Light Absorption:- Accessory Pigments Extend the Range of Light Absorption
In addition to chlorophylls, thylakoid membranes contain secondary light-absorbing pigments, or accessory pigments, called carotenoids. Carotenoids may be yellow, red, or purple. The most important are -carotene, which is a red-orange isoprenoid, and the yellow carotenoid lutein (Fig. 19–40c, d). The carotenoid pigments absorb light at wavelengths not absorbed by the chlorophylls (Fig. 19–41) and thus are supplementary light receptors. Experimental determination of the effectiveness of light of different colors in promoting photosynthesis yields an action spectrum (Fig. 19–44), often useful in identifying the pigment primarily responsible for a biological effect of light. By capturing light in a region of the spectrum not used by other organisms, a photosyn thetic organism can claim a unique ecological niche. For example, the phycobilins in red algae and cyanobacteria absorb light in the range 520 to 630 nm (Fig. 19–41), allowing them to occupy niches where light of lower or higher wavelength has been filtered out by the pigments of other organisms living in the water above them, or by the water itself.
FIGURE 19–44 Two ways to determine the action spectrum for photosynthesis. (a) Results of a classic experiment performed by T. W. Englemann in 1882 to determine the wavelength of light that is most effective in supporting photosynthesis. Englemann placed cells of a filamentous photosynthetic alga on a microscope slide and illuminated them with light from a prism, so that one part of the filament received mainly blue light, another part yellow, another red. To determine which algal cells carried out photosynthesis most actively, Englemann also placed on the microscope slide bacteria known to migrate toward regions of high O2 concentration. After a period of illumination, the distribution of bacteria showed highest O2 levels (produced by photo synthesis) in the regions illuminated with violet and red light. (b) Results of a similar experiment that used modern techniques (an oxygen electrode) for the measurement of O2 production. An action spectrum (as shown here) describes the relative rate of photo synthesis for illumination with a constant number of photons of different wavelengths. An action spectrum is useful because, by comparison with absorption spectra (such as those in Fig. 19–41), it suggests which pigments can channel energy into photosynthesis.
