NCERT Grounding
NCERT Class 11 Biology, Chapter 11, Section 11.4 ("How many types of pigments are involved in photosynthesis?") establishes the canonical four-pigment model through paper chromatography of leaf extracts. The text states explicitly: "A chromatographic separation of the leaf pigments shows that the colour we see in leaves is not due to a single pigment but due to four pigments: Chlorophyll a (bright or blue green), chlorophyll b (yellow green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange)."
"Though chlorophyll is the major pigment responsible for trapping light, other thylakoid pigments like chlorophyll b, xanthophylls and carotenoids… also absorb light and transfer the energy to chlorophyll a."
NCERT Biology Class 11, Section 11.4
Section 11.5 further specifies that each photosystem has all the pigments (except one molecule of chlorophyll a) forming the light harvesting or antenna system, while the single chlorophyll a molecule forms the reaction centre — the photochemically active core that initiates electron transport.
The Four Pigments — Structure and Function
All four pigments reside in the thylakoid membranes of the chloroplast. They are organized into two functional pools within each photosystem: the large antenna (light harvesting) complex and the single reaction-centre chlorophyll a. The four pigments differ in molecular structure, colour, absorption wavelengths, and role in photosynthesis.
Organizing principle: Chlorophyll a is the only pigment capable of directly donating an excited electron to the primary electron acceptor. Every other pigment transfers absorbed energy to chlorophyll a by resonance (Förster) transfer and is therefore called an accessory pigment.
Chlorophyll a
430 & 680 nm
Absorption peaks (blue-violet + red)
Colour: Blue-green (bright green in chromatogram)
Role: Primary photosynthetic pigment; forms reaction centre (P700 in PS I, P680 in PS II)
Key fact: The most abundant plant pigment on Earth; only pigment that directly participates in the light reaction
NEET Trap — reaction centre onlyChlorophyll b
450 & 640 nm
Absorption peaks (blue + orange-red)
Colour: Yellow-green
Role: Accessory pigment; broadens the spectral range of light harvesting; transfers energy to chlorophyll a
Key fact: Differs from chl a by –CHO group instead of –CH₃ at C-7; more polar than chl a
Accessory — not a reaction centreXanthophylls
~450 nm
Absorption peak (blue region)
Colour: Yellow
Role: Accessory pigment; photoprotective — quenches excess energy and scavenges singlet oxygen; energy transfer to chl a
Key fact: Oxygenated carotenoids (contain –OH groups); less non-polar than carotene → lower Rf
Carotenoid class — oxygenatedCarotenoids (β-carotene)
450–480 nm
Absorption peaks (blue region)
Colour: Yellow to yellow-orange
Role: Accessory + photoprotection; quenches triplet chlorophyll and singlet O₂ preventing photo-oxidation of chl a
Key fact: β-carotene is a precursor of Vitamin A; gives carrot its characteristic colour; most non-polar pigment → highest Rf
Highest Rf in chromatographyAbsorption Peaks — Comprehensive Table
| Pigment | Chromatogram Colour | Peak 1 | Peak 2 | Type | Rf Order |
|---|---|---|---|---|---|
| Chlorophyll a | Blue-green (bright) | ~430 nm (blue-violet) | ~680 nm (red) | Primary + reaction centre | 3rd (second-lowest) |
| Chlorophyll b | Yellow-green | ~450 nm (blue) | ~640 nm (orange-red) | Accessory | 4th (lowest — most polar) |
| Xanthophylls | Yellow | ~450 nm (blue) | — | Accessory + photoprotective | 2nd |
| Carotenoids (β-carotene) | Yellow-orange | 450–480 nm (blue) | — | Accessory + photoprotective | 1st (highest — most non-polar) |
Antenna Complex vs Reaction Centre
Each photosystem is architecturally divided into two components: the light harvesting complex (LHC), also called the antenna complex, and the reaction centre. This distinction is among the most frequently tested concepts from this section in NEET.
