Download Lec 8: Photosynthesis: Light reaction

Lec 8: Photosynthesis: Light reaction
Reference material
Biochemistry 4th edition, Mathews, Van Holde, Appling, Anthony‐Cahill. Pearson ISBN:978‐0‐13‐800464‐4
Lehninger Principles of Biochemistry 4th edition, David L. Nelson, Michael M. Cox. W. H. Freeman ISBN:978‐0716743392
Photosynthesis:
‧Provides carbohydrates for energy production
‧Fixes CO2
‧The major source of atmospheric O2
The Reverse of Respiration
Previously, we talked about how to get energy from sugars (which is organic carbon)… Now, we need to think about how to get carbon, energy, and electrons, from non‐organic carbon sources.
As you learn more about metabolism, try to answer how an organism receive its source of 1. Carbon
2. Energy
3. Electrons
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The carbon cycle in nature: ‧Carbon dioxide and water are combined through photosynthesis to form
carbohydrates. ‧In both photosynthetic and nonphotosynthetic organisms these
carbohydrates can be reoxidized to regenerate CO2 and H2O.
‧Part of the energy obtained from both photosynthesis and fuel oxidation is
captured in ATP.
16 - 3
• Photosynthesis requires a reductant (source of electrons),
usually H2O, to reduce CO2 to the carbohydrate level.
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Different photosynthetic organisms can use different source of electrons
Photosynthesis converts CO2 into central metabolites, which are then used to make everything else (lipids, sugars, amino acids, nucleic acids, etc)
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Photosynthesis can be split into 2 parts: Light and Dark Reactions
The two subprocesses of photosynthesis: ‧The overall process of photosynthesis is divided
into light reactions and dark reactions. ‧The light reactions, which require visible light as
an energy source, produce reducing power (in the form of NADPH), ATP, and O2.
‧The NADPH and ATP drive the so‐called dark
reactions, which occur in both the presence and the absence of light and fix CO2 into carbohydrates.
A photosynthetic prokaryote:
Photosynthetic bacteria such as
cyanobacteria have many
membrane folds
Photosynthesis (light reaction) occurs
at these membranes
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•
Chloroplasts are the photosynthetic organelles of green plants
and algae.
like a “cyanobacteria cell” in a plant or algae cell. Much like how
mitochondria is like a “bacteria cell” in a eukaryotic cell.
Energy of light (Electromagnetic radiation)
Recall that energy of EM wave: E = hν = hc/λ
h = planck’s constant 6.626 x 10‐34 J s
c = speed of light 3 x 108 m/s
Typically we want to know the energy in 1 mole of light of particular wavelength…
For example, Red light with wavelength 650 nm,
E = (6.626 x 10‐34 J s)(3 x 108 m/s)/(650 x 10‐9 m) = 3.06 x 10‐19 J
1 mole of light at this wavelength = (3.06 x 10‐19 J)(6.02 x 1023 mol‐1) = 184000 J/mol
184 kJ/mol
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The energy of photons: ‧The graph shows energy per mole of photons as a function of wavelength, compared with
energies of several chemical bonds. ‧Light in the ultraviolet range has enough energy to break many chemical bonds directly.
‧Visible light can break some weak bonds.
‧Light in the long‐wavelength portion of infrared region of the spectrum causes only heat‐
producing molecular vibrations.
Some photosynthetic pigments: ‧Chlorophylls a and b are the most abundant
plant and algal pigments, whereas b‐carotene and phycocyanin are examples of accessory
pigments. ‧Phycocyanin and the related phycoerythrin are open‐chain tetrapyrroles that are covalently attached to phycobiloproteins via a sulfhydryl group, and they are abundant in aquatic photosynthetic organisms. ‧These pigments absorb strongly in the 500–600 nm range, wavelengths that can efficiently pass through water. ‧There are also bacteriochlorophylls, which differ slightly in structure. Remember that conjugation in chemical structure absorbs light
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Comparing natural light harvesting pigments with light hitting earth surface
Two modes of energy transfer following photoexcitation: a) In resonance transfer molecule I transfers its excitation energy to an identical
molecule II, which rises to its higher energy state as molecule I falls back to the
ground state. Resonance transfer is extraordinarily fast. (more like energy transfer)
b) In electron transfer an excited electron in molecule I is transferred to the slightly
lower excited state of molecule II, making molecule I a cation and molecule II an
anion.
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Resonance transfer of energy in a light-harvesting complex:
• The excitation energy originating in a
photon of light wanders from one
antenna molecule to another until it
reaches a reaction center.
• There an electron is transferred to a
primary electron acceptor molecule, and
the energy is trapped.
Overview of the “Z-scheme” – transfer electrons from H2O to NADPH (in
the process, generating proton gradient)
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The Light Reactions
• Two photosystems, linked in series, are are involved in the
photosynthetic light reactions in algae, cyanobacteria, and
higher plants.
• In each of the two photosystems, the primary step is transfer
of a light-excited electron from a reaction center (P680 or
P700) into an electron transport chain.
• The ultimate source of the electrons is the water molecules
producing O2.
• The final destination of the electrons is the molecule of
NADP+, which is thereby reduced to NADPH.
