Photosynthesis Flashcards
What are some photosynthetic organisms?
Photosynthetic organisms - bacteria, algae and plants
Cyanobacteria can bloom on a lake
These organisms make a considerable contribution to CO2 fixation
Photosynthesis may be oxygenic or non-oxygenic, depending on the electron donor (mainly in bacteria) i.e. water of hydrogen sulfate
What is necessary for absorption of photons?
The principal light harvesting molecules are (bacterio)chlorophylls
Chlorophyll a gives a plant it’s green colour, and this is the most abundant one
Similar structure to haem (cyclic tetrapyrrole) with Mg in the middle
It can absorb light and delocalise electrons around the structure and transfers the energy across to adjacent chlorophyll molecules = Mg not involved in redox
There are other pigments e.g. carotenoids
What are the steps of photosynthesis? and protein complexes involved?
Steps: Light harvesting Photochemical charge separation Electron transport Oxidation of electron donor Proton pumping Reduction of electron acceptor
Protein complexes: Light harvesting protein complexes Photosystem II (PSII) Cytochrome b6f Photosystem I (PSI)
Describe the absorption spectrum of chlorophyll a and b?
Chlorophyll a and b have different absorption/emission - due to different side chains, which leads to different absorption properties
They absorb well in the blue and red ends of the spectrum but not in the green
The pigments are bound to proteins that affect their precise absorbance characteristics
What is the principal of light energy, within photosynthesis?
Light energy causes excitation of electrons within the conjugated double bond system of chlorophyll
This excitation energy (not electrons) can be transferred to adjacent chlorophyll molecules and eventually it reaches the reaction centre, which is part of a photosystem
Therefore light energy can be harvested from a large surface area (antenna chlorophylls or light-harvesting complex (LHC))
What are all the ways in whihc an electronically excited molecule can dissipate its energy?
Internal conversion - converted to kinetic energy and then heat for motion
Fluorescence
Excitation transfer - transfers the energy to unexcited molecules with similar electronic properties (charge separation)
Therefore light energy can be harvested from a large surface area
Give an overview of light into chemical energy in bacteria?
The photochemistry of charge separation occurs and is harnessed within protein complexes called photosynthetic reaction centres
The relatively simple photosynthetic reaction centre of the photosynthetic bacterium Rhodopseudomonas viridis reveals the blueprint for photochemistry
Describe the structure of Rhodopseudomonas viridis reaction centre?
4 polypeptides L (red) M (blue) H chain (white) and cytochrome subunit (C, yellow)
L and M are structural
Bacteriochlorophyll is like chlorophyll of plants but has minor structural difference that gives rise to a longer absorption max (near 1000nm)
Bacteriopheophytin is bacteriochlorophyll without the Mg2+ ion
The special pair of chlorophylls are where the light-induced charge separation occurs
Quinone, haems and non haem iron from the mitochondrial electron transport - they perform similar electron and hydrogen atom transport functions here
What is the mechanism of electron transfer in bacteria?
- The energy arrives, after jumping from chlorophyll to chlorophyll, at a ‘special’ pair of chloroplasts called P960
Named as 960 nm is the peak wavelength to excite the electrons the most - The energy of excitation causes an electron to hop off and go onto a nearby molecule - bacterial pheophytin (similar to chlorophyll but doesn’t contain Mg in the middle)
- A separate electron from the heme group of the cytochrome subunit to reduce the reaction center P960 again (heme is oxidised)
This fast simultaneous reaction allows this process to be unidirectional - The first electron then jumps to plastoquinone (Qa)
Similar to ubiquinone in mitochondria e.g. Lipophilic and can diffuse within the membrane - The electron is passed to an exchangeable plastoquinone (Qb), and here it picks up to protons from the stroma to reduce the quinone
- The cycle repeats with a second electron - producing a molecule of quinone reduced in its QH2 form
Describe the reoxidiation of QH2 by the respiratory chain?
The QH2 formed by the bacterial reaction centre is re-oxidised by complex III (bc1) of the respiratory chain and the cytochrome subunit re-reduced by cytochrome c2
QH2 being oxidised results in protons being released to the periplasmic side of the membrane and the e- go to cytochrome c2
These electrons are then donated to the cytochrome subunit of the reaction center
This is a cyclic process that produces a proton gradient anaerobically in the presence of light
Therefore there is a respiratory chain as well as a reactive chain within the same membrane
Give an overview of photosynthesis in chloroplasts?
Photons fall on the protein complexes, the energy is used to split water to produce oxygen
The electrons that come from splitting water, are passed down the redox potential gradient
This energy is used to move protons form the stroma to the cytosol so they can be used to form ATP
Here we are reducing NADP+ to NADPH
Net reaction 2H20 + 2NADP+ → O2+ 2NADPH + 2H+
Describe the structure of photosystem II?
14 subunits of 19 sit in the thylakoid membrane, sticking out on both sides into the thylakoid lumen and the stroma
Core proteins - D1 and D2 (part of the reaction center)
On the thylakoid lumen side it has accessor proteins which turn the oxygen evolving complex, with the aid of the manganese centre
Describe the first stage of photosythesis (LDR)?
Photosystem II
Within the oxygen evolving complex 2 molecules of water are bound
- The energy arrives, after jumping from chlorophyll to chlorophyll, at a ‘special’ pair of chloroplasts called P680
○ Named as 680 nm is the peak wavelength to excite the electrons the most - The energy of excitation causes an electron to hop off and go onto a nearby molecule - pheophytin (similar to chlorophyll but doesn’t contain Mg in the middle)
- The electron then jumps to plastoquinone (Qa)
○ Similar to ubiquinone in mitochondria e.g. Lipophilic and can diffuse within the membrane - The electron is passed to an exchangeable plastoquinone (Qb), and here it picks up to protons from the stroma to reduce the quinone
The electrons from water reduce the reaction centre back again
How does photolysis of water reduce the RC?
The oxygen evolving complex (OEC) breaks down water into 1/2 O2 2H+ and 2e-
OEC cycles between 5 different states S0 through S4 (very high reduction potentials) and O2 is released between S4-S0
The electrons produced reduce the reaction center and the H+ ions contributer to the transmembrane proton gradient
The oxygen evolving complex contains 4 Mn ions that are oxidised by P680+ one step at a time (2+, 3+, 4+, 5+)
There are 4 photons required to generate one O2 molecule
Give a comparison of PSII to R. viridis RC?
PSII is functionally equivalent to the bacterial reaction centre
D1 and D2 are homologous to L and M
The electron transfer pathway is similar, from the special pair to a (bacterio)pheophytin to a bound quinone to an exchangeable quinone
The plant photosystem has additional subunits that act as chlorophyll a binding proteins. These increase the efficiency with which light energy is absorbed
The plant PSII uses water as the electron donor and evolves oxygen
