5.2.1—
Morphology in Situ

Mitochondria in living cells are highly pleomorphic, as shown by phase contrast microcinematography by Hongladarom et al., (1965). Pleomorphism is reflected

also in thin section electron microscopy, in which mitochondria appear as roughly circular profiles as well as highly elongated or irregular cross sections (Fig. 5.1a). The circular sections may represent transverse or oblique sections through an otherwise elongated organelle. The diameter of the elongated mitochondrion appears to be about 0.4 to 0.5 µm, while the length may be several micrometers long. Although rods or apparent spheres are the most common profiles seen, sections derived from branched or cup shaped organelles have also been discerned (Bagshaw et al., 1964). The recent analysis of serial sections of yeast cells by Hoffman and Avers (1973), which showed that yeast contains a single, giant, branched mitochondrion, suggests that the irregular cross sections of mitochondria of other cells might also be sections of a single branched organelle.

The mitochondrion consists of a double membrane system with an inner convoluted membrane enclosing the matrix, and surrounded by a smooth outer membrane (Fig. 5.1a, 5.1b). High resolution electron micrographs of material

Figure 5.1a
Mitochondria in phloem parenchyma cells of a maize leaf. Magnification bar = 1 m m.
(Micrograph courtesy of O. E. Bradfute and Diane C. Robertson.)

fixed with glutaraldehyde and post-fixed with osmium tetroxide show the tripartite nature of both the inner and outer membranes. Each membrane has a thickness of approximately 9 nm (Baker et al., 1968).

Figure 5.1b
Isolated mitochondria from mung bean hypocotyls. Mitochdria have been
suspended in 0.3 M  mannitol prior to fixation. Magnification × 26,000.
(Micrograph courtesy of W. D. Bonner, Jr.)