Photosynthesis

The Sun - Ultimate Energy

• 1.5 x 1022 kJ falls on the earth each day

• 1% is absorbed by photosynthetic organisms and transformed into chemical energy

• 6CO2 + 6H2O C6H12O6 + 6O2

• 1011 tons (!) of CO2 are fixed globally per year

• Formation of sugar from CO2 and water requires energy

• Sunlight is the energy source!

Photosynthesis: Light Reactions and Carbon

Fixation• The light reactions capture light energy and

convert it to chemical energy in the form of reducing potential (NADPH) and ATP with evolution of oxygen

• During carbon fixation (dark reactions) NADPH and ATP are used to drive the endergonic process of hexose sugar formation from CO2 in a series of reactions in the stroma

Light: H2O + ADP + Pi + NADP+ + light O2 + ATP + NADPH + H+

CF: CO2 + ATP + NADPH + H+ Glucose + ADP + Pi + NADP+

Sum: CO2 + light Glucose + O2

Chloroplast• Inner and outer membrane = similar to

mitochondria, but no ETC in inner membrane.• Thylakoids = internal membrane system.

Organized into stromal and granal lammellae.• Thylakoid membrane - contains

photosynthetic ETC• Thylakoid Lumen – aqueous interior of

thylkoid. Protons are pumped into the lumen for ATP synthesis

• Stroma – “cytoplasm” of chloroplast. Contains carbon fixation machinery.

• Chloroplasts possess DNA, RNA and ribosomes

Conversion of Light Energy to Chemical Energy

• Light is absorbed by photoreceptor molecules (Chlorophylls, carotenoids)

• Light absorbed by photoreceptor molecules excite an electron from its ground state (low energy) orbit to a excited state (higher energy) orbit .

• The high energy electron can then return to the ground state releasing the energy as heat or light or be transferred to an acceptor.

• Results in (+)charged donor and (–)charged acceptor = charge separation

• Charge separation occurs at photocenters. • Conversion of light NRG to chemical NRG

Photosynthetic Pigments

Chlorophyll• Photoreactive,

isoprene-based pigment • A planar, conjugated

ring system - similar to porphyrins

• Mg in place of iron in the center

• Long chain phytol group confers membrane solubility

• Aromaticity makes chlorophyll an efficient absorber of light

• Two major forms in plants Chl A and Chl B

Accessory Pigments

• Absorb light through conjugated double bond system • Absorb light at different wavelengths than Chlorophyll• Broaden range of light absorbed

Carotenoid

Phycobilin

Absorption Spectra of Major Photosynthetic

Pigments

Harvesting of Light and Transfer of Energy to

Photosystems• Light is absorbed by

“antenna pigments” and transferred to photosystems.

• Photosystems contain special-pair chlorophyll molecules that undergo charge separation and donate e- to the photosynthetic ETC

Resonance Transfer• Energy is transfer through antenna

pigment system by resonance transfer not charge separation.

• An electron in the excited state can transfer the energy to an adjacent molecule through electromagnetic interactions.

• Acceptor and donor molecule must be separated by very small distances.

• Rate of NRG transfer decreases by a factor of n6 (n= distance betwn)

• Can only transfer energy to a donor of equal or lower energy

Photosynthetic Electron Transport and

Photophosphorylation • Analogous to respiratory ETC and oxidative

phosphorylation

• Light driven ETC generates a proton gradient which is used to provide energy for ATP production through a F1Fo type ATPase.

• The photosynthetic ETC generates proton gradient across the thylakoid membrane.

• Protons are pumped into the lumen space.

• When protons exit the lumen and re-enter the stroma, ATP is produced through the F1Fo ATPase.

Photosynthetic ETC

Eukaryotic Photosystems

• PSI (P700) and PSII (P680) • PSI and PSII contain special-pair

chlorophylls• PSI absorbs at 700 nm and PSII absorbs

at 680 nm • PSII oxidizes water (termed “photolysis") • PSI reduces NADP+ • ATP is generated by establishment of a

proton gradient as electrons flow from PSII to PSI

Z-Scheme

The Z Scheme• An arrangement of the electron

carriers as a chain according to their standard reduction potentials

• PQ = plastoquinone • PC = plastocyanin • "F"s = ferredoxins

• Ao = a special chlorophyll a

• A1 = a special PSI quinone

• Cytochrome b6/cytochrome f complex is a proton pump

P680(PSII) to PQ Pool

Excitation, Oxidation and Re-reduction of

P680• Special pair

chlorophyll in P680 (PS II) is excited by a photon

• P680* transfer energy as a e- to pheophytin A through a charge separation step.

