Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Photosynthesis
Thylakoid membranes contain two types of photochemical reaction systems

Light-absorbing pigments of the thylakoid membranes are organized into functional sets, or ensembles, as illustrated in Fig. 23-8, B. In spinach METABOLISM/14.html">Chloroplasts, these ensembles, termed Photosystems, comprise about 200 chlorophyll molecules and approximately 50 carotenoid molecules. Such ensembles are capable of absorbing light across the entire visible spectrum; however, absorption is particularly intense in two regions: 400 to 500 nm and 600 to 700 nm (Fig. 23-10). All pigment molecules within a given photosystem absorb photons, but only a single molecule in each ensemble is capable of converting light energy into chemical energy. This specialized energy-transducing pigment molecule, consisting of a chlorophyll molecule bound to a specific protein, is known as the photochemical reaction center. All other pigment molecules in such a photosystem, or ensemble, are referred to as light-harvesting or antenna molecules. Their function is to absorb light energy and rapidly transfer it to individual reaction centers, where the photochemical act takes place (Fig. 23-11).

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Fig. 23-10. Absorption spectrum and photochemical action spectrum of a green leaf. The absorption spectrum illustrates the fraction of absorbed light energy as a function of wavelength, whereas the photochemical action spectrum demonstrates the dependence of relative photosynthetic efficiency on wavelength. Generally speaking, visible light of any wavelength can stimulate Photosynthesis, yet peak photosynthetic efficiency is achieved at wavelengths of 400-500 and 600-700 nm. For comparison, the absorption spectrum of pure chlorophyll a is shown, which exhibits relatively weak Absorption in the 500-600 nm range. Certain photosynthetic Cells contain accessory pigments that absorb intensely in this region, thereby complementing the absorption profiles of the chlorophylls.

The thylakoid membranes of higher plant chloroplasts contain two distinct types of photosystems, each possessing its own Complement of light-harvesting chlorophyll and carotenoid molecules and its own photochemical reaction center. Photosystem I, which is maximally activated by the longer-wavelength portion of the spectrum, is characterized by a high ratio of chlorophyll a to chlorophyll b. Photosystem II, activated maximally by light at wavelengths shorter than 680 nm, contains a relatively higher proportion of chlorophyll b and occasionally chlorophyll c. The thylakoid membranes of a single spinach chloroplast harbor many hundreds of both types of photosystems. As we will discuss later, these Two Photosystems fulfill different Functions. Nevertheless, one important generalization can be made immediately: all oxygen-evolving photosynthetic cells—that is, those of Higher Plants and cyanobacteria—contain both Photosystems I and II, whereas non-oxygen-evolving photosynthetic Bacteria possess only Photosystem I.

Fig. 23-11 Schematic representation of the photosystem surface within the thylakoid membrane. It resembles a mosaic composed of several hundred antenna chlorophyll and carotenoid molecules oriented in a specific manner within the membrane. An exciton absorbed by one of the antenna molecules migrates rapidly via the pigment molecules toward the P700 reaction center, with its pathway indicated by red arrows. While all antenna molecules are capable of absorbing light, only the molecule acting as the reaction center can transform the exciton energy into a flow of electrons.