Presenter: Angelie Claire G. Manliguez
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Stephen Hales
Joseph Priestly
Jan Ingen-Housz
Father of Plant Physiology
Experiment and Observation
with Different kinds of Air
500 experiement on
Purification of Air
The first stage of photosynthesis is to capture of light energy
from the sun.
Green plants
Green Algae
Cyanobacteria
Calvin cycle
➢ STROMA
LIGHT REACTION
➢ THYLAKOID
Conversion of light
energy to chemical
energy
Reaction that
synthesis glucose
⊷ Epidermis (upper and lower)
⊷ Palisade cells – elongated,
⊷
A
B
cylindrical cells.
Spongy mesophyll- irregular
shape, or somewhat
isodiametric because of
prominent air spaces
between cells.
Monocotyledon leaf is lacks the
distinction between palisade and
spongy mesophyll (A,B)
The structure of leaves shown in cross-section. (A,C) A dicotyledonous leaf Acer sp. (B,D) A
monocotyledonous leaf (Zea mays), showing a section between two major veins. (A,B: From
T. E. Weier et al., Botany, 6th ed. New York, Wiley, 1982. Used by permission of authors.)
SIEVE EFFECT
Multiple layers of
photosynthetic cells is
one way of increasing the
probability that photons
passing through the first
layer of cells will be
intercepted by successive
layers
(A) Photon strikes a chloroplast and is absorbed by chlorophyll. (B) The sieve effect—a photon passes through the first layer of
mesophyll cells without being absorbed. It may be absorbed in the next layer of cells or pass through the leaf to be absorbed by
another leaf below. (C) The planoconvex nature of epidermal cells creates a lens effect, redirecting incoming light to chloroplasts
along the lateral walls of the palisade cells. (D) The light-guide effect. Because the refractive index of cells is greater than that of air,
light reflected at the cell–air interfaces may be channeled through the palisade layer(s) to the spongy mesophyll below.
PHOTOSYNTHESIS IS AN OXIDATION–REDUCTION PROCESS
Carbon dioxide + water  sugar + oxygen + water
C.B. VAN NIEL, 1920
Formulate
photosynthesis
equation
Oxygenic (i.e., oxygen-evolving) photosynthesis in green plants;
Anoxygenic (i.e., non-oxygen-evolving) photosynthesis in the sulfur bacteria
Ferricyanide
 Hill’s experiments confirmed the redox nature of
green plant photosynthesis and added further support
for the argument that water was the source of evolved
oxygen.
 Using either CO2 or H2O labeled with O, a heavy
isotope of oxygen, the label was only supplied with
water (H2O) and was not supplied as carbon dioxide in
the developed oxygen. If evolved O2 comes from
water, two water molecules must be involved in
reducing each CO2 molecule.
PSII and PS I are named for the order
of which they were discovered, not for
the order in which they participate in
photosynthesis
P700- primary
electron donor;
P680 - strongest
oxidizing agent
Photosystem I
Photosystem II
1. PHOTOSYSTEMS ARE MAJOR COMPONENTS OF THE
PHOTOSYNTHETIC ELECTRON TRANSPORT CHAIN
The core antenna for
photosystem II consists of two
chlorophyll-proteins (CP)
known as CP43 and CP47.
These two CP complexes each
contain 20 to 25 molecules of
chlorophyll a. The core
antenna chlorophyll a absorb
light but do not participate
directly in photochemical
reactions.
Photoexcitation is the first step in photosynthesis
The energy electrons are passed from
the reaction center of photosystem II
to an electron transport
When light energy is absorbed by
pigments and passed inward to the
reaction center, the electron is
boosted to a very high energy level
and transferred to an acceptor
molecule. The special pair’s missing
electron is replaced by a new
electron from PS II (arriving via
electron transport chain.
⊷ The light harvesting complexes enhance photosynthetic efficiency under low
light, that is, under conditions where light limits photosynthesis.
⊷ Schematic of the photosynthetic electron transport chain depicting the
arrangement of PSI, PSII, and the cytochrome b6/f complex in the thylakoid
membrane is presented in. A fourth complex—the CF0-CF1 coupling factor or
ATP synthase—is also shown.
⊷ On a slower time scale of
Reaction proteins D1 and D2 play their
role in binding and orienting the PSII
reaction center's specific redox carrier
in order to lower the probability of
charge recombination between P680+ and
Pheo−.
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microseconds, the electron is
passed from QA to
plastoquinone(PQ), resulting in
the formation of [P680+ Pheo
QA]. PQ is a quinone that binds
transiently to a binding site (QB)
that is on the stromal side of the
D1 reaction center protein. The
reduction of PQ to plastoquinol
(PQH2) decreases its affinity for
the binding site on the D1
polypeptide. The (PQH2) is thus
released from the reaction
center, to be replaced by another
molecule of PQ.
⊷ PQH2 diffuses from the QB site and becomes part of the PQ pool present in the
thylakoid membrane. Since PQ requires two electrons to become fully reduced to
PQH2, reduction at the QB site is considered a two-electron gate.
⊷ The photon flux to which a leaf can be exposed may
reach 2000 μmol photons m−² s− ¹. This means that
about 10¹⁹ charge separations per second may occur over
a leaf surface area of 1 cm ².
PSII and the OEC
exhibit oxygenic
photosynthesis,
that is, a
photosynthetic
process that
generates
molecular oxygen
(O2)
2H2O
4Hᶧ
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O2
⊷ The electrons that reduce P680+ are most immediately
supplied by a cluster of four manganese ions associated
with a small complex of proteins called the oxygenevolving complex (OEC).
The oxidation of two moles of water generates one mole
of oxygen, four moles of protons, and four moles of
electrons. It has been determined that only one PSII
reaction center and OEC is involved in the release of a
single oxygen molecule.
Photosynthetic Bacteria as anoxygenic
⊷ They do not contain chloroplast
⊷ The bacterial reaction center contains the bacteriochlorophyll a, P870, rather than
P680. Excitation of P870 does not generate a sufficiently positive redox potential to
oxidize water.
⊷ Thus, the evolution of PSII and its associated OEC was a major factor determining
the global distribution of oxygenic photosynthetic organisms that fundamentally
changed the development of all life on Earth
3. THE CYTOCHROME COMPLEX AND
PHOTOSYSTEM I OXIDIZE PLASTOQUINOL
⊷ The cytochrome complex also contains an additional redox component called the
Rieske iron-sulfur (FeS) protein—iron-binding proteins in which the iron complexes
with sulfur residues rather than a heme group as in the case of the cytochromes.
⊷ Plastocyanin (PC) – is a small
peripheral protein that is able to diffuse
freely along the lumenal surface of the
thylakoid membrane.
⊷ Ferredoxin is another FeS-protein that
is soluble in the stroma. Ferredoxin in
turn is used to reduce NADP+, a
reaction mediated by the enzyme
ferredoxin-NADP+-oxidoreductase.
PC PS I
The reaction center
chlorophyll P700
become
photooxidized to
P700+ and gives its
electron to a
molecule of
chlorophyll a
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Ferredoxin  NADP
Fd-NADP+ reductase
then uses ferredoxin
to reduce NADP+.
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⊷ The quantum requirement for oxygen
evolution is defined as the number
photons required to evolve one
molecule of O2.
⊷ The evolution of one molecule of O2 by
P680+ generates four electrons that are
eventually transferred through PSI to
reduce 2NADP+ to 2NADPH. Thus, to
transfer four electrons from H2O to
NADP+ requires 8 photons. Therefore
the minimal theoretical quantum
requirement for O2 evolution is 8
photons/molecule of O2 evolved.
⊷ Light-driven production of ATP by chloroplasts is
known as photophosphorylation.
⊷ Formation of ATP in association with noncyclic
electron transport is known as noncyclic
photophosphorylation.
⊷ Noncyclic photophosphorylation results in
production of ATP and NADPH, whereas cyclic
photophosphorylation does not generate
NADPH
⊷ A key to energy conservation in photosynthetic electron transport and the
accompanying production of ATP is the light-driven accumulation of protons
in the lumen. There are two principal mechanisms that account for this
accumulation of protons: the oxidation of water, in which two protons are
deposited into the lumen for each water molecule oxidized, and a PQcytochrome proton pump. The energy of the resulting proton gradient is then
used to drive ATP synthesis in accordance with Mitchell’s chemiosmotic
hypothesis.