Antenna Complex (LHC)
~300
pigment molecules per photosystem
- Contains all four pigments: chl a, chl b, xanthophylls, carotenoids
- Functions by absorbing photons across a broad spectrum
- Transfers excitation energy to the reaction centre by resonance (Förster) transfer — no electrons are moved here
- Acts as a funnel: hundreds of molecules collect energy and pass it to one special chl a
- Increases photosynthetic efficiency in low-light environments
Reaction Centre
1
special chlorophyll a molecule
- Contains only one special chlorophyll a molecule (P700 in PS I; P680 in PS II)
- Undergoes actual photochemical reaction: excited electron is ejected to the primary electron acceptor
- In PS II: P680 absorbs red light at 680 nm; electron excitation initiates water splitting
- In PS I: P700 absorbs red light at 700 nm; excited electron ultimately reduces NADP⁺ to NADPH
- The photochemical step that converts light energy into chemical energy occurs here
NCERT (Section 11.5) states: "Each photosystem has all the pigments (except one molecule of chlorophyll a) forming a light harvesting system also called antennae. The single chlorophyll a molecule forms the reaction centre." The reaction centre chlorophyll a in PS I is called P700 because it has an absorption maximum at 700 nm; in PS II it is called P680 because its absorption maximum is at 680 nm.
Figure 1. Schematic of a photosystem. Hundreds of antenna pigment molecules (chlorophyll a, b, xanthophylls, carotenoids — shown as coloured circles) absorb photons and transfer excitation energy by resonance to the single reaction centre chlorophyll a (P700 in PS I; P680 in PS II — black circle). The reaction centre then ejects a high-energy electron to the primary electron acceptor, the only step that constitutes direct participation in the light reaction.
Paper Chromatography and Rf Values
Paper chromatography is the experimental technique used to separate the four leaf pigments. A concentrated leaf extract is spotted on chromatography paper and developed with a non-polar solvent (e.g., petroleum ether : acetone mixture). Pigments migrate at rates determined by their polarity relative to the stationary phase (paper, polar) and the mobile phase (solvent, non-polar).
The Rf (retardation factor) is defined as the ratio of the distance travelled by the pigment to the distance travelled by the solvent front. A more non-polar pigment interacts less with the polar paper and therefore travels farther, yielding a higher Rf value.
Chromatography band order (top to bottom — highest Rf to lowest)
-
Band 1
Carotene (β-carotene)
Yellow-orange. Highest Rf. Most non-polar pigment — no oxygen-containing groups. Travels farthest.
Rf ≈ 0.98 -
Band 2
Xanthophylls
Yellow. Second highest Rf. Oxygenated carotenoids (–OH groups) make them slightly more polar than carotene.
Rf ≈ 0.71 -
Band 3
Chlorophyll a
Blue-green. Third band. Less polar than carotene and xanthophylls but less polar than chlorophyll b.
Rf ≈ 0.65 -
Band 4
Chlorophyll b
Yellow-green. Lowest Rf. Most polar pigment due to additional –CHO group. Travels shortest distance.
Rf ≈ 0.45
Distinct pigment bands
A standard leaf chromatogram resolves into exactly four bands. NEET questions sometimes ask how many bands appear — the answer is always four, not two (as students sometimes confuse with the two pigment classes: chlorophylls and carotenoids).
Reaction centre pigment
Only one pigment — chlorophyll a — directly participates in the photochemical step. All others are accessory pigments, regardless of how efficiently they absorb light.
Why accessory pigments matter
The absorption spectrum of chlorophyll a shows strong peaks only in the blue (~430 nm) and red (~680 nm) regions. The green region (around 500–600 nm) is reflected — which is why leaves appear green. Without accessory pigments, this large portion of the solar spectrum would be wasted.
Chlorophyll b absorbs at 450 nm and 640 nm, filling part of the gap. Xanthophylls and carotenoids absorb in the 450–480 nm blue range. Together, the four pigments cover a much wider portion of the visible spectrum, improving the efficiency of light capture in natural conditions. This is the fundamental reason plants possess accessory pigments.
