Photosynthesis: Light Reactions II Flashcards
PET: Photosynthetic Electron Transport
- occurs in the thylakoid membranes of chloroplasts during photosynthesis
- both Photosystems I and II are utilized to split water to get electrons (light dependent reactions)
- electron transport helps establish a proton gradient that powers ATP production and also stores energy in the reduced coenzyme NADPH. this energy is used to power the Calvin Cycle to produce sugar and other carbohydrates.
Discovery of Photosystems: experiment
Emerson - Arnold experiment
- > give brief flashes of light to Chlorella and measure how much oxygen is produced
- > Chlorella – single-celled green algae
- > Conclusion: increase amount of photon/light energy, increased oxygen production
Emerson-Arnold experiment
O2 evolution saturates with light
measure of chl content : 2500 chl per O2 evolved
8-10 photons per O2 evolved
Original hypothesis: 1 oxygen per photon does not support the data (4 photons to remove 4 electrons to produce 1 O2)
Results show 8-10 photons
-> suggests maybe 2 photons are required per
electron produced from H2O
Emerson enhancement effect
action spectrum vs absorption spectrum
found a red drop
- the increase in the rate of photosynthesis after chloroplasts are exposed to light of wavelength 670 nm (deep red spectrum) and 700 nm (far red spectrum)
- the effect was early evidence that two photosystems, processing different wavelengths, cooperate in photosynthesis.
Follow up experiment
- used two wavelengths of light
- if two wavelengths of light are used, then high rate of photosynthesis suggests two separate events cooperative events (PS I and PS II interact )
rate of photosynthesis: both lights on – there is more photosynthetic activity than the sum of far-red light on and red-light on. Why? - unexpected findings: 8-10 photons required per O2 produced
Photosystem
- has an antenna (energy transfer): pigment molecules (such as chlorophyll a and carotenoids) absorb light from the sun, energy (not electron) is transferred to a neighboring molecule. Some molecules go to ground state and give off heat instead of transferring energy, while other molecules make it through the reaction center and convert a photon of observed light into a flow of electrons
Electron donor here is water - has a reaction center (electron transfer): a special molecule of chl a that can transfer an electron (P680)
Reaction Center
P680: special molecule of chlorophyll A that can absorb 680 nanometers (photosystem pigment of PS II)
-> Absorption of a photon by P680 leads to the excited form of the pigment, called P680. P680 but not ground‐state P680 gives up an electron to another molecule, plastiquinone.
PS I and PS II
This model is consistent with Emerson-Arnold experiment and Emerson Enhancement Effect
- > Two photons per electron; many pigments per electron
- > The photosystems work cooperatively
- > Light energy converted to chemical energy (NADPH, ATP)
- > ATP produced from H+ gradient
Photosystem II
- first protein complex
- located in the thylakoid membrane
- pigments: chl a, b, carotenoids
>absorbs light
> tries to transfer energy to reaction centers/
sometimes it doesn’t happen because energy gives
off heat or fluorescence instead
~200 chl
~50 carotenoids - reaction center: where the energy of light is converted into the motion of energized electrons
special chl a: P680 - transfers electron - H2O oxidation
> Pheophytin (Pheo) – electron acceptor for PSII
colorless chl a
H2O is electron donor
Pheo is electron acceptor - electrons flow from water to Pheo
H2O (2 e-, 2 H+, 1/2 O2) —–light—> P680 ——> Pheo - electrons transferred from P680 to Pheo to PQ
PS II Summary
specialized protein complex that uses light energy to oxidize water, resulting in the release of molecular oxygen into the atmosphere, and to reduce plastoquinone, which is released into the hydrophobic core of the photosyntheticmembrane
PQ: an electron acceptor which accepts 2 excited e– from PSII and 2H+ from the stroma to become plastoquinol (PQH2).
- has two molecules of P680 chlorophyll a at its reaction center, makes ATP and uses electrons from light
Photosystem I
- contains chl a, b
~ 50 β - carotenes
~200 chl (less chl b than PS II)
- Why? Chl b absorbs different wavelengths of light
than chl a
- reaction center: P700
- Fe-S proteins are acceptors
electron donor —–light—> P700 —–> Fe-S protein
Ferrodoxin: the final electron carrier in the light-dependent reactions, which accepts an electron from PSI to become reduced ferredoxin, before reducing NADP+ in the reaction catalysed by ferredoxin NADP+ reductase
PS I Summary
- 2nd photosystem in the photosynthetic light reactions
- integral membrane protein complex that uses light energy to produce the high energy carriers ATP and NADPH
- P700 reaction center, makes NADPH, does not take place first
PS II and PS I in thylakoids
- not randomly distributed in thylakoids
> PS II in stacked regions
> PS I in unstacked regions - What are the consequences of the spatial segregation of the photosystems? Not near each other, but are mobile in the lipid bilayer
> There are several PSI and PSII in the thylakoids
> This process converts light energy to chemical energy.
Non-cyclic electron transport
- whole process that uses both PS II (P680) and PS I (P700) acting in series (they are linked)
- moving energy from water (high energy compound) to NADPH (low-energy compound)
> Electrons transported from H2O to NADP+ - Light energy converted to chemical energy
- ATP produced from H+ gradient
> ATP generated in this process will provide the energy for the synthesis of glucose during the Calvin cycle (light independent reactions).
oxygen-evolving complex (OEC)
2H2O →4 e- + O2 + 4H+ (oxidation of water)
- “water-splitting complex”
How is water oxidized to produce electrons, protons, and oxygen? Happens in PSII
> PS II transfers one electron at a time
> ANS: each PS II is independent and stores
“oxidizing equivalents” (4 PSII do NOT cooperate)
