1. Lecture 3 - Photosynthetic pigments
in relation to their functions
2. Photosynthesis
• It is process by which green plants convert
light energy into chemical energy
• Synthesis of organic compounds (usually
carbohydrates along with oxygen) with the
help of sunlight.
• Sometime called – carbon assimilation
• Photosynthetic apparatus in green plants –
chloroplasts
• Photosynthetic pigments – (1) Chlorophyll (2)
Carotenoids (3) Phycobilins
• All the photosynthetic pigments were located
in grana portions of chloroplast.
4. • Chlorophyll & carotenoids – insoluble in water
& can be extracted only with organic solvents
• Phycobilins – soluble in water
• Carotenoids – include carotenes &
xanthophylls (carotenols)
• Different pigments – absorb light of different
wavelength – show characteristic absorption
peak in vivo and in vitro
• They show the property of flurescence
5. Chlorophylls
• In photosynthesis of a green plant - light is
collected primarily by chlorophylls pigments
that absorb light at a wavelength below 480
nm and between 550 and 700 nm.
• When white sunlight falls on a chlorophyll
layer, the green light with a wavelength
between 480 and 550 nm is not absorbed, but
is reflected. This is why plant chlorophylls and
whole leaves are green.
• There are different classes of chlorophylls.
6. • The basic structure is a ring made of four pyrroles, a
tetrapyrrole, which is also named porphyrin.
• Mg++ is present in the center of the ring as the central atom.
• Mg++ is covalently bound with 4 N-atoms of the tetrapyrrole
ring.
• A cyclopentanone is attached to ring c.
• At ring d a propionic acid group forms an ester with the
alcohol phytol.
• Phytol consists of a long branched hydrocarbon chain with
one C-C double bond. It is derived from an isoprenoid, formed
from four isoprene units
7. • Chlorophyll always occurs bound to proteins.
• In ring b, chl-b contains a formyl residue instead of the methyl
residue in chl-a.
• This small difference has a large influence on light absorption.
• In plants- the ratio chl-a to chl-b is about three to one.
• Chlorophyll a – molecular formula- C55H72O5N4Mg
• Chlorophyll b – molecular formula- C55H70O6N4Mg
• Only chl-a - constituent of the photosynthetic reaction centers -
therefore - regarded as the central photosynthesis pigment.
• In a wide range of the visible spectrum - chl-a does not absorb
light. This non absorbing region - named the “green window.”
• The light energy – first absorbed by chl-b – transferred very
efficiently to chl-a.
• In this way chl-b - enhances the plant’s efficiency for utilizing
sunlight energy.
8. • Chlorophyll – formed from protochlorophyll in
light
• Protochlrophyll – lacks 2 hydrogen atoms at 7th &
8th carbon atoms in IV pyrrole ring.
Light
Protochlrophyll Chlrophyll
2H
• This reaction was catalyzed by
NADPH:protochlorophyll oxidoreductase
9. Carotenoids(Yellow orange pigment)
Carotenes:
• Consist of an open chain conjugated double
bond system ending on both sides by
“ ionone” rings
• They are hydrocarbons with general formula
C40H56
• Example – βcarotene, Lycopene
10. Xanthophylls(Carotenols)
• Similar to carotenes but differ in having 2 oxygen
atoms in the form of hydroxyl, carbonyl or carboxyl
groups attached to the ionone rings.
• General formula – C40H56O2
11. Major role of carotenoids
• Absorbs light energy and transfers to
Chlorophyll a
• Prevents the photodynamic damage with in
the photosynthetic apparatus
• photodynamic damage – It is caused by
oxygen molecules in their singlet stage (very
reactive) which oxidize organic compounds
such as chlorophyll and thus makes it unfit for
photosynthesis.
12. Phycobillins (Red & Blue Pigments)
• These consist of open conjugated system of
four pyrrol rings and lack Mg and the phytol
chain.
• Eg. Phycoerythrobilin
