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Electron transport between the weak reductant produced by P680 and the weak oxidant
produced by P700 is coupled to the vectorial transport of protons into the interior of the space
enclosed by the photosynthetic (thylakoid) membrane, and ultimately to ATP synthesis. The
photochemical reactions are thus directly or indirectly coupled to all major reactions
producing or utilizing NADPH or ATP (Foyer et al., 1990). Principal among these are the
reactions of electron transport and CO2 fixation. Since photochemistry and fluorescence are
both competing for excited state energy, variations in any of the coupled reactions may
potentially show up as a change in the yield of fluorescence.
The fact that linear electron transport from water to NADP requires that P700 and P680
function in series does not mean that P700 and P680 occur in a 1: 1 stoichiometry in thylakoid
membranes. In green algae and higher plants, the P680 and P700 reaction center complexes
are widely separated in the membrane, connected only by pools of mobile electron carriers
(Ort, 1986). P680:P700 ratios may vary between species and with environmental conditions
(Falkowski et al., 1981).
Antenna pigments and excitation transfer. Although a reaction center pigment is fully
capable of direct absorption of light energy, its physical size limits the rate at which it can
absorb light in a typical terrestrial or aquatic environment. Even in maximum sunlight, the
rate of photochemical turnover of the reaction centers without antennae would be insufficient
to support a phototrophic metabolism. Nature has overcome this problem by coupling the
absorption of hundreds of light-harvesting antenna pigments to each reaction center. This
coupling occurs in the form of highly efficient excited state energy transfer among the antenna
pigments. (The mobile excited state is often called an excitation or exciton.) This coupling
of antenna pigments effectively increases the area through which the reaction center can
absorb light energy and results in a proportional increase in the rate of photochemical
turnover. Each type of reaction center has associated with it a specific antenna of defined size
and composition. P700, its associated electron transport components and antenna form an
aggregate called photosystem I (PS I), while the corresponding aggregate containing P680 is
called PS II.
Electron transport between the weak reductant produced by P680 and the weak oxidant
produced by P700 is coupled to the vectorial transport of protons into the interior of the space
enclosed by the photosynthetic (thylakoid) membrane, and ultimately to ATP synthesis. The
photochemical reactions are thus directly or indirectly coupled to all major reactions
producing or utilizing NADPH or ATP (Foyer et al., 1990). Principal among these are the
reactions of electron transport and CO2 fixation. Since photochemistry and fluorescence are
both competing for excited state energy, variations in any of the coupled reactions may
potentially show up as a change in the yield of fluorescence.
The fact that linear electron transport from water to NADP requires that P700 and P680
function in series does not mean that P700 and P680 occur in a 1: 1 stoichiometry in thylakoid
membranes. In green algae and higher plants, the P680 and P700 reaction center complexes
are widely separated in the membrane, connected only by pools of mobile electron carriers
(Ort, 1986). P680:P700 ratios may vary between species and with environmental conditions
(Falkowski et al., 1981).
Antenna pigments and excitation transfer. Although a reaction center pigment is fully
capable of direct absorption of light energy, its physical size limits the rate at which it can
absorb light in a typical terrestrial or aquatic environment. Even in maximum sunlight, the
rate of photochemical turnover of the reaction centers without antennae would be insufficient
to support a phototrophic metabolism. Nature has overcome this problem by coupling the
absorption of hundreds of light-harvesting antenna pigments to each reaction center. This
coupling occurs in the form of highly efficient excited state energy transfer among the antenna
pigments. (The mobile excited state is often called an excitation or exciton.) This coupling
of antenna pigments effectively increases the area through which the reaction center can
absorb light energy and results in a proportional increase in the rate of photochemical
turnover. Each type of reaction center has associated with it a specific antenna of defined size
and composition. P700, its associated electron transport components and antenna form an
aggregate called photosystem I (PS I), while the corresponding aggregate containing P680 is
called PS II.