PHOTOSYNTHETIC PIGMENTS

🌱 Comprehensive Notes on Photosynthetic Pigments

🌞 What are Photosynthetic Pigments?

Definition: Molecules that absorb light energy and transfer it to the photosynthetic machinery for conversion into chemical energy.

Role:

Broaden the absorption spectrum.
Capture solar radiation.
Protect photosystems from excess light damage.

TABLE:1 Photosynthetic Pigments, their distribution & absorption peaks

Pigment	Distribution in Plant Kingdom	Absorption Peaks (nm)
Chlorophyll _a	Universal in all oxygenic photosynthetic organisms (plants, green algae, red algae, cyanobacteria)	430–435 nm (blue), 662–665 nm (red)
Chlorophyll _b	Green algae, higher plants (absent in cyanobacteria, red algae, brown algae, diatoms)	453–460 nm (blue), 642–650 nm (red)
Chlorophyll _c (c1, c2, c3)	Brown algae, diatoms, dinoflagellates, chrysophytes	447–452 nm (blue), 580–585 nm (red-orange)
Chlorophyll _d	Certain red algae, cyanobacteria	710 nm (far-red)
Chlorophyll f	Cyanobacteria (specialized, found in shaded/infrared environments)	720–740 nm (infrared)
Carotenoids (β-carotene, lutein, violaxanthin, etc.)	Widely distributed in higher plants, green algae, many other algae, cyanobacteria	400–500 nm (blue-violet)
Xanthophylls (oxygenated carotenoids: lutein, zeaxanthin, fucoxanthin, peridinin)	Lutein in higher plants and green algae; fucoxanthin in brown algae & diatoms; peridinin in dinoflagellates	450–540 nm (blue-green)
Phycobilins (phycoerythrin, phycocyanin, allophycocyanin)	Red algae, cyanobacteria (organized in phycobilisomes)	Phycoerythrin: 495–570 nm (green-yellow); Phycocyanin: 610–620 nm (orange-red); Allophycocyanin: 650–655 nm (red)

📈ABSORPTION SPECTRA

The visible part of the spectrum of electro-magnetic radiation (that ranges from wavelength 390 nm to 760 nm) which is absorbed by the photosynthetic pigments of organisms, is called absorption spectra.

Figuer 1: Absorption spectrum of the photosynthetic pigments showing their respective range of absorption of different wavelengths of light

🍀CHLOROPHYLL PIGMENTS🍀

Chlorophyll a

Chemical Formula → C₅₅H₇₂O₅N₄Mg
Molecular Structure →
Porphyrin ring with magnesium (Mg²⁺) at the center
Long phytol tail (hydrophobic, anchors pigment in thylakoid membrane)
Contains a methyl group (-CH₃) at C3 position
Functions →
Primary photosynthetic pigment
Present in reaction centers P680 (PSII) and P700 (PSI)
Directly converts light energy into chemical energy

Chlorophyll b

Chemical Formula → C₅₅H₇₀O₆N₄Mg
Molecular Structure →
Similar porphyrin ring with Mg²⁺
Phytol tail
Contains an aldehyde group (-CHO) at C3 position instead of –CH₃
Functions →
Accessory pigment in higher plants, green algae
Broadens absorption spectrum by capturing blue light
Transfers captured energy to chlorophyll a

TABLE:2 Differences between Chlorophyll _a and Chlorophyll _b

Feature	Chlorophyll a	Chlorophyll b
Chemical formula	C₅₅H₇₂O₅N₄Mg	C₅₅H₇₀O₆N₄Mg
Side group at C3	Methyl (-CH₃)	Aldehyde (-CHO)
Reflecting Color	Blue-green	Yellow-green
Absorption peaks	430–435 nm (blue), 662–665 nm (red)	453–460 nm (blue), 642–650 nm (red)
Function	Primary pigment (reaction center)	Accessory pigment (light harvesting)
Occurrence	Universal in all oxygenic photosynthesizers	Only in green algae & higher plants

pictuer 2 — **Figure 2: Different type of Chlorophylls**.

