Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 17

For light energy to be stored by photosynthesis, it must first be absorbed by one of the pigments associated with the photosynthetic apparatus. Photon absorption creates an excited state that eventually leads to charge separation in the reaction center. Not every pigment carries out photochemistry; the vast majority function as antennas, collecting light and then delivering energy to the reaction center where the photochemistry takes place. The antenna system is conceptually similar to a satellite dish, collecting energy and concentrating it in a receiver, where the signal is converted into a different form (Fig. 1.3). Energy transfer in antenna systems is discussed in more detail in Chapter 5.

The antenna system does not do any chemistry; it works by an energy transfer process that involves the migration of electronic excited states from one molecule to another. This is a purely physical process, which depends on a weak energetic coupling of the antenna pigments. In almost all cases, the pigments are bound to proteins in highly specific associations. In addition to chlorophylls, common antenna pigments include carotenoids and open‐chain tetrapyrrole bilin pigments found in phycobilisome antenna complexes.

Figure 1.3 (a) Schematic diagram illustrating the concept of antennas in photosynthetic organisms. Light is absorbed by a pigment, which can be either a type of chlorophyll or an accessory pigment. Energy transfer takes place in which the excited state migrates throughout the antenna system and is eventually delivered to the reaction center where electron transfer takes place, creating an oxidized electron donor and a reduced electron acceptor. (b) Radio telescope as an analogy to the photosynthetic antenna. The dish serves as the antenna, collecting energy and delivering it to the receiver, which transduces it into a signal.

Source: NASA.

Antenna systems often incorporate an energetic and spatial funneling mechanism, in which pigments that are on the periphery of the complex absorb at shorter wavelengths and therefore higher excitation energies than those at the core. As energy transfer takes place, the excitation energy moves from higher‐ to lower‐energy pigments, at the same time heading toward the reaction center.

Antenna systems greatly increase the amount of energy that can be absorbed compared with a single pigment. Under most conditions, this is an advantage, because sunlight is a relatively dilute energy source. Under some conditions, however, especially if the organism is subject to some other form of stress, more light energy can be absorbed than can be used productively by the system. If unchecked, this can lead to severe damage in short order. Even under normal conditions, the system is rapidly inactivated if some sort of photoprotection mechanism is not present. Antenna systems (as well as reaction centers) therefore have extensive and multifunctional regulation, protection, and repair mechanisms.

The number of antenna pigments associated with each reaction center complex varies widely, from a minimum of a few tens of pigments in some organisms to a maximum of several thousand pigments in other types of organisms. The pigment number and type largely reflect the photic environment that the organism lives in. Smaller antennas are found in organisms that live in high intensity conditions, while the large antennas are found in environments where light intensity is low.