P h o to sy n t ~

(The Pigments)

Th e aut otr oph ic pla nts syn the size

eno rmo us am oun ts of Later on in 1772, Priestley, an Eng
org ani c foo d wit h the hel p of lish minister, gave us
the ligh t ene rgy ava ilab le some idea ofthe gas exchange talcing
fro m sun. Car boh ydr ate s pro duc place in photosynethsis.
ed thro ugh photosynthesis He discovered that CO2 containing 'inj
con stit ute the bas ic raw ma ter ure d' air (phlogiston)
ials , wh ich dir ect ly or wo uld get pur ifie d (dephlogi.sto
ind irec tly giv e rise to all the organi n) if kept in contact with
c components of virtually gre en min t pla nts for som e month
all pla nts and animals. The entire hum s. Thu s he discovered that
anity depends upo n the oxy gen wa s pro duc ed by gre en
pre par ed foo d of plants. Eve ry plants. Priestley, however,
yea r som e 200 billion ton s did not rec ogn ise the rol e of eith
of car bon go thro ugh the photos er carbon dioxide or light
ynthetic process. It is one in photosynthesis.
of the mo st ma ssiv e che mic al eve
nt going on the earth. It
has bee n est ima ted tha t pla nts tak In 177 9, lng enh ous z, wh o wa
e up 7 x 1011 tons of CO s a phy sici an to the
to pro duc e rou ghl y 5 x 10 11 ton 2 em per or ofAu stri a , got interested
s of solid pla nt material. in Priestley's papers and
Ap pro xim ate ly 90 per cen t of the rep orte d tha t pla nts 'pu rifi ed' the
wo rld' s photosynthesis is air only in the presence
car ried out of ma rin e and freshw ofl igh t. He fou nd tha t the sam e
ater algae. tiss ue made air 'impure' in
dark. He also wro te tha t onl y the
gre en parts of the plant
pro duc ed the pur ifyi ng age nt (ox
HISTORICAL ygen), while non -green
tissue contaminated the air. Thu s,
Ingenhousz recognised the
Fro m the tim e of Ar isto tle unt participation of cholorophyll and
il the 17t h cen tury it wa s ligh t in the photosynthetic
gen era lly bel iev ed tha t pla nts process. He per for me d abo ut 500
der ive d all the ir nut riti on experiments to show that
fro m the pla nt and ani ma l deb the pla nts pur ifie d the air. I
ris of the soil. In the ear ly
sev ent een th cen tur y J.B . van Jea n Sen ebi er (17 82) a min iste
Be lm ont (15 77- 164 4), r of Geneva, was the
a Be lgi an phy sic ian wh o cul tiva first to rec ogn ise tha t ''fix ed" air
ted a wil low pla nt in a (CO i) wa s essential in
con tain er for five yea rs wit h eno photosynthesis. He pub lish ed his
ugh wa teri ng con clu ded researches on the influence
tha t it wa s wa ter and soi l wh ich of carbon dioxide upo n vegetation
con trib ute d to the gro wth in 1800. He thought that
of the pla nt. In 169 9, Wo odw the oxygen wh ich was liberated from
ard pro pou nde d the vie w the plants in the process
tha t veg eta ble s we re not for me cam e directly from the car bon dio
d of wa ter but of a cer tain xide, wh ich wa s absorbed
pec uli ar terr est rial ma tter , wh ich by the plants. He also discovered
wa s abs orb ed alo ng wit h tha t red wavelengths of the
visible spe ctru m of light was mo
wa ter. st effective in the process.
It wa s left to Ste phe n Ha les (17 In 1804, de Saussure confirmed the
27), an English clergy- finding oflngenhoUSZ
ma n an illu stri ous contemporary regarding the gas exchange of the
of Issac Newton, ofte n two types: one in light
(photosythesis) and another in dark
fi ~e d to as 'fat her of pla nt phy n~s_s (respiration). He
siology' to poi nt out tha t also discovered that water was also
re e
gre enp Iant s ma y get par t of their nourish
ment thro ugh the ir
utih zed in the process.
. Du tro che t ( 183 7) con firm ed
tha t the gre en part
lea ves tirom the air and sunlight. (chlorophyll) was essential for pho
tosythesis.
 esis 1(The Pigments) 157

,,.,, . S40), (lenil8 ll agricultural chemist, reported that .