Chlorophyll C

Chemical Formula → C₃₅H₂₈O₅N₄Mg
Molecular Structure →
Porphyrin ring with Mg²⁺
Lacks phytol tail (unlike chlorophyll a & b) → makes it more polar
Exists as c₁, c₂, c₃ variants
Functions →
Accessory pigment in brown algae, diatoms, dinoflagellates, chrysophytes
Broadens light absorption (blue-green region)
Transfers energy to chlorophyll a

Chlorophyll d

Chemical Formula → C₅₄H₇₀O₆N₄Mg
Molecular Structure →
Similar to chlorophyll a but with a formyl group (-CHO) at C3 position of ring A
Long phytol tail for membrane anchoring
Functions →
Found in some red algae and cyanobacteria (Acaryochloris marina)
Absorbs far-red light (~710 nm) → adaptation to low-light aquatic environments
Transfers energy to chlorophyll a in reaction centers

Chlorophyll f

Chemical Formula → C₅₅H₇₀O₆N₄Mg
Molecular Structure →
Similar porphyrin structure with Mg²⁺
Has a formyl group (-CHO) at C2 position of ring A
Long phytol tail present
Functions →
Discovered in cyanobacteria (2010, Australia)
Absorbs infrared light (720–740 nm) → allows photosynthesis beyond visible light
Helps organisms survive in deep-shaded or infrared-rich environments

🥕CAROTENOID PIGMENTS🥕

Carotenoids are natural pigments found in plants, algae, and some bacteria and fungi, responsible for the yellow, orange, and red colors of many fruits and vegetables.

Light Absorption & Color →
Absorb 400–550 nm (violet to green light)
Cause bright yellow, orange, red colors
Dominant in autumn leaves of 15–30% tree species
Structure →
Tetraterpenoids → 40 carbons (from 4 terpene units of 10C each)
Polyene hydrocarbon chain with 9–11 double bonds, may end in rings, may ± oxygen atoms
Conjugated double bonds → high reducing potential
Able to transfer electrons
Polyene hydrocarbon chain,
Usually lipophilic (long unsaturated chains)
Electron Transfer Mechanisms →
Singlet–Singlet transfer: carotenoid → chlorophyll (photosynthesis)
Triplet–Triplet transfer: chlorophyll → carotenoid (photoprotection, ROS control)
Types of Carotenoids →
Carotenes: unoxygenated hydrocarbons (α-carotene, β-carotene, lycopene)
Xanthophylls: oxygenated carotenoids (lutein, zeaxanthin)

🍂Carotenes
Unoxygenated (oxygen-free) carotenoids composed of only carbon and hydrogen
Empirical formula: C₄₀H₅₆
Colors: orange, red
Examples: α-carotene, β-carotene, lycopene
(β-carotene is the most common carotene. It’s both ends are cyclicised. It absorbs light mostly between 400-500 nm.
Lycopene is carotene found in tomato fruit. It has open ends).

pictuer 3 — **Figure 3: Structure of carotenoid pigments**

🍁Xanthophylls
Yellow-colored oxygenated carotenes
Empirical formula: C40H56Oₓ (x = 1–8 oxygen atoms)
Examples: Lutein → hydroxylated α-carotene, Zeaxanthin → hydroxylated β-carotene

🧠Mind Dumps (Quick Recall Tools)

⚡ Core Pigment Facts
Chl a = universal, primary pigment.
Chl b = accessory in plants/green algae.
Chl c, d, f = specialized adaptations.
Carotenoids = photoprotection + accessory absorption.
Phycobilins = aquatic advantage pigments.

🎯 Mnemonic for Chlorophyll Variants
“Always Be Clever During Finals” → a, b, c, d, f.

⚡ Absorption Peaks Quickie
Chl a → Blue 430 nm, Red 662 nm.
Chl b → Blue 453 nm, Red 642 nm.
Carotenoids → Blue-Violet 400–500 nm.
Phycobilins → Green–Red 495–655 nm.

🍁

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