Pigments in Plants

 

Pigments

Pigments are “molecules that absorb specific wavelengths (energies) of light and reflect all others.” A plant pigment is any type of colored substance produced by a plant. In general, any chemical compound that absorbs visible radiation between about 380 nm (violet) and 760 nm (ruby-red) is considered a pigment.

There are many different plant pigments, and they are found in different classes of organic compounds. Plant pigments give color to leaves, flowers, and fruits and are also important in controlling photosynthesis, growth, and development. In plants, algae, and cyanobacteria, pigments are the means by which the energy of sunlight is captured for photosynthesis.

The selective absorption of different wavelengths determines the colour of a pigment. For example, the chlorophylls of higher plants absorb red and blue wavelengths, but not green wavelengths, and this gives leaves their characteristic green colour.

There are follwing basic classes of pigments.

Type #1. CHLOROPHYLL

Chlorophyll’s name is derived from ancient Greek: chloros = green and phyllon = leaf. Chlorophyll is the pigment that gives plants their green color, and is an essential component of photosynthesis

History:-

Chlorophyll was first isolated and named by Joseph Bienaimé  and Pierre Joseph Pelletier in 1817.In 1883 German physiologist Julius Van Sachs showed that chlorophyll is not scattered all around in plant cell but it is found in special structures called chloroplast. He proved that chlorophyll involved in photosynthesis. The presence of magnesium in chlorophyll was discovered in 1906.

The general structure of chlorophyll a was elucidated by Hans Fischer in 1940. In 1960, when most of the stereo-chemistry of chlorophyll a was known, Robert Burns Woodward published a total synthesis of the molecule. In 1967, the last remaining stereo chemical elucidation was completed by Ian Fleming, and in 1990 Woodward and co-authors published an updated synthesis.

Chlorophyll f (C₅₅H₇₀O₆N₄Mg) was announced to be present in cyanobacteria and other oxygenic microorganisms that form stromatolites in 2010.

Occurrence:

There are five types of chlorophylls occur in plants other than bacteria— a, b, c, d and e. Out of these only two chlorophylls occur in the chloro­plasts of higher plants, a and b. The amount of chlorophyll b is roughly one fourth of total chloro­phyll content.

Chlorophyll a is found in all photo­synthetic plants except bacteria. Hence, it is termed as universal photosynthetic pigment.Bacteria possess two types of related pigments— bacteriochlorophyll (further of several sub types) and bacterioviridin (= chlorobium chlorophyll).

Properties of chlorophyll;-

• Chlorophylls are fat soluble green pigments.
• These are chlorines which absorb blue region and reflect green light.
• They are responsible for the green colour of algae and other higher plants.
• chlorophyll is one of the best antioxidants.

Chemical Structure:

Chemists have identified more than 1,000 different, naturally occurring chlorophylls. All chlorophylls are classified as metallo–tetrapyrroles. a tetrapyrrole is simply four pyrroles joined together. Chlorophyll has a tadpole like configuration with a head called porphyrin and a tail made up of long chain alcohol called phytol (Fig.1).

Porphyrin head is made up of four pyrrole rings which are linked by methine bridges (—CH=). The skeleton of each pyrrole ring is made up of 5 atoms— 4 carbon and one nitrogen. The latter lies towards the centre. Chlorophyll has a molecular formula of (C55H72O5N4Mg). Molar mass of 893.51g/mol, density = 1.079g/cm² , melting point  =152.3^oC.

A non ionic magnesium atom is held in the centre of porphyrin head by nitrogen atoms of pyrrole rings (through two covalent and two coordinate bonds).

The external carbon atoms of the pyrrole rings have been given specific numbers, 1-8. Carbon atoms 1, 3, 5 and 8 have methyl groups (^__CH₃). Carbon atom 2 possesses a vinyl group (—CH = CH₂) while carbon atom 4 has an ethyl group (— CH₂ — CH₃). Carbon atom 6 is attached to the next methine group by a fifth isocyclic ring called cyclopentanone.

Carbon atom 7 is connected to the phytol tail through a propionic acid residue. Phytol is an insoluble long chain of carbon and hydrogen atoms with a formula of C₂₀H₃₉OH. It anchors the chlorophyll molecule into the lipid part of the thylakoid membrane. Chlorophyll without its Mg-core is colourless and is called phaeophytin. It is the early electron acceptor.

Types of Chlorophyll

Chlorophyll a;-

• Chlorophyll-a is the primary pigment for photosynthesis in plants and occurs in all photosynthetic organisms except photosynthetic bacteria.
• It is a specific form of chlorophyll, used in oxygenic photosynthesis, where it occurs in both reaction centres (RC) and in all light-harvesting complexes (LHC) , Because of its role as a primary electron donor in electron transport chain .
• It absorbs most energy wavelengths of violet blue and orange red light.
• Soluble in a number of organic solvents but it is more soluble in petroleum ether.
• Chlorophyll a is also transfer resonance energy in the antenna complex ,ending in the reaction centres where specific chlorophylls p680 and p700 are located.
• Chlorophyll a is bluish-green in the pure state. It has an empirical formula of C₅₅H₇₂O₅N₄Mg and molecular weight of 893.
• Bacte­riochlorophyll a has an empirical formula of C₅₅H₇₄O₆N₄Mg and molecular weight of 911.