capable of producing chlorophyll even indarkn ess. In this
·
case th fl l d
JJeblg ~e ofcarbon in plants was carbon dioxide of the ' e na re uctton ofprotochlorophyll to chloro h 11
p. y l
woul~ appear to be chemical, rather than a photo chem1ca
sole;t the oxygen was released from CO2• .
reaction.
and R bert Mayer announced the law ofconservation
Granick. (1954 ) and Shemin (l 956 ) have exp 1amed.
rnt84S, : in 1848 he pointed out that the energy used th
of tner'P ~ animals in their metabolism is obtained from e synt~es1s of chlorophyll. According to them glycine
and succmyl Co-A condense to fonn unstable a._ammo .
b)'Ptants an His ideas of organic synthesis and energy A k d' . -
on decar boxyl ation f:
solar ene~• n remains qualitatively comp lete and true ..,- etoa 1p1c acid,
I' .
which orm
8 0 . o . l Gassm an (1967 ) and Bogor ad
1J'811sforrn -amm o evu m1c acid.
~todaY· (1967) have suggested that the synthesis ofo-aminolevulini
. ault (1860--65) conducting an experiment on
80055 acid requi res the prese nce of light. Two molecules 0;
~:quality of carbon dioxide absorbed and oxygen
o-aminolevulinic acid condense to form a monopyrrole
v~JUJlle: in light showed that plants obtained their total
porphobilinogen in the presence of the enzyme o-amino-
giv~e nts of carbon from carbon dioxide of air.
levulinase (o-aminolevulinic acid dehydrase ).
~achs (1877) and his pupils established the foundation Four mole cules of porph obilin ogen give rise to
modem. views ofphotosynthesis rest. He was
upon Which our uroporphyrinogen m under the influence of the enzymes
the first to discover that green chloroplasts are the organs uroporphyrinogen synthetase and urophyrinogen III
where carbon dioxide is used up and oxygen is released.
cosynthetase which act as catalysts. It is decarboxylated
Secondly, Sachs found that starch was the first visible to copr opor phyr inoge n III in the prese nce of the
product of photosynthesis .. enzym e uropo rphyr inoge n decarboxylase. It then gives
Englemann (1888) gave the action spectrum of photo- rise to proto porp hyrin ogen IX in the prese nce of
synthesis. copr opor phyr inoge n III oxida ted decarboxylase.
Warburg (1919) was the first to use the green alga Proto porph yrino gen IX is oxidised to protoporphyrin
chlorella for study of photosynthesis. IX whic h incor porat es magn esium to produ ce Mg-
protoporphyrin IX. The latter takes up a methyl group
from S-adenosyl methionine in the presence ofthe enzyme
THE PHOTOSYNTHETIC PIGMENTS
.J Mg-protoporphyrin methyl easterase to form Mg-proto-
The photosynthetic produ cts are energ y-rich orgx · nic porphyrin IX monomethyl easter. It is then converted
compounds. The poten tial chem ical energ y of t ese into protochlorophyllide which takes up a phytol group
COl!lpounds comes from the light energy. to form protochlorophyll. It gains hydrogen to become
chlorophyll.
abso ~ten ~to be effective in photosynth~sis must e
by a suitable pigment. This vital role is perfo rm~
Acco rding to some the imme diate precu rsor of
by thegreen .
pigment, chlorophylls, in plants. . \ chlor ophyl l is beliv ed to be chlorophyllide and not the
proto chlor ophy ll. Gass man and Bogorad (1967) and
SYnthesis• of Chlorophyll Akoy unog lon and Slegelman (1968) have found that
proto chlor ophy llide is reduc ed to chlorophyllide a in
Cb!oraphyU .
the p11esence of light. However, in gymnosperms, some
~l'Otochloro;~ normally fonned from a precw-sor called
111
tbea&sen YII. The latter differs from the chlorophyll ferns, and many algae light is not absolutely essential for
ring.,See,m:~ftwo hydrogen atoms in one of its pyrro le chlor ophyl l synthesis. In the last step the esterification
Proroc1.,_ -"61' grown in darkness p11oduce small amou nt of of a phyto l group to chlorophyllide a takes place to form
to I' "IUl'oPhyII If .
to Igh~ the P . such et1olated seedlings are transferred chlorophyll a in the presence ofthe enzyme chlorophyllase.
ch10r0phy11rotochorophYII is• quantitatively converted Chlor ophyl l a is believ ed to give rise to chlorophyll b
a. Gynmospenn seedl~gs, however, are· (Fig. 11.1 ).
I
 160
Plant Physiology
The chlorophylls are Primari\
Chlorophyll Pigments thylakoids. The chlorophyll m yl located within the grana
There are at least seven types of chlorophylls known: . o ecu\es t
between the protem and lipid 1 °nn a monolayer
chlorophylls a, b, c, d and e, bacteriochlorphyll and ayers of th
the thylakoids. The hydrophil:tc heads ofe hmembranes of
bacterioviridin. All theses chlorophyll molecules contain molecules are embedded within th . t e chlorophyll
a tetrapyrrole skeleton formed into a ring, with an atom . .. . e protein layer while th
hpoph1hc tads are located within the lipid 1ayer. e
of magnesium in the centre of the ring. A so-called pyrrole
molecule contains a skeleton of five atoms, four of carbon
and one nitrogen and the five are arranged in a ring. Four such The Absorption Spectrum
pyrroles arranged in a ring form the 'head' ofa chlorophyll
The portion of the electromagnetic spectrum which
molecule. Attached to this porphyrin ring at one point is
participates in photosynthesis is from 300 to 900 nm. In green
an alcohol (phytol) 'tail', a long chain of linked carbons.
plants only the visible spectrum (400-750 nm) is effective
Relatively minor variations in the kinds and groupings of
in photosynthesis. Photosynthetic green bacteria can absorb
other atoms joined to this head and tail skeleton account
for the differences among different kinds of chlorophylls.
wavelengths from 375-800 nm while purple photosynthetic
Chlorophylls a and b are the two most abundant bacteria absorb 300-950 nm (Fig. 11.3).
chlorophylls. Chlorophyll
a is found in all the
autotrophic plants except
the photosynthetic H3C C2Hs H3C ~Hs
bacteria. Chlorophyll b is
H M H
absent in the blue-green, H
H3C H3C H3C
brown and red algae. The CH3 CH3
H
other chlorophylls ( c, H H
d, e) are found only in CH2H--~

