Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 35

Eukaryotic photosynthetic organisms all contain the subcellular organelle called the chloroplast. An overview diagram of the photosynthetic complexes in the chloroplasts of a variety of photosynthetic eukaryotes is shown in Fig. 2.2 (right panel). Chloroplasts are one of a larger group of organelles known as plastids, some of which carry out other functions, such as starch or pigment storage in flowers and fruits. As discussed above, a variety of evidence clearly shows that chloroplasts originated by a process called endosymbiosis, in which a cyanobacterial‐like cell was initially a symbiont with a protoeukaryotic cell and then eventually became a semiautonomous but essential part of the host cell (Margulis, 1993). The chloroplast contains DNA, which is organized and regulated in a manner typical of bacteria, not eukaryotes. This DNA encodes a number of chloroplast proteins that function in photosynthesis and chloroplast‐localized ribosomal protein synthesis machinery. After the initial endosymbiotic event, a significant degree of genetic transfer to the nucleus took place, so the chloroplast no longer contains enough information to be completely free of the nucleus. The majority of chloroplast proteins are therefore coded for by nuclear DNA, which is transcribed into RNA, the proteins synthesized on cytoplasmic ribosomes and then imported into the chloroplast. In addition, the plastid is the site of the early steps in lipid biosynthesis for the entire cell, so essential cellular components are also exported from the chloroplast. This division of labor requires a sophisticated control and regulation mechanism, which is discussed in more detail in Chapter 10. Mitochondria also originated by endosymbiosis, but in this case the symbiont was a proteobacterium instead of a cyanobacterium.

In addition to the primary endosymbiosis, which formed the first photosynthetic eukaryote, there is abundant evidence that there have been several secondary endosymbioses, in which a eukaryotic photosynthetic organism underwent a second endosymbiosis and in some cases even tertiary endosymbiosis (Keeling, 2013). Some of the classes of algae discussed below originated via this mechanism. The evolutionary relationships among all the types of photosynthetic organisms and the complex history of the various groups of eukaryotic photosynthetic organisms are discussed in more detail in Chapter 12.

An electron micrograph of a typical higher plant chloroplast is shown in Fig. 2.5. A schematic diagram of the chloroplast is shown in Fig. 2.6. The chloroplast has dimensions of a few microns, slightly larger than the size of a typical bacterium. It is surrounded by a chloroplast envelope, made up of a double membrane with two complete bilayers separated by an intermembrane space. The region inside the inner chloroplast envelope membrane is called the stroma. The stroma is like the cytoplasm of the chloroplast and contains numerous soluble enzymes, in particular the enzymes involved in carbon fixation. An extensive internal membrane system inside the chloroplast, called the thylakoid membrane, contains chlorophyll and the electron transport system that carries out the initial light energy capture and storage. In higher plants, these thylakoids are pressed together in multiple places to form collections of very densely packed membranes, called grana, which are in turn connected by other membranes that are not pressed together. These membranes are known as stroma lamellae. In cyanobacteria and many algae, the thylakoid membranes are not found together in densely stacked grana, but are instead associated in stacks of two or a few membranes. As we will learn in more detail in Chapter 7, the components of the photosynthetic apparatus in algae and plants are not uniformly distributed in the thylakoid membranes. Photosystem II is localized primarily in the grana membranes, whereas Photosystem I is found mostly in the stroma lamellae. The thylakoid membranes appear in many pictures to be arranged like a stack of coins. However, in reality, they are highly interconnected, and actually form one or a few interconnected membranes, as shown in Fig. 2.7 (Daum and Kühlbrandt, 2011). Like all biological membranes, the thylakoid is intrinsically asymmetric, with the components arranged with a particular vectorial orientation in the membrane. This results in an overall sidedness to the thylakoid membrane system. The side of the thylakoid that is toward the stroma is called the stromal side, whereas the enclosed space that is in contact with the opposite side of the thylakoid is called the lumen. This distinction between the two sides of the thylakoid membrane is a crucial point, as many of the functions of the chloroplast components rely on the presence of a membrane system that is osmotically intact and impermeable to ions.

Figure 2.5 Electron micrograph of chloroplast from tobacco.

Source: Courtesy of Kenneth Hoober.

Figure 2.6 Schematic diagram of a chloroplast, showing the inner and outer envelope membranes, the thylakoid membranes – which are divided into grana and stroma lamellae – and the nonmembraneous stroma, containing soluble enzymes.

Source: Taiz et al. (2018)/Oxford University Press.