Photosynthesis Pigments

1.0Where Does Photosynthesis Take Place?

Photosynthesis does take place in the green leaves of plants but it does so also in other green parts of the plants. There is a clear-cut division of labour within the chloroplast.

Usually, the chloroplasts align themselves along the walls of the mesophyll cells, such that they get the optimum quantity of the incident light.

2.0How Many Pigments Are Involved In Photosynthesis?

Photosynthetic pigments are special molecules those absorb, transmit and reflect different colours of light from the visible spectrum of sunlight. Pigment appears in the colour which it reflects and uses the colour which it absorbs.

Types of Photosynthetic Pigments

1. Chlorophylls

• Green colored pigment.
• Light is required for their synthesis.
• Soluble in organic solvents.

Chlorophylls are of following types –

$Chl–a,Chl–b,Chl–c,Chl–d,Chl–e$

$Chl$ – a is an universal pigment, which is found in all O₂ liberating photosynthetic organisms. Its color is blue green in chromatogram.

$Chl$ – b is an accessory photosynthetic pigment found in euglenoids, green algae and higher plants. Its color is yellowish green in chromatogram.

Chlorophyll – a C₅₅H₇₂O₅N₄Mg 

Chlorophyll – b C₅₅H₇₀O₆N₄Mg 

2. Carotenoids

Carotenoids are yellow to yellow orange colour pigments which occur only inside the chromoplast and occur along with chlorophylls inside chloroplast. These pigments are universal in occurrence (except eubacteria) and insoluble in water.

Functions of Carotenoids

(1) They are accessory pigments and make photosynthesis more efficient by absorbing different wavelengths of light.

(2) β-carotene acts as a precursor of Vitamin-A

β-Carotene   ⎯⎯⎯⎯⎯⎯→   Vitamin-A

3. Phycobilins

They are hot water-soluble pigment. They lack Mg and phytol tail.

Types:

(i) Phycocyanin – Blue

(ii) Phycoerythrin – Red

(iii) Allophycocyanin – Light blue

They occur exclusively in BGA and Red algae as an accessory pigment.

Absorption Spectrum

PAR (Photosynthetically Active radiation)–400-700nm

Graphical presentation of the absorption of different wavelength of light by a particular pigment.

3.0Rate of Photosynthesis

Absorption

Graph (c) show that the wavelengths at which there is maximum absorption by chlorophyll a, i.e., in the blue and the red regions, also show higher rate of photosynthesis.

It resembles roughly the absorption spectra of chlorophyll a and b.  

Hence, we can conclude that chlorophyll–a is the chief pigment associated with photosynthesis.

These graphs a, b and c together show that most of the photosynthesis takes place in the blue and red regions of the spectrum; some photosynthesis does take place at the other wavelengths of the visible spectrum. It happens because other thylakoid pigments like chlorophyll b, xanthophylls and carotenoids, which are called accessory pigments, also absorb light and transfer the energy to chlorophyll a. Indeed, they not only enable a wider range of wavelength of incoming light to be utilised for photosynthesis but also protect chlorophyll a from photo-oxidation. 

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