Chlorophyll b;-

• Chlorophyll b is olive green in the pure state with an empirical formula of C₅₅H₇₀O₆N₄Mg and molecular weight of
• Chlorophyll b (Chl b) is distinguished from Chi a by a 7-formyl instead of the 7-methyl substitutent. Its structure has been established by chemical correlation with Chl a; the stereochemistry and esterifying alcohol of both pigments are identical.
• It is more soluble then chlorophyll a in organic solvents because of carbonyl group, but is more soluble in 92% methyl alcohol.
• It absorbs blue light.
• In land plants, the light harvesting antenna complex around photosystem II contain the majority 50% of chlorophyll b.
• Hence, in shade adopted chloroplast which have an increased ratio of chl b then chl a. This is adaptive as increase in chl b increase in range the wavelength absorbed by the shade chloroplast.

 

Chlorophyll c;-

• This form of chlorophyll is found in certain algae.
• It has a blue greenish colour and is an accessory pigment.
• It absorbs light of 447-452 nm.
• It is soluble in organic solvents.
• Like chl a and chl b it helps in absorbing light and passing a quanta of excitation through a light harvesting antenna to photosynthetic reaction center.
• It is divided into C1 and C2_{………………}C_8.

Chlorophyll d;-

• It is a form of chlorophyll identified by Harlod Strain and Winsten in 1943.
• It is present in marine algae and cyanobacteria and is used by them for the capturing of sunlight for photosynthesis.
• Chl d differs from Chl a by the presence of a 3-formyl group.
• It absorbs far-red light at a wavelength of 710 nm.
• It is soluble in organic solvents.
• Its molar mass is 895.462 g/mol.

Chlorophyll e;-

• Its molecular formula is: C₅₄H₇₀O₆N₄Mg
• It is present in algae (xanthophyceae).
• It is a rare type of chlorophyll found in a few algae like Tribonema,Vaucheria.

Chlorophyll f;-

• It is the type of chlorophyll that absorbs further red (infrared light) than the other chlorophyll.
• Its molecular formula is: C₅₅H₇₀O₆N₄Mg
• Its molar mass is 907.4725 g/mol.
• In 2010, it has been reported that from Stromatolites

 

Biosynthesis:

  Several investigators have unfolded some of the suggested steps are as follow:

(a) Succinyl COA, an intermediate of the Krebs cycle, combines with glycine amino acid to form δ- amino -ketoadipic acid as unstable compound. This loses CO₂ to yield aminolevulinic acid. The presence of cofactors pyridoxal phosphate and iron are essential. The enzyme δ- aminolevulinic acid synthetase (-ALAS) catalyses it. As mentioned earlier, iron deficiency causes chlorosis of young leaves. Light is shown to mediate the condensation of these two compounds.

(b) In the next step two molecules of δ-aminolevulinic acid condense, and the process is mediated by the enzyme δ-aminolevulinic acid dehydrase, to form porphobilinogen. In this reaction there is a fusion of two molecules.

(c) Then 4 molecules of porphobilinogen condense to form uroporphyrinogen III. Four ammonium ions are lost in this reaction and the process is mediated by the enzyme uroporphyrinogen-Isynthetase and uroporphyrinogen III cosynthetase.

(d) The four acetic acid substitutes of uroporphyrinogen-III yield coproporphyrinogen-III and the reaction is catalyzed by uroporphyrinogen decarboxylase.

(e) Under aerobic conditions, coproporphyrinogen-III, in the presence of coproporphyrinogen oxidative decarboxylase gives rise to protoporphyrinogen IX.

(f) Protoporphyrinogen IX undergoes oxidation and thus protoporphyrin IX is formed. It takes magnesium to form Mg protoporphyrin IX. Mg protoporphyrin methyl esterase catalyzes the addition of a methyl group of Mg protoporphyrin IX. It may be mentioned that the methyl group is donated by S-adenosyl methionine.

(g) In the next step, Mg protoporphyrin IX mono-methylester is converted to protochlorophyllide.

(h) A phytol group is added to protochlorophyllide to produce protochlorophyll. Once it was believed that protochlorophyll is the immediate precursor of chlorophyll a. However, recent evidences suggest that the immediate precursor of chlorophyll a is chlorophyllide a. When the etiolated seedlings are subjected to light, protochlorophyllide is reduced to form chlorophyllide a. The light is essentially required for this conversion.

(i) In the final step esterification of a phytol group to chlorophyllide a occurs and so chlorophyll a is produced. Enzyme chlorophyllase  is involved in the process.

In gymnosperms, some ferns, and many algae, chlorophyll can be synthesized in die dark solely through enzymatic activity. On the other hand, it is believed that chlorophyll b is formed from chlorophyll a. Some of the minerals like manganse, potassium, zinc, copper, magnesium, iron, and nitrogen are essential for the synthesis of chlorophyll.

When absent or deficient they cause chlorosis. Chlorophyll formation is also dependent upon genetic factors as well. Absence of the gene(s) essential for its formation in the genetic constitution, produces seedlings from the germinating seeds which lack chlorophyll.

These are known as “albinos”.