Chapter 15 (part1)
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, isoprenebased 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
Carotenoid
Phycobilin
• Absorb light through conjugated double bond system
• Absorb light at different wavelengths than Chlorophyll
• Broaden range of light absorbed
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 rereduced 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 semiquinone 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 PQformed. 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