Download photosynthesis

PHOTOSYNTHESIS
Dr. C. P. Upadhyaya
Outline
• How is solar energy captured and transformed into
metabolically useful chemical energy?
• What are the general properties of photosynthesis?
• How is solar energy captured by chlorophyll?
• What is the molecular architecture of photosynthetic
reaction centers?
• What is the quantum yield of photosynthesis?
• How does light drive the synthesis of ATP?
• How is carbon dioxide used to make organic
molecules?
• How does photorespiration limit CO2 fixation?
The Sun - Ultimate Energy
 1.5 x 1022 kJ of sunlight energy 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.
Definition of Photosynthesis
• An anabolic, endergonic, carbon dioxide (CO2) requiring
process that uses light energy (photons) and water (H2O)
to produce organic macromolecules (glucose).
Where does the Photosynthesis takes place?
 Photosynthesis occurs in thylakoid membranes of
Chloroplasts.
 This Chloroplast has paired folded structures called lamellae
that stack to form “Grana“.
 The soluble portion of the chloroplast is known as "stroma“.
 The interior of the thylakoid vesicles is the "thylakoid lumen"
 Chloroplasts possess DNA, RNA and ribosomes.
Where does photosynthesis take place?
Oxygen
(O2)
Guard Cell
Leaves
Guard Cell
Carbon Dioxide
(CO2)
A. stoma
Nucleus
Location:
1.
a.
b.
Leaves
Stoma
Mesophyll cells
Cell Wall
Chloroplast
Central Vacuole
B. Mesophyll cells
The Chloroplast
Schematic diagram of an idealized choloplast.
Electron micrograph of a
chloroplast
Photosynthesis Consists of Both Light Reactions and
Dark Reactions
 Light reactions are associated with the thylakoid membranes,
capture light energy and convert it to chemical energy in the form of
reducing potential (NADPH) and ATP with evolution of oxygen.
 Dark reactions (also called carbon reaction) occurs in the stroma, use
NADPH and ATP to drive the endergonic process of hexose sugar
formation from CO2 in a series of reactions in the stroma.
Chlorophyll Molecules
• Located in the thylakoid membranes.
• Chlorophyll have Mg+ in the center.
• Chlorophyll pigments harvest energy (photons) by absorbing
certain wavelengths (blue-420 nm and red-660 nm are most
important).
• Plants are green because the green wavelength is reflected, not
absorbed.
Chlorophyll is a photoreactive, isoprene-based pigment.
Mg2+ is coordinated in the center of the planar conjugated ring
structure.
A long chain alcohol, Phytol, group confers membrane solubility.
Aromaticity makes chlorophyll an efficient absorber of light.
Light absorption promotes an electron to a higher orbital,
enhancing the potential for transfer of this electron to a suitable
acceptor.
Chemical structure of Chlorophyll?
Since the absorption
spectra for a and b differ,
plants that possess both
can harvest a wider
spectrum of incident energy.
Accessory light-harvesting pigments increase the
possibility for absorption of light
Structures of representative
accessory light-harvesting
pigments in photosynthetic cells.
(a) β-Carotene, an accessory
light-harvesting pigment in
leaves. (b) Phycocyanobilin, a
blue pigment found in
cyanobacteria.
Question:
• During the fall, what causes the leaves to
change colors?
In addition to the chlorophyll pigments, there are other
pigments present.
During the fall, the green chlorophyll pigments are
greatly reduced revealing the other pigments.
Carotenoids are pigments that are either red or
yellow.
1. Light Reactions
The first step in photosynthesis : the light absorption
The photosynthetic unit consists of several hundred light-capturing chlorophylls
plus a pair of special chlorophylls in the reaction center.
Light is captured by one of the "antenna chlorophylls" and routed from one to the
other until it reaches the reaction center that is photochemically active.
The Reaction Center is the Place where the photoexcitation of electrons and
electron transfer Occurs.
The basis of photosynthesis is transduction of
light energy into chemical energy.
Photon absorption raises chlorophyll (Chl) to Chl*.
Electron transfer from Chl* to an adjacent
molecule A, producing oxidized Chl (Chl•+) and
reduced A (A-).
Oxidation of A- eventually culminates in reduction of
NADP+ to NADPH.
Photosynthetic Units Consist of Many Chlorophyll
Molecules but Only a Single Reaction Center
• The photosynthetic unit consists of several
hundred light-capturing chlorophylls plus a pair of
special chlorophylls in the reaction center.
• Light is captured by one of the "antenna
chlorophylls" and routed from one to the other
until it reaches the reaction center chlorophyll
dimer that is photochemically active.
• Oxidation of chlorophyll leaves a cationic free
radical, Chl•+, whose properties as an electron
acceptor are important to photosynthesis.
What Kinds of Photosystems Are Used to Capture
Light Energy?
