Week 8 Johnson Lecture 3 Flashcards
Why are antennas needed?
To capture and concentrate light energy.
What effect do antennas have on reaction centres?
increases RC excitation rate by 2 orders of magnitude
What is the fate of electrons in the excited states?
Electrons in S2 state will lose their energy via vibrational relaxation, but will still stay at the S2 state. This happens on a femto time scale. Internal conversion occurs which is the movement of electrons from S2 state to S1 state and happens on a pico time scale. This changes the blue photon to red. From S1 to S0, this happens slower, because electrons are more stable. They occur on a nano timescale. This occurs slowly enough to allow competition of alternative channel de-excitation. Förster resonance energy transfer (FRET) occurs if 2 chlorophylls are in close proximity, with excited state energy levels that overlap S0 which can pass on photons to nearby chlorophylls.
What is FRET?
Förster resonance energy transfer (FRET) is when 2 chlorophylls are close enough to pass on their photons to the other chlorophyll. The efficiency of FRET varies with the sixth power of the distance, so only works with very short distances.
What is the general structure of pigment binding sites?
Each pigment binding site in LHCII is unique
- The environment (amino acid sequences) of each pigment affects the π-electron system and so the pigment’s excited state properties including energy, spectra and excited state lifetime
- Range of binding site energies broadens the spectral cross- section further and creates directionality in energy flow.
How is directionality provided in energy movement?
Chlorophylls closer to the reaction centre (RC) have excited state at lower energies than those further out in the antenna- so excitation energy cascades downhill towards the RC by FRET, where it is ‘trapped’ as an electron transfer reaction till it reaches the reaction centre
