Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 19

The essence of photosynthetic energy storage is the transfer of an electron from an excited chlorophyll‐type pigment to an acceptor molecule in a pigment–protein complex called the reaction center. The initial, or primary, electron transfer event is followed by separation of the positive and negative charges by a very rapid series of secondary chemical reactions. This basic principle applies to all photosynthetic reaction centers, although the details of the process vary significantly from one system to the next.

Figure 1.5 Schematic diagram of the noncyclic electron transfer pathway found in oxygenic photosynthetic organisms. The upper diagram (a) is an energetic picture of the electron transport pathway, incorporating the major reactions of photosynthesis into what is called the Z‐scheme of photosynthesis. The lower diagram (b) is a spatial picture, showing the major protein complexes whose energetics are shown in the Z‐scheme, and how they are arranged in the photosynthetic membrane. Neither view alone gives a complete picture, but together they summarize much information about photosynthetic energy storage.

Source: (a) Hohmann‐Marriott and Blankenship (2011) (p.532)/Annual Reviews. Reproduced with permission of Annual Reviews. (b) Courtesy of Dr. Jonathan Nield.

In some organisms, one light‐driven electron transfer and stabilization is sufficient to complete a cyclic electron transfer chain. This is shown schematically in Fig. 1.4b, in which the vertical arrow represents energy input to the system triggered by photon absorption, and the curved arrows represent spontaneous, or downhill, electron transfer processes that follow, eventually returning the electron to the primary electron donor. This cyclic electron transfer process is not in itself productive unless some of the energy of the photon can be stored. This takes place by the coupling of proton movement across the membrane with the electron transfer, so that the net result is a light‐driven difference of pH and electrical potential, or electrochemical potential gradient across the two sides of the membrane. This electrochemical potential gradient, called a protonmotive force, is used to drive the synthesis of ATP.

The more familiar oxygen‐evolving photosynthetic organisms have a different pattern of electron transfer. They have two photochemical reaction center complexes that work together in a noncyclic electron transfer chain, as shown in Fig. 1.5. The two reaction center complexes are known as Photosystems I and II. Electrons are removed from water by Photosystem II, oxidizing it to molecular oxygen, which is released as a waste product. The electrons extracted from water are transported via a quinone and the cytochrome b₆ f complex to Photosystem I and, after a second light‐driven electron transfer step, eventually reduce an intermediate electron acceptor, NADP⁺ to form NADPH.

Protons are also transported across the membrane and into the thylakoid lumen during the process of the noncyclic electron transfer, creating a protonmotive force. The energy in this protonmotive force is used to make ATP (see Chapter 8).

Reaction centers and electron transfer processes in anoxygenic bacteria are discussed in more detail in Chapter 6, while these processes in oxygenic photosynthetic organisms are discussed in more detail in Chapter 7.