• The oxidized P680+ is re-reduced by e- derived from the oxidation of water

Oxygen evolution by PSII

• Requires the accumulation of four oxidizing equivalents

• P680 has to be oxidized by 4 photons

• 1 e- is removed in each of four steps before H2O is oxidized to O2 + 4H+

• Results in the accumulation of 4 H+ in lumen

Electrons are passed from Pheophytin to Plastoquinone

• Plastoquinone is analagous to ubiquinone

• Lipid soluble e- carrier• Can form stable semi-

quinone intermediate• Can transfer 2

electrons on at a time.

Transfer of e- from PQH2 to Cytbf Complex (another Q-

cycle)• Electrons must be transferred one at a time to Fe-S group.

• Another Q-cycle• First PQH2 transfers one

electron to Fe-S group, a PQ- formed. 2 H+ pumped into lumen

• A second PQH2 transfers one electron to Fe-S group and the one to reduce the first PQ- to PQH2. 2 more H+ pumped into lumen

• 4 protons pumped per PQH2. Since 2 PQH2 produced per O2 evolved 8 protons pumped

Terminal Step in Photosynthetic ETC

• Electrons are transferred from the last iron sulfur complex to ferredoxin.

• Ferredoxin is a water soluble protein coenzyme

• Very powerful reducing agent.

• Ferredoxin is then used to reduce NADP+ to NADPH by ferredoxin-NADP+ oxidoreductase

• So NADP+ is terminal e- accepter

Photophosphorylation

Photophosphorylation• Light-Driven ATP Synthesis • Electron transfer through the

proteins of the Z scheme drives the generation of a proton gradient across the thylakoid membrane

• Protons pumped into the lumen of the thylakoids flow back out, driving the synthesis of ATP

• CF1-CFo ATP synthase is similar to the mitochondrial ATP synthase

Chloroplast CF1CFo ATPase

• Similar in structure to mitochondrial F1Fo ATPase

• CF1 domain (ATP synthesis) extends into the stroma.

• Many of the protein subunits are encoded by the chloroplast genome

Chloroplast Proton Motive Force (p)• What contributes more to PMF, or pH?

• In the light pH=3

• is negligible due to counter ion movement in and out of the lumen

• G for export of one mole H+ across thylakoid membrane = -17 kJ/mole

• Go’ for ATP formation = 30.5 kJ/mole

• Since 12 moles of protons gives –200 kJ of energy

• Experiment show that 3 ATPs are generated per mole of O2 produced

Energy Balance Sheet• 8 photons (4 e-) generate 1 oxygen

and 2 NADPH

• Photosynthetic ETC pumps between 8 and 12 protons across thylakoid membrane to generate proton gradient (pH ~3.5).

• Photophosphorylation produces 3 ATPs per O2 produced

Non-cyclic photosynthetic ETC

cyclic photosynthetic ETC

•NADPH and ATP produced

•Involves both PSI and PSII

•only ATP produced

•Involves only PSI

Cyclic Photosynthetic ETC• Involves only PSI

• Reduced ferredoxin transfers e- to Cytobf complex which then re-reduces Plastocyanin and finally the oxidized P700 of PSI

• No NADPH produce.Only ATP• Levels of NADP+ thought to

regulated this process. • Low NADP+ activates cyclic ETC• Observed in vitro.

Arrangement of photosystems in thylakoid

membrane

• PSII primarily present in granal lamellae

• Light harvesting antennae complexes (LHC) are also present in the granal lamellae.

• Under low light conditions LHCs are closely associated with PSII, Under high light condition the 2 disassociate.

• PSI and ATPase are in the stroma lamellae.

• Physical separation suggest that mobile electron carrier must be involved (i.e. PQ and Plastocyanin)

Arrangement of photosystems in thylakoid

membrane