◊-t·"
algae and in combination
with chlorophyll a.
Chlorophyll a possesses
-CH3 , a methyl . group
CH2
?
which· is replaced by- I
CH
CHO, an aldehyde group II
in chlorophyll b. The T-CH3
structures of chlorophyll TH2
a, chlorophyll b and TH2
bacteriochlorophyll are TH2
given in Figure 11.2. Hy-CH3
The molecular formulae
fH2
of the chlorophylls are CH
given bel?w: . I 2
TH2
Chlorophyll a: Hy-cH3
C55 H120sN4Mg yH2
yH2
Chlorophyll b:
TH2
C55 H1006N4Mg
/C,H
Both the chlorophylls CH3CH3
a and b have hydrophilic Chlorophyll a Chlorophyll b
Mg-porphyrin bead ~d Fig. 11.2 Structures of Chloroph Bacterlochlorophyll 8
a Jipophilic phytol tail. YII a, Chlorophyll b
and Bacterlochlorophyll a
 161

107 10• 1011 1013 1015

10• 108 104 102

Fl~. 11.3 The electromagnetic spectrum

using both wave frequen
wavelength (A) . . cy (u) and
in cm. Vanous portions
of t~e s~ectrum are shown ' an the visible
portion is expanded to indicate the region
tha! appears to the human eye to have
- - - - - - - - - Low energy
various colors

453

t of different
hotosynthesis 1+-- +- - - - Chlorophyll a _ _ _ _~
6 62
tregions of the
ofthe evolution
by the motile 1--- - - Chlorophyll b - --..

