Photosynthetic Pigments

Chloroplasts look green because of the photosynthetic pigments they contain — specialised molecules that absorb light energy at specific wavelengths. OCR specification module 5.2.1(e) and practical activity group 11 (use of chromatography) require you to know the main pigments, understand absorption and action spectra, and be able to use chromatography to separate pigments and calculate Rf values. This lesson links physical chemistry (absorption of photons) to biological function (exciting electrons in photosystems) and includes a required practical.

Key Definitions:

• Pigment — a coloured molecule that absorbs certain wavelengths of visible light and reflects or transmits others.
• Absorption spectrum — a graph showing the absorbance of different wavelengths of light by a pigment.
• Action spectrum — a graph showing the rate of photosynthesis at different wavelengths of light.
• Chromatography — a technique for separating mixtures of substances based on differences in solubility and affinity for a stationary phase.
• Rf value — the ratio of the distance travelled by a substance to the distance travelled by the solvent front.

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Why Photosynthesis Needs Pigments

A photon of light only becomes useful biologically when it is absorbed by a pigment molecule. When absorbed, the photon transfers its energy to an electron in the pigment, raising it to a higher energy level ("excitation"). This high-energy electron is then harvested by the photosystem and passed down the electron transport chain.

Without pigments, chloroplasts would be transparent to visible light and no photosynthesis could occur. Different pigments absorb different wavelengths — together they allow plants to capture a broad range of the visible spectrum.

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The Main Photosynthetic Pigments

OCR expects you to know four main pigments found in higher plants. They fall into two groups: primary (chlorophyll a) and accessory (chlorophyll b and carotenoids).

Pigment	Colour	Absorbs	Role	Primary/Accessory
Chlorophyll a	Blue-green	Red (~670 nm) and blue-violet (~430 nm)	Direct involvement in the light reaction at the reaction centre of PSI and PSII	Primary
Chlorophyll b	Yellow-green	Red (~650 nm) and blue (~475 nm)	Harvests light and passes energy to chlorophyll a	Accessory
Carotene (β-carotene)	Orange	Blue-violet (~450 nm)	Harvests light energy; protects against photo-oxidative damage	Accessory
Xanthophyll	Yellow	Blue-violet (~450 nm)	Harvests light; also involved in photoprotection	Accessory

Chlorophyll a is essential — without it no photosynthesis can occur. Accessory pigments broaden the range of wavelengths captured and funnel energy to chlorophyll a in the reaction centre. Together, hundreds of pigment molecules plus a central chlorophyll a pair form a photosystem (a light-harvesting antenna complex with a reaction centre).

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Absorption and Action Spectra

An absorption spectrum tells you how much light of each wavelength a pigment absorbs. An action spectrum tells you how much photosynthesis occurs at each wavelength.

flowchart LR
    W[White light] --> P[Pigments]
    P -->|Absorb blue & red| E[Excited electrons]
    P -->|Reflect green| G[Green colour we see]
    E --> PS[Photosystem reaction centre]
    PS --> ETC[Electron transport chain]

• Both spectra peak in the red (~670 nm) and blue-violet (~430 nm) regions.
• Both show a trough in the green (~550 nm) region — this is why leaves look green: green light is poorly absorbed and instead reflected.
• The action spectrum closely matches the combined absorption spectrum of all the pigments. This match is strong evidence that the pigments are responsible for photosynthesis.
• The action spectrum is slightly higher in the green region than the chlorophyll a absorption spectrum alone — because accessory pigments (carotenoids, chlorophyll b) absorb some wavelengths chlorophyll a cannot.

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Why Plants Look Green

Plants reflect and transmit the wavelengths of light they do not absorb efficiently. Since chlorophylls absorb strongly in red and blue but weakly in green, green light is reflected into your eyes — hence leaves appear green. This sometimes surprises students: the green you see is the light the plant has "wasted", not the light it is using.

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Required Practical: Chromatography of Leaf Pigments

OCR practical activity group 11 requires you to separate the pigments in a leaf extract by paper or thin-layer chromatography (TLC) and calculate Rf values.