Photosynthetic pigments

Pigments in photosynthesis

In plain words: a leaf is a team of light-catchers. Chlorophyll a is the captain, catching mostly blue and red light. The other pigments catch colours that chlorophyll a misses and pass the energy on to it, and they also shield it from damage by too much light.

Paper chromatography of leaf pigments

Leaves show many shades of green, even on the same plant. To see why, the leaf pigments can be separated by paper chromatography. NCERT describes four main pigment bands or groups, rather than four individual chemical species (page 137):

A note on the names: the carotenoids are a family with two classes, the carotenes and the xanthophylls (Science in School, 2019); NCERT itself lists “carotenoid pigments like carotene, xanthophylls and others” (Chapter 8, page 97). So the xanthophylls are themselves carotenoids, and the yellow-orange band that NCERT calls “carotenoids” is the carotene band. In an NCERT-based question, use NCERT’s four names.

How the separation works, beyond the NCERT text: a spot of leaf extract is placed on a pencil line near the bottom of a paper strip, and the lower end of the strip is dipped in a solvent, such as petroleum ether and acetone (9:1), with the spot kept above the solvent surface. As the solvent rises, each pigment is carried up at its own speed, so the pigments separate into bands. On a typical paper chromatogram the order from the top is carotene, xanthophyll, chlorophyll a and chlorophyll b (Mohanlal Sukhadia University, Udaipur, practical notes); on silica thin-layer plates the xanthophylls run below the chlorophylls (Science in School, 2019), so the order depends on the paper or plate and the solvent. These flammable solvents require a supervised laboratory set-up away from ignition sources.

Absorption spectrum and action spectrum

• An absorption spectrum shows how much light of each wavelength a pigment absorbs. NCERT Figure 11.3a gives the absorption spectra of chlorophyll a, chlorophyll b and the carotenoids (page 137).
• An action spectrum shows the rate of photosynthesis, measured by the release of O_₂, at each wavelength (NCERT Figure 11.3b, page 137).
• Chlorophyll a absorbs most in the blue and the red regions, and these are also the wavelengths with the higher rates of photosynthesis. That is why chlorophyll a is taken to be the chief pigment (page 137).
• But the two spectra do not overlap one-to-one (NCERT Figure 11.3c). Most photosynthesis takes place in the blue and red regions, yet some takes place at the other wavelengths of the visible spectrum too (page 138).

Beyond the NCERT text: in NCERT Figure 11.3c photosynthesis falls away steeply above about 680 nm, although chlorophyll a still absorbs some light there. Measured per unit of light absorbed (the quantum yield), photosynthesis drops sharply in this far-red light; this is called the red drop (Emerson and Lewis, American Journal of Botany, 1943). NCERT notes that cyclic photophosphorylation also occurs when only light beyond 680 nm is available (Section 11.6.2, page 140).

Chlorophyll a and accessory pigments

Chlorophyll is the major pigment responsible for trapping light. The other pigments of the thylakoids, chlorophyll b, xanthophylls and carotenoids, are called accessory pigments (NCERT page 138; for the thylakoids, see plastids). They:

• absorb light and transfer the energy to chlorophyll a;
• let the plant use a wider range of wavelengths of the incoming light for photosynthesis; and
• protect chlorophyll a from photo-oxidation (page 138).

In each photosystem the reaction centre is a single chlorophyll a molecule in NCERT’s account (beyond the NCERT text, a pair of chlorophyll a molecules, as the light reaction topic explains), and all the other pigments form the light-harvesting antenna. The reaction-centre chlorophyll a absorbs at 680 nm in PS II (P680) and at 700 nm in PS I (P700), a little beyond the red peak in NCERT Figure 11.3a (NCERT Section 11.5, page 138).

This is why the action spectrum is broader than the absorption spectrum of chlorophyll a alone: light absorbed by the accessory pigments, for example in the blue-green region where the carotenoids absorb strongly, still drives photosynthesis.

Why leaves look green, beyond the NCERT text: the chlorophylls absorb red and blue light strongly and green light much less (NCERT Figure 11.3a), so more of the green light is reflected, and green is the colour that reaches our eyes (Science in School, 2019).

