (wavelength = 700 nm)

Photosystem II:• chlorophyll molecules of the reaction center are a form of chlorophyll a, known

as P680 (for its optimal absorption)

When a chlorophyll molecule in the antenna complex is excited, energy is rapidlytransferred from one molecule to the next until it reaches chlorophyll molecules inthe reaction center

These chlorophyll molecules transfer the excited electrons to the electron transportchain