·a. The amount of

· um in blue and
II (Fig. 11.4).
ent absorption peaks
reports of different forms 4 oo 500 . 600 100
·on peaks at 660, 670, 680, Fig. 11.5 Absorption spectrum of chlorophyll a and chlorophyll b
ometers. These variations '-...... .,..._ .
'-l!!~-p~gments in green sulphur bacteria are called
vironmental changes. The

. .
liyll b occur at 453 and chlorobrnm chlorophylls 650 and 660 because its
642 absorption peaks are in the red region at 650 and 660 run.

r ~e two pigments diff~r.

ell m petroleum ether while
They are always associated with bacteriochlorophyll a. The
pigment system of Chromatium is composed of 3 forms of
bacteriochlorophyll which absorbwavelengths of 800 run,
850 run and 890 run. The bacteriochlorophyll which absorbs
eaks of absorption at light at 890 run is called B 890. It is present in the ratio of
one B 890 to every 50 bacteriochlorophyll molecules. It is

Spirogyra cell
Splral choroplat

••• • • •••
• • • •• •••
•
• •••• •• ••
•• •• • ••• ••• •• •• • •
••••••
• • •• • ••• ••••• •• ••••••••••
• •••••

600 7.00

wavelength of llght (nm)

8 nd red colour
on filament of Splrogyre 1howlng the release of maximum oxygen In blue
 - \QO - ~t11e11t ., ..,caJ &c I'll, ..,.,,_.
llsL h , of Bio,.;i,.,,.11
. .010/og· ,,.,., IC

162

1E 100
(.)
r: :- -- -- -- -- -- -- -- -- -- -~ Plant Physiology

~~
E
'E
Q)
'i3
II=
8

400 500 700 800

Wavelength In mµ
Flg.11.6 Absorption spectrum of bacterlochlorophyll
a in ether
the reaction centre like P 700 of PSI. Green sulphur bacteria
appears yellow in colour. a-carotene is present in ve
has chlorophyll 770 as the reaction centre.
small amounts in certain species. During autumn seaso~
The strong similarity between the absorption and chlorophylls degenerate and the more stable carotenoid;
action spectrum of photosynthesis shows that the rate of become visible as orange and yellow colours in the leaves.
photosynthesis is proportionate to the light energy absorbed The carotenoids are located in the chloroplast membranes or
by the chlorophyll molecules. within the chromoplasts (Fig. 11.7). The carotenoids perform
The distribution of pigments and their absorption peaks two types of functions in the green plants. They trap light
are given in Table 11.1. energy and transfer it to the chlorophyll a particularly in
algae and to some extent in higher plants. In higher plants
Table 11.1 Distribution of pigments and their absorptions
peaks this function is performed by lutein of the xanthophylls
and ~-carotene. Carotenoids are lipid soluble. Lutein
and zeaxanthin are hydroxylated forms of a-carotene and
~-carotene, respectively.
All green plants
!lll lm Bm
All grccn plants except 480

• .l....'\ ..,·\
diatoms brown, red ...
and blue green algae
Chic Diatoms and brown .. . ''

.. .
,l \
algae '

Cbld Some red algae 740

1 ''

Protochlorophyll Etiolated plants

Bacterioviridin Green sulphur bacteda 750or760
Bacteriocblorophyll Purple sulphurbacteria 800,850
and89 0
\
CAROTENOIDS 300 400 500 600 700
The carotenoids are the main accessory pigments in Wavelenght (nm)
photosynthesis. They transfer the light energy to chlorophyll Fig. 11.7 Absorption spectra of a-carotene and ~rote
ne
for photosynthesis. The carotenoids are widely distributed
in plants. They occur in bacteria, algae and higer plants. At high light intensities the entire cell apparatus is oxidised
by atmospheric oxygen into carbon dioxide. This process is
Carotenoids occur in roots of carrot and tomato fruits. They
termed photo-oxidation which is as good as combustion.
include orange carotenes and yellow xanthophylls. The
Chlorophyll mutants are actually carotenoid mutants.
latter is oxygenated carotenes. They absorb wavelengths
The carotenoids (Ji-carotene) protect_ the Pho,tosyntbestic
400 run to S00 run because of which they are orange in apparatus from this type of destruction b~ trapping and
co 1OUr• Of the carotenes j3-caroten e is the abundant and dissipating the excess excitation energy which Would have
most important type. It absorbs blue l'gh1 t, and, therefore, otherwise converted molecular oxygen to a hi&hly reactive
 (The pigments) 163
u,esiS 1
~ . SUJ)CIOxide (02). The dissipation of excess
,,Ji 111uiageJ11° ofbeat is facilitated by xanthophyll cycle.
PHYCOBILINS X
~inibefollll • oflong chains ofcarbon atoms linked Engelmann found blue green light to be very effective in
d dollble bonds with six-carbon rings increasing the rate of photosynthesis ofbrown and red algae.
In fact the red algae gave the best result in green light. Since
i.e. they contain carbon chlorophylls hardly absorb green light it became obvious
known as carotenols) that some other accessory pigment was involved. Several
oxygen, carbon and workers have since then demonstrated the role ofphycobilins
bCated in chloroplasts and carotenoids in photosynthesis. Th~ irradiation of
named after carrot in carotenoids causes fluorescence of chlorophyll suggesting
on ofthe carotenoids the transfer of energy by accessory pigments to chlorophyll
e Table 11.2. a during photosynthesis.

