Biomedical Engineering Reference
In-Depth Information
1.3 Pigments and Light Absorption
The capacity of plants to absorb light covers a wide range of wavelengths,
between 350 and 700 nm, which include almost 60% of incident sunlight
spectrum. All the pigments involved in light absorption, which are grouped
in to two big classes, chlorophylls and carotenoids, are located within the
thylakoids. Chlorophylls, named
a
or
b
, because of two different chemical
forms, are organo-metallic compounds in which a substituted porphyrin ring
structure coordinates a magnesium atom in its center. A phytol chain, neces-
sary for their localization within the lipid membrane, constitutes one of the
lateral chains of the porphyrin. These pigments strongly absorb red and blue
light, while they scatter green light, thus giving reason for plant leafs color [6].
The absorption of a photon drives chlorophylls to an excited state, con-
verting electromagnetic energy into electronic excitation; red light determines
excitation to the first singlet excited state, while blue light causes excitation
to the second singlet state (Fig. 1.2).
The photosynthetic apparatus makes the job of converting such molecular
excitation into different forms of stable free energy. Electronic excitation of
molecules decays to ground state in the nanosecond time scale, and the reac-
tions that convert the excitation energy must be fast enough to successfully
compete with decay.
The higher singlet-excited states, as well as the excited vibrational states,
decay by internal conversion to the lowest vibrational state of the lowest ex-
cited singlet, in the time range of femtosecond, which is much faster than the
Blue
2nd excited singlet state
Absorption
Red
Energy
1st excited singlet state
Fluorescence
Ground state
Fig. 1.2.
Light absorption and energy decay scheme