Solved examples

Common mistakes to avoid

Quick revision summary

• Pigments absorb light at specific wavelengths; NCERT’s paper chromatogram describes four main labels: chlorophyll a (blue green), chlorophyll b (yellow green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange).
• Chlorophyll a absorbs most in the blue and red regions, where photosynthesis is also highest, so it is the chief pigment.
• The action spectrum and the chlorophyll a absorption spectrum do not overlap one-to-one: some photosynthesis happens at other wavelengths.
• Accessory pigments (chlorophyll b, xanthophylls, carotenoids) absorb light, pass the energy to chlorophyll a, widen the usable wavelengths and protect chlorophyll a from photo-oxidation.

Questions students ask

What is the difference between chlorophyll a and chlorophyll b?

Chlorophyll a is the chief pigment of photosynthesis, while chlorophyll b is an accessory pigment that absorbs light and transfers the energy to chlorophyll a (NCERT pages 137–138). In a paper chromatogram chlorophyll a is bright or blue green and chlorophyll b yellow green, and only chlorophyll a forms the reaction centres, P700 and P680 (NCERT pages 137–138). Chemically, beyond the NCERT text, they differ in one side group: chlorophyll b has a formyl group (–CHO) where chlorophyll a has a methyl group (–CH_₃) (Schliep and others, Plant, Cell & Environment, 2013), which makes chlorophyll b more polar, so it runs below chlorophyll a on a typical paper chromatogram.

What is the difference between carotenoids, carotenes and xanthophylls?

Carotenoids are a family of yellow, orange or red pigments with two classes: the carotenes, which are hydrocarbons, made of carbon and hydrogen only, and the xanthophylls, which are oxygenated derivatives (beyond the NCERT text; Petibon and Wiesenberg, Frontiers in Plant Science, 2022). So the xanthophylls are themselves carotenoids, and when NCERT’s leaf chromatogram lists xanthophylls (yellow) and carotenoids (yellow to yellow-orange) separately, its “carotenoids” band is the carotenes (page 137). Both kinds are accessory pigments: they absorb light that chlorophyll a absorbs poorly, pass the energy to it and protect it from photo-oxidation (NCERT page 138).

How do accessory pigments protect chlorophyll a from photo-oxidation?

Accessory pigments, chiefly the carotenoids, protect chlorophyll a from photo-oxidation by taking away excess light energy before it can do harm; NCERT states only that they “protect chlorophyll a from photo-oxidation” (page 138). Beyond the NCERT text, in strong light an excited chlorophyll that cannot pass its energy on can turn into a long-lived triplet state, which hands its energy to oxygen and forms singlet oxygen, a highly reactive form that destroys pigments and damages the thylakoid membranes. Carotenoids quench this triplet chlorophyll and also scavenge singlet oxygen, losing the energy harmlessly as heat (Young, Physiologia Plantarum, 1991).

Where are the photosynthetic pigments located in a chloroplast?

The photosynthetic pigments are located in the thylakoid membranes of the chloroplast: NCERT says chlorophyll pigments are present in the thylakoids (Chapter 8, page 98) and calls chlorophyll b, xanthophylls and carotenoids “thylakoid pigments” (page 138). There they are bound to proteins in two light harvesting complexes, within Photosystem I and Photosystem II (NCERT page 138; see light harvesting complexes). Cyanobacteria have no chloroplasts. NCERT Chapter 8 calls pigment-containing prokaryotic membranes chromatophores (page 91); modern cyanobacterial terminology usually calls their photosynthetic membranes thylakoids. Do not equate them with mesosomes, the historical structures discussed in the prokaryotic-cell lesson.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. What colour is chlorophyll b in a chromatogram of leaf pigments?

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   Yellow green (NCERT page 137).

2. Q2. What does an action spectrum of photosynthesis plot against wavelength?

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   The rate of photosynthesis, measured by O_₂ release (NCERT Figure 11.3b, page 137).

3. Q3. Name the accessory pigments.

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   Chlorophyll b, xanthophylls and carotenoids (NCERT page 138).