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2-44 Mitochondria and plastids are organelles of energy generation in eukaryotic cells

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  • Mitochondria are found in almost all eukaryotic cells and convert high-energy electrons into ATP.
  • Plastids are factories for photosynthesis, converting light energy into high-energy electrons and ATP.
  • Both of these structures trace their ancestry back to free-living prokaryotes.

Mitochondria are involved in energy generation through respiration. Mitochondria have no fixed shape, but often look like short rods in transmission electron micrographs when viewed along their long axis (Figure 2-73). Each mitochondrion contains two membranes. The outer membrane is smooth and serves as a selective barrier. The inner membrane is highly convoluted and folded and contains high numbers of membrane complexes. Nutrients are oxidized inside the mitochondria by catabolic enzymes and the high-energy electrons extracted are donated to a respiratory chain in the inner membrane. These enzymes then create a proton gradient and this gradient is then used to synthesize ATP. ATP leaves the mitochondria and it serves as a source of energy for the rest of the cell's machinery.

Figure 2-73 Mitochondria structure

Mitochondria structure

Mitochondria are rod-shaped structures that resemble the shape of common bacteria. They contain two membranes, similar to what is found in gram-negative bacteria, and 70S ribosomes. Energy generation occurs at the inner membrane.

Plastids are specialized organelles involved in metabolism that are unique to plants and come in several forms. Amyloplasts are starch storage containers found in some plants. Chloroplasts are oval-shaped structures inside of plant and algal cells that contain an outer and inner membrane as shown in Figure 2-74. The outer membrane serves a similar function to the outer membrane of mitochondria, while the inner membrane consists of a network of stacks of membranous disks, called thykaloids, which are attached together by narrow tubes of membrane. The thykaloid membranes are the centers of photosynthesis in eukaryotes. They contain enzyme complexes that capture light and produce ATP and high-energy electrons that are used to form sugar from carbon dioxide.

Figure 2-74 Chloroplast structure

Chloroplast structure

Chloroplasts are the site of the light reactions of photosynthesis. Light striking the chloroplast is converted into a proton motive force and this is used to generate energy. Chloroplasts contain two membranes, again similar to gram-negative bacteria, and the chloroplast itself is a relative of cyanobacteria.

Mitochondria and chloroplasts each have a single circular chromosome and large numbers of ribosomes that are of bacterial (70S) and not eukaryotic (80S) form. The presence of two lipid bilayers, a circular chromosome and 70S ribosomes is consistent with the evolutionary hypothesis that we have already explained near the beginning of this chapter.

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