The second function of accessory pigments — particularly carotenoids — is photoprotection. Under high-intensity illumination, chlorophyll a can form reactive triplet states and generate singlet oxygen, which is damaging to the photosynthetic machinery. Carotenoids quench these reactive species, preventing photo-oxidation (photobleaching) of chlorophyll a. NCERT Section 11.4 states this explicitly: accessory pigments "not only enable a wider range of wavelength of incoming light to be utilised for photosynthesis but also protect chlorophyll a from photo-oxidation."
Worked Examples
In a paper chromatography experiment on a leaf extract, four bands are observed. Band A is at the top of the chromatogram and Band D is at the bottom. Which of the following correctly identifies the bands?
(1) A = Chlorophyll a, B = Chlorophyll b, C = Xanthophylls, D = Carotene
(2) A = Carotene, B = Xanthophylls, C = Chlorophyll a, D = Chlorophyll b
(3) A = Xanthophylls, B = Carotene, C = Chlorophyll b, D = Chlorophyll a
(4) A = Carotene, B = Chlorophyll a, C = Xanthophylls, D = Chlorophyll b
Answer: (2). The band at the top of the chromatogram has the highest Rf value, meaning it travelled the farthest. Carotene is the most non-polar pigment and has the highest Rf. The order from top to bottom (highest to lowest Rf) is: Carotene → Xanthophylls → Chlorophyll a → Chlorophyll b. Chlorophyll b is the most polar pigment and stays closest to the origin. Options (1), (3), and (4) incorrectly place the pigments.
Suppose a plant lacked chlorophyll a but had high concentrations of chlorophyll b and carotenoids. Would it be capable of carrying out photosynthesis? Justify your answer with reference to the antenna complex and reaction centre.
Answer: No. Chlorophyll b and carotenoids are accessory pigments. They absorb light and transfer the energy by resonance to the reaction centre chlorophyll a, but they cannot themselves act as the reaction centre. The photochemical step — ejection of a high-energy electron to the primary electron acceptor — can only be performed by the special chlorophyll a molecule (P700 or P680) at the reaction centre. Without chlorophyll a, no electron transport chain can be initiated, ATP and NADPH cannot be produced, and carbon fixation cannot occur. The plant would be unable to carry out photosynthesis despite having functional antenna pigments. This is an NCERT Exercise Question 5 (Section 11.5).
Match the pigment with its correct absorption peak and chromatogram colour:
P. Chlorophyll a — (i) 450 nm + 640 nm, yellow-green
Q. Chlorophyll b — (ii) 430 nm + 680 nm, blue-green
R. Carotene — (iii) 450–480 nm, yellow-orange
S. Xanthophyll — (iv) ~450 nm, yellow
Select the correct match.
Answer: P–(ii), Q–(i), R–(iii), S–(iv). Chlorophyll a absorbs at 430 nm and 680 nm and appears blue-green. Chlorophyll b absorbs at 450 nm and 640 nm and appears yellow-green (it has a longer-wavelength second peak than chlorophyll a's red peak). Carotene absorbs in the 450–480 nm range and is yellow-orange — it gives carrot its colour. Xanthophylls are oxygenated carotenoids absorbing near 450 nm and appear yellow.
Common Confusion & NEET Traps
Chlorophyll a
680 nm
Second absorption peak (red)
- Blue-green colour; bright green in chromatogram
- Absorption peaks at ~430 nm and ~680 nm
- Forms the reaction centre: P680 (PS II) and P700 (PS I)
- Primary photosynthetic pigment — directly participates in light reaction
- Less polar → higher Rf (third from bottom)
- Has –CH₃ group at C-7 of the porphyrin ring
Chlorophyll b
640 nm
Second absorption peak (orange-red)
- Yellow-green colour
- Absorption peaks at ~450 nm and ~640 nm
- Does NOT form the reaction centre — purely accessory
- Transfers absorbed energy to chlorophyll a; never ejects an electron directly
- More polar → lowest Rf (travels least in chromatogram)
- Has –CHO group at C-7 (replacing –CH₃ of chl a) — increases polarity