The Light Reactions
• At two stages in the electron transport process, protons are
released into the thylakoid lumen.
• Some of the protons come from the H2O that is broken down,
some come from the stroma.
• This transfer of protons into the lumen produces a pH gradient
across the thylakoid membrane.
• Thus, ATP and reducing power in the form of NADPH are the
products of the light reactions.
• These compounds are exactly what is needed to drive the
syntheses carried out in the dark reactions.
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The Light Reactions
Z‐scheme: Photosystem II first split water to O2 a
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Structure of photosystem II:
Oxygen evolving complex (OEC) cluster in PSII contain coordinated Mn ions. These Mn ions participates directly in the Water splitting reaction (by changing their redox state. A model for the catalytic function of the oxygenevolving
complex (OEC) cluster in PSII:
• S0 - S4 represent the different oxidation states that the ligated metal
cluster cycles through as e- and H+ are abstracted from the H2O molecules.
• In the first four transitions, light energy is used to oxidize P680 to P680+, which
in turn oxidizes the metal-oxo cluster of the OEC.
• The transition is light-independent and releases H2O. Thus, four photons are
required to oxidize two H2O to one O2.
The detailed mechanism is on page 686 of text book
Biochemistry 4th edition, Mathews, Van Holde, Appling, Anthony‐Cahill. Pearson 16 - 22
ISBN:978‐0‐13‐800464‐4
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Net effect of PSII….
•
The system has in effect stripped four electrons from the four hydrogen
atoms in two water molecules.
•
The oxygen produced diffuses out of the chloroplast.
•
The four protons that are produced from the two water molecules are
released into the thylakoid lumen, helping to generate a pH difference
between the lumen and stroma.
•
We may summarize the reaction carried out by photosystem II as
follows:
Further “transported” to cyt b6f and to PSI
Photosystem II
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Plastoquinone:
Just as we saw in electron transport chain in respiration, quinones are electron mediators.
Z‐scheme: Photosystem II first split water to O2 a
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Structure of the cytochrome b6f complex:
Structure of the cytochrome b6f complex compare with
complex III of electron transport chain in mitochondria:
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Cytochrom b6f complex
Z‐scheme: Photosystem II first split water to O2 a
Ao is a special chlorophyll acceptor in PSI
A1 is Phylloquinone
Fx, FA, FB are ion sulfur clusters
Fd = Ferredoxin is a Soluble iron sulfur protein – in the stroma
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The structure of plant photosystem I:
Electron transfer at photosystem I:
Plastocyanin is a small (10kDa) protein containing Cu ion Electrons from cyt b6f is transferred to plastocyanine, which then enters PSI
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Photosystem I
Converted to NADPH
in stroma (or cytosol for cyanobacteria)
Summary view of the light reactions as they occur in the thylakoid:
NET: 2 H2O + 2 NADP+ + 8 hν  2 H+ + 2 NADPH + O2
In the process, pumped 12 H+ to lumen
Experimental measurements show 3 ATP per O2 evolved
Analogous to mitochondrial respiration ETC, 1 ATP is produced as 4 H+ move across ATPase
pH 5
pH 8
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Arrangement of components of the two photosystems on the
thylakoid membrane:
• The membrane layers in the interior of the granum are rich in photosystem II.
• The stroma lamellae and the top and bottom surfaces of the granum are rich
in photosystem I and ATP synthase particles, allowing NADP reduction and
ATP generation to occur at or near these stroma-facing surfaces.
Comparison between proton movements in different biological system
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Photophosphorylation and oxidative phosphorylation shares the same electron carriers
Cyclic electron flow:
• When levels of NADP+ are low
and levels of NADPH are high,
electrons from the P700 center
are returned to it via the
cytochrome b6f complex.
• There is no NADP+ reduction, but
protons are pumped across the
membrane and therefore ATP is
generated.
How many photons are necessary to make 1 ATP?
Each photon excites 1 electron. Each electron translocates 2 H+ to lumen
4 H+ give 1 ATP…
Uncoupled ATP and NADPH generation  Flexibility
Therefore… 2 photons (for 2 electrons) are required per ATP
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Cyclic electron flow in purple bacterial photosynthesis
• This process somewhat resembles the
photosystem II reactions in the thylakoid, with a reaction center and a membrane‐
bound cytochrome complex.
• However, there is only one kind of reaction
center, water is not split, and electron flow
is cyclic.
Photoreaction center of purple bacterial photosynthesis
Reaction center bacteriochlorophylls absorb max at 870 nm. (therefore called P870) This is an example of anoxygenic photosynthesis, since it does not split water to O2.
These microbes can use ATP generated from this process to strip electrons from H2S, H2, etc
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Different photosynthetic organisms can use different source of electrons
Some archea use a single protein to pump H+ across membrane
Bacteriorhodopsin is a protein containing rentinal (aldehyde derivative
of vitamin A) and pumps protons across membrane when exposed to light
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Protons move across membrane due to photo‐induced conformational change of rentinal prosthetic group
Example of artificial photosynthesis: • In this solar cell, absorbed light energy drives the splitting of water into oxygen
and protons, forming H2 fuel.
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