Photosynthetic Pigments: Carotenoids, Absorption Spectrum and Action Spectrum

Alongside the chlorophylls, every green leaf also contains a second broad class of pigments called carotenoids, which are yellow to orange in colour and are usually masked in an intact leaf by the much more abundant, strongly green chlorophylls -- becoming visible to the eye only in autumn or on ripening, once chlorophyll breaks down faster than the carotenoids do. Carotenoids are conventionally divided into two closely related groups: carotene, a pure orange hydrocarbon pigment with the empirical formula C₄₀H₅₆, and xanthophyll, a yellow, oxygen-containing derivative of carotene with the empirical formula C₄₀H₅₆O₂.

Functionally, carotenoids serve two distinct roles in the photosynthetic apparatus. First, as accessory (or secondary) pigments, they absorb light at wavelengths that chlorophyll a and b absorb only weakly, and then transfer this captured energy, by a resonance process, to chlorophyll a at the reaction centre -- effectively widening the overall window of usable light and increasing the efficiency of light capture across the visible spectrum. Second, carotenoids play an important photoprotective role: under conditions of very high light intensity, they can absorb and safely dissipate excess light energy that would otherwise generate damaging reactive oxygen species within the chlorophyll molecules, protecting the photosynthetic membranes from photo-oxidative injury.

The relationship between a pigment's ability to absorb light and its ability to actually drive photosynthesis is captured by two related, but conceptually distinct, kinds of graph. The absorption spectrum of a pigment is obtained by extracting the pigment in a solvent and measuring, at each individual wavelength of visible light, how strongly that isolated pigment absorbs the incoming light. For chlorophyll a and b together, this produces a characteristic curve with two prominent peaks -- one in the blue-violet region (roughly 430-450 nm) and a second in the red region (roughly 640-680 nm) -- separated by a deep trough in the green region (roughly 500-560 nm). This trough is the direct chemical explanation for the everyday observation that healthy leaves look green: green wavelengths are largely reflected or transmitted rather than absorbed, while the blue and red wavelengths that are strongly absorbed are the ones actually put to photosynthetic use.

The action spectrum, by contrast, is obtained not from an isolated pigment extract but from a living, functioning leaf or cell, and it plots the actual measured rate of photosynthesis (commonly assessed as the rate of O2 evolution or CO2 uptake) produced when the organism is illuminated separately with each individual wavelength of light. This is essentially the type of measurement Engelmann achieved, ingeniously, using the oxygen-seeking behaviour of aerobic bacteria clustering around his illuminated alga rather than any chemical instrument, as described in the section on early experiments. …