Light, time and micro-organisms
15 > pts > 9
Femtoseconds to nanoseconds. The first time domain
The energy of an electron in an atom or molecule can be described from quantum mechanics as possessing one of a relatively small number of discrete values. Absorption of a single quantum of light depends on the availability of an electron whose permitted energy change falls within the range determined by the energy of the light quantum that induces it. The energy of the quantum is proportional to the frequency of the radiation, and inversely proportional to its wavelength.
Light absorption and the movement of the electron between energy levels occur on a femtosecond time scale. Internal conversion may occur between one energy level and a lower one, and is accompanied by release of energy as heat. There are then four processes that may then take place. The first is thermal, or non-radiative, de-excitation. This is the usual route for atoms and molecules absorbing specific energies without further events of direct biological relevance. The remaining three processes that follow absorption can occur at different rates, and all are important properties of photosynthetic systems.
Fluorescence is important for what it tells us. Fluorescence is the re-emission of a quantum of light, and occurs in picoseconds to seconds. Chlorophyll and bacteriochlorophyll have an inherently high, and variable, yield of fluorescence. Fluorescence emission from chlorophyll occurs with a lifetime typically measured in picoseconds, and the fluorescence lifetime of chlorophyll depends on its physical environment as well as on the rate of the competing processes of energy transfer and photochemistry.
The next route for the falling electron is energy transfer, which is a sub-picosecond phenomenon. This is the fate of excitation of most chlorophyll molecules bound to protein in photosynthetic membranes.
The function of light-harvesting pigments is to collect light energy over a far larger area than would be possible if each molecule were required itself to participate in storage of that energy in chemical form. The reason for this constraint is the relatively long time required for regeneration of the ground state by the final route for de-excitation, namely photochemistry. Although the chemical structure of the molecules involved may be the identical, the division of function between light-harvesting pigments (carrying out energy transfer) and reaction centre pigments (carrying out photochemical charge separation) is a fundamental feature of all photosynthetic systems.In reaction centres, the chlorophyll or bacteriochlorophyll molecules (P) involved in the primary photochemical reaction of photosynthesis receive excitation energy from their chemically identical light-harvesting antenna pigment molecules, but the reaction centre pigments themselves are held by histidine ligands in an environment close to an electron donor (D) and acceptor (A), such that the excited state (P*) returns to the ground state (P) via an oxidised species (P+), the electron being lost to the acceptor, thus:
DPA ---> DP*A ---> DP+A-In the third state (DP+A-) the excitation energy is said to have been "trapped" by photochemistry. The rise-time of the absorption change (a photochemical bleaching) that reports on the generation of P+ has been timed at 4 ps (pts = 11.4) for reaction centres of purple bacteria, and this is synchronous with the reduction of the acceptor, bacteriophaeophytin. Subsequent events are again determined by the kinetics of a number of competing reactions, but the "useful" reaction is forward electron transfer from A- to a secondary electron acceptor, a quinone, which takes 200 ps (pts = 9.7). Re-reduction of P+ to restore the ground state by the donor (in purple bacteria, a c-type cytochrome), together with movement of the electron from the first to a second quinone, takes about 200 ms (pts = 3.7). The second quinone accepts a second electron by the same route, and moves on to provide electrons to the Q-cycle. The sequence of events in the primary photochemical reaction of photosynthesis...
|Imaging chlorophyll fluorescence|