The breakdown of sucrose to pyruvate releases less than 25% of the total energy in sucrose; the remaining energy is stored in the two molecules of pyruvate. The next two stages of respiration (the citric acid cycle and oxidative phosphorylation—i.e., electron transport coupled to ATP synthesis) take place within an organelle enclosed by a double membrane, the mitochondrion (plural mitochondria).
In electron micrographs, plant mitochondria—whether in situ or in vitro—usually look spherical or rodlike (Figure 11.5), ranging from 0.5 to 1.0 |m in diameter and up to 3 |m in length (Douce 1985). With some exceptions, plant cells have a substantially lower number of mitochondria than that found in a typical animal cell. The number of mitochondria per plant cell varies, and it is usually directly related to the metabolic activity of the tissue, reflecting the mitochondrial role in energy metabolism. Guard cells, for example, are unusually rich in mitochondria.
The ultrastructural features of plant mitochondria are similar to those of mitochondria in nonplant tissues (see Figure 11.5). Plant mitochondria have two membranes: a smooth outer membrane that completely surrounds a highly invaginated inner membrane. The invaginations of the inner membrane are known as cristae (singular crista). As a consequence of the greatly enlarged surface area, the inner membrane can contain more than 50% of the total mitochondrial protein. The aqueous phase contained within the inner membrane is referred to as the mitochon-drial matrix (plural matrices), and the region between the two mitochondrial membranes is known as the intermembrane space.
Intact mitochondria are osmotically active; that is, they take up water and swell when placed in a hypo-osmotic medium. Most inorganic ions and charged organic molecules are not able to diffuse freely into the matrix space. The inner membrane is the osmotic barrier; the outer membrane is permeable to solutes that have a molecular mass of less than approximately 10,000 Da (i.e., most cellular metabolites and ions, but not proteins). The lipid fraction of both membranes is primarily made up of phospholipids, 80% of which are either phosphatidylcholine or phos-phatidylethanolamine.
Like chloroplasts, mitochondria are semiautonomous organelles because they contain ribosomes, RNA, and
FIGURE 11.5 Structure of plant mitochondria. (A) Three-dimensional representation of a mitochondrion, showing the invaginations of the inner membrane that are called cristae, as well as the location of the matrix and intermembrane spaces (see also Figure 11.10). (B) Electron micrograph of mitochondria in a mesophyll cell of Vicia faba. (Photo from Gunning and Steer 1996.
DNA, which encodes a limited number of mitochondrial proteins. Plant mitochondria are thus able to carry out the various steps of protein synthesis and to transmit their genetic information. Mitochondria proliferate through the division by fission of preexisting mitochondria and not through de novo biogenesis of the organelle.
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