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PHOTOSYNTHETIC PIGMENTS
Light can work in chloroplasts only if it is absorbed. Pigments are the substances that absorb visible
light(380-750 nm in wave length). Diferent pigments absorb light of diferent wave lengths (colours),
and the wave lengths that are absorbed disappear. An instrument called Spectrophotometer is
used to measure relative abilities of diferent pigments to absorb diferent wavelengths of light.
A graph plotting absorption of light of diferent wave lengths by a pigment is called absorption
spectrum of the pigment.
Thylakoid membranes contain several kinds of pigments, but chlorophylls are the main
photosynthetic pigments. Other, accessory photosynthetic pigments present in the chloroplasts
include yellow and red to orange carotenoids; carotenes are mostly red to orange and xanthophylls
are yellow to orange. These broaden the absorption and utilization of light energy.
Chlorophylls
There are known many diferent kinds of chlorophylls. Chlorophyll a, b, c and d are found in
eukaryotic photosynthetic plants and algae, while the other are found in photosynthetic bacteria
and are known as bacteriochlorophylls.
Chlorophylls absorb mainly violet-blue and orange-red wave lengths. Green, yellow and indigo
wave lengths are least absorbed by chlorophylls and are transmitted or relected, although the
yellows are often masked by darker green colour. Hence plants appear green, unless masked by
other pigments A chlorophyll molecule has two
main parts : One lat, square,
light absorbing hydrophilic head
and the other long, anchoring,
hydrophobic hydrocarbon tail.
The head is complex porphyrin
ring which is made up of 4 joined
smaller pyrrole rings composed
of carbon and nitrogen atoms. An
atom of magnesium is present in
the centre of porphyrin ring and is
coordinated with the nitrogen of
each pyrrole ring. That is
why magnesium deiciency causes
yellowing in plants.
Haem portion of haemoglobin
is also a porphyrin ring but
containing an ironatom instead of
magnesium atom in the centre.
Long hydrocarbon tail which is
attached to one of the pyrrole rings
is phytol (C20 H39). The chlorophyll
molecule is embedded in the
hydrophobic core of thylakoid
membrane by this tail.
Chlorophyll a and chlorophyll b
difer from each other in only one
of the functional groups bonded
to the porphyrin; the methyl group
(-CH3
) in chlorophyll a is replaced
by a terminal carbonyl group
(-CHO) in chlorophyll b.
The molecular formulae for chlorophyll a and b are:
Chlorophyll a : C55 H72 05
N4
Mg Chlorophyll b : C55 H70 06
N4
Mg
Due to this slight diference in their structure, the two chlorophylls show slightly diferent absorption
spectra and hence diferent colours. Some wave lengths not absorbed by chlorophyll a are very
efectively absorbed by chlorophyll b and vice-versa. Such diferences in structure of diferent
pigments increase the range of wave lengths of the light absorbed. Chlorophyll a is blue-green
while chlorophyll b is yellow-green.
Of all the chlorophylls, chlorophyll a is the-most abundant and the most important photosynthetic
pigment as it takes part directly in the light-dependent reactions which convert solar energy
into chemical energy. It is found in all photosynthetic organisms except photosynthetic bacteria.
Chlorophyll a itself exists in several forms difering slightly in their red absorbing peaks e.g. at 670,
680, 690, 700 nm.
Chlorophyll b is found alongwith chlorophyll a in all green plants (embryophytes) and green algae.
Chlorophylls are insolube in water but souble in organic solvents, such as carbon tetrachloride,alcohol
etc.
Carotenoids-accessory pigments
Carotenoids are yellow and red to orange pigments that absorb strongly the blueviolet range,
diferent wave lengths than the chlorophyll absorbs. So they broaden the spectrum of light that
provides energy for photosynthesis.
These and chlorophyll b are called accessory pigments because they absorb light and transfer
the energy to chlorophyll a, which then initiates the light reactions. It is generally believed that the
order of transfer of energy is:
Some carotenoids protect chlorophyll from intense light by absorbing and dissipating excessive
light energy, rather than transferring energy to chlorophyll. (Similar carotenoids may be protecting
human eye).
carotenoids chloroph