./ • C~3/CH3

A ~ ~ ~,CH2
Hz CCH=CHC=CHCH=CHC=CHCH=CHCH=CCH =CHCH=CCH=CHC
! I CH3
H2 Cr
~CH3
!3-Carotene
I
CH3
I
CH3
II
/C CH2
CH3 'c~3
I

/CH3 C~3/CH3
/c"'\. yH3 yH3 /c,
CH2 CCH=CHC=CHCH=CHC=CHCH=CHCH=CCH =CHCH=CCH=CHC CH2
I I CH3 I I I I
CH2 C a.-Carotene CH3 CH3 C CH2
'\./'
~~ ~~~
/ /'-

~ ~~
' '/ 3
)\ CH3 CH3 C
c H2 CICH=CH6=CHCH=CH6=CHCH=CHCH=CCH =CHCH=CCH=CHC~
1 /CH3 I I I
'c~
I
~ /c, Lutein CHa CH3 ~ CHOH
~ CH2 CH3 cii'3 'c~
CH3

r
'\
7(°/
2
o
yHa yH3
v i a CHC=CHCH=CHC=CHCH=CHCH=CCH=CHCH=CC H g;J.C/
yH3 yH3 '<
CH3C~

c~
~ 'cHa Vlolaxanthln :} 6HOH
2
H CHa ' c ~

CH3
;q-o
yH2 '-dcH =C
6 I H3
%Ha yHa yHa yHa .
H -CHCH=CHC=CHCH=CHCH=CCH =CHCH=CCH=CHCbH !H2
'<
CH3 CH3

1-f\
0 H CH3
' Neoxanthln
• I
C HOH
H
2
b"c~
H
Flg.11 8 Th
• e molecular structures of the major carotenolds of higher plants
 164

Table 11.2 The carotenoids

I. Carotenes
a.-carotene Many ,_,_
....."_,
and-:....1...... Jn.recl algae and a group of green algae called Siphonales In h
--~
\ exane,at420,440,470
\ ~ Mam Ciltotene of all oth~~
I
In hexane, at 425,450,
480
II y-Carotene ~or~o tgreen In hexane, at 440, 460,
495
I II. Carotenoh (Also called 'Xanthophylls')
\
Luteol Majot carotenolof ~ algae and red algae In ethanol at 425,445, 475
Violaxantbol second major carotmoi oft
In ethanol, at 425,450,475
\ ofdiatotns ancllwown algae In hexane, at 425,450,475
le bacteria In hexane, at 464, 490, 524

In blue-gre en and red algae

some additional pigments known as
phycobilins are present. They are
also tetrapyrroles like chlorophylls
but the four joined pyrrole rings
form a straight chain (Fig. 11.9).
HO
h ch ct ic h
N H N H
Biliverdin {blue-green)
H H N OH

Like anthocyanins they also mask the M E M P P MM E

green colour ofthe chlorophylls. They
i,
I
are, however, intimately associated
with the chlorophylls. The light
absorbed by them can be used in
photosythesis. Phycobilins include
red coloured phycoerythrins and
HOUCDCDDOO H H H2
Mesobilirubin {orange)
H H

blue coloured phycocyanins found in

red and blue-green algae respectively.
Phycoerythrin absorbs the green light
the best. Phycocyanin absorb blue
light the best.
Phycobilins are supplementary
Hohct1cbctloo H H _ _ .
Mesoblllrubln {orange)
H2 H

accessory light harvesting pigments.

