Carotenoids are found in all known native photosynthetic organisms in
hundreds of chemically distinct structures (Chapter 10). These molecules
have extended delocalized πelectrons that result in strong absorption bands
in the green region. Carotenoids are accessory pigments in the absorption
of sunlight that transfer the energy to the chlorophylls. Another function
is a process termed photoprotection in which the carotenoids quench
harmful excited states of chlorophylls. Carotenoids also are involved in
the regulation of energy transfer through the xanthophyll cycle.
Energy transfer and light-harvesting complexes
One of the early outcomes of the experiments by Emerson and Arnold
was that a saturating light flash produces only one oxygen molecule for
about 2500 chlorophyll molecules. Although no mechanism for energy
transfer was known at that time, this observation gave rise to the ideal
of a photosynthetic unit, consisting of many pigments in which the light
energy was trapped before conversion. This concept was disputed by
several scientists, including James Franck and Edward Teller, but even-
tually the weight of experimental evidence led to its acceptance.
The reason for the development of a system of large pigments in the
capture of sunlight can be understood from an energetic basis. The num-
ber of photons of light available for energy capture is relatively low. Full
sunlight has an intensity, I, of about 1800μEm−^2 s−^1 in the visible region,
corresponding to 10^21 photons m−^2 s−^1 or equivalently 10 photons Å−^2 s−^1.
For a molecule with a size of about 10 Å and an extinction coefficient
of 10^5 M−^1 cm−^1 , only about 10 photons are estimated to be absorbed per
second. Due to the low amount of light energy available, photosynthetic
organisms optimize the capture of light by the presence of pigments that
have been specifically designed for their ability to capture light energy
and transfer that energy to specialized centers that perform the electron-
transfer reactions. This role of the light-harvesting protein complexes is
very similar to the function of large antenna dishes for radiowaves, which
funnel the radiowaves to a central location where it is transformed into
electrical energy.
Organisms use many different types of pigment and protein to per-
form the energy capture. In some cases the antennae are peripheral units
and in other organisms the antennae are integral to the central protein
that performs the photochemistry. The absorption spectra of the antenna
varies in different photosynthetic organisms but in all cases the energy is
funneled from the higher-energy pigments to the lowest-energy pigments.
In the cell membranes of purple bacteria, the antennae form large ring
structures with a diameter of approximately 65 Å (Figure 20.2). The ring
for one type of antenna, called light-harvesting complex II, is composed
of nine (sometimes eight) pairs of two-protein subunits arranged in a pseudo