Whereas in procaryotes the enzymes involved in adenosine triphosphate generation (see Chapter 6) are associated with the plasma membrane, in eucaryotes they are found in specialised organelles called mitochondria. These are generally rod-shaped and may be present in large numbers. They are enclosed by a double membrane, the inner surface of which is folded into finger-like projections called cristae. Respiratory enzymes are located on the increased surface area this provides, while other metabolic reactions take place in the semi-fluid matrix (Figure 3.15) (see also Chapter 6).
The mitochondrial cristae of algae, fungi and protozoans each have their own characteristic shapes. Until very recently, a few primitive protozoans, such as Giardia, appeared to lack mitochondria completely, and were thought to represent an intermediate stage in the evolution of the eucaryotic condition. Recent research, however, has shown them to possess highly reduced remnants of mitochondria, which have been given the name mitosomes. It seems that such organisms did, after all, once possess mitochondria, but have subsequently lost much of their function - an example of so-called reductive evolution.
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