HohcbDD~
The phycobilins in association with
proteins constitute phycobilinosomes
in cyanoba cteria and red algae
They absorb light in the range of
H H H H2 H
520-530 nm which are not absorbed
Mesoblllrubln {orange)
by chlorophylls and thus supplement
them in trapping maximum energy of M = CH2; P = -CHrCHr COOH; V = -CH=CH ; E= -CHr-CH
2 2
visible spectrum.
Fig. 11.9 Molecular structure of phycobillns
 7

(The pigments) 165

1
~ b lls and carotenoids are soluble in THE ROLE OF LIGHT
;tc,
A ... clJloroP tvcobilins are soluble in hot water. The
lfll"' ttheP ·~ . all h . d It has been observed that full sunlight is inhibitory for
of phycoerythrin, op ycocyamn an
11,&U
:Mleli;,- in Figure 11. l 0 photosynthesis. The leaves are oriented in such a manner that
the sunlight intensity is effectively reduced to a level when
_____,.__.r--t, .#..,\ i ·".
I
I I
I
/ •
'
photosynthesis becomes most effecient. In direct sunlight
the leaves lie at an acute angle to the rays. In the shade, on
••
'i
I • the other hand , the leaves lie at right angles to the general
••• \/
••
:,
direction of rays. A major amount of incident light, about 80
• per cent is absorbed, 10 per cent is reflected and IO per cent
/
.A.• is transmitted. These three things vary with the wavelengths
•
I ; I (Fig. 11 '. 11 ). It has been stated earlier that the pigments of
,_.J
•
i
I
I
I
Absorption
I
I
I
I
I
I
I
I
I
I
I
I
\ \
400 500 600 700
Wavelength/nm
1(l.jlll~........- ........---.-___;:"-r-----.----.----.----1
Rg.11.10 Absorption spectrum of three phycobilins. 300 400 500 600 700 800 900 1000 1200
U.V.----Visible light----1---- Infra-red--
Phyoocyanobilin and phycoerythrobilin are chromophores
Fig. 11.11 Reflection, absorption and transmission of light by
whichbind to apoproteins to form the pigments phycocyanin, green leaves in relation to wavelength
phycoerytbrin and allophycocyanin. Their structure is so
designed that it causes 95% of energy transfer. photosynthetic process absorb light only in certain regions of
the spectrum and transmit the remaining wavelengths. While
The distribution of phycobilins in the plant kingdom is
shown in the Table 11 .3. the chlorophylls absorb both the shorter (blue and violet) as
well as the longer waves (orange and red) the carotenoids
absorb shorter wavelengths. The phycoerythrin absorbs blue,
ANTHOCYANINS green and yellow colours and the phycocyanin absorbs the
The colour ofl • longer wavelengths. Together they absorb most of the visible
__ eaves 1s modified in certain plants due to the
''"OCIICC of Purpl . light.The photosynthetic bacteria absorbs even the infra-red
Conned b e pigment called anthocyanins. They are
.
rings ofatoms, the nngs . . . dm
b emgJome . light. Part of the radiant energy absorbed by chlorophyll
f-Otnptex Yserveral
ways Anth . is used in producing a chemical change, that is, its effect
~in the · ocyaruns are soluble in water, hence they
t,L_ vacuolar sa Of th . . its photochemical. Part of the radiant energy is re-emitted
~anYPart. P e cells. This pigment does not
Ill ti.. Ill photos th . as light , which is called fluorescence. It has been found
"JC C}'top1as YD es1s. Anthocyanin is not present
Ill. that absorption of light by the chloroplast is not uniform
over the spectrum. The chlorophylls are responsible for the

Table 11.3 The distribution of phycobillins In the plant kingdom

166

absorption of the red and the blue components of light an d PJ

the carotenoids are effective in absorbing the green colour. sl
It has been definitely shown that chlorophyll a is the k~y w
substance in the photochemical reaction of photosynthesis,
which indicates transfer of energy from the ca ro te no id s
to chlorophyll a. Such transfer has been demonstrated in
a variety of different kinds of plants, but it seems to be of
special biological advantage in algae, e.g., re d se aw ee ds
f~
le
growing in deep water. Green light, wh ich pe ne tra tes fa rth
es t a.
in cle ar water, is no t well ab so rb ed by ch lo ro ph yl l bu
t is al:
ab so rb ed by sp ec ial ac ce ss or y ph ot os yn th eti c pi gm
en ts lo:
which transfer the energy to chlorophyll a fo r ph ot os vn th es
1s_