⊷ Thus, for each pair of electrons passing from plastoquinone through the
Rieske FeS-center and cytochrome f to plastocyanin, four protons are
translocated from the stroma into the lumen of the thylakoid.
⊷ a pair of electrons passing through noncyclic electron transport would be
expected to yield two ATP molecules for every NADPH produced
(2ATP/NADPH).
Comparison of cyclic and non-cyclic
photophosphorylation
Cyclic
Photophosphorylation
Non-Cyclic
Photophosphorylation
Pathway of ѐ
Cyclic
Non-cyclic/ Linear
First ѐ donor
PS I
Water
Last ѐ acceptor
PS I
NADP+
Products
ATP only
ATP, NADPH, O2
Numbers of Photosystem
involved
PSI Only
PS I & PS II
LATERAL HETEROGENEITY IS THE UNEQUAL
DISTRIBUTION OF THYLAKOID COMPLEXES
⊷ Refers to the uneven
distribution of PSI and PSII
and ATP synthesis complex
on thylakoid membranes.
⊷ The PSI/LHCI complexes and the CF0-CF1 ATPase are located
exclusively in nonappressed regions of the thylakoid; that is, those
regions where the membranes are not paired to form grana.
PSII/LHCII complex are located in appressed regions.
⊷ Three mobile carriers : PQ, PC, Fd
 Plastoquinone is a hydrophobic molecule and is
consequently free to diffuse laterally within the lipid
matrix of the thylakoid membrane.
 Plastocyanin is a small (10.5 kDa) peripheral
copper-protein found on the lumenal side of
the membrane.
•
PS II/ LCHII
PS I/ LCHI
Ferredoxin is a small (9 kDa) iron-sulfur protein, is found on the
stroma side of the membrane. It receives electrons from PSI and,
with the assistance of the ferredoxin-NADP oxidoreductase, reduces
NADP+ to NADPH.
⊷ A major light harvesting
Plectonema boryanum
pigment-protein complex
found in chloroplast
thylakoid membranes of
plants and green algae,
the light harvesting
complex of cyanobacteria
is an extrinsic pigmentprotein complex called a
phycobilisome which is
bound to the outer,
cytoplasmic surface of
cyanobacterial thylakoids.
⊷ Phycobilisomes (PBSs) are rod-shaped
chromoproteins called phycobiliproteins
which may constitute up to 40 percent of
the total cellular protein.
⊷ The phycobiliproteins usually associated
with PBS include allophycocyanin (AP),
phycocyanin (PC), and phycoerythrin (PE).
⊷ In addition to PBS, PSII of cyanobacteria
include the Chlorophyll a core antenna
CP47 and CP43 similar to that found in
eukaryotic organisms.
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⊷ Two major classes of such herbicides are derivatives of urea,
such as monuron and diuron, and the triazine herbicides,
triazine and simazine.
⊷ There they bind to the QB binding site of the D1 protein in PSII
(also known as the herbicide-binding protein).
⊷ The herbicide interferes with the binding of plastoquinone to
the same site and thus blocks the transfer of electrons to
plastoquinone. Because of its action in blocking electron
transport at this point, DCMU is commonly used in laboratory
experiments where the investigator wishes to block electron
transport between PSII and PSI.
⊷ The triazine herbicides are used extensively to control weeds in
cornfields, since corn roots contain an enzyme that degrades
the herbicide to an inactive form.
⊷ The availability of herbicide-resistant
genes together with recombinant DNA
technology has stimulated considerable
interest in the prospects for developing
additional herbicide-resistant crop plants.
⊷ Another class of herbicides are the
bipyridylium viologen dyes—paraquat
which act by intercepting electrons on
the reducing side of PSI.
The overuse of herbicides promote
herbicide tolerance in weeds, which
exacerbates the weed problem in
the long term.
Note. Viologen herbicides are also highly
toxic to animals, their use is banned or
tightly regulated in many jurisdictions.
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