![]() All of these reactions are confined to the chloroplast. As the final step in this series of reactions, high-energy electrons are loaded (together with H +) onto NADP +, converting it to NADPH. During the electron-transport process, H + is pumped across the thylakoid membrane, and the resulting electrochemical proton gradient drives the synthesis of ATP in the stroma. The chlorophyll obtains its electrons from water (H 2O), producing O 2 as a by-product. In the photosynthetic electron-transfer reactions (also called the “light reactions”), energy derived from sunlight energizes an electron in the green organic pigment chlorophyll, enabling the electron to move along an electron-transport chain in the thylakoid membrane in much the same way that an electron moves along the respiratory chain in mitochondria. The many reactions that occur during photosynthesis in plants can be grouped into two broad categories: 1. Purine and pyrimidine synthesis, most amino acid synthesis, and all of the fatty acid synthesis of plants takes place in the plastids, whereas in animal cells these compounds are produced in the cytosol.Ĭhloroplasts Capture Energy from Sunlight and Use It to Fix Carbon Plants have also used their plastids to compartmentalize their intermediary metabolism. It is important to realize that plastids are not just sites for photosynthesis and the deposition of storage materials. In some plants, such as potatoes, the amyloplasts can grow to be as large as an average animal cell. A common form of leucoplast is the amyloplast ( Figure 14-33B), which accumulates the polysaccharide starch in storage tissues-a source of sugar for future use. They are little more than enlarged proplastids. Leucoplasts are plastids present in many epidermal and internal tissues that do not become green and photosynthetic. (B) Three amyloplasts (a form of leucoplast), or starch-storing (more.) Note the double membrane the inner membrane has also generated the relatively sparse internal membranes present. (A) A proplastid from a root tip cell of a bean plant. When exposed to light, the etioplasts rapidly develop into chloroplasts by converting this precursor to chlorophyll and by synthesizing new membrane pigments, photosynthetic enzymes, and components of the electron-transport chain. If a leaf is grown in darkness, its proplastids enlarge and develop into etioplasts, which have a semicrystalline array of internal membranes containing a yellow chlorophyll precursor instead of chlorophyll. ![]() Proplastids develop according to the requirements of each differentiated cell, and the type that is present is determined in large part by the nuclear genome. In addition, each is enclosed by an envelope composed of two concentric membranes.Īs discussed in Chapter 12 (see Figure 12-3), all plastids develop from proplastids, small organelles in the immature cells of plant meristems ( Figure 14-33A). Most notably, all plastids in a particular plant species contain multiple copies of the same relatively small genome. Plastids are present in all living plant cells, each cell type having its own characteristic complement. The Chloroplast Is One Member of the Plastid Family of OrganellesĬhloroplasts are the most prominent members of the plastid family of organelles. Nevertheless, the fundamental mechanisms involved in light-driven ATP synthesis in chloroplasts are very similar to those that we have already discussed for respiration-driven ATP synthesis in mitochondria. The many differences between chloroplasts and mitochondria are thought to reflect their different bacterial ancestors, as well as their subsequent evolutionary divergence. Mitochondria are also generally believed to be descended from an endocytosed bacterium. In plants, the products include a low-molecular-weight sugar (usually sucrose) that is exported to meet the metabolic needs of the many nonphotosynthetic cells of the organism.īiochemical and genetic evidence strongly suggest that chloroplasts are descendants of oxygen-producing photosynthetic bacteria that were endocytosed and lived in symbiosis with primitive eucaryotic cells. The immediate products of photosynthesis, NADPH and ATP, are used by the photosynthetic cells to produce many organic molecules. Chloroplasts perform photosynthesis during the daylight hours. ![]() In plants and algae, which developed much later, photosynthesis occurs in a specialized intracellular organelle-the chloroplast. As we see in this section, it is thought that the evolution of cyanobacteria from more primitive photosynthetic bacteria eventually made possible the development of abundant aerobic life forms. (CH 2O) n + nO 2], they also liberate into the atmosphere the oxygen required for oxidative phosphorylation. ![]()
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