Molecular Mechanisms of Photosynthesis. Robert E. Blankenship
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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.
2.6.1 Algae
Algae are a large group of eukaryotic organisms (Graham et al., 2008). Most of them are pigmented and carry out oxygenic photosynthesis. They are either unicellular, and therefore usually microscopic in size, or colonial, containing many cells. The colonial algae are macroscopic in size and can sometimes form huge structures that may look like plants but are quite distinct. There are many different groups of algae, which are usually distinguished by their pigment compositions and morphological features. In aquatic habitats, algae are the dominant photosynthetic life forms, although they are also found on land, including habitats as seemingly unlikely as the surface of snowfields and the hairs of polar bears. Many competing systems of algal classification are in use. We will not attempt to enumerate all the myriad types, but will instead just list some of the most commonly studied types in terms of their photosynthetic properties. In general, the algae all have rather similar electron transport chains, but widely variable antenna complexes from one group to another.
The green algae (chlorophytes) are the most widely studied, because their properties are the closest to higher plants. They contain both chlorophyll a and chlorophyll b as photopigments. They are certainly the evolutionary precursors to plants. The red algae (rhodophytes) are mostly marine organisms that contain chlorophyll a and phycobilisomes, antenna complexes similar to those found in most cyanobacteria. They often have a complex life cycle. The green and red algae, plus one other group (the glaucophytes), are thought to be primary endosymbionts, in that they arose from a single endosymbiotic event. All other algal groups are the result of additional endosymbiotic events, in which a eukaryotic alga was itself incorporated into an organism to form a new type of chimeric cell that in many cases retained the photosynthetic capability of the endosymbiont. Most of these secondary and in some cases tertiary endosymbiotic events involved the red algal line and the complex history of this group includes a dizzying array of gain, loss, and regain of photosynthesis. Many of these organisms contain chlorophyll c as an accessory pigment. The chromoalveolate hypothesis proposes that most of the non‐green eukaryotic algae have been derived from secondary endosymbiosis of red algae and subsequent events (Cavalier‐Smith, 1999).
Figure 2.7 Electron microscopic tomographic surface representation of the thylakoid network within a ruptured chloroplast. The different views are of the same thylakoid network from different angles.
Source: Daum and Kühlbrandt (2011). Reproduced with permission of Oxford University Press.
2.6.2 Plants
Plants