Lecture 6 - Light Driven Proton Transport Flashcards
What is photosynthesis?
The reaction centre (Photosensitive component) converts energy of irradiation into potential (redox) energy. Electron transport is coupled to H+ pumping which generates the H+ electrochemical gradient which is used to synthesise ATP.
It can be cyclic or non cyclic both involve the absorption of a photon to excite a photosensitive component to release a higher energy electron however the fate of this electron differs.
Describe cyclic electron transport
Cyclic electron transport is a process in photosynthesis where electrons are circulated in a closed loop, leading to the pumping of protons across a membrane. Here’s a breakdown of the key steps:
Photon Absorption and Electron Release:
The photosensitive component (P870) absorbs a photon at 870nm, entering an excited state.
In this excited state, P870 releases an electron due to its more negative redox potential.
Electron Transfer:
The released electron is transferred first to bacteriopheophytin.
Charge Separation:
The electron is then passed to a bound quinone (Q) on the N side of the membrane.
This transfer results in a “charge separation” with a positively charged component (P+) on one side and a negatively charged quinone on the other side of the membrane.
Second Photon Absorption and Quinone Reduction:
Another photon is absorbed by P, repeating the process.
This results in a full (two-electron) reduction of the quinone on the N side.
The fully reduced quinone takes protons from the N side and is released into the membrane pool.
Electron Transport to Complex III:
The electrons then move to complex III in the respiratory chain.
In this step, the Q cycle occurs, leading to the pumping of four protons for every two electrons.
Return of Electrons:
Electrons are moved back across the membrane to the P side, allowing them to be accepted by cytc2 (cytochrome c2).
Closure of the Loop:
The electrons are finally returned to the photosensitive component (P870 chlorophyll) via the mobile cytc2.
This cyclic process does not involve the net production of reducing equivalents or the generation of oxygen. Instead, it serves to generate a proton gradient across the membrane, which can be utilized for ATP synthesis.
Cyclic electron transport is a vital process in photosynthesis, particularly in cyclic photophosphorylation, which occurs in the light-dependent reactions of photosynthesis. Here’s a summary of the key steps involved in cyclic electron transport:
Photon Absorption and Electron Release: The photosensitive component (P870 chlorophyll) absorbs a photon, leading to the release of an electron from P870 due to its more negative redox potential.
Electron Transfer: The released electron is transferred to bacteriopheophytin.
Charge Separation: The electron is then passed to a bound quinone (Q) on the N side of the membrane, resulting in a charge separation across the membrane.
Second Photon Absorption and
Quinone Reduction: Another photon is absorbed by P870, repeating the process. This leads to the full reduction of the quinone on the N side, which then takes protons from the N side and is released into the membrane pool.
Electron Transport to Complex III: The electrons move to complex III in the respiratory chain, where the Q cycle occurs, leading to the pumping of four protons for every two electrons.
Return of Electrons: Electrons are moved back across the membrane to the P side, where they are accepted by cytochrome c2 (cytc2).
Closure of the Loop: Finally, the electrons are returned to the photosensitive component (P870 chlorophyll) via the mobile cytc2, completing the cyclic process.
This cyclic process does not result in the net production of reducing equivalents or the generation of oxygen. Instead, it serves to generate a proton gradient across the membrane, which can be utilized for ATP synthesis through the process of chemiosmosis.
Describe the strucutre of the reaction centre for cyclic photosyntesis in purple bacteria (Rhodobacter)
The reaction centre is composed of three subunits which work together to hold the redox components in a fixed position. The photosensitive component is on the P-side and the quinones are on the N-side
Redox centres:
* 4 chlorophylls (2 form P870)
* 2 bacteriopheophytins
* 2 quinones
What are pheophytins?
They along with cholorophyll have a tetrapyrrole strucute which have either magnesium (chlorophyll) or protons (phenophytin) chelated to them
Pheophytins become deprotonated when they are oxidised and electrons are donated from this deprotonation event.
Describe how the structure of cholorophyll enables it to carry out its function.
The reaction involves stabilisation of the tetrapyrrole structure by magnesium.
The chlorophylls absorb photons (light) as they possess conjugated bonds (i.e. double bonds that alternate to single bonds and back again). The electrons in these pyrrole rings are not localised to a particular atom but shared in the ring (hence conjugated double bonds) – so the electrons are in a molecular orbit rather than an atomic orbit. When photons are absorbed, electrons transition to a higher energy orbital. In most molecules, the electrons then return back to the lower energy orbit and the energy is released as heat. However, if an electron acceptor is near, the electron can move to a new acceptor molecule leaving the donating chlorophyll with a positive charge.
Describe the structure of quinones?
Quinones have a unifying structure; an aromatic ring structure that is able to accept and donate electrons. The different quinones have different length side chains in their structure.
What is the function of light harvesting complexes in the reaction centre.
Light Harvesting Complexes (LH Complexes):
Accompany the reaction center in the photosynthetic membrane.
Act as antennae to collect photons and increase the efficiency of photon absorbance.
Enhance photon absorbance up to 100 times.
Consist of two types differing in the composition of carotenoid and chlorophyll pigments.
Photon Absorption and Energy Transfer:
Upon absorbance of photons, energy is transferred and shared between neighboring pigments in the LH complexes.
This energy transfer occurs via a mechanism called delocalization excitation coupling.
Delocalization excitation coupling refers to the overlap of electrons in each pigment as they become excited.
Resonance Energy Transfer:
Energy is readily shared among the pigments until a threshold is reached to induce long-range transfer.
Long-range transfer occurs through resonance energy transfer.
Resonance energy transfer involves the transfer of energy associated with vibrating electrons as a virtual photon.
Describe non-cyclic electron transport.
Electron Transport in Photosynthesis:
Non-cyclic Electron Transport:
Stoichiometry of 6 protons/2 electrons.
Involves two Reaction Centers (RCs): P680 and P700.
Photon absorption by P680 initiates the process.
Excited state passes the electron through pheophytin and quinones A and B.
Electron transfer to complex bf (analogous to complex III).
Operation of the Q cycle at complex bf.
Electron transfer to plastocyanin (Pc).
Electrons donated to P700 instead of cycling back to P680.
Photon absorption by P700.
Donation of electrons to chlorophylls A0 and A1, then to ferredoxin.
Ferredoxin donates electrons to an NADP+ reducing enzyme for NADPH production.
NADPH utilized in the Calvin cycle for sugar synthesis in dark reactions.
Cyclic Electron Transport:
Occurs when NADPH is replete.
Involves transferring electrons from ferredoxin back to P700.
Allows for continued ATP production without net NADPH generation.
Water Splitting and Oxygen Evolution:
The manganese center is the site of water splitting.
Manganese adopts different oxidation states (+1 to +7).
Accepts electrons from water splitting.
Donates electrons one at a time to P680 as required.
P680 has a more positive redox potential than water, enabling it to split water into molecular oxygen, protons, and electrons.
These electrons reduce P+ back into the P state.
Describe and explain some of the differences between mitochondria and choloroplasts
- The lumen of the mitochondria is the N-side and the protons are pumped out of the lumen in chloroplasts the lumen of the thylakoid membranes is the P-side due to protons being pumped into the lumen
- The magnitudes of the gradients are equal however they have opposite polarity and the composition of the gradient is different
○ In mitochondria the electrical gradient is more significant
○ In chloroplasts the chemical gradient is more significant
- The magnitudes of the gradients are equal however they have opposite polarity and the composition of the gradient is different
The reasons for this is because the inner membrane of the mitochondria is a large surface area (invaginated membrane) and therefore has a large capacity to store charge across that membrane. The thylakoid membranes are not invaginated and thus the limited surface area limits the storage of charge.
The thylakoid membranes therefore have selectively “leaky” membranes (to a range of ions) which dissipates the electrical gradient (e.g. chloride ions in or magnesium ions out give an equal but opposite movement of charge to the protons).
In mitochondria, the membrane use electroneutral transporters to transport nutrients into the lumen which are necessary for ATP synthesis (e.g. H+/PO42- symporter). This dissipates the H+ chemical gradient but not the electrical gradient.
