Mireille Consalvey PhD Thesis - University of St Andrews

Chdpt(1111 1. -, -- I--, -- -- I, ----, -- -- 11
---- ---- -- -- -
1) To neighbouring molecules as molecular motion (heat)
2) Through fluorescence: emitting another photon in the red band (lower
energy, longer wavelength)
3) Through passing energy to a neighbouring molecule and boosting another
electron to a higher orbital, enabling this molecule to reach an excited state
4) Through driving a chemical reaction.
These processes are competitive and a decrease in one marks an increase in
another.
On each thylakold disk, hundreds of pigment molecules are found in
compact aggregations called antennae complexes. These complexes work as an
antenna, channelling energy towards a reaction centre chlorophyll a protein
complex. There are two types of reaction centre chlorophyll a, P700 and P680.
P700 is pigment 700 and most strongly absorbs photons with a wavelength
around 700 nm, whilst P680 absorbs photons around 680 nm. Surrounding the
reaction centre chlorophyll is an electron acceptor and an electron donor: this
aggregation is termed the photosystem. The P700 photosystem is called
Photosystem I (PSI) and the P680 photosystem is called Photosystem 11 (PSII).
The light reactions have been characterised as aZ pathway (Figure 1.1).
When a photon strikes P680 an electron is elevated to an excited state. The
molecule is highly unstable and the electron is passed to phaeophytin. The
electron is then transferred to quinone acceptor QA and then to quinone acceptor
QB. The electron remains here. At the same time, the now oxidised P680 is
reduced by the electron donor, Yz (tyrosine in the reaction centre protein DI).
H20 is split by the oxygen evolving complex and re-reduces Yz+ whilst releasing
II