Oxygenic phototrophs have two distinct photosystems:
PSI (P700) and PSII (P680).
• PSI systems have a maximal red light absorption at
700 nm and use ferredoxins as terminal electron
acceptors.
• PSII have a maximal red absorption at 680 nm and
use quinones as terminal electron acceptors.
• All chlorophyll is protein-bound – as part of either PSI
or PSII or light-harvesting complexes (LHCs)
PSI and PSII Participate in the Overall Process of
Photosynthesis
•
•
•
•
•
What do PSI and PSII do?
PSI provides reducing power in the form of NADPH.
PSII splits water, producing O2, and feeds the electrons
released into an electron transport chain that couples PSII
to PSI.
Electron transfer between PSII and PSI pumps protons for
chemiosmotic ATP synthesis.
Essentially, electrons flow from H2O to NADP+, driven
by light energy absorbed at the reaction centers.
Light-driven phosphorylation of ADP to make ATP is
termed photophosphorylation.
The Pathway of Photosynthetic Electron
Transfer Is Called the Z Scheme
The electron carriers are arranged as a chain,
according to their standard reduction potentials
• Such an arrangement resembles the letter “Z”
• Thus the pathway name – the Z scheme
• Its components:
• 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
The Z Scheme
The Z scheme of
photosynthesis.
(a) A diagrammatic
representation.
(b) The functional
relationships
among PSI,
PSII, the cyt bf
complex, and
the ATP
synthase in the
thylakoid
membrane.
More about Z scheme
• Electrons from PSII Are Transferred to PSI via the Cytochrome b6f
Complex.
• The cytochrome b6f complex is a large multimeric protein
possessing 26 transmembrane α-helices.
• This complex is homologous to the cytochrome c complex of
mitochondria.
• The purpose of this complex is to mediate the transfer of electrons
from PSII to PSI and to pump protons across the thylakoid
membrane.
• Plastocyanin (PC in the Z scheme) is a small copper-containing
protein that carries electrons from cytochrome b6f to PSI.
• The copper in PC cycles between the reduced Cu+ and oxidized Cu2+
states in this transfer.
How Does PSII Generate O2 From H2O?
• The oxidation of H2O to O2 is chemically difficult.
• The oxygen-evolving complex (OEC) is a large globular protein
domain on the lumenal side of the PSII.
• The active site of the OEC contains a cube-like metal cluster of four
Mn ions, one Ca+2 ion, and five O atoms bridging the Mn atoms.
• Electron removal from H2O molecules at the cluster (one from each
Mn) facilitates O2 formation.
What Is the Quantum Yield of
Photosynthesis?
• The quantum yield of photosynthesis is defined
as the amount of product formed per equivalent
of light input i.e., the amount of O2 evolved per
photon.
• Four photons per reaction center drive the
evolution of 1 O2, reduction of 2 NADP+, and the
translocation of 12 H+.
• Current estimates suggest that 3 ATPs are
formed for every 12 H+ translocated.
How Does Light Drive the Synthesis of ATP?
Photophosphorylation:
• The transduction of the electrochemical gradient
into the chemical energy of ATP is carried out by
the chloroplast ATP synthase, more properly
called the CF1CF0-ATP synthase.
• 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.
Cyclic Photophosphorylation Generates
ATP but Not NADPH or O2
• In cyclic photophosphorylation, the photo-excited electron removed
from P700 returns to P700 in a pathway indicated in picture
• Cyclic Photophosphorylation depends only on PSI, not on PSII
Primary
Electron
Acceptor
SUN
ee-
ePhotons
P700
e-
Accessory
Pigments
Photosystem I
ATP
produced
by ETC
B. Noncyclic Electron Flow
 Occurs in the thylakoid membrane, Uses PS II and PS I, P680
reaction center (PSII) - chlorophyll a, P700 reaction center (PS I) chlorophyll a, Uses Electron Transport Chain (ETC), Generates
O2, ATP and NADPH
Primary
Electron
Acceptor
Primary
Electron
Acceptor
SUN
Enzyme
Reaction
2e-
ETC
2e-
Photon
H 2O
2e-
2e-
2e- P700
NADPH
ATP
P680
Photon
1/2O2 + 2H+ Photosystem II
Photosystem I
Noncyclic Electron Flow
• ADP +
(Reduced)
P
• NADP+ + H


ATP
NADPH
(Reduced)
• Oxygen comes from the splitting of H2O, not CO2
H2O 
(Oxidized)
1/2 O2 + 2H+
Chemiosmosis
• Powers ATP synthesis.
• Located in the thylakoid membranes.
• Uses ETC and ATP synthase (enzyme) to make ATP.
• Photophosphorylation: addition of phosphate to ADP to
make ATP.
Next Lecture:
About Calvin cycle (Synthesis of glucose